Guidelines for Planning and Construction of
Roads in Cyclone Prone Areas
CRRI Report – July 2013
Sponsored by
National Dis...
DISCLAIMER
All the data and technical information furnished in this report are based on the literature review and
discussi...
FOREWORD
India has long coastline of about 7500 km including its island territories. Thriving cities and ports
have been b...
Draft Preparation Team at CSIR-CRRI
Dr.S.Gangopadhyay
Director, CSIR-CRRI
Shri Sudhir Mathur
Chief Scientist & Advisor
Shr...
CONTENTS
Page No
Chapter – 1 Introduction 1
Chapter – 2 Destructions Caused by Cyclones 4
Chapter – 3 Planning of Road Net...
LIST OF FIGURES
Figure No. Title Page No
1.1 Wind and Cyclone Zones in India (Ref: NDMA) 2
1.2 Cyclone Hazard and PMSS Map...
LIST OF TABLES
Table
No.
Title Page
No
1.1 Classification of Cyclones 2
2.1 Storm Intensity and Expected Damages 5
4.1 Pro...
LIST OF ABBREVIATIONS
AOS Apparent Opening Size of Geotextile (Also known as O95)
BIS Bureau of Indian Standards
CBP Concr...
1
Chapter – 1
INTRODUCTION
A tropical cyclone is a storm system characterised by a large low pressure centre and numerous
...
2
Table – 1.1 Classification of Cyclones
Type of Disturbances Associated Wind Speed in the Circulation
Low Pressure Area L...
3
Fig – 1.2 Cyclone Hazard and PMSS Map
(Ref: BMPTC)
4
Chapter – 2
DESTRUCTIONS CAUSED BY CYCLONES
There are three elements associated with a cyclone, which cause destruction....
5
2.2 Strong Winds/ Squall
Cyclones are known to cause severe damage to infrastructure through high speed winds and gusts....
6
floods. Rainwater on the top of the storm surge may add to the fury of the storm. Rain is the serious
problem for the pe...
7
low elevation are likely to fail as a result of excessive longitudinal or transverse movement of bridge
deck. Under the ...
8
Chapter – 3
PLANNING OF ROAD NETWORK IN CYCLONE PRONE AREAS
As in any other part of the country, in a similar manner, th...
9
should be preferably Two-Lane or atleast they should be of intermediate lane width. While planning
the alignment of the ...
10
h) Road top level/ alignment are to be decided after taking into account high flood levels and flooding
pattern. While ...
11
CHAPTER – 4
CONSTRUCTION OF ROAD EMBANKMENTS
Successful performance of an embankment depends as much on adopting standa...
12
(b) After clearing the site, limits of the embankment are to be marked by fixing batter pegs and
marking toe lines on b...
13
Fig 4.1 – Rill Erosion in Road Embankment
Fig 4.2 – Deep Cut in Road Embankment Due to Erosion
Fig 4.3 – Severe Erosion...
14
4.4.1 Slope protection by simple vegetative turfing
Vegetation is ideal for erosion control because it is relatively in...
15
infertile slopes. The approximate thickness of mulch cover should be about 2.5 cm. The organic mulch
covering the soil ...
16
Table – 4.1 Property Requirements for 3-D Mat
3-D Mat Property Specified value* Test Method
Minimum Tensile Strength 2 ...
17
Band drains are generally installed by displacement methods. The mandrels used with band drains are
hollow and normally...
18
minimise settlement and prevent deep seated shear failure. However stone column technique would be
comparatively costli...
19
Chapter – 5
SEA EROSION CONTROL TECHNIQUES & RIVER BANK PROTECTION
Coastal beach erosion occurs in various forms around...
20
armour layer. The function of inner filter layer is to prevent soil particles from being eroded and to allow
free escap...
21
Fig – 5.1 Provision of Sea Wall and Concrete Tetrapods for Sea Erosion Protection at Mumbai
Fig – 5.2 Protection Agains...
22
Fig – 5.4 Beach Protection Using Boulder Revetment at Paradip, Odisha
Fig – 5.5 Another View of Boulder Revetment at Pa...
23
5.2.4 Soft Structural/ Non-structural measures – Artificial nourishment of beaches
Beach nourishment may be adopted for...
24
failure. Selection of geotextile can be carried out as per IRC SP:59. A typical rip-rap gradation for
coastal revetment...
25
perform, however for long term applications, the performance requirements of the geotextile container
are more severe. ...
26
applications where the geotextile tube in its filled condition is of prime importance. When the geotextile
tube has bee...
27
Fig 5.9 – Applications of Geotextile Tubes
Fig 5.10 – Typical Use of Multiple Geotextile Tubes for Coastal Embankment C...
28
5.4.2 Geotextile containers
Geotextile containers, as their name may imply, are large volume containers that are filled...
29
fill liquefaction and loss of shape of the geotextile bags. To ensure that the contained fill is maintained
in its dens...
30
satisfactory and cost effective solution. Gabions can be made from either polymeric material or double
twisted steel wi...
31
important. The selection of filter fabric with correct opening size depends on the percentage of finer
material availab...
32
Offshore breakwaters also may be constructed to protect beaches or coastlines from erosion by wave
activity. In such ca...
33
expanse, substratum and salinity of soil as well as water. Out of 45 mangrove species occurring in
India, some are true...
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
Guidelines for Planning and Construction of Roads in cyclone Prone Areas
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National Disaster Management Authority approached CSIR- CRRI to prepare the ‘Guidelines for Planning and Construction of Roads in Cyclone Prone Areas’.

This task was jointly undertaken by a team from Geotechnical Engg Division and Bridges and Structures Division of CSIR-CRRI.

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Guidelines for Planning and Construction of Roads in cyclone Prone Areas

  1. 1. Guidelines for Planning and Construction of Roads in Cyclone Prone Areas CRRI Report – July 2013 Sponsored by National Disaster Management Authority New Delhi Geotechnical Engineering Division Central Road Research Institute is an ISO 9001 Institution
  2. 2. DISCLAIMER All the data and technical information furnished in this report are based on the literature review and discussions held with expert members/field engineers and site visits undertaken by CSIR-Central Road Research Institute (CSIR-CRRI) team. The responsibility of CSIR-CRRI is limited to the technical and scientific matters contained in this report. All the procedural/ legal/ operational matters would be responsibility of implementing agencies who would be using this report.
  3. 3. FOREWORD India has long coastline of about 7500 km including its island territories. Thriving cities and ports have been built on our coasts. Road network is very vital for providing connectivity to these population centres. However, road infrastructure in coastal region faces constant threat due to tropical cyclones. With the technological advancement, many new products and techniques are now available for civil engineers to provide protection to road infrastructure against cyclone impact. Keeping in view these issues, National Disaster Management Authority (NDMA) approached CSIR- CRRI to prepare the ‘Guidelines for Planning and Construction of Roads in Cyclone Prone Areas’. This task was jointly undertaken by a team from Geotechnical Engg Division and Bridges and Structures Division of CSIR-CRRI. The project team is grateful to NDMA for giving us an opportunity to work on this task. Special thanks are due to Prof Prem Krishna, Prof D.K.Paul and Dr.S.Arunachalam who reviewed the draft many times and provided valuable suggestions and comments. Acknowledgements are due to Hon’ble members of ‘Disaster Management Committee’ and ‘Earthwork, Embankment and Ground Improvement Committee’ of Indian Roads Congress, New Delhi and also to Prof.M.R.Madhav, Member, Research Council, CSIR-CRRI for many useful comments/suggestions received from them. The draft report was presented in three workshops held at Visakhapatnam, Bhubaneswar and at New Delhi and received suggestions/ comments from various officers and engineers of state disaster management authorities and Public Works Departments. These reviews/comments/ suggestions immensely helped in improving the draft. CSIR-CRRI Team expresses special thanks to all of them. (Dr.S.Gangopadhyay) Director, CSIR- CRRI
  4. 4. Draft Preparation Team at CSIR-CRRI Dr.S.Gangopadhyay Director, CSIR-CRRI Shri Sudhir Mathur Chief Scientist & Advisor Shri U.K.Guru Vittal Head, Geotechnical Engg Division (Project Leader) Dr. Lakshmy Parameswaran Chief Scientist Dr. Rajeev Garg Head, Bridges and Structures Division (Technical Assistance: Shri J.Ganesh and Dr.Pankaj Gupta) Expert Committee for Review Dr.Prem Krishna, FNAE Honorary Visiting Professor, Department of Civil Engineering Indian Institute of Technology, Roorkee – 247667 (Uttarakhand) Dr.D.K.Paul Dean of Faculty Affairs & Professor, Department of Earth Quake Engg & Head, Centre for Excellence in Disaster Mitigation and Management Indian Institute of Technology, Roorkee – 247667 (Uttrakhand) Dr.S.Arunachalam Formerly Advisor (M), SERC, Chennai Director, Wind Engineering Application Centre Jaypee University of Engineering & Technology A.B. Road, P.B. No. 1, Raghogarh, Dist: Guna (M.P.) - 473226
  5. 5. CONTENTS Page No Chapter – 1 Introduction 1 Chapter – 2 Destructions Caused by Cyclones 4 Chapter – 3 Planning of Road Network in Cyclone Prone Areas 8 Chapter – 4 Construction of Road Embankments 11 Chapter – 5 Sea Erosion Control Techniques & River Bank Protection 19 Chapter – 6 Road Pavements in Cyclone Prone Areas 37 Chapter – 7 Mitigation Measures for Culverts and Bridges 42 Chapter – 8 Road Traffic Operations During Evacuation 55 References 59 Annexure – I (Technical Specifications for Geotextile Tubes) 61
  6. 6. LIST OF FIGURES Figure No. Title Page No 1.1 Wind and Cyclone Zones in India (Ref: NDMA) 2 1.2 Cyclone Hazard and PMSS Map (Ref: BMPTC) 3 2.1 Effect of Cyclone ‘Aila’ on Embankment 7 2.2 Another View of Eroded Embankment due to Cyclone ‘Aila’ in West Bengal 7 4.1 Rill Erosion in Road Embankment 13 4.2 Deep Cut in Road Embankment Due to Erosion 13 4.3 Severe Erosion of Road Embankment 13 4.4 A Type of Polymeric Vertical Drain (Band Drain) 17 5.1 Provision of Sea Wall and Concrete Tetrapods for Sea Erosion Protection at Mumbai 21 5.2 Protection Against Sea Erosion by Retaining Wall and Gabions – Puri Konark Road 21 5.3 Another View of Protection Works – Puri Konark Road 21 5.4 Beach Protection Using Boulder Revetment at Paradip, Odisha 22 5.5 Another View of Boulder Revetment at Paradip, Odisha 22 5.6 Masonry Sea Wall Constructed at Sea Coast at Koteshwar, Kori Creek, Gujarat 22 5.7 Typical Components of a Geotextile Tube 25 5.8 Geotextile Tube Application for Coastal Protection 26 5.9 Applications of Geotextile Tubes 27 5.10 Typical Use of Multiple Geotextile Tubes for Coastal Embankment Construction 27 5.11 Geotextile Tubes with Gabions as Armour Protection layer 27 5.12 View of geotextile Tubes Covered with Armour Protection Layer of Gabions 28 5.13 Shore Reclamation Using Geotextile Bags 29 5.14 Geotextile Bag 29 5.15 Use of Gabions for River Bank Protection 30 6.1 Construction of Roller Concrete Pavement for Rural Roads 38 6.2 Problem of Sand Dunes Encroaching Road Pavement 41 6.3 Close up View of Black Top Pavement Abraded by Sand 41 7.1 Cable Restrainer 49 7.2 Cable Restrainer Installed in Longitudinal Direction 49 7.3 Examples of Connecting the Beam Ends of Adjacent Spans 49 7.4 Connection of Deck to the Substructure 51 7.5 Cable restrainer between superstructure and substructure 51 7.6 Typical Details of a Restrainer 52 7.7 Tying the Restrainer from the Girders Around the Pier 53 7.8 Reaction Block/Stopper 54 7.9 Seat Extension to Accommodate Large Longitudinal Displacements 54
  7. 7. LIST OF TABLES Table No. Title Page No 1.1 Classification of Cyclones 2 2.1 Storm Intensity and Expected Damages 5 4.1 Property Requirements for 3-D Mat 16 5.1 Basic Engineering Parameters for Geotextile Tubes filled with Sand 26 5.2 Geotextile Hydraulic Property Requirements under Different Regimes 31 7.1 Hourly Mean Wind Speed and Pressure at 10m Level for Cyclone Resistant Design of Bridges Situated Within 60 km off the Coast 44 7.2 Transverse Wind Forces Due to Cyclone Acting on Unit Exposed Frontal Area of Bridge Deck at 10m Level (Plain Terrain) 45 7.3 Qualitative Damage State Descriptions for Typical Cyclone Induced Bridge Damage (FEMA, 2003) 47
  8. 8. LIST OF ABBREVIATIONS AOS Apparent Opening Size of Geotextile (Also known as O95) BIS Bureau of Indian Standards CBP Concrete Block Pavement CDO Central Dense Overcast (Area immediately surrounding eye region of cyclone) CRZ Coastal Regulation Zone DPR Detailed Project Report ICBP Interlocking Concrete Block Pavement JGT Jute Geotextile MDD Maximum Dry Density MDR Major District Road MORD Ministry of Rural Development, Government of India MoRTH Ministry of Road Transport and Highways, Government of India NH National Highways NRRDA National Rural Roads Development Agency ODR Other District Roads OH Organic Soil having High Liquid Limit OI Organic Soil having Medium Liquid Limit OL Organic Soil having Low Liquid Limit OMC Optimum Moisture Content PCMS Portable Changeable Message Signs PMSS Probable Maximum Storm Surge Pt Peat PVD Polymeric Vertical Drain/ Prefabricated Vertical Drain RCCP Roller Compacted Concrete Pavement RECP Rolled Erosion Control Product SH State Highways TRM Turf Reinforcement Mats VR Village Roads WMO World Meteorological Organisation WPS Wireless Priority Service
  9. 9. 1 Chapter – 1 INTRODUCTION A tropical cyclone is a storm system characterised by a large low pressure centre and numerous thunderstorms that produce strong winds and flooding rain. Tropical cyclones feed on heat released when moist air rises, resulting in condensation of water vapour contained in the moist air. The term ‘tropical’ refers to both the geographic origin of these systems, which form almost exclusively in tropical regions of the globe, and their formation in maritime tropical air masses. The term ‘cyclone’ refers to such storms’ cyclonic nature, with counter clockwise rotation in Northern Hemisphere and clockwise rotation in the Southern Hemisphere. Depending on its location and strength, a tropical cyclone is called by many other names, such as hurricane, typhoon, tropical storm, cyclonic storm, tropical depression and simply cyclone. While tropical cyclones can produce extremely powerful winds and torrential rain, they are also able to produce high waves and damaging storm surges. They develop over large bodies of warm water, and lose their strength if they move over land. This is the reason for coastal regions receiving a significant damage from a tropical cyclone, while inland regions are relatively safe from their effect. Heavy rains, however, can produce significant flooding inland, and storm surges can produce extensive coastal flooding up to 40 kilometres from the coastline. Although their effects on human populations can be devastating, tropical cyclones can also relieve drought conditions. They also carry heat and energy away from the tropics and transport it toward temperate latitudes, which make them an important part of the global atmospheric circulation mechanism. As a result, tropical cyclones help to maintain equilibrium in the earth’s troposphere, and to maintain a relatively stable and warm temperature worldwide. A strong tropical cyclone usually harbours an area of sinking air at the centre of circulation. This area is called ‘eye of the cyclone’. Weather in the eye is normally calm and free of clouds, although sea may be extremely violent. The eye is normally circular in shape, and may vary in size from 3 km to 370 km in diameter. Surrounding the eye is the region called ‘Central Dense Overcast (CDO)’, a concentrated area of strong thunderstorm activity. Curved bands of clouds and thunderstorms trail away from the eye in a spiral fashion. These bands are capable of producing heavy bursts of rain and wind, as well as tornadoes. If one were to travel between the outer edge of a hurricane to its centre, one would normally progress from light rain and wind, to dry and weak breeze, then back to increasingly heavier rainfall and stronger wind, over and over again with each period of rainfall and wind being more intense and lasting longer. 1.1 Classification of Tropical Cyclones Tropical cyclones with an organised system of clouds and thunderstorms with a defined circulation, and maximum sustained winds of 61 kmph or less are called ‘tropical depressions’. Once the tropical cyclone reaches wind speed of more than 61 kmph, they are typically called a ‘tropical storm’ and assigned a name. When maximum sustained winds reach a speed of 119 kmph, such a cyclone is called a ‘severe cyclonic storm’. The criteria followed by the Meteorological Department of India to classify the low pressure systems in the Bay of Bengal and in the Arabian Sea as adopted by the World Meteorological Organisation (WMO) are given in Table 1.1. Cyclones affect both Bay of Bengal and the Arabian Sea. The areas affected by cyclone in India are shown in Fig 1.1 and 1.2. 1.2 Scope of These Guidelines These guidelines cover various aspects related to planning and construction of road infrastructure in cyclone prone areas, mainly dealing about preparedness in the eventuality of a cyclone disaster.
  10. 10. 2 Table – 1.1 Classification of Cyclones Type of Disturbances Associated Wind Speed in the Circulation Low Pressure Area Less than 17 knots (< 31 kmph) Depression 17 to 27 knots (31 to 49 kmph) Deep Depression 28 to 33 knots (50 to 61 kmph) Cyclonic Storm 34 to 47 knots (62 to 88 kmph) Severe Cyclonic Storm 48 to 63 knots (89 to 118 kmph) Very Severe Cyclonic Storm 64 to 119 knots (119 to 221 kmph) Super Cyclonic Storm 120 knots and above (222 kmph and above) Source: India Meteorological Department Fig – 1.1 Wind and Cyclone Zones in India (Ref: NDMA)
  11. 11. 3 Fig – 1.2 Cyclone Hazard and PMSS Map (Ref: BMPTC)
  12. 12. 4 Chapter – 2 DESTRUCTIONS CAUSED BY CYCLONES There are three elements associated with a cyclone, which cause destruction. These have been described below: 2.1 Storm Surge Cyclones are associated with high-pressure gradients and consequent strong winds. These, in turn, lead to storm surges. A storm surge can be defined as an abnormal rise of sea level near the coast caused by a severe tropical cyclone; as a result of which, sea water inundates low lying areas of coastal regions drowning human beings and live-stock, eroding beaches and embankments, destroying vegetation and reducing soil fertility. Storm surge is the single major cause of devastation from tropical storms. Storm surge is formed due to pushing of sea water towards shore by the force of the winds swirling around the storm. In addition, wind driven waves are superimposed on the storm tide. This advancing surge may happen to combine with the high tides to create the hurricane storm tide, which can increase the average water level to 4.5 m or more. The level of surge in a particular area is also determined by the slope of the continental shelf. Storm surge is inversely proportional to the depth of sea water. A shallow slope off the coast will allow a greater surge to inundate coastal communities. Communities with a steeper continental shelf will not see as much surge inundation, although large breaking waves can still present major problems. Vulnerability to storm surges is not uniform along Indian coasts. The following segments of Indian coast are most vulnerable to high surges: a) North Odisha and West Bengal Coasts b) Andhra Pradesh coast between Ongole and Machilipatnam c) Tamilnadu Coast, south of Nagapattinam The west coast of India is less vulnerable to storm surges than the east coast of India in terms of height of storm surge as well as frequency of occurrence. However, the following segments of western coast are vulnerable to significant surges: a) Maharashtra coast, north of Harnai and adjoining south Gujarat coast and the coastal belt around the Gulf of Mumbai b) The coastal belt around the Gulf of Kutch The world’s highest recorded storm surge was about 12.5m (about 41 ft) and it was associated with the Backergunj cyclone in 1876 near the Meghna estuary in present-day Bangladesh. The Probable Maximum Storm Surge (PMSS) is the highest along the West Bengal coast where it ranges from 9 m to 12.5 m. It reduces to about 3.8 m in Khurda district, Orissa, increasing again to about 8.2 m along the south Andhra Pradesh coast in Krishna, Guntur and Prakasam districts. A small region in south Tamil Nadu around Nagapattinam coast also has higher PMSS of about 8.4 m. Along the west coast; the PMSS varies from about 2 m near Thiruvananthapuram to around 5 m near the Gulf of Khambat in the Saurashtra region of Gujarat. Expected storm surge height in metres along India’s coastline is shown in Fig 1.2.
  13. 13. 5 2.2 Strong Winds/ Squall Cyclones are known to cause severe damage to infrastructure through high speed winds and gusts. Very strong winds which accompany a cyclonic storm, damage installations, dwellings, communication systems, trees, etc., resulting in loss of life and property. A tropical cyclone damages and destroys structures in two ways. First, many homes are damaged or destroyed when the high speed wind simply lifts the roof of the dwellings. High speed wind moving over the top of the roof creates lower pressure on the exposed side of the roof relative to the attic side. The higher pressure in the attic lifts the roof. Once lifted, the roof acts as a sail and is blown clear of the dwelling. With the roof gone, the walls are much easier to be blown down by the hurricane wind. The second way that wind destroys buildings can also be a result of the roof becoming airborne. The wind picks up the debris (i.e. wood, metal siding, toys, trash cans, tree branches, etc.) and sends them hurling at high speeds into other structures. Based on observations made during damage investigations, researchers have concluded that much of the damage in windstorms is caused by flying debris. Brief details about damages caused by winds of different speed are given in Table 2.1 (Ref: Saffir-Simpson Hurricane Scale – Management of Cyclones: NDMA Guidelines – http://nidm.gov.in/PDF/guidelines/cyclones.pdf) Table – 2.1 Storm Intensity and Expected Damages Scale No (Category) Sustained winds (kmph) Damage Storm Surge in m 1 119 – 153 Minimal: Unanchored mobile homes, vegetation and signs 1.2 to 1.5 2 154 – 177 Moderate: All mobile homes, roofs, small craft and flooding 1.6 to 2.4 3 178 – 209 Extensive: Small buildings, low lying roads cut-off 2.5 to 3.6 4 210 – 249 Extreme: Roofs destroyed, trees down, roads cut-off, mobile homes destroyed, beach homes flooded 3.7 to 5.5 5 250 or more Catastrophic: Most buildings destroyed, vegetation destroyed, major toads cut-off, homes flooded More than 5.5 The vertical wind shear in a tropical cyclone environment is also important. Wind shear is defined as the amount of change in the wind velocity direction or speed with increasing altitude. The damages produced by winds are extensive and cover areas occasionally greater than the areas of heavy rains and storm surges which are in general localised in nature. The impact of the passage of the cyclone eye, directly over a place is quite different from that of a cyclone that does not hit the place directly. The latter affects the location with relatively unidirectional winds i.e. winds blowing from only one side, and the lee side is somewhat protected. An eye passage brings with it rapid changes in wind direction, which imposes torques and can twist the vegetation or even structures. Part of structures that were loosened or weakened by the winds from one direction are subsequently severely damaged or blown down when hit upon by the strong winds from the opposite direction. A partial eye passage can also do considerable damage, but damage would be less than a total eye passage. 2.3 Torrential Rains and Inland Flooding Torrential rainfall (more than 30 cm/hour) associated with cyclones is another major cause of damage. It also creates problems in post cyclone relief operation. Unabated rain gives rise to unprecedented
  14. 14. 6 floods. Rainwater on the top of the storm surge may add to the fury of the storm. Rain is the serious problem for the people who become shelterless due to a cyclone. Heavy rainfall resulting from a cyclone would be usually spread over a wide area. As a result, soil erosion also occurs on a large scale. Heavy rains inundate the low-lying ground and cause softening of the soil due to soaking. This contributes to weakening of the embankments, leaning of utility poles or even collapse of pole type structure. Heavy and prolonged rains due to cyclones cause river floods and submergence of low lying areas. River floods occur when the runoff from torrential rains, brought on by landfall of cyclones reach the rivers. Even after the wind has diminished, the flooding potential of cyclonic storms remains for several days. Most of the fatalities due to flooding occur because people underestimate the power of moving water and purposely walk or drive into flooding conditions. It is common to think that stronger the storm the greater the potential for flooding. However, this is not always the case. A weak, slow moving tropical storm can cause more damage due to flooding than a more powerful fast moving hurricane. In addition to the storm surge, tropical cyclones usually cause flash flooding. Flash floods are rapidly occurring events. This type of flood can begin within a few minutes or hours of excessive rainfall. The rapidly rising water can reach heights of 10 m or more and can roll boulders, rip trees from the ground, and destroy buildings and bridges. Urban area floods are also rapid events although not quite as severe as a flash flood. Still, streets can become swift-moving rivers and basements can become death traps as they fill with water. The primary cause is due to the conversion of fields or woodlands to roads and paved parking lots. It may be mentioned that all the three factors mentioned above occur simultaneously and, the rescue and relief operations for distress mitigation become difficult. So, it is imperative that advance action is to be initiated for relief measures before commencement of adverse weather conditions due to cyclones. 2.4 Effect of Cyclones on Road Infrastructure From the above discussion, it becomes apparent that cyclonic storms affect human habitations and infrastructure in multiple ways. Providing road connectivity in cyclone prone areas emerges as a vital tool for undertaking rescue and rehabilitation operations. It is also obvious that road infrastructure created in cyclone prone areas need to be designed and constructed to withstand the onslaught of cyclonic storms. The first step would be to identify the road stretches which are vulnerable to effect of cyclonic storm. Principle mode of destruction of road embankments and pavements due to cyclonic storms would be through erosion caused due to storm surge and flooding. The storm water causes damage to the road pavement surface, washes away portions of road at many locations, even breaching portions of embankment. Because of these problems, such road stretches in cyclonic areas become bottle neck and hamper relief operations. Embankments and road pavements are not much susceptible to damage due to winds. However wind forces affect design of bridge structures. Wind forces also affect certain road furniture like sign boards, electric and telephone poles and trees planted along roadside causing disruptions to traffic flow by uprooting of trees, falling branches of trees, electric/ telephone poles falling in roadway, etc. Keeping these points in view, identified vulnerable road stretches in cyclone prone areas would have to be designed/ constructed to ensure that damage due to storm surge/flooding/winds are minimised or totally alleviated. Impact of a cyclone on road infrastructure may lead to (a) Damage to the roads due to storm surge / flooding (b) Hindrance to traffic movement due to deposition of debris left on roadway (c) Unseating / drifting of bridge superstructure due to storm surge / flooding (d) damages to the bridges due to debris impact. Impact damages can occur due to barge impact, boats, oil rigs, uprooted trees, boulders etc. The impact damage is manifested in the form of span misalignment, damage to fascia girder, fender, pier or pile damage. During cyclone occurrence, bridges may fail due to unseating of individual span, depending on the connection between the bridge deck and pier. The bridge decks with
  15. 15. 7 low elevation are likely to fail as a result of excessive longitudinal or transverse movement of bridge deck. Under the storm surge, the bridge decks are subjected to buoyant forces and pounding action of waves. Bearings also suffer damages due to the unseating/drifting of bridge deck. In some bridges, shifting of span due to lateral wave and wind forces often causes damages to the abutments, pier caps, or girders. Damage to parapets on bridge decks, scouring of bridge foundations, erosion of abutment, etc are seen after cyclone disaster. Further details regarding damages caused to bridges due to cyclone are given in Chapter 7. While designing road structure in cyclone prone areas, the above mentioned factors are to be considered and suitable remedial measures described in subsequent chapters are to be provided. Further it is to be noted that usually a package of remedial or protection works are usually fashioned to suit individual site conditions. CHAPTER – 3 Fig 2.1 – Effect of Cyclone ‘Aila’ on Embankment Fig 2.2 – Another View of Eroded Embankment due to Cyclone ‘Aila’ in West Bengal
  16. 16. 8 Chapter – 3 PLANNING OF ROAD NETWORK IN CYCLONE PRONE AREAS As in any other part of the country, in a similar manner, the hierarchy of road infrastructure in cyclone prone area would comprise of various categories like National Highways (NH), State Highways (SH), District Roads and Rural Roads. National and State Highways are the arterial roads connecting major cities, ports, state capitals, industrial centres, etc. These roads should provide uninterrupted road communication throughout their length. District roads are intended to act as important roads within a district serving areas of production and markets and connecting with each other or with the main highways of a district. They are further sub-divided into two categories – Major District Roads (MDR) and Other District Roads (ODR). Village Roads (VR) provide connectivity between villages or habitations and District Roads. The term ‘Rural Roads’ is used to denote both ODR and VR. All these types of roads are very important in the overall road network of cyclone affected areas. Effective road connectivity ensures fast deployment of men, materials and machinery to cyclone affected areas and also ensures speedy evacuation of people from vulnerable places to safer areas in the face of an impending disaster threat. Hence the need for development of a reliable road network in the vulnerable areas is very vital to ensure coordination of relief and response in the event of a cyclone. Designation of arterial roads like National Highways and State Highways are based on traffic volume and importance of cities/ towns/ ports to be connected by them. Planning a rural road network in a district is carried out in our country based on guidelines provided by Ministry of Rural Development (MORD) and the Indian Roads Congress (IRC). As per the MORD Guidelines, all-weather road access is to be provided to all villages/habitations of population greater than 500 people. The Operations Manual of MORD states that an all weather road is defined as one which is negotiable during all weathers, with some permitted interruptions. Essentially this means that at cross-drainage structures, the duration of overflow or interruption at one stretch shall not exceed 12 hours for ODRs and 24 hours for VRs in hilly terrain, and 3 days in the case of roads in plain terrain. The total period of interruption during the year should not exceed 10 days for ODRs and 15 days for VRs. As per MORD Guidelines, population criteria for providing all weather connectivity has been kept equal to 250 in case of hill States (North-Eastern states, Sikkim, Himachal Pradesh, Jammu & Kashmir and Uttarakhand), desert areas and tribal areas. The most important issue with the road construction is the alignment of the road. Many issues of drainage, inundation and breaching of road embankment can be tackled at planning stage by choosing the best possible alignment. But in many projects it may not be possible to change the alignment of existing track due to problems like land acquisition, forest area, etc. These are important issues but such issues need to be taken care of by respective state Governments. Moreover, road projects involve huge investment. Hence, it is crucial to give adequate attention at the planning and design stage itself so as to achieve better and economical alignment. For planning road network in cyclone prone areas, following additional points need to be considered: a) For planning higher category of roads like NH, SH and MDRs, ‘20 Year Road Development Plan’ and ‘Vision 2020 – Road Development Document’ published by IRC/ MoRTH are to be considered. While such arterial roads are necessary to connect main cities and towns, considerations of traffic to be catered and trade/economic importance of cities being connected are also equally essential. Additionally in case of cyclone prone areas, arterial roads required for evacuation in the event of cyclone occurrence need to be identified. Upgradation of such arterial evacuation routes to NH/SH/MDR category depending upon their importance and population being catered by such routes needs to be considered. Width of the important arterial evacuation routes (SH or MDR)
  17. 17. 9 should be preferably Two-Lane or atleast they should be of intermediate lane width. While planning the alignment of the rural link roads, it is imperative to connect existing and proposed cyclone shelters in addition to providing connectivity to habitation/ village. b) Rural link roads in cyclone prone areas are very crucial for evacuation and rescuing of people. Similar to hilly areas and tribal regions, in areas prone for severe cyclone impact (coastal belt of 25 km from the sea), population criteria for providing all weather connectivity can be kept equal to 250 for a habitation to be connected by an all weather road. All weather roads, as already pointed out, may experience interruptions to the traffic due to submergence of a bridge for periods extending from 24 hours to 72 hours. However, roads built in cyclone prone areas need to be designed to reduce the duration of traffic interruption due to flooding. This duration of disruption for roads identified for evacuation (belonging to ODR and VR category) should be preferably not more than 3 hours even after highest flooding expected in that region. c) The geometric design standards for rural roads are to be followed as per IRC SP – 20, ‘Rural Roads Manual. Roads are always associated with culverts and bridges as the terrain demands, to make them fit throughout the year. Geometric design of NH and SH are to be carried out as per IRC: 73. While selecting the bridge site, factors like (i) permanency of the channel, (ii) presence of high and stable banks (iii) narrowness of the channel and average depth compared to maximum depth, straight reach of the stream, freedom from islands in both upstream side and downstream side, possibility of right angled crossings, good approaches, etc., are to be given adequate attention so as to keep them functional in the event of any disaster. d) Concerned state agencies who are in charge of the road project should prepare beforehand ‘Hazard Zonation Maps’ of suitable scale, showing extent of cyclone/ flood hazard expected in that region. These maps should indicate vulnerable roads and bridges and risk assessment is to be carried out. In case of failure of bridge/road pavement during a cyclone event, alternate routes should be identified for evacuation/ rescue and relief on these maps. Missing links/ additional infrastructure needs should also be marked on these maps so that they can be attended to during planning process. These state agencies may take assistance from State Disaster Management Authorities and local bodies for preparation and validation of such maps. e) Construction of roads is to be taken up in such a manner that roads are atleast 500 m away from seashores / coastal regulation zones (CRZ). Intensive protective works to prevent erosion towards seashore side of the road should be planned. f) The most important consideration for construction of road in the cyclone prone area would be its alignment avoiding inundation of the road under cyclonic rain. Adequate cross drainage works should be provided to prevent such occurrence. Therefore, a survey along the most probable route is needed to ascertain the highest flood level that had occurred during its past history. The free board allowance for different categories of roads is indicated in section 5.6. Provision of minimum free board as per section 5.6 will ensure connectivity even after the cyclonic storm. g) Rigid pavements are preferable over flexible pavements as there is no appreciable variation in temperature to cause significant thermal stress and resulting distress. Cement concrete pavements withstand flooding/ waterlogging in a better manner than bituminous pavements. Techno-economics of adopting cement concrete pavements vis-a-vis flexible pavement needs to be undertaken before making a final choice.
  18. 18. 10 h) Road top level/ alignment are to be decided after taking into account high flood levels and flooding pattern. While fixing the road top levels, special care will have to be taken to cater for rapid changes in underground water table and consequent movement of the soil moisture. This can be achieved by designing and constructing an efficient drainage system. Keeping the road levels above the high flood levels and highest water table need to be ensured. For provision of storm water drainage for roads in urban areas, IRC SP: 50, ‘Guidelines on Urban Drainage’, can be referred to. i) Preparation of a Detailed Project Report (DPR) for each of the proposed road is a pre requisite for proper evaluation of the project and it ensures timely completion and avoids time and cost over runs.
  19. 19. 11 CHAPTER – 4 CONSTRUCTION OF ROAD EMBANKMENTS Successful performance of an embankment depends as much on adopting standards of good compaction in construction as on careful pre investigations leading to selection of appropriate borrow material and design features of the embankment. Soil is the primary construction material for embankment and also for road subgrade. So soil and construction material survey forms the basic step for preparation of DPR for any road project. While carrying out soil survey along proposed road alignment, representative samples should be collected wherever there is a visible change in soil type. In case the same type of soil continues, at least three representative samples from each kilometre length of road alignment should be collected for laboratory testing. 4.1 Material Specifications for Embankment and Subgrade The material used in embankments, subgrades, earthen shoulders and miscellaneous backfills shall be soil, moorum, gravel, a mixture of these or any other suitable material approved by the engineer. Such material shall be free from organic materials like logs, stumps, roots, rubbish or any other ingredient likely to deteriorate or affect the stability of the embankment/subgrade. The following material shall be considered unsuitable for embankment:  Materials from swamps, marshes and bogs, peat, log, stump and perishable material, any soil classified as OL, OI, OH or Pt in accordance with IS: 1498  The fill soil to be used should have liquid limit less than 70 and plasticity index less than 45  Materials having salts which may result in leaching in the embankment Expansive clay exhibiting marked swell and shrinkage properties (‘free swell index’ exceeding 50 when tested as per IS: 2720–Part 40) shall not be used as fill material. Where expansive clay with free swell index less than 50 is used as a fill material, subgrade and top 500 mm portion of the embankment below subgrade shall be non-expansive in nature. The soil to be used as embankment fill or subgrade should also meet maximum dry density and other requirements as specified in MoRTH Specifications (in case of NH/SH works) or MORD Specifications (in case of rural roads). 4.2 Design of Embankments The cyclone impact occurs in the form of erosion of road embankments. Apart from preventing erosion, the designer has to ensure stability of road embankments. For details regarding design of road embankment IRC: 75 can be referred to. Failure of embankments may be due to either inadequate bearing capacity or due to deep seated shear failure. The objective of the stability analysis is to ensure that embankment does not face any risk of shear failure. Generally in the slip circle method failure plane is assumed to be circular. A particular circle gives the minimum factor of safety. Calculation of factor of safety of different circles until the critical circle is located is a very time consuming process. Available software may provide quick solutions. 4.3 Important Considerations for Embankment Construction (a) Fill material should conform to MoRTH/ MORD Specifications depending on road classification. Borrow pit excavation should be located at a distance atleast 5 m away from the toe of embankment. Top soil should not be used as fill material. Top soil should be spread back on the excavated land or used for covering the side slopes of the embankment.
  20. 20. 12 (b) After clearing the site, limits of the embankment are to be marked by fixing batter pegs and marking toe lines on both sides at regular intervals as guides. Where ever feasible, stagnant water, if any, from the roadway (embankment foundation area) should be removed. (c) After removing the topsoil / unsuitable material, the ground surface should be loosened upto a minimum depth of 150 mm by ploughing or scarifying and compacted to the specified density. For embankment construction over ground not capable of supporting equipment, successive loads of embankment fill material should be spread in a uniformly distributed layer of adequate thickness to support equipment and to construct the lower portion of the embankment. In case of soft sub-soil areas (marine clay sub-soil), ground improvement measures may be necessary to prevent failure of embankment. Expert advice should be obtained in such cases and specified foundation treatment should be carried out in a manner and to the depth as specified. Brief details of ground improvement techniques are given in section 4.5. (d) The soil should be spread over the entire width of the embankment in layers not exceeding required loose layer thickness. The moisture content of the fill material spread for compaction should be within ±2 per cent of the optimum moisture content of the soil. Clayey soils should be compacted at moisture content slightly higher than OMC (upto 2 per cent above OMC). (e) Each layer of fill material should be compacted using rollers to meet the specified compaction requirements. Adequate quality control and field tests as per MoRTH/ MoRD specifications are needed to ensure this. (f) The top 50 cm of the embankment (in case of NH and SH) or 30 cm (in case of rural roads) which forms the subgrade should be built to specification requirements of the subgrade. For further details/ specifications, reference may please be made to ‘MoRTH Specifications for Road and Bridge Works’ or ‘MoRD Specifications for Rural Roads’ and ‘IRC 36 – Recommended Practice for Construction of Earth Embankment and Subgrade for Road Works’. 4.4 Embankment Slope Protection against Soil Erosion Road embankments experience a high degree of damage due to erosion from torrential rains which accompany cyclones and hence erosion protection of embankment slopes should receive special attention in such areas. Soil erosion is the process of detachment and transportation of soil particles by wind or water. Cohesionless soil particles may get blown away by wind (Aeolian) erosion. However erosion due to surface run-off would be the principal cause for failure of road embankments in the aftermath of a cyclone disaster. The kinetic energy of falling raindrops causes detachment of soil particles which are subsequently carried away by surface run-off. Nature of soil and impact of rain drops are determinant factors in the erosion process. Silty and sandy types of soils are more susceptible to erosion than clayey soils. Distress in the form of rills to gullies and finally to erosion ditches develop when intensity of rainfall is high and the slope is steep. These problems will impair slope stability if not controlled with proper protective measures. The surface protection of embankment against action of rain and wind is usually achieved by promoting vegetation growth. When embankments are constructed using non-cohesive material, cover of 0.3 to 0.6 m thick cohesive material can be given. In case of high embankments, a system of kerb channel and median drains coupled with chutes should be provided to drain off the rain water from the road embankments. Different engineering measures which may be adopted for erosion protection of roads built in cyclone prone areas are briefly described below. For more details, IRC: 56, ‘Recommended Practices for Treatment of Embankment and Roadside Slopes for Erosion Control’ can be referred to.
  21. 21. 13 Fig 4.1 – Rill Erosion in Road Embankment Fig 4.2 – Deep Cut in Road Embankment Due to Erosion Fig 4.3 – Severe Erosion of Road Embankment
  22. 22. 14 4.4.1 Slope protection by simple vegetative turfing Vegetation is ideal for erosion control because it is relatively inexpensive to establish and maintain and it presents aesthetically appealing look. Vegetation on the embankment side slopes provides adequate canopy interception to the falling rain drops and saves the soil from splash erosion, while the mass of litter and Rhizomes act as speed breakers for running water on the slope. Mechanical function of plant is to reinforce the soil by binding the loose soil particles with its fibrous root system. However, planting of tree species which grow considerably big/tall should not be permitted alongside the road in cyclone prone areas. During cyclones, such trees may get uprooted/ braches may snap which may cause obstruction to movement of traffic and may even lead to accidents. Moreover, roots of big trees may tend to loosen the structure of the embankment when shaken by wind storm which would cause cracks in the embankment. Shrubs, thorny bushes and short grass growing on the slope of embankments provide good protection against erosion and such vegetation should be promoted. Tree plantation should be carried out in areas beyond road land (Right of way) width. Generally the side slopes and unpaved shoulders in the top portion of the embankment should be turfed with grass sods and this turfing should extend beyond the toe on the country-side and the river side by 6.0 meters and 3.0 meters respectively. This is as per existing practices of some cyclone prone states. Simple vegetative turfing method should be adopted where the soil has enough nutrients and the environmental conditions are conducive to promote vegetation growth. The density of sowing is of great importance. In general, while sowing a mixture of grass and legume plants, seed rate would be normally 15 gm/m2. Prior to sowing, the soil surface should be adequately prepared. On highly erodible slopes where seeding or sprigging is liable to be washed down before they have had time to take root. In such circumstances, it is advisable to go for special techniques such as the ones recommended in the succeeding paragraphs. 4.4.2 Transplantation of readymade turfs of grass ‘Sodding’ technique which involves bodily transplantation of blocks of turfs of grass (with 5-8 cm of soil covering the grass roots) from the original site to the barren slopes to be treated can be adopted in locations where ensuring grass growth would require considerable time. The sod to be used for transplantation should consist of dense, well-rooted growth of permanent and desirable grasses, indigenous to the locality where it is to be used, and it should be practically free from weeds or other undesirable matter. Thickness of the sod should be as uniform as possible, with some 50-80 mm or so of soil covering the grass roots depending on the nature of the sod, so that practically all the dense root system of the grasses is retained in the sod strip. The completed embankment side slopes should be scarified to a depth of about 25 mm and application of fertiliser/ manure should be carried out. After the sods have been laid in position, the surface shall be cleaned of loose sod, a thin layer of top soil shall be scattered over the surface of top dressing and the area thoroughly moistened by sprinkling with water. For further details MoRTH Specifications for Road and Bridge Works, Clause 307 and 308 can be referred to. 4.4.3 Application of mulch The term ‘mulch’ refers to any loose or soft organic material, e.g. straw with cowdung or wood shavings mixed with cowdung or saw dust and dung mixture, etc laid down on the slopes to protect the roots of plants. In the case of embankments which are less than 3 m high, where the severity of the erosion problem is not of a high order, the mulch application would be very helpful for vegetation growth even in
  23. 23. 15 infertile slopes. The approximate thickness of mulch cover should be about 2.5 cm. The organic mulch covering the soil slopes can be held in place and made resistant to being washed downhill or being blown away by pegging them down with bamboos, at suitable intervals, in a grid pattern. Cellulose based fibrous mulches can be hydraulically spray applied with the seed. These ‘spray-on’ mulch systems (also called Hydro-mulching or Hydro-seeding) are somewhat more resistant to erosion than dry applied systems but they are relatively costlier also. 4.4.4 Promotion of vegetative turfing by using jute/ coir netting Growth of appropriate vegetation on exposed soil surface is facilitated by use of natural (agro based) geotextiles such as open weave jute geotextiles (JGT) or coir netting. Such nettings laid on slopes provides a cover over exposed soil lessening the probability of soil detachment and at the same time reduces the velocity of run-off, the main agent of soil erosion. Natural geotextiles bio-degrade within one to three years. In spite of this, agro based geotextiles facilitate rapid growth of dense vegetation during its service life. Once dense vegetation develops on the slope, plant cover would prevent erosion and it would be self sustaining. Hence biodegradability of jute/ coir nettings cannot be considered as a drawback in areas which experience adequate precipitation to ensure green vegetation cover throughout the year. For more details and specifications of this technique, IS: 14986 ‘Guidelines for application of Jute Geotextile for rain water erosion control in road and railway embankments and hill slopes’, IS: 15869 ‘Open weave coir Bhoovastra-Specification’ and IS 15872 ‘Application of coir geotextiles (coir woven Bhoovastra) for rain water erosion control in roads, railway embankments and hill slopes-Guidelines’ may be referred to. 4.4.5 Erosion control using two dimensional (2–D) synthetic geogrids/ Geosynthetic nettings Geosynthetic nettings/ geogrids can be used for promoting vegetation growth on barren slopes in a manner similar to biodegradable nettings. Under erratic weather conditions, successful vegetation growth and its sustenance depends on un-seasonal rainfall and hence longer life of reinforcing material would be required for ensuring vegetation growth apart from contribution from the mesh towards reduction in velocity of surface runoff. Agro based nettings may fail to provide erosion prevention in areas which experience repetitive change in climate, prolonged drought in particular. Use of polymer geogrid mesh provides a permanent protection as it is not biodegradable, long lasting and has almost unfailing success rate for vegetation growth, year after year. 4.4.6 Three dimensional erosion control mat / Rolled erosion control products Relying upon vegetation growth alone may be sometimes very unpredictable and unreliable as it may be extremely difficult to achieve 100 per cent vegetation coverage, leaving exposed areas vulnerable to erosion. Furthermore, vegetation may sometimes dry up or become diseased, reducing its erosion control capability. Reinforced vegetation (or reinforced grass) is a better method that can be adopted for enhancing slope stability and erosion control. Such erosion control products are usually three dimensional mats, having multi-filamented materials of specified thickness. Such materials are known as Rolled Erosion Control Products (RECPs)/ 3-D Mats and also as ‘Turf Reinforcement Mats (TRM). While mats made using natural fibres last for one to two years, polymeric mats are used in situations where such products are required to last for a longer time. 3-D mats having a wide ranging variety of strength are available. The material used for manufacturing these mats also varies. Hence following general specifications are given (Table 4.1) for guidance. However, field conditions like harsh areas/ high survivability requirements may warrant use of 3-D mats with tensile strength as high as 35 kN/m or even more.
  24. 24. 16 Table – 4.1 Property Requirements for 3-D Mat 3-D Mat Property Specified value* Test Method Minimum Tensile Strength 2 kN/m ASTM D 5035 UV Stability (Min % tensile strength retention) 80% ASTM D 4335 (500 hour exposure) Minimum thickness 6.5 mm ASTM D 6525 Mass per unit area (Minimum) 250 gm/ m2 ASTM D 3776 * Minimum Average Roll Values, machine direction only for tensile strength test 4.4.7 Preformed polymer geosynthetic cells or webs Often, embankments are to be constructed in areas where vegetation may be difficult to establish and erosion problem might be severe due to water bodies. It may also be not possible to mitigate potential erosive forces that are likely to overcome the strength of the root system. In such cases ‘Geosynthetic Cells’ can be adopted. However, geosynthetic cells would be relatively more costly than all other techniques outlined above. 4.5 Ground Improvement Techniques Often problems like slip failure of road embankment or high degree of unevenness of road pavements which occur in coastal roads can be traced to inadequate consolidation of clayey sub-soil found in such locations. Such problems in the coastal and delta areas arise due to low shear strength and high compressibility of soft clay sub-soils which are commonly referred to as marine clays. In severe cases, road embankments may even fail or pavement surface may experience unacceptable levels of settlements stretching over considerable period of time. Improvement of the load response behaviour of such soft sub-soil becomes necessary if the embankments are to be built economically and serviceability levels are to be kept high. Accelerating the consolidation process by providing vertical drains has been widely adopted for road embankment construction in such marine clay areas. 4.5.1 Ground improvement using vertical drains Vertical drains have been in use for more than half a century to promote rapid consolidation of thick soft clay deposits like marine clays, where preloading alone will be insufficient. Sand drains were the earliest type of vertical drains used for consolidation of soft clay layer. Installation of sand drains is usually done by drilling boreholes in soft clay and back filling the borehole using sand of specified gradation. The major problem in this case would be formation of cavities due to bulking of sand. Polymeric vertical drains (PVD) which are also known as ‘Band drains’ have now virtually replaced sand drains/ sand wick technique for ground improvement. 4.5.2 Band drains (PVD) Band drains consists of a plastic/polymeric core formed to create channels or paths which are surrounded by a thin geotextile filter jacket. Typically the size of band drains is 10 cm in width and 3 to 9 mm in thickness. The primary use of band drains is to accelerate consolidation and to greatly decrease the settlement time of embankments over soft soils. By doing so, band drains also accelerate the rate of strength gain of the in-situ soils. Band drains are used in consolidation situations where soil to be treated is a moderate to highly compressible soil with low permeability and fully saturated in its natural state. The soil should be either normally consolidated or under consolidated prior to loading. The loading should exceed maximum past consolidation pressure for the band drains to be beneficial.
  25. 25. 17 Band drains are generally installed by displacement methods. The mandrels used with band drains are hollow and normally rectangular or trapezoidal in cross section. The mandrel covers and protects the band drain material during installation. All installation methods employ some form of anchoring system (generally using a disposable end shoe) to hold the drain in place when mandrel is withdrawn. Commonly used methods employ an installation mast (called ‘Stitcher’) which contains the material reels, mandrel and provision for providing installation force. Added to this is a carrier, which is a crawler excavator or crawler crane, depending somewhat on the depth of installation. Usually for drain installation depth upto 20 m, the mast can be mounted on a crawler excavator. Drains requiring depth greater than 20 m most often require an installation mast mounted to a crane to provide stability. The most important criteria for method of installation is the size of the installing mandrel. The mandrel should be kept to a minimum size, usually not greater than 80 cm2 unless larger size is required for penetrating to greater depth. Although equipment is available to work over slopes, a level granular surface containing no large obstructions is ideally required for band drain installation. Sufficient head room is also required for its installation. A thumb rule for head room required would be 3 m longer than depth of installation. Band drains have been installed upto 60 m depth, by using specialised equipment. It is essential to recognise that band drains serve no structural function. By providing a shorter drainage path, it provides a faster release of excess pore pressure, thereby resulting in faster settlement and quicker strength gain through consolidation. For sites with a stability problem, the soil will initially have the same strength with or without the band drains installed. Further band drains do not play any role in secondary consolidation. Therefore in cases where secondary consolidation is expected to be significant, it is necessary to provide excess surcharge and/or extended waiting periods prior to final construction. It is not recommended to install band drains where pre-drilling is necessary for installation. A drainage layer of coarse sand or gravel is provided above the ground to drain off water from band drains. Generally sand layer is provided for a thickness of 0.5 to 1.0 m. 4.5.3 Stone columns Stone columns comprise of boreholes of designed diameter made at specified distance apart in the soft soil, which are then back filled using stone aggregates and compacted. The diameter of stone columns varies from about 0.4 m to 0.7 m and their spacing varies from 1.5 m to 3.5 m. This method is used in soft subsurface soils to both accelerate settlement and provide sufficient increase in strength to Fig – 4.4 A Type of Polymeric Vertical Drain (Band Drain)
  26. 26. 18 minimise settlement and prevent deep seated shear failure. However stone column technique would be comparatively costlier than providing polymeric vertical drains. Hence stone column technique is selectively adopted to support structures which are sensitive to large amount of settlement or in cases where it is also required to increase the bearing capacity of the sub-soil. At locations where undisturbed shear strength of clayey soil (Su) is lower than 15 kPa, providing stone columns may result in considerable wastage of stone aggregates and Geosynthetic encased stone columns may be adopted in such places. IS 15284 (Part 1) provides guidelines for design and construction of stone columns. 4.5.4 Instrumentation and monitoring Field instrumentation such as piezometers, settlement platform, settlement gauges and inclinometers are used to monitor performance of band drains and possibly control the rate of embankment construction and/or surcharge. It is important that both the designer and the instrumentation personnel have a full appreciation of the instrumentation being installed. Generally settlement measuring devices of different types like settlement platforms, deep settlement points or horizontal deflection devices are used to measure only the rate and total amount of consolidation. An inclinometer is used to measure horizontal deflection with depth and as a warning device against potential failure. The pore pressure devices (piezometers) are used for both calculation of achieved consolidation rate and excessive build up of pore pressure which are an indication of potential failure. Proper selection of instrumentation devices and the frequency of monitoring a project are important. For simple projects where stability is of no concern, and time is not the critical factor, only surface settlement platforms, which are relatively easy to install, are needed. In situations where stability is critical, pore pressure measurements and measurements of horizontal deformations (using inclinometer) are also necessary. The monitoring can be done daily or once in two/three days during loading period depending on rate of loading. The periodicity of taking the readings from the instruments can then be reduced to once a week or ten days gradually after loading is over. The design of PVD system and monitoring of consolidation using instrumentation are a specialised job and hence advice of geotechnical consultants is to be obtained in these tasks. 4.6 Embankment Construction in Waterlogged Areas When embankment construction is to be undertaken through an existing pond, dewatering and slush removal should be taken up before placing the embankment fill. In case dewatering is not considered to be feasible and embankment is to be constructed under water, only acceptable granular material shall be used. Acceptable granular material should consist of well graded, hard durable particles with maximum particle size not exceeding 75 mm. The material should be non plastic having coefficient of uniformity not less than 10. The material placed in standing water shall be deposited by end tipping without compaction. Other methods which can be adopted in water logged areas include – Depressing the water table by using geotextile wrapped aggregate drains (also known as trench drains), raising the embankment height and providing a capillary cut off. Custom made synthetic drains made of polymeric materials are also available which can be used in place of aggregate trench drains. For more details regarding embankment construction in waterlogged/ salt infested areas or in areas where ground water table is very high, IRC: 34, ‘Recommendations for road construction in areas affected by waterlogging, flooding and/or salts infestation’ may be referred.
  27. 27. 19 Chapter – 5 SEA EROSION CONTROL TECHNIQUES & RIVER BANK PROTECTION Coastal beach erosion occurs in various forms around the world. This phenomenon gets more acute during cyclones and in-turn causes damage to infrastructure facilities including roads. This is due to severity of waves and storm surge which result in coastal erosion. The basic approach to mitigate coastal erosion related problems is to provide suitable cover to the soil. The measures to control coastal erosion can be categorised as structural and soft/ non-structural. These can be taken up together or separately also. Structural measures used for arresting coastal erosion are sea wall, revetment (rock armour, gabion mattress or precast concrete block revetment systems), offshore breakwater, groynes, etc. Soft measures generally adopted to prevent coastal erosion are artificial nourishment of beaches, vegetative cover such as mangrove plantation, etc. Instead of providing rock armour layer, latest and environmental friendly technologies which make use of geosynthetics for construction of armour protection layer can also be adopted. 5.1 Wave Generation in Sea Waves are caused by a disturbance of the water surface. Such disturbances become more prominent during cyclones because of wave surge and high speed winds. Most waves are generated by wind. After waves are formed, they can propagate across the surface of the sea for thousands of miles. When waves break on a shoreline or coastal structure, they have fluid velocities and accelerations that can impart significant forces. The wave period of individual waves remains constant through the transformations until breaking but the direction of propagation and the wave height can change significantly. As a wave moves into shallower water the wavelength decreases and the wave height increases. Waves break at two general limits:  In deepwater, waves can become too steep and break when the wave steepness defined as, H/L, approaches 1/7 (where H = Height of the wave i.e., distance between crest of the wave and water surface, L = Wave length defined as distance between two successive wave crests).  In shallow water, waves break when they reach a limiting depth (d) of water. This depth-limited breaking is important in the design of coastal revetments protecting highways. For an individual wave, the limiting depth is roughly equal to the wave height and lies in the range given below: 0.8 < Maxd H       > 1.2 ........... Equation 5.1 A practical value of wave height which can be considered when there is mild sandy slope offshore is: Maxd H       ≈ 0.8 ........... Equation 5.2 5.2 Systems for Protection of Coastline Against Sea Erosion The systems adopted for protection against water erosion comprise of two different parts – the outer revetment or armour layer to absorb the hydraulic energy of velocity of water flow and/or the wave energy; and the inner part of filter layer. Revetment systems in the form of rip-rap blocks, prefabricated concrete elements or gabion mattresses or RCC/stone masonry walls are most commonly used as
  28. 28. 20 armour layer. The function of inner filter layer is to prevent soil particles from being eroded and to allow free escape of internal water simultaneously. Conventionally several layers of granular material with well designed grain size distribution and thicknesses are used for this purpose. Geotextiles can be successfully adopted to replace such granular filter material. They are now being increasingly adopted owing to various technical advantages, cost benefits, ease of installation, faster completion of the project and superior long term performance of the system. Fig 5.1 to 5.6 show photos of protection measures adopted at various locations in India for protection of sea coast. 5.2.1 Bulkheads and revetments The distinction between revetments, seawalls, and bulkheads is one of functional purpose. Revetments are layers of protection on the top of a sloped surface to protect the underlying soil. Seawalls are designed to protect beach against large wave forces. Bulkheads are designed primarily to retain the soil behind a vertical wall in locations with less wave action. Bulkheads are mostly adopted where wave heights are very small. Seawalls are more common where wave heights are quite large. Revetments are often common in intermediate situations such as on bay or lake shorelines. Seawalls can be rigid structures or rubble-mound structures specifically designed to withstand large waves. Vertical sheet pile seawalls with concrete caps are common but require extensive marine structural design. A more common seawall design type is a rubble-mound that looks very much like a revetment with larger stones to withstand the design wave height. Thus, the two terms, seawalls and revetments, can be used interchangeably with the former typically used for the larger wave environments. 5.2.2 Seawall Seawall is useful in case of protection of specific area from erosion due to waves and storm surges. Seawalls are constructed along the coast adopting stone masonry technique or using reinforced cement concrete. Seawall can be constructed using gabions also when wave heights are low, typically less than about 1.0 m. Seawalls constructed using gabions are permeable and flexible; thereby they would be able to withstand differential settlement without loss of its structural integrity. Provision of filter layer behind the seawall is essential to prevent piping of sand and subsequent destabilisation of structure. Sometimes a combination of sea wall constructed using masonry or reinforced cement concrete is further protected on sea side using gabions or concrete blocks/ tetrapods. Design of the masonry or gabion seawalls is to be carried out in a manner similar to design of retaining walls, to ensure stability against overturning, sliding, excessive foundation pressure (bearing capacity failure) and water uplift. Additionally ‘Wave flume studies’ may also have to be adopted to arrive at satisfactory design of stone, rock and/or concrete armour units. 5.2.3 Breakwater Breakwaters are coastal structures constructed to protect an area from the effects of waves. Breakwaters are adopted to protect a ship berthing area, to train and prevent silting of the entrance of river mouths or to prevent erosion of coastlines. However, adverse effects are observed on down drift side and it should be avoided unless their main purpose is to protect a specific area at the cost of adjoining areas. An off-shore breakwater may be constructed to prevent beaches or coastlines from erosion by wave activity. The off-shore breakwaters are submerged structures located at certain distance offshore in order to dissipate wave energy before they reach shoreline. The broken waves would not be having the energy to erode the beach or coastline and the coastline may even increase in extent as a result of accretion. It is an expensive option and needs regular maintenance.
  29. 29. 21 Fig – 5.1 Provision of Sea Wall and Concrete Tetrapods for Sea Erosion Protection at Mumbai Fig – 5.2 Protection Against Sea Erosion by Retaining Wall and Gabions – Puri Konark Road Fig – 5.3 Another View of Protection Works – Puri Konark Road
  30. 30. 22 Fig – 5.4 Beach Protection Using Boulder Revetment at Paradip, Odisha Fig – 5.5 Another View of Boulder Revetment at Paradip, Odisha Fig – 5.6 Masonry Sea Wall Constructed at Sea Coast at Koteshwar, Kori Creek, Gujarat
  31. 31. 23 5.2.4 Soft Structural/ Non-structural measures – Artificial nourishment of beaches Beach nourishment may be adopted for protection and beach development. Combination of nourishment of beaches with seawall/ groynes will create beach in front of protected area and eliminate leeside erosion. 5.2.5 Vegetation cover Plantation of mangroves and palm trees can be taken up for beach protection. Vegetation cover can restrict sand movement and erosion. 5.2.6 Artificial reef balls A reef ball is a designed artificial reef used to restore ailing coral reefs and to create new fishing and scuba diving sites. Reef balls are the only type of artificial reefs that can be floated and towed behind a boat. Reef balls are made of a special grade marine environment resistant concrete and are designed to mimic natural reef systems. They are also used widely to create habitats for fish and other marine and fresh water species. Reef balls are made in many sizes to best match the natural reef system which is being mimicked. Out of these measures, depending on the techno-economic viability, any suitable measure can be adopted, while a combination of these measures usually gives optimum results. 5.3 Design of Coastal Rip-rap Against Wave Attack Rip-rap can be used for protection from four different types of hydraulic situations: direct rainfall impacts, overland flow, stream or river currents, and waves. This section addresses only wave attack. IRC:89 provides procedures for the design of riprap revetments for channel bank protection on larger streams and rivers where the active force of the flowing water exceeds the bank material’s ability to resist movement. Brief details of the same are provided in section 5.6. Flow in a stream or river is unidirectional and typically aligned parallel to the banks. Waves produce oscillatory velocities and accelerations that can be in almost any direction on a revetment. In such situations, it is recommended that ‘Hudson’s equation’ be used to estimate stone size for revetments subject to wave action. This involves determining the design wave height (as per equation 5.1) and using Hudson’s equation to size the stones to be used for rip-rap. This approach can lead to designs with larger stones and narrower stone gradations than designs for non-wave situations. The difference is due to the higher forces caused by waves. Situations where riverine and wave flows are also significant, the design engineer should consider both design approaches and develop a conservative design. A simplified version of Hudson’s equation for calculating required median weight for the outer, or armour layer, stones is: θcot 280H W 3 50  ........... Equation 5.3 Where, W50 = Median weight of armour stone in kgs H = Design wave height in m Θ = Slope The range of recommended slopes for revetments is up to 2:1 (horizontal:vertical) or flatter. Hence cotΘ would be equal to 2 for a 2:1 slope and cotΘ=3 for a 3:1 slope. Apart from armour stone, either graded aggregate filter layers or preferably geotextile needs to be placed below armour to prevent piping
  32. 32. 24 failure. Selection of geotextile can be carried out as per IRC SP:59. A typical rip-rap gradation for coastal revetment with a median weight W50 = 350 kgs, will have 50 per cent of stones weighing between 100 kgs to 350 kgs, 30 per cent weighing between 350 kgs to 700 kgs, and 20 per cent weighing between 700 kgs to 1350 kgs. Thus, the recommended coastal revetment gradation precludes the smaller stones and allows for some larger stones as compared to gradation adopted for river bank protection. These smaller stones are typically not included in coastal revetments because of their tendency to move in response to wave action. Further it may be noted that the construction of a revetment, while it protects the upland, does not address the underlying cause of erosion. The depths at the toe of the revetment will likely increase if the erosion process continues. The presence of a revetment or seawall can increase the vertical erosion at its base. A common practice to overcome toe erosion is to extend revetment beyond the slope inside water and provide toe protection. A commonly proposed alternative to rubble mound revetments is a concrete block revetment, which are also known as 'Tetrapods'. Some of these have physical interlocking between individual blocks also. Many such concrete blocks which have a patented shape are also available. These are essentially unreinforced concrete objects designed to resist the wave action. If the intensity of wave action is severe, then additional layers of armour protection would be required. In such cases, ‘Tetrapods’ can be placed over stone blocks. 5.4 Use of Geosynthetic Products as Revetment Coastal and waterways protection applications comprised the earliest use of geosynthetics. Over the last 40 years, there have been numerous coastal and waterway protection projects that have utilised geosynthetics. Geosynthetics can be used as components of coastal and waterway protection measures in two different ways – they can be used as filters within coastal and waterway protection structures and they can also be used to create revetment systems (containers) to act as mass-gravity protection works. During the construction of structural measures to control sea erosion, problem generally faced is the non availability of construction materials like big size boulders, sand, etc., within reasonable and cost effective distance. This problem can be sorted to a great extent by using geosynthetic revetment systems. The most universal and widely used geotextile containers are well- known, ubiquitous sand bags which are seen world over for shoring up flood defences in times of natural calamity. The dominant geosynthetic material used for making revetment systems is geotextiles, which are robust and permeable materials. Three types of geoetextile revetment systems differentiated by geometrical shape are available. They are geotextile tubes, geotextile containers and geotextile bags. Geotextile tubes are tubular containers that are filled in-situ on land or in water. Geotextile containers are large volume containers that are filled above water and then deposited into the submarine environment. Geotextile bags are small volume containers that are filled on land or above water and then pattern-placed either near water or below water level. The geotextile revetment systems have the following advantages: 1. They are resistant to chemical attacks occurring in usage, especially to alkalies and acids. 2. Geotextiles are quite durable when exposed to elements of nature like – Sun, precipitation, etc. However, ultraviolet radiation reduces their strength in long term. Hence they need to be treated to enhance their ultraviolet resistance if they are going to be exposed to sun during their service life. 3. They are resistant to organic attacks like bacteria and fungus and are not attractive to rats or termites. Geotextile containers behave as mass-gravity elements that can resist hydraulic forces. For these applications, the geotextile skin should have specific mechanical, hydraulic and durability requirements. Distinction must be made between those applications where the geotextile containment is required for only temporary use and those applications that require long term performance. For temporary works the requirements of the geotextile container itself is fairly basic as it only has a short life over which it has to
  33. 33. 25 perform, however for long term applications, the performance requirements of the geotextile container are more severe. With regard to long term performance, distinction must also be made according to type of hydraulic environment acting on the geotextile container. For example, the action of still water, or intermittent water flows, will have a different effect on the geotextile container than the action of breaking waves. 5.4.1 Geotextile tubes Geotextile tubes are large cylindrical structures made using high strength woven geotextile material which are then filled with dredged material in-situ. Geotextile tubes may be used for a range of coastal and waterway protection applications where barrier type, mass-gravity, structures are required. The dredged material is usually pumped in a slurry form from nearby area and consists of a mixture of sandy soil and water. The geotextile tube, being permeable, enables the excess water to pass the geotextile skin while the fill attains a compacted, stable mass within the tube. For coastal and waterway applications the type of fill used is sand, or a significant percentage would be sand. The reasons being – sand can be compacted to a good density by hydraulic means, sand has good internal shear strength which gets further improved by the presence of confining geotextile tube skin, and this type of fill once compacted, will not undergo settlement, which would change the shape of the filled up geotextile tube The tube is filled by direct coupling to a hydraulic pumping system conveying dredged material. Designed with appropriately sized openings called ‘Filling Ports’, the geosynthetic tubes retains fill material while allowing water to permeate through tube wall. After dewatering typically very little consolidation will occur in case of pure sands while it may be as much as 70 per cent in case of tube that has been filled with fine grained organic material. Openings called, fill ports are provided in geotextile tubes at a spacing of about 8 to 10 m for filling dredged material. Special high strength seaming techniques are adopted in their manufacturing process to resist pressure during pumping action. Geotextile tubes permanently trap granular material in both dry and underground construction. Geotextile tubes are generally about 1 m to 3 m in diameter, though they can be custom made to any size depending on their application. Geotextile tubes ranging in diameters from 1.5 m to 5.0 m are available for coastal and waterway protection applications. Stacking of geotextile tubes one over other can also be made to construct structures of higher heights. Geotextile tubes may be used for a range of coastal and waterway protection applications where barrier type, mass gravity structures are required. Geotextile tubes can be used for construction of groynes, off-shore breakwater, etc. When geotextile tubes are used as off-shore breakwater structures, the dimensions of geotextile tubes are to be chosen in such a way that waves break over the geotextile tubes. Geotextile tubes are normally described in terms of either a theoretical diameter, D or a circumference, C. While these two properties represent the fundamental characteristics of geotextile tubes they are not of direct interest when it comes to engineering parameters for coastal and waterway protection Fig – 5.7 Typical Components of a Geotextile Tube
  34. 34. 26 applications where the geotextile tube in its filled condition is of prime importance. When the geotextile tube has been filled with sand, it assumes an oval shape. The width of the oval tube and its height are of importance from engineering performance point of view. Table 5.1 lists relationships between the fundamental geotextile tube characteristics and engineering parameters. The relationships are applicable to geotextile tubes that have a maximum strain of about 15 per cent, low unconfined creep, and are filled to maximum capacity with sand. It is also assumed that the foundation beneath the tube is a flat, solid surface. Geotextile tubes are used for revetments where their contained fill is used to provide stability. They have been used for both submerged as well as exposed revetments (Fig 5.8). For submerged revetments, the geotextile tube is covered by local soil and is only required to provide protection when the soil cover has been eroded during the periods of intermittent storm activity. Once the storm is over, the revetment is covered by soil again either naturally or by maintenance filling. For exposed revetments, the geotextile tube is exposed throughout its required design life. To prevent erosion of the foundation soil in its vicinity, and undermining of geotextile tube revetment, it is common practice to install a scour apron. This scour apron usually consists of a geotextile filter layer that passes beneath the geotextile tube and is anchored at the extremity by a smaller sized geotextile tube, called anchor tube. Table – 5.1 Basic Engineering Parameters for Geotextile Tubes filled with Sand Engineering parameter In terms of theoretical diameter, D In terms of tube circumference, C Maximum filled height, H H ≈ 0.6 D H ≈ 0.19 C Filled width, W W ≈ 1.4 D W ≈ 0.45 C Base contact width, b b ≈ 0.9 D b ≈ 0.29 C Cross sectional area, A A ≈ 0.65 D2 A ≈ 0.07 C2 Average vertical stress at base, σ σv ≈ 0.72 γ D (γ = Density of the fill) σv ≈ 0.24 γ C (γ = Density of the fill) Revetments using multiple-height geotextile tubes are also constructed. Here the geotextile tubes are staggered horizontally to achieve the required stability. Considerable care should be exercised during construction of these types of revetments to ensure the water emanating from hydraulic filling of upper geotextile tubes does not erode the soil and undermine the lower geotextile tubes in the multiple-height revetment structure. In a similar manner, geotextile tubes can be used for constructing offshore breakwaters, protection dykes, containment dykes and groynes as shown in Fig 5.9. Fig 5.8 – Geotextile Tube Application for Coastal Protection
  35. 35. 27 Fig 5.9 – Applications of Geotextile Tubes Fig 5.10 – Typical Use of Multiple Geotextile Tubes for Coastal Embankment Construction Fig 5.11 – Geotextile Tubes with Gabions as Armour Protection layer
  36. 36. 28 5.4.2 Geotextile containers Geotextile containers, as their name may imply, are large volume containers that are filled above water and then positioned and placed at reasonable water depth. Geotextile containers are made from high strength woven geotextiles or a combination of woven and non woven geotextiles (depending on the fill characteristics) which are filled with sand/ dredged material. The volumes of these containers more commonly range from 100 m3 to 700 m3, although containers as large as 1000 m3 have also been installed. To facilitate the installation of geotextile containers, an efficient and practical installation system is required. To date, this has been achieved by using split bottom barges. This entails, the filling of the geotextile container in a split bottom barge. The container is then sealed and the barge is positioned at the correct dumping location. The split bottom of the barge is then opened and the container is deposited on the seabed. Geotextile containers are used as mass-gravity structural components in coastal and waterway protection applications such as offshore breakwaters, containment dykes, artificial reefs, slope buttressing, etc in a manner similar to geotextile tubes. 5.4.3 Geotextile bags Geotextile bags are made from high strength woven and nonwoven geotextiles or a combination of these can be used (depending on the fill characteristics) which are filled with sand/ dredged material. Geotextile bags are used at sea shores or bunds adjacent to rivers which are to be protected from erosion, especially during emergency situations. Geotextile bags have also been used as revetments, breakwaters, etc to build structural erosion protection measures during normal periods also (Fig 5.13). Geotextile bags provide stability and prevent erosion. Geotextile bags are filled off-site and then installed to the geometry required in a similar manner to geotextile containers. For best performance, they have to be filled to maximum volume and density with sand in an identical manner to geotextile tubes. However, geotextile bags have two major differences to other geotextile containment techniques – they can be manufactured in a range of shapes, and they are installed in a pattern-placed arrangement that greatly improves their overall stability and performance. Geotextile bags ranging in volume from 0.05 m3 to about 5 m3, which are pillow shaped or box shaped or mattress shaped are available, depending on the required application. Filling geotextile bags with dry sand becomes more difficult as the volume of the bag increases, but filling task can be efficiently done by using sand+water mixture (hydraulically filling the sand into a bag). Filled density and volume are important from the view point of maximising the stability, but it is also important from the view point of minimising the effects of Fig 5.12 – View of geotextile Tubes Covered with Armour Protection Layer of Gabions
  37. 37. 29 fill liquefaction and loss of shape of the geotextile bags. To ensure that the contained fill is maintained in its dense state, the geotextile skin should have adequate tensile strength. One major advantage of geotextile bags is that these small volume units can be used to construct hydraulic and marine structures that require adherence to designed geometrical shape accurately. This makes them preferable to large volume units such as geotextile containers when specific slope and height tolerances are to be attained. Another advantage of small volume units of geotextile bags is that maintenance and remedial works can be carried out easily by replacing the failed bags. This is much simpler than carrying out remedial works on large volume containment units. 5.4.4 Gabions Gabions, which are mesh like structures filled with relatively small size stones, are an attractive alternative to large boulder stones for various erosion control and scour protection applications. Gabions by holding the small stones together, function like large boulders but at the same time facilitates easy construction and offer a flexible structure. Thereby gabions provide a technically Fig 5.14 – Geotextile Bag Fig 5.13 – Shore Reclamation Using Geotextile Bags
  38. 38. 30 satisfactory and cost effective solution. Gabions can be made from either polymeric material or double twisted steel wires having zinc+polymer coating. Gabions are generally available in a prefabricated collapsible form with the bottom and four sides held together by appropriate binding and with a flip open top lid. Filled with stones, the gabion becomes a large, flexible and permeable building block using which a broad range of structures can be built. Because of their inherent flexibility, gabion structure can yield to earth movement and retain their full efficiency while remaining structurally sound. They are quite unlike rigid or semi-rigid structures, which may suffer complete failure when even slight changes occur in their foundation. Besides the above, gabions can be easily lifted by cranes, they are suitable for underwater construction and several gabions can be tied together to create continuous, integral structures. The pervious structure of gabions gradually absorbs the heavy wave impact than an impervious structure. IS 16014 provides specifications for zinc+polymer coated steel wire gabions. Compared to steel wire gabions, polymeric gabions have advantages like superior corrosion resistance, ability to withstand acidic and alkaline environment, excellent durability, excellent flexibility to take the shape of ground contour, etc. However, these gabions due to their very high flexibility, may not be as much amenable to construction of retaining structures as compared to steel wire rope gabions. 5.5 Geotextiles as Filters Below the revetments (either stone/rock or concrete armour units), filters are invariably required to prevent soil washout. Traditional granular filters usually consist of several layers of stone aggregates. If the water forces are strong enough and the soil to be protected is fine grained, then upto four layers of granular materials may be required to satisfy the hydraulic design requirements. Hence, this kind of relatively complex structures can be expensive and difficult to construct. Furthermore, granular/ aggregate filters are difficult to place on steep slopes, cannot always be installed in tidal zones and laying process demands reliable and expert supervision. Geotextiles can be used as substitutes for one or more granular under layer materials below revetments. Geotextiles offer many advantages over granular filter materials:  They enable design flexibility with regard to the choice of the size of the granular material in the layer immediately adjacent to the geotextile filter.  They are easier to install to specific geometrical configurations than granular materials – in many cases below water level.  In general, in-situ quality control test requirements for geotextiles are nominal. Where geotextiles are used as filters for coastal and waterway protection, their primary function is to prevent the erosion of soil through the protection structure and thus prevent instability. In case of geotextile filters, hydraulic characteristics like apparent opening size and permittivity are most Fig 5.15 – Use of Gabions for River Bank Protection
  39. 39. 31 important. The selection of filter fabric with correct opening size depends on the percentage of finer material available in bed material. In order to fulfil its function, the geotextile material has to be robust enough to resist mechanical stresses applied to it during installation. Secondly, the geotextile material must have required hydraulic properties in order to perform as a filter material. Thirdly, the geotextile must have adequate durability to maintain its mechanical and hydraulic properties throughout the design life of revetment. The criteria for selection of filter fabric can be based on IRC SP: 59. As the weight of the stones/ drop height increases, thicker geotextile having greater mass per unit area would be required. Another important property would be trapezoidal tear strength. Normally, geotextiles having trapezoidal tear strength varying from 200 to 600 N (ASTM D 4533) are used in coastal works. When determining the appropriate hydraulic properties for the geotextile revetment filter consideration needs to be given to the critical hydraulic regime that will act on the revetment structure over its design life. Table 5.2 list the geotextile filter hydraulic properties requirements according to the type of hydraulic regime. When several different hydraulic regimes occur at the same location then the most critical hydraulic regime (1 being the least critical and 3 being the most critical in Table 5.2) should be chosen for design. While installing geotextile filter, it is to be first laid out on the soil surface prior to placing stones and rocks. For good long term performance, the geotextile filter should be covered with an adequate thickness of granular material to ensure that it remains protected from the effects of long term exposure to ultra-voilet (UV) rays. The minimum thickness of stone coverage above the geotextile filter to protect against UV radiation should be atleast two times the maximum stone size in the rock armour layer above the geotextile filter. During installation, it may be inevitable that the geotextile filter would be exposed to UV rays and this condition may extend, depending upon pace of construction. To cover such eventualities, the UV stability of requirement of geotextile to be used should meet the specification requirements as per IRC SP:59. The geotextile filter coverage beneath the revetment armour layer should extend beyond the zone of erosion. This would ensure that revetment structure will remain stable throughout the life of the structure. Table – 5.2 Geotextile Hydraulic Property Requirements under Different Regimes 1 Water current flows parallel to revetment face Non-dispersive soil O95 ≤ 0.35 mm Dispersive soil d15 ≤ O95 ≤ d85 2 Gradual reversing water flows d15 ≤ O95 ≤ d85 3 Impacting wave activity d15 ≤ O95 ≤ d50 1. d15, d50, and d85 are percentile particle size fractions to be protected 2. O95 is apparent opening size (AOS) of the geotextile filter (ASTM D 4751) 5.5.1 Geotextile filters for breakwaters For rubble mound and caisson wall breakwaters geotextile filters are placed on top of the sea bed prior to construction of the breakwater. In this location, the primary role of geotextile filter is to prevent erosion of sea bed and the undermining of the breakwater. To facilitate installation on the sea bed, the geotextile filter is usually prefabricated onsite into a fascine mattress structure. This technique involves the fabrication of geotextile filter into large sheets on land and attaching an interconnecting grid of fascines, bamboo or timber. The resulting mattress is then pulled into the water and floated into place and sunk on the sea bed. This technique has proved to be an efficient and cost effective means of installing geotextile filters on the sea bed. The tensile stresses imposed on the geotextile filter during fascine mattress installation procedure are relatively high. Consequently, woven geotextiles with wide- width tensile strengths ranging from 80 kN/m to 200 kN/m are normally used for this type of application.
  40. 40. 32 Offshore breakwaters also may be constructed to protect beaches or coastlines from erosion by wave activity. In such cases, the breakwaters would be submerged structures that force the waves to break when passing thus, expending much of their wave energy. The broken waves would not have the energy to erode the beach or coastline and the coastline may even extend outwards into the sea as a result. 5.5.2 Geotextile filters for containment dykes To reclaim land from sea, it is common to first construct a containment dyke around the extremity of the reclamation area. Soil or sand fill is then dry dumped or hydraulically pumped into the containment area to form dry land. The function of the containment dyke is to prevent loss of the placed soil or sand fill into the surrounding water. The nature of the containment dyke is slightly different depending on whether the reclamation occurs in relatively deep water or in shallow water. Where land reclamation occurs in relatively deep water, the size of the containment dyke is fairly large and may require two or more stages to complete the structure. Commonly, the dyke consists of a rubble mound of dumped rock with a geotextile filter placed across the base of the dyke. The role of the geotextile filter is to prevent the loss of reclamation fill through the rubble mound dyke and the erosion of the sea bed beneath the rubble mound. The geotextile filter across the base of the dyke can also prevent the loss of the rubble mound material into the sea bed if the foundation is soft. For permanent protection, a rock armour layer may be placed on the outside of the rubble mound depending on the water forces acting on the structure. Where land reclamation occurs in relatively shallow water, the containment dyke is normally constructed in a single stage. Commonly, the bund consists of a rubble mound with geotextile filter placed across the base of the dyke. Again for permanent protection, a rock armour layer may be placed on the outside of the rubble mound depending on the water forces acting on the structure. It is not uncommon for the base geotextile filter beneath the containment dyke to have different properties on different faces. Normally, the base geotextile filter is installed in a manner similar to the breakwater structure which may require a fascine mattress approach to installation. This imparts relatively high tensile stresses on the geotextile filter during installation, and consequently woven geotextile filters with wide width tensile strengths between 80 kN/m and 200 kN/m are usually used for this purpose. 5.6 Mangrove Cultivation Among soft/ non-structural measures for coastal protection, mangrove cultivation is one of the most effective techniques. Mangrove is a group of typical tropical and specialised trees growing in the saline and brackish water system. The mangrove trees are highly productive, economical and most importantly they protect the shoreline from erosion and cyclonic impact. The mangroves are angiosperms, with about 45 species found in India. They have special characters like viviparous germination, pneumatophores, prop or knee roots and salt glands. These trees form a thick forest belt on the deltas, along major estuaries, and fringe the estuarine banks, as well as backwaters. This unique tree resource is useful for tannin extraction, paper and pulp, firewood, timber, charcoal, fodder and several other by-products. The mangrove swamps are rich in the larvae of many economically important fishes, prawns, crabs and bivalves. These are the most suitable area for feeding, breeding and nursery grounds of these marine organisms and hence important for aquaculture purposes. Afforestation of coastal areas suitable for mangrove cultivation would go a long way for preventing soil erosion. Mangrove trees generally prefer soft, clay mud for their growth. These species show different salinity tolerance limits. The expanse of mangrove forest depends on the intertidal
  41. 41. 33 expanse, substratum and salinity of soil as well as water. Out of 45 mangrove species occurring in India, some are true mangrove while others are considered as 'associated' flora. The most dominant mangrove species found along the east and west coast of India are listed below: Rhizophora mucronata R. apiculata Bruguiera gymnorrhiza B. parviflora Sonneratia alba S. caseolaris Cariops tagal Heretiera littoralis Xylocarpus granatum X. molluscensis Excoecaria agallocha Lumnitzera racemosa Avicennia officinalis A. marina The species mentioned above are available easily and their seedlings (propagules) or seeds are also available in considerable quantity in mangrove forest. Mangrove seeds (fruits and seedlings) are always available in small quantity throughout the year. The main fruiting or seedling season, however, start from June to September, when plenty of seedlings of all the Rhizophoraceae, Avicennia and other types can be collected. Only mature seedlings of these mangrove species should be collected for afforestation or nursery purpose. The seedlings of rhizophoracious trees have a podlike structure with tapering end of varying sizes and with typical morphological characters. Avicennia fruits are triangular in shape while Sonneratia is globular. It is however, always advisable to store these seedlings partially immersed (pointed end in water) in seawater. There are two ways of planting the mangrove seedlings  Direct planting in the swamp  Raising seedling in the nursery Seedlings which are healthy, non-infected and fully matured should only be used for planting. Any intertidal area (between the high tide and low tide) where mangroves are absent and the substratum is of soft clay or mud and is inundated by regular tidal waters every day, are suitable for direct mangrove planting. Along the Gujarat coast and West Bengal, where intertidal expanse is very large with highest tidal amplitude of 6 to 8 m, the upper limit of 1 m tidal water level has to be selected. After selecting the area to be planted, planting of seedlings may be undertaken according to the length of the propagules. Rhizophora mucronata or Rhizophora apiculata whose seedlings are the longest should always be planted towards the waterfront, these can be followed by Kandelia, Ceriops, Bruguiera, Avicennia, Lumnitzera, etc. Species with smallest seeds like Sonneratia should come to the landward side of the intertidal expanse, followed by species of grasses. Direct planting method has to be used in open areas. Nursery technique method is useful where the mangrove species are not available in plenty. This also has advantages like selected species can be grown in large numbers. Mangrove nurseries can be developed in the upper part of the intertidal region where seedlings can be grown in polyethylene bags supported with bamboos. The mangrove nursery may be located near the estuary or sea where seawater or estuarine water is available. The nursery may be on the open ground or in the low lying protected areas where seawater reaches. The collected and selected seedlings are inserted in the polyethylene bags filled with mangrove soil. If the nursery is on the raised ground then the perforations in the bags are not needed, but the nurseries in the low lying area need the perforations in the polyethylene bags. Care should be taken to cut open the polythene bags at the base before

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