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Pile foundation

  1. 1. 9/21/2013 Deep Foundation  Shallow Foundations (Spread Footings) - Bearing Capacity - Settlement  Deep Foundations - Load Capacity (bearing and friction) - Settlement - Negative Skin Friction Necessity of Deep Foundations    The loads are so high that there is not enough plan area to accommodate the size of the foundation required Where Water Table is high (dewatering is required) The presence of adjacent buildings in congested built-up areas imposes restrictions on open excavation and calls for construction of walls to restrain displacement of existing buildings. 1
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  3. 3. 9/21/2013 Necessity of Deep Foundations...  2. To resist uplift or overturning forces.  3. To control settlements when spread footings are on marginal or highly compressible soil.  4. To control scour problems on bridge abutments or piers.  5. In offshore construction to transmit loads through the water and into the underlying soil.  6. To control earth movements, such as landslides. BATTER PILE (RAKER PILE) – The pile which is installed at an angle to the vertical. 3
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  7. 7. 9/21/2013 Pile Foundations  Piles are relatively long and slender members used to transmit foundation loads through soil strata of low bearing capacity to deeper soil or rock having a higher bearing capacity.  Pile resistance is comprised of - end bearing - shaft friction  For many piles only one of these components is important. This is the basis of a simple classification End Bearing Piles PILES SOFT SOIL ROCK 7
  8. 8. 9/21/2013 Friction Piles PILES SOFT SOIL Strength increases with depth 8
  9. 9. 9/21/2013 NEGATIVE SKIN FRICTION For end bearing and skin friction to develop the pile must move downwards in relation to the soil. There are, however, occasions when after a pile has been installed, the soil surrounding the pile begins to move downwards in relation to the pile. When this occurs, the soil exerts a downward drag on the pile. This downward drag is called negative skin friction. Consider a soil profile underlain by a hard stratum. 9
  10. 10. 9/21/2013 NEGATIVE SKIN FRICTION NEGATIVE SKIN FRICTION Consider what happens on account of the following two events: 1. A fill is placed at the ground surface above the soft clay – the fill will induce the development of excess pore water pressures in the soft clay and with time they will dissipate, the effective stress will increase and the soft clay will consolidate. As it consolidates it will move downwards in relation to the pile since the pile is resting on firm stratum. 2. At this site for season the ground water table is lowered – the lowering of the ground water table has the effect of increasing the effective stress in the soft clay and it will consolidate and move downwards in relation to the pile. 10
  11. 11. 9/21/2013 NEGATIVE SKIN FRICTION In both these situations, the effect of downward movement of the soft clay in relation to the pile will be two folds: 1. The skin friction in soft clay helping to resist the load from the superstructure will be wiped out 2. The downward movement of the soil will impose a drag equal to the skin friction in the downward direction which will have to borne by end bearing. Qult = Qb – Qs End bearing will have to support not just the load of the superstructure but also the load on account of negative skin friction acting on the pile surface. Loads applied to Piles  Combinations of vertical, horizontal and moment loading may be applied at the soil surface from the overlying structure  For piles in jetties, foundations for bridge piers, tall chimneys, and offshore piled foundations the lateral resistance is an important consideration  M H For the majority of foundations the loads applied to the piles are primarily vertical  V The analysis of piles subjected to lateral and moment loading is more complex than simple vertical loading because of the soilstructure interaction. 11
  12. 12. 9/21/2013 Individual Piles Method of Estimating Load Capacity  Load Test  Dynamic Formula  Static Analysis Axial Pile Capacity – Pile Load Test Approach Fig. 20.22 Arrangements for conducting a Pile Load Test 12
  13. 13. 9/21/2013 a. Dense Sand or stiff clay; safe design load = Ultimate load/Factor of safety b. Loose sand or soft/medium stiff clay – No clear failure load; Limiting settlement is used. A typical criteria states that the safe design load shall be taken as the lower of: 1. Half the load at which pile settlement is 10% of pile diameter 2. 2/3rd of load at which pile settlement is 12 mm Fig. 20.23 Results from Pile Load Tests 13
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  15. 15. 9/21/2013 IS2911: Definition-old code- check as per new code  3.14 Test Pile  A pile which is selected for load testing and which is subsequently used as a part of the foundation. The test pile may form a working pile itself, if subjected to routine load test up to 1.5 times the safe load.  3.15 Working Pile  A pile forming part of the foundation system of a given structure.  3.16 Trial Pile  One or more piles, which are not working piles, may be installed if required to assess the load-carrying capacity of a pile. These piles are tested either to their a) ultimate load capacity or b) to 2 times the estimated safe load. 15
  16. 16. 9/21/2013 Steps in Rational Pile Design and Selection  Adequate Subsurface Investigation  Soil Profile Development  Appropriate Lab/Field Testing  Selection of Soil Design Parameters  Static Analysis  Applied Experience Ultimate Bearing Capacity - Static Formula Method (Qu = Qp + Qs) Qu = Ultimate Bearing Capacity Qs = fAs Embedded f = Unit Frictional Resistance =D Length AS = Shaft Area qP = Unit Bearing Capacity AP = Area of Point QP = qPAP 16
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  18. 18. 9/21/2013 Types of Pile  There are many piling systems  The pile installation procedure varies considerably, and has an important influence on the subsequent response  Two main groups can be identified - Displacement (Driven) piles - Non-displacement (Bored) piles 18
  19. 19. 9/21/2013 Types of Displacement Piles Displacement Large Preformed Small Formed in-situ Hollow tube, or H-section Steel (Precast) Solid Concrete, or Timber Hollow tube Closed end Steel or Concrete Screw Tube former withdrawn void filled with concrete Types of Bored Piles Bored Piles (Non-displacement) Unsupported during Construction Supported during Construction Permanently by Casing Temporarily by casing By Drilling Mud Void filled with Reinforced Concrete 19
  20. 20. 9/21/2013 Types of Pile Material Concrete Steel Pipe Timber Steel H Pre-cast Concrete Composite Is 2911: DESIGN AND CONSTRUCTION OF PILE FOUNDATIONS – CODE OF PRACTICE-Part 1 –Concrete Piles  Section 3 Driven precast concrete piles  Section 1 Driven cast-in-situ concrete piles  Section 2 Bored cast-in-situ concrete piles  Section 4 Precast piles in pre-bored holes 20
  21. 21. 9/21/2013 Precast Driven Pile: (IS 2911: Part1-Section 3 Driven precast concrete piles  The pile constructed in concrete in a casting yard and subsequently driven into the ground when it has attained sufficient strength. Piles are inserted into the soil by the following methods:  1. Driving using a pile hammer.  2. Driving using a vibratory device.  3. Jacking the pile.  4. Drilling a hole (pre-drilling) and inserting a pile into it.  5. Screw into the ground. 21
  22. 22. 9/21/2013 Driven precast concrete piles 22
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  25. 25. 9/21/2013 Precast Concrete Plies 25
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  31. 31. 9/21/2013 SEGMENTAL PRECAST RCC PILES  Wherever final pile length is so large that a single length precast pile unit is either uneconomical or impracticable for installation, the segmental precast RCC piles with a number of segments using efficient mechanical jointing could be adopted.  Excessive whipping during handling pre-cast pile may generally be avoided by limiting the length of pile to a maximum of 50 times the least width. As an alternatives segmental precast piling technique could be used. 31
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  34. 34. 9/21/2013 3.1 Driven Cast-in-situ Pile (IS 2911: Part1-sec1)  The pile formed within the ground by driving a casing of uniform diameter (displacement piles), subsequently filling the hole with reinforced concrete.  For displacing the subsoil the casing is driven with a plug or a shoe at the bottom. When the casing is left permanently in the ground, it is termed as cased pile and when the casing is taken out, it is termed as uncased pile. The steel casing tube is tamped during its extraction to ensure proper compaction of concrete. Fig. 27.11 Driven cast-in-situ pile encased in a mandrel driven thin steel shell 34
  35. 35. 9/21/2013 Fig. 27.12 An uncased driven cast-in-situ pile 35
  36. 36. 9/21/2013 An uncased driven cast-in-situ pile of compacted concrete 36
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  39. 39. 9/21/2013 SMALL DISPLACEMENT PILE BORED CAST IN SITU PILE (IS 2911: Part1-sec2) -A pile formed within the ground by excavating or boring a hole within the ground (non displacement ), with or without the use of a temporary casing and subsequently filling it with plain or reinforced concrete. When the casing is left permanently it is termed as cased pile and when the casing is taken out it is termed as uncased pile. In installing a bored pile, the sides of the borehole (when it does not stand by itself) is required to be stabilized with the aid of, a temporary casing, or with the aid of drilling mud of suitable consistency. 39
  40. 40. 9/21/2013 Cast-in-situ pile: reinforcement insertion followed by concreting Cast-in-situ pile: concreting followed by reinforcement insertion 40
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  43. 43. 9/21/2013 Drilling Methods 1) Dry method 2) Casing method possible only for competent soil profiles needed for caving soils 3) Slurry (or “wet”) method Selection of the drilling method depends on the nature of the ground 43
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  52. 52. 9/21/2013 Difference between “Driven and cast in situ” piles and “Bored and cast in situ” piles?  Driven cast in-situ piles are displacement piles in which a hole is formed by driving a metallic shell or a casing into the ground while bored and cast-in situ piles are non displacement piles in which hole is formed by boring (i.e. excavating soil by auger etc.). Advantages of bored and cast-in situ  Very little displacement & no risk of heave.  Soil can be checked & inspected.  Length of pile can be readily varied at site.  Piles of great length up to 50m can be made.  Large diameter piles with enlargement 2-3 time diameter of shaft is possible.  Piles can be installed without much noise, vibration  Piles can be installed with limited head room.  Feasible in strata with cobbles and boulders. 52
  53. 53. 9/21/2013 Disadvantages of bored and cast-in situ  1. Installation of cast-in-situ piles requires careful supervision and quality control of all the materials used in the construction.  2. The method is quite cumbersome. It needs sufficient storage space for all the materials used in the construction.  3. The advantage of increased bearing capacity due to compaction in granular soil that could be obtained by a driven pile is not produced by a cast-in-situ pile.  4. Construction of piles in holes where there is heavy current of ground water flow or artesian pressure is very difficult. Precast piles -Advantages       1. Piles can be precast to the required specifications. 2. Piles of any size, length and shape can be made in advance and used at the site. As a result, the progress of the work will be rapid. 3. A pile driven into granular soil compacts the adjacent soil mass and as a result the bearing capacity of the pile is increased. 4. The work is neat and clean. The supervision of work at the site can be reduced to a minimum. The storage space required is very much less. 5. Driven piles may conveniently be used in places where it is advisable not to drill holes for fear of meeting ground water under pressure. 6. Drivens pile are the most favored for works over water such as piles in wharf structures or jetties. 53
  54. 54. 9/21/2013 Precast piles -Disadvantages       1.Precast or prestressed concrete piles must be properly reinforced to withstand handling stresses during transportation and driving. 2. Advance planning is required for handling and driving. 3. Requires heavy equipment for handling and driving. 4. Since the exact length required at the site cannot be determined in advance, the method involves cutting off extra lengths or adding more lengths. This increases the cost of the project. 5. Driven piles are not suitable in soils of poor drainage qualities. If the driving of piles is not properly phased and arranged, there is every possibility of heaving of the soil or the lifting of the driven piles during the driving of a new pile. 6. Where the foundations of adjacent structures are likely to be affected due to the vibrations generated by the driving of piles, driven piles should not be used. 54
  55. 55. 9/21/2013 Auger Cast-in-situ Piles Non-displacement-Bored 55
  56. 56. 9/21/2013 CFA piles are typically installed with diameters ranging from 0.3 to 0.9 m and lengths of up to 30 m Advantage: No casing is required for stablizing the hole Fig. 27.17 Auger cast-in-situ pile 56
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  59. 59. 9/21/2013 Precast Piles in Pre-bored Holes (IS 2911: Part1-Section 4 Precast piles in prebored holes A pile constructed in reinforced concrete in a casting yard and subsequently lowered into prebored holes and the annular space around the pile ground is grouted through grouting duct. Grouting Duct - A circular hole kept in the center of a precast pile for the purpose of grouting the annual space in the borehole around the pile.  Small and large diameter piles (as per IS2911)  Piles of 600mm or less in diameter are commonly known as small diameter piles while piles greater than 600mm dia are called large diameter piles. The following nominal diameters (in mm) are commonly used in piling: 450, 500, 600, 750, 800, 900, 1000, 1100, 1200 and upto 2000 mm. 59
  60. 60. 9/21/2013 6.6 Spacing of Piles{2911-(part1sec1,2,3,4):2010}  The center to center spacing of piles is considered from two aspects, viz.,  a) practical aspects of installing the piles; and  b) the nature of the load transfer to the soil and possible reduction in the bearing capacity of piles group.  C)Nature of load transfer to the soil and possible reduction in the load capacity of pile group. 6.6 Spacing of Piles: for End Bearing Pile{2911-(part1-sec1,2,3,4):2010}  In case of piles founded on hard stratum and deriving their capacity mainly from end bearing the minimum spacing shall be 2.5 times the diameter of the circumscribing circle corresponding to the cross-section of the shaft.  In case of piles resting on rock, the spacing of 2 times the said diameter may be adopted.  NOTE – In the case of piles of non-circular cross-section, diameter of the circumscribing circle shall be adopted. 60
  61. 61. 9/21/2013 6.6 Spacing of Piles: for Friction Pile {2911-(part1-sec1,2,3,4):2010}  Piles deriving their bearing capacity mainly from friction shall be spaced sufficiently apart to ensure that the zones of soils from which the piles derive their support do not overlap to such an extent that their bearing values are reduced.  Generally the spacing in such cases shall not be less than 3 times the diameter of the shaft. 6.6 Spacing of Piles:old version of code  In case of loose sand or filling closer spacing may be possible since displacement during the piling may be absorbed by vertical and horizontal compaction of the strata. Minimum spacing in such strata may be two times the diameter of the shaft. 61
  62. 62. 9/21/2013 BORED COMPACTION PILE - A bored cast in situ pile with or without bulb(s) in which the compaction of surrounding ground and freshly filled concrete in pile bore is simultaneously achieved by suitable method. If the pile with bulb(s), it is known ‘underreamed bored compaction pile’. UNDER-REAMED PILE A bored cast in situ or bored compaction concrete pile with an enlarged bulb(s) made by either cutting or scooping out the soil or by any other suitable process. 62
  63. 63. 9/21/2013 Fig. 27.16 Bored cast-in-situ under-reamed pile 63
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  65. 65. 9/21/2013 UNDER-REAMED PILES Under-reamed piles are bored cast in-situ and bored compaction concrete types having one or more bulbs formed by enlarging the borehole for the pile stem. •These piles are suited for expansive soils which are often subjected to considerable ground movements due to seasonal moisture variations. These also find wide application in other soil strata where economics are favorable. UNDER-REAMED PILES When the ground consists of expansive soil, for example, black cotton soils, the bulb of under-reamed pile provide anchorage against uplift due to swelling pressure, apart from the increased bearing, provided topmost bulb is formed close to or just below the bottom of active zone. 65
  66. 66. 9/21/2013 All Dimension in mm All Dimension in mm 66
  67. 67. 9/21/2013  In deep deposits of expansive soils the minimum length of piles, irrespective of any other considerations, shall be 3.5 m below ground level.  If the expansive soil deposits are of shallow depth and overlying on nonexpansive soil strata of good bearing or rock, piles of smaller length can also be provided.  In recently filled up grounds or other strata or poor bearing the piles should pass through them and rest in good bearing strata.  The minimum stem diameter of under-reamed pile can be 200 mm up to 5m depth in dry conditions, that is strata with low water table.  The minimum stem diameter for piles up to 5 m depth in strata with high water table within pile depth, shall be 300 mm for normal underreamed pile and 250 mm for compaction underreamed pile.  For piles of more than 5 m depth, the minimum diameter in two cases shall be 375 mm and 300 mm respectively.  The minimum diameter of stem for strata consisting of harmful constituents, such as sulphates, should also be 375 mm. 67
  68. 68. 9/21/2013 The diameter of under-reamed bulbs may vary from 2 to 3 times the stem diameter, depending, upon the feasibility of construction and design requirements. In bored cast in-situ under-reamed piles and under-reamed compaction piles, the bulb diameter shall be normally 2.5 and 2 times the stem diameter respectively. For piles of up to 300 mm diameter, the spacing of the bulbs should not exceed 1.5 times the diameter of the bulb. For piles of diameter greater than 300 mm, spacing can be reduced to 1.25 times the bulb diameter. 68
  69. 69. 9/21/2013 From NBC 2005 The topmost bulb should be at a minimum depth of two times the bulb diameter. In expansive soils it should also be not less than 2.75 m below ground level. The minimum clearance below the underside of pile cap embedded in the ground and the bulb should be a minimum of 1.5 times the bulb diameter. 69
  70. 70. 9/21/2013 Clayey Soils — For clayey soils, the ultimate load carrying capacity of an underreamed pile may be worked out from the following expression: Clay 70
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  72. 72. 9/21/2013 SAFE LOAD TABLE The safe bearing, uplift and lateral loads for under-reamed piles given in Table 1 apply to both medium compact (l0<N <30) sandy soils and clayey soils of medium (4<N < 8) consistency including expansive soils. The values are for piles with bulb diameter equal to two-and-a-half times the shaft diameter. 72
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  75. 75. 9/21/2013 For dense sandy (N>=30) and stiff clayey (N >=8) soils, the safe loads in compression and uplift obtained from Table 1 may be increased by 25 percent. For piles in loose (4< N <10) sandy and soft (2< N <4) clayey soils, the safe loads should be taken 0.75 times the values shown in the Table. For very loose (N < 4) sandy and very soft (N < 2) clayey soils the values obtained from the Table should be reduced by 50 percent. CONCRETE: Bored and Driven castin-situ piles including under-reamed piles    The minimum grade of concrete to be used for cast-insitu piles shall be M-25 and the minimum cement content shall be 400 kg/m3 (Table5-IS456, M-25 mini. Is 300kg/m3)in all conditions. For piles up to 6 m deep, concrete with minimum cement content 350 kg/m3 without provision for under-water concreting may be used under favourable non-ggressive subsoil condition and where concrete of higher strength is not needed structurally or due to aggressive site conditions. The concrete in aggressive surroundings due to presence of sulphates, etc, shall conform to provision given in IS : 456-2000. 75
  76. 76. 9/21/2013 MINIMUM CLEAR COVER  The minimum clear cover over the longitudinal reinforcement shall be 50 mm. In aggressive environment of sulphates etc, it may be increased to 75 mm. Fig. 27.18 A micro pile 76
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