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1. International Journal of Civil Engineering and CIVIL ENGINEERING AND6308 INTERNATIONAL JOURNAL OF Technology (IJCIET), ISSN 0976 – (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME TECHNOLOGY (IJCIET) ISSN 0976 – 6308 (Print) ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December, pp. 101-115 © IAEME: www.iaeme.com/ijciet.asp Journal Impact Factor (2013): 5.3277 (Calculated by GISI) www.jifactor.com IJCIET ©IAEME EARTHQUAKES AND DAMS IN INDIA: AN OVERVIEW K. Jagan Mohan1, R. Pradeep Kumar2 1 2 (Asst. Professor, CED, MGIT, Hyderabad-500075, India) (Professor of Civil Engineering, Earthquake Engineering Research Centre, IIIT-H, Hyderabad-500032, India) ABSTRACT Dam is one of the biggest structures built on the Earth. It is known as a life line structure, as it serves the purpose of irrigation, hydro-electric power generation, flood control, domestic and industrial water supply etc., which are important for human existence. This makes dam as a reliable structure. For this reason, dam should always be designed for highest safety, resisting worst forces of nature. India is a country with over 5,100 large dams. India is also a seismically active country with over 1,040 active faults. Earthquake events like 1988 Bihar, 1991 Uttarkashi, 1993 Killari, 1997 Jabalpur, 1999 Chamoli, 2001 Bhuj, 2002 Andaman, 2004 Sumatra, 2005 Kashmir, and 2011 Sikkim have caused enormous loss of life and property in the country. Also events like 1992 Landers, 1994 Northridge, 1995 Hyogoken-Nanbu and few other events that took place around the world proved how devastating an earthquake could be, particularly if it is near-field. Near-field ground motions could cause more damaging effects on structures, as they were observed to differ dramatically from the characteristics of their far-field counterparts. The propagation of fault rupture towards a site at very high velocity causes most of the seismic energy from the rupture to arrive in a single or multiple large long period pulse of motion, which occurs at the beginning of the record. This characteristic of near-field ground motions could cause damage to a wide range of structures including dams. Several dams that were built in India, which are in highly seismic zones are prone to near-field ground motions. In this regard, behavior of a dams subjected to near-field ground motion should be studied using discrete element modeling, where initiation and propagation of cracks can also be observed. Keywords: Dam, earthquake, near-field earthquake, numerical modeling, discrete element modeling. 101
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME 1. INTRODUCTION Dams are impressive constructions in our world and it is a fascinating chapter of our history to investigate their origin. The history shows, that these constructions are not innovations of nowadays, because the first predecessors have existed even 6000 years before our modern times. Throughout the world, histories of dams have been successful in upholding and enhancing the quality of life. At present, the oldest dams believed to be known are very few. A dam is a barrier or structure across a stream, river, or a waterway for the purpose of confining and controlling the flow of water. Depending upon requirements, construction of a dam can vary in size and material from small earthen embankments to massive concrete structures. Primary purpose of dams being irrigation, hydro-electric power generation, and flood control, domestic and industrial water supply etc. makes these structures as one of the life line structures. As such, dams are cornerstones in the water resources development of river basins. Dams are now built to serve several purposes and are therefore known as multipurpose. With rapid growth of population in India and the consequent demand over water for various purposes, it has now become necessary not only to construct new dams with revised design procedures which can sustain worst forces of nature but also to rehabilitate and maintain existing ones. However, due to lack of technology, people in the past have failed in retaining and rehabilitating the dams. However, there is no unique way to store huge water other than dams. This is why in the present world with new growing technologies; we see different shapes in dams. Some dams are tall and thin, while others are short and thick. And even dams are made from a variety of materials such as rock, earth, concrete etc. varying from small earthen embankments to massive concrete structures. Considering all these parameters, to reach the needs of humans and their activities, construction of dams has become the most important and necessary item which can't be ignored from very beginning of planning for a dam to selection of site, until its construction and maintenance. Natural hazards like earthquake, landslide, cyclone, flood, drought, etc., are quite common in different parts of India. These can create catastrophe leading to the loss of life, property damage and socio-economic disturbances. Such losses have grown over the years due to increase in population and misuse of natural resources. Among all these natural hazards earthquakes are one of the worst and it is also known that it is impossible to prevent earthquakes from occurring. However, the disastrous effects of these can be greatly minimized. This can be achieved through scientific understanding of their nature, causes, and areas of influence. By identifying the areas, population and structures vulnerable to hazards, earthquake disaster mitigation and preparedness strategies to those would reduce miseries to mankind. The study of life line structure like dam is thus required to design resisting worst forces of nature. One such force of nature which could cause failure of dam is an earthquake. 1.1 Causes of dam failures By the end of 20th century, there are over 45,000 large dams built in 150 countries (International Commission on Large Dams – ICOLD). No doubt, the dams provide the mankind with sufficient benefits. However, if any dam breaks or breaches, the large volume of water stored in the reservoir gets suddenly released and flows in the downstream valley resulting in a catastrophe. Thus the analysis of "Dam Failure" has attained significance in concerns of Dam Safety. Every structure which is built will have a life time and so for dams. However, the failures can also occur before the structures life time with more than a few reasons. Occurrences of failures reveal that depending on the type of dam, the causes of failures are of several types. However, the maximum number of failures can be seen in earthen dams as concrete 102
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME masonry dams are stronger because of material properties. A study of dam failures in the world has revealed the percentage distribution of dam breaks and its featured causes of failures. Table 1: Causes of failures of dams around the world with percentage Cause of failure % Cause of failure Foundation problems 40 Inadequate spillways 23 Poor Construction 12 Uneven Settlement 10 High pore pressure 5 Acts of war 3 Embankment slips 2 Defensive materials 2 Incorrect operations 2 Earthquakes <1 Even though the failure of dams caused by earthquakes is < 1%, they still remain a serious threat as they are capable to completely break the dam with the energy released from the event. 1.2 Performance of concrete gravity dams subjected to earthquakes The ﬁrst failure of a dam due to earthquake reported in the literature was Augusta Dam, Georgia, during the 1886 Charleston, South Carolina earthquake. However, the milestone in the seismic analysis of dams turned after the 1967 Koyna earthquake in India where damage was caused to the upstream and downstream side of the concrete gravity dam and 1971 San Fernando earthquake in California where damage was caused to embankment dams (San Fernando dams) and also to an arch-gravity dam (Pacoima dam). Although such ground motions caused problems to dams, no serious damages were observed. However, during some earthquake events, concrete gravity dams were uprooted when blind faults which were lying below the dam body turned active. These very few events have shown that the earthquake hazard continues to be a serious threat to dams, as the failure of a full reservoir concrete gravity dam could cause catastrophe on the downstream. In the epicentral area of the earthquake, a number of concrete gravity dams have experienced ground shaking. However, only about 20 dams have been subjected to 0.3g PHGA or higher without apparent damage. Some of these concrete dams performance to earthquakes are tabulated below. 103
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME Table 2: Concrete dams subjected to significant shaking (PHGA > 0.3g) [Courtesy: USSD Proceedings 2012] Dam (completed) Country Ht. (m) Lower Crystal Springs (1890) USA 47 Koyna (1963) India 103 Williams (1895) USA 21 Bear Valley (1912, 1988) Gohonmatsu (1900) Shih-Kang (1977) Mingtan (1990) USA Japan 28 33 Taiwan 21.4 Taiwan 82 Kasho (1989) Japan 46.4 Uh (___) Japan 14 Takou (2007) Japan 77 Miyatoko (1993) Japan 48 Gibraltar (1920, 1990) USA 52 Pacoima (1929) Ambiesta (1956) Rapel (1968) USA Italy Chile 113 59 111 Techi (1974) Taiwan 185 Shapai (2003) China 132 Hsinfengkiang (1959) Sefid Rud (1962) China 105 Iran 106 Dist. to fault (km) Concrete Gravity Dams San Francisco 0.4 Apr 18, 1906 Koyna 3.0 Dec11, 1967 Loma Prieta 9.7 Oct 17, 1989 Landers 45.0 Jun 28, 1992 Earthquake name and date Big Bear Jun 29, 1992 14.5 Kobe 1.0 Jan 17, 1995 Chi Chi 0.0 Sep 21, 1999 Chi Chi 12.0 Sep 21, 1999 Western Tottori 3.0-8.0 Oct 6, 2000 Western Tottori 1.0-3.0 Oct 6, 2000 Tohoku 109.0 Mar 11, 2011 Tohoku 135.0 Mar 11, 2011 Concrete Arch Dams Santa Barbara Jun ? 29, 1925 Mag. PHGA (g) 8.3 0.52 to 0.68 (est.) 6.5 0.63 (cc) Cracks on both faces 7.1 0.6 (est.) No damage 7.4 0.18 6.6 0.57 7.2 0.83 7.6 7.6 7.3 1.16 9.0 0.38 9.0 0.32 6.3 > 0.3 (est.) 6.6 0.6 to 0.8 6.8 0.53 20.0 6.5 0.36 45.0 7.8 0.31 232.0 7.8 0.302 85.0 7.6 0.5 20.0 Wenchuan May 12, 2008 0.54 18.0 Northridge Jan 17, 1994 Gemona-Friuli May 6, 1976 Santiago Mar 3, 1985 Maule Feb 27, 2010 Chi Chi Sept 21, 1999 7.3 5.0 San Fernando Feb 9, 1971 8.0 0.25 to 0.50 (est.) Concrete Buttress Dams Reservoir 1.1 6.1 Mar 19, 1962 Manjil Jun 21, 1990 0.51 h 0.53 v 0.4 to 0.5 (est.) Near dam site 7.7 0.54 0.71 (est.) Remarks Not the slightest crack Multiple arch modified to gravity dam in 1988. No damage, except slight displacement of crest bridge girders. No damage on this masonry dam Vertical disp. of 9 m, Rupture of concrete. No damage Cracks in control building at crest Small crack at spillway base Cracking of gate-house walls at crest. No damage No damage. Modified in 1990 with RCC. No cracks in arch. Open joint between arch and thrust block. Open joint (2”) between arch and thrust block No damage Damage to spillway and intake tower. Dam performed well. Cracked pavement. Local cracking of curb at dam crest. No Damage Horizontal cracks in top part of dam Horizontal cracks near crest, minor disp. of blocks Notes: Legend: Ht.=height, est.=estimated, Dist.=distance, Mag.=magnitude (ML or MB for less than 6.5 and MS above 6.5), cc=cross canyon, h=horizontal, v=vertical. PHGA=Peak Horizontal Ground Acceleration, disp.=displacement 104
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME Table 2 illustrates about the worldwide performance of concrete (Arch, Buttress & Gravity) dams subjected to ground motions > 0.3g. From the table it can be concluded that concrete dams have performed well when subjected to high intensity accelerations. There might be several reasons why concrete dams have performed well and consistently well than that predicted by design or analysis when shaken by an earthquake. However, the dams present in highly seismic zones are always under threat as some dams have performed less than what was expected. Several factors like magnitude, epicentral distance, PHGA, range of frequency can solely vary the performance of dams subjected to earthquakes. A thorough understanding on the ground motions should be studied for that respective area before the construction of dam. For this huge number of strong motion recorders should be placed at or near the dam sites, which would increase our knowledge over the performance of severely shaken concrete dams and that knowledge could be applied in designing future dams. The most significant factors other than magnitude to be considered in determining the response of concrete dams are the epicentral distance to the dam, PHGA and also the spectral acceleration at the natural frequency of the dam. PHGAs get amplified from the base of the dam to the crest and peak accelerations at the crest would be greater when the reservoirs are full. The epicentral distance also has got its effect when the high velocity energy pulse to hit the dam, causing near-field earthquake effect on dam. Also if the natural frequency of the dam matches with the frequency of the ground motion there would be questions raised over the performance of concrete dams. Even though there are several potential failure modes like foundation problems, settlement, base sliding etc., general accepted failure mode for concrete dams during earthquake is cracks in the concrete of the dam body. Most of the concrete dams listed in Table 2 when subjected to severe shaking were observed with cracks in the concrete and that too at the change in location of geometry. While concrete dams are designed to withstand severe shaking and have performed well in the past, it should not be considered as a positive sign of their performance in the future. Utmost care in design and construction practices should be taken and special attention towards possible faults located near the dam should be given. Before analyzing the structure for earthquakes, it is first important to know the prevalent seismic hazard in India. 2. PREVALENT SEISMIC HAZARD IN INDIA USGS estimates that around 5 lakh earthquakes hit the Earth every year, 1 lakh of those can be felt, and very few cause damage [http://earthquake.usgs.gov/learn/facts.php]. Moreover, in IndianSubcontinent, particularly the north-eastern and north-western regions are the most earthquakesprone regions of the world. 1988 Bihar earthquake, 1991 Uttarkashi earthquake, 1993 Killari earthquake, 1997 Jabalpur earthquake, 1999 Chamoli earthquake, 2001 Bhuj earthquake, 2002 Andaman earthquake, 2004 Sumatra earthquake, 2005 Kashmir earthquake, 2011 Sikkim earthquake are some of the worst hit earthquakes, which cumulatively have caused over 1 lakh death toll. Seismic zonation map clearly shows that India is highly vulnerable to earthquake hazard. During last 100 years, India has witnessed more than 650 earthquakes of magnitude ≥ 5.0 [Kamalesh Kumar, 2008]. In addition to very active northern and north-eastern range, the recent events of 1993 Killari (Maharashtra) and Jabalpur (Madhya Pradesh) in the Peninsular India have started raising doubts as the disasters caused by these earthquakes are alarmingly increasing. Earthquake events reporting from the Himalayan mountain range, Andaman and Nicobar Islands, Indo-Gangetic plain as well as from peninsular region of India belongs to subduction category and a few events had also been under intra-plate category. 105
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME Figure 1: Seismic zonation map of India Himalayan Frontal Arc (HFA) ranging about 2,500 km long extending from Kashmir in the west to Assam in the east undergoes subduction process, making it one of the most seismically active regions in the world. The Indian plate came into existence after initial rifting of the southern Gondwanaland in late Triassic period. Later the force from spreading of the Arabian Sea on either side of the Carlsberg ridge caused to continue drifting since mid-Jurassic to late Cretaceous time to finally collide with the Eurasian plate [Kamalesh Kumar, 2008]. This led to the formation of Himalayan mountain range and the present day seismicity in this region is due to the continuous collision between Indian and Eurasian plates. Some of the most important earthquakes that have occurred during the past century in Himalayan Frontal Arc are tabulated below in Table 3. Table 3: Important earthquakes in Himalayan Frontal Arc (Kamalesh Kumar (2008), http://gbpihed.nic.in) Place Year Magnitude Casualty Kangra Valley April 4, 1905 8.6 >20,000 Bihar-Nepal border January 1, 1934 8.4 >10,653 Quetta May 30, 1935 7.6 about 30,000 North Bihar 1988 6.5 1000 approximately Uttarkashi October 20, 1991 6.6 >2,000 Chamoli March 29, 1999 6.8 >150 Hindukush November 11, 1999 6.2 no death reported Sikkim September 18, 2011 6.9 about 111 The Peninsular India which was once considered as a stable region has started to experience the earthquakes in increased number because of intra-plate mechanism. Even though the magnitudes of these are less and recurrence intervals are larger than those of the HFA, it started to create panic among the inhabitants in this region. Some of the most important earthquakes that have occurred in Peninsular India in the past are tabulated below in Table 4. 106
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME Table 4: Important earthquakes in Peninsular India (Kamalesh Kumar (2008), http://gbpihed.nic.in) Place Year Magnitude Casualty Kachchh June 16, 1819 8.5 No record Jabalpur June 2, 1927 6.5 — Indore March 14, 1938 6.3 — Bhadrachalam April 14, 1969 6.0 — Koyna December 10, 1967 6.7 >200 Killari (Latur) September 30, 1993 6.3 >10,000 Jabalpur May 22, 1997 6.0 >55 Bhuj January 26, 2001 7.6 >20,000 North-eastern region of India which is one of the six most seismically active regions of the world lies at the junction of the Himalayan arc to the north and the Burmese arc to the east. Eighteen large earthquakes with magnitude ≥ 7.0 occurred in this region during the last hundred years (Kayal, 1998). High seismicity in the north-eastern region may be attributed to the collision tectonics in the north (Himalayan arc) and subduction tectonics in the east (Burmese arc). Some of the most important earthquakes that have occurred in this region of India in the past are tabulated below in Table 5. Table 5: Important earthquakes in Northeastern region of India (Kamalesh Kumar (2008), http://gbpihed.nic.in) Place Year Magnitude Remark Numerous earth fissures Cachar March 21, 1869 7.8 and sand craters Shillong Plateau June 12, 1897 8.7 About 1542 people died Sibsagar August 31, 1906 7.0 Property damage Myanmar December 12, 1908 7.5 Property damage 4500 sq km area Srimangal July 8, 1918 7.6 suffered damage S-W Assam September 9, 1923 7.1 Property damage Railway lines, culverts Dhubri July 2, 1930 7.1 and bridges cracked Assam January 27, 1931 7.6 Destruction of Property N-E Assam October 23, 1943 7.2 Destruction of Property Upper Assam July 29, 1949 7.6 Severe damage About 1520 people died. One of the largest Upper Assam August 15, 1950 8.7 known earthquake in the history Indo-Myanmar border August 6, 1988 7.5 No casualty reported. Seismologists seem not to believe that the frequency in the occurrence of earthquakes has increased. Unfortunately, earthquakes of higher magnitudes which use to occur in uninhabited areas or virtually uninhabited areas have hit some thickly populated areas. Consequently, they have killed thousands of people. Increase in the loss of life and property damage is due to increasing vulnerability of human civilization to these hazards. This can be understood by the fact that Kangra event of 1905 (MW=8.6) and Bihar-Nepal of 1934 (MW=8.4) killed about 20,000 and 10,653 people 107
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME respectively. On the other hand 1897 and 1950 events of the northeast (MW=8.7 each) caused death to about 1542 and 1520 people. This is because Kangra and Bihar-Nepal events struck in densely populated areas of Indo-Gangetic plain. On the other hand, the north-eastern region was thinly populated in 1897 and 1950 [Kamalesh Kumar, 2008]. The concentration of population has become denser since the time when such major earthquake occurred in these regions, creating more alarming situation and the devastation it would become if such event occurs now. There are several examples, where high number of casualties and deaths occurred when the event occurred during early morning hours and quite opposite when they occurred during the day time even when the epicenter is too near to the inhabited areas. These examples clearly tell that the time of occurrence of the event and the epicenter also matters, to quantify loss of life and damage to property. 2.1 Large dams in India By the time India got independence in 1947, there were less than 300 large dams in India. At present this number has grown to about 5,100 [National Register of Large Dams – 2009], with 181 dams of national importance. Among 5100 dams, more than half of them were built between 1971 and 1989. As of now India ranks fourth in the world in dam building, after USA, Russia and China. While some of these dams were built primarily for flood control, water supply, and hydro-electric power generation, the primary purpose of most large dams (96%) remains irrigation in India. The bar chart of Figure 2 gives the state-wise distribution of large dams in India. Figure 3 – figure 5 describes the decade wise distribution of large dams in India, state-wise completed dams and statewise under construction dams in India. Figure 2: Bar chart showing State – wise distribution of large dams (existing and ongoing) in India [NRLD – 2009] Figure 3: Distribution of large dams in India - decade wise [NRLD – 2009] 108
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME Figure 4: Pi graph showing State – wise distribution of large dams (completed) in India [NRLD – 2009] Figure 5: Pi graph showing State – wise distribution of large dams (under construction) in India [NRLD – 2009] 2.2 Importance of seismic study on large dams in India India is a seismically active country, with history of major earthquakes occurred in the past. North-eastern and north-western parts of India are seismically very active as the Indo-Australian plate is sub-ducting under Eurasian plate at this region. 1967 Koyna earthquake, 1988 Bihar earthquake, 1991 Uttarkashi earthquake, 1993 Killari earthquake, 1997 Jabalpur earthquake, 1999 Chamoli earthquake, 2001 Bhuj earthquake, 2002 Andaman earthquake, 2004 Sumatra earthquake, 2005 Kashmir earthquake, 2011 Sikkim earthquake are the major earthquakes in the recent past, which resulted in catastrophes, with loss of life and property. These earthquakes also witnessed the failures of different range of structures from small buildings to major dams. The 1967 Koyna earthquake because of reservoir induced seismicity caused damage to the upstream and downstream side of dam with lot of cracks. However, there was no flooding. The 2001 Bhuj earthquake also resulted in failure of large number of earth dams due to liquefaction. However, there was no severe flooding as the region was under drought since 2 year during the time. There were several other occasions where the dams in India have performed poorly because of earthquakes. 109
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME Figure 6: National Importance Dams of India on Seismic Zonation Map Figure 7: National Importance Dams of India placed on Fault & Seismic Zonation map of India The well defined and documented seismic sources, published in the Seismotectonic Atlas2000 are the work done by Geological Survey of India. Seismotectonic details in this atlas include geology, rock type, fault orientation with length, lineaments with lengths, shear zones with length and seismic earthquake events. Geological survey of India has compiled all the available geological, geophysical and seismological data for the entire India and has published a seismotectonic map in 2000. Using a software package OpenJUMP GIS, seismic zonation of India, fault data given by GSI and National Importance Dams given by NRLD are integrated. Fig. 7 shows the seismic zonation map of India, with faults and national importance dams. Fig. 8 shows the seismic zonation map of India with national importance dams and top 100 active faults. The top 100 active faults are selected based on the energy released around these faults in the events of earthquakes from 1064 AD – 2009 AD. 110
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME Figure 8: National Importance Dams of India on Seismic Zonation Map plotted with top 100 active faults 3. SPECIAL FOCUS ON EARTHQUAKE EFFECTS An earthquake is a result of sudden release of strain energy from the rocks in the earth crust, which in turn manifests themselves by shaking and sometimes displacing the ground on the earth’s surface. The amount of energy released by an earthquake is so huge that it can collapse any structure in its vicinity if its magnitude is very high. If the structure lies very close to the source of an earthquake, energy released at the beginning of the event might cause more damage, than to the structure which lies far. These additional characteristics can be observed very close to the epicenter and their intensity reduces to the farther distances depending upon the rupture magnitude and also the properties of the soil. These are thus known as near-field earthquakes. Even though the percentage cause of failure of dams due to earthquakes are very less, there is a possibility that the dam might be very close to an active fault or inactive fault which might become active, causing a rupture which produces the effects of near-field earthquake on dam and might lead to the failure of the dam. The catastrophe that a dam can make over other causes of failures is very high in terms of loss of property and life. Earthquakes alone have got several effects on structures; however, they are not limited to shaking and ground rupture. Landslides, avalanches, fires, soil liquefaction, tsunami, floods, and tidal forces are few of secondary effects. Therefore the study of a structure subjected to an earthquake is necessary and for one of the life line structures like dam subjected to near-field effects, it is compulsory to design it as an earthquake resistant irrespective of seismic sector it is present in. 3.1 Near Field Earthquakes Near-Field earthquake is caused by shear dislocation that begins at a point on the fault and spreads at a velocity that is almost as large as the shear wave velocity. The propagation of fault rupture toward a site at very high velocity causes most of the seismic energy from the rupture to arrive in a Single Large Long Period Pulse of motion which occurs at the beginning of the record [Somerville and Graves 1993]. This pulse of motion represents the cumulative effect of almost all of the seismic radiation from the fault. The radiation pattern of the shear dislocation on the fault causes this large pulse of motion to be oriented in the direction perpendicular to the fault, causing the strikenormal peak velocity to be larger than strike-parallel peak velocity. 111
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME • • Strike – normal refers to the horizontal component of motion normal to the strike of the fault. Strike – parallel refers to the horizontal component of motion parallel to the strike of the fault. Figure 9: Fault Normal and Fault Parallel components of 1994 Northridge earthquake [Paul Somerville, 2005] 3.2 Important features of near-fault ground motions Near fault ground motion comprise of velocity pulse. And the two main causes being directivity and fling • • Large velocity pulse Two causes of large velocity pulses – Directivity – Fling 3.2.1 Directivity: It is related to the direction of rupture front. It is a two-sided velocity pulse due to constructive interference of shear waves generated from parts of the rupture located between the site and epicenter. It occurs at sites located close to the fault however, away from the epicenter or near the epicenter depending on the wave propagation. The two kinds of directivity are a) Forward Rupture Directivity b) Backward Rupture Directivity a) Forward Directivity: This occurs when these conditions are met. When shear wave velocity coincides with the rupture velocity, the rupture propagates toward the site (site away from the epicenter), and when the direction of slip on the fault is aligned with the site Forward Rupture Directivity effect occurs. It is readily met in strike-slip faulting. And not all near fault locations will experience forward rupture directivity in an event. b) Backward Directivity: A backward directivity effect occurs when the rupture propagates away from the site (site near the epicenter). 112
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME Figure 10: 1992 Lander’s earthquake, showing the Forward and Backward Directivity region [Paul Somerville, 2005] 3.2.2 Fling: It is a one-sided velocity pulse due to tectonic deformation. It is related to the permanent tectonic deformation at the site. Fling occurs at sites located near the fault rupture, independent of the epicenter location. 4. IMPORTANCE ON STUDY OF NEAR-FIELD EARTHQUAKE EFFECTS ON DAMS The Northridge–1994 (Mw 6.7) and Hyogoken-Nanbu-1994 (Mw 6.8) earthquakes have revealed that near-field ground motions have very damaging effects on structures, if they were not adequately taken into account with seismic design guidelines. Beginning with Landers earthquake (Mw 7.3) of 1992, strong motion data began to be recorded from near-field stations located within a few kilometers of the plane of fault rupture. These ground motions were observed to differ dramatically from their far-field counterparts. They were characterized by distinct large amplitude single or multiple pulses, large velocity pulses, forward rupture directivity and larger ratio of vertical-to-horizontal components ratio (V/H), which was viewed as damaging criteria. Other records from U.S (for example Pacoima Dam site) and Japan show similar pattern. The Near-Field pulse-like velocity and displacement time histories associated with a strong earthquake can greatly affect a wide range of different types of structures. Concerns about the seismic safety of concrete dams have been growing during recent years, partly, because the population at risk in locations downstream of major dams continues to expand and also because it is increasingly evident that the seismic design concepts in use at the time most existing dams were built were inadequate. Since the Northridge and Hyogoken-Nanbu (Kobe) earthquakes, there has been much discussion about the adequacy of design practice of concrete dams. The hazard posed by large dams has been demonstrated since 1928 by the failure of many dams of all types and in many parts of the world. However, no failure of a concrete dam has resulted from earthquake excitation; in fact the only complete collapses of concrete dams have been due to failures in the foundation rock supporting the dams. The 1999 Chi-Chi, Taiwan earthquake (MW 7.6) has witnessed collapse of Shih-Kang dam, which is 50 km from the epicenter. This dam has failed due to differential thrust fault movement. Over two thirds of the dam body were uplifted about 9 m vertically and displaced 2 m horizontally. The dam experienced horizontal accelerations up to 0.5g. However, the damage was confined to only two bays overlying the fault rupture. The reservoir slowly drained through the failed bays, without causing major ﬂooding. This example clearly shows how disastrous an earthquake could be if an active fault is lying near the dam site. 113
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME Figure 11: Surface faulting caused major damage to Shii-kang Dam (Image Source from internet) In fact, lots of major dam sites are located alongside major active faults and so could be subjected to near-field ground motions from large earthquakes. Knowledge of ground motion in the near-field region of large earthquakes is limited by the scarcity of recorded data. The near-field of an earthquake (also called near-source or near-fault region) is the region within which distinct pulse-like particle motions are observed due to a coherent release and propagation of energy from the fault rupture process. For damaging earthquakes, the near-field region may extend several kilometers outward from the projection on the ground surface of the fault rupture zone and its extension to the surface, particularly in the direction of rupture propagation. The near-field ground motions are characterized by high peak acceleration (PGA), high peak velocity (PGV), high peak displacement (PGD), pulse-like time history, and unique spectral content. The nature of near-field ground motions differs significantly from that of far-field ground motions. Therefore it is crucial that these near-field effects be identified and thoroughly understood, and that appropriate mitigation measures are found to deal with these special ground motions. 5. NUMERICAL METHOD Numerical methods for the analysis of structures can be broadly classified in to two. The first one is based on continuum mechanism. Finite Element Method (FEM) [Jr. William Weaver, James M. Gere, 1966] is one such example. However, it cannot perform the analysis up to collapse because of limitations that exist in representation of cracks and separation distance between elements. FEM can answer only one question “will the structure fail or not?” it can’t tell how the structure collapse On the other hand, second category of numerical methods is based on discrete element methods, like Extended Discrete Element Method [Williams J.R, Hocking G, and Mustoe G.G.W, 1985; A.A. Balkema, Rotterdam, 1985] for nonlinear analysis of structures. This method can track the behavior from zero loading to total collapse of structure. However, this method is less accurate than FEM in small deformation range. So this can answer only the second question “how does the structure collapse?” To follow total structural behavior from small deformation range to complete collapse, a unique, efficient and accurate technique is required. Tagel Din Hatem (1988) gave a new method of analyzing the structural behavior from zero loading, crack initiation & propagation, separation of structural members till the total collapse with reliable accuracy, and with relatively simple material models. The method is now known as “Applied Element Method” (AEM) and is widely in usage. 114
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME 6. CONCLUSIONS In the country with 5,100 large dams and 1,040 active faults covering 57% of land mass making prone to earthquakes, there is always a possibility that a severe earthquake in highly seismic zones might affect the performance of dam. In this regard, a discrete element modeling has to be carried out where the behavior of structure can be observed from zero loading, crack initiation to complete collapse of structure. 7. REFERENCES        Dam Safety Organization, Central Water Commission “National Register of Large dams”, 2009. Earthquake facts, “http://earthquake.usgs.gov/learn/facts.php”, United States Geological Survey (USGS). Abdolrahim Jalali, Tatsuo Ohmachi, “Aspects of Concrete Dams Response to Near-Field Ground Motions”, The 12th World Conference on Earthquake Engineering, 2000. Kamalesh Kumar, a text book on “Basic Geo-technical Earthquake Engineering”, 2008. Larry K Nuss, Norihisa Matsumoto, Kenneth D Hansen, “Shaken but not Stirred – Earthquake Performance of Concrete Dams”, USSD Proceedings, 2012. Paul G Somerville, “Engineering Characterization of Near-Fault Ground Motions”, NZSEE conference, 2005. Najmobaidsalim Alghazali and Dilshad A.H. Alhadrawi, “Mathematical Model of RCC Dam Break Bastora RCC Dam as a Case Study”, International Journal of Civil Engineering & Technology (IJCIET), Volume 4, Issue 2, 2013, pp. 1 - 14, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316.  R Pradeep Kumar, K Meguro, “Applied Element Simulation of non-Linear Behaviour of DipSlip Faults for Studying Ground Surface Deformations”, Seismic Fault-induced Failures, pp. 109114, January 2001.  Stevel L Kramer, a text book on “Geo-technical Earthquake Engineering”, 2008.  Tagel-Din-Hatem, “A New Efficient Method for Nonlinear, Large Deformation and Collapse analysis of Structures”, PhD Thesis (1998), The University of Tokyo, Japan.  T K Dutta, a text book on “Seismic Analysis of Structures”, 2010.  Ming Narto Wijaya, Takuro Katayama, Ercan Serif Kaya and Toshitaka Yamao, “Earthquake Response Of Modified Folded Cantilever Shear Structure With Fixed-Movable-Fixedsub-Frames”, International Journal of Civil Engineering & Technology (IJCIET), Volume 4, Issue 4, 2013, pp. 194 - 207, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316.  Tatsuo Ohmachi, Abdolrahim Jalali., “Fundamental Study on Near-Field Effects on Earthquake Response of Arch Dams”, Earthquake Engineering and Engineering Seismology, Volume 1, Number 1, pp. 1 – 11, September 1999.  Vidula S. Sohoni and Dr.M.R.Shiyekar, “Concrete–Steel Composite Beams of a Framed Structure for Enhancement in Earthquake Resistance”, International Journal of Civil Engineering & Technology (IJCIET), Volume 3, Issue 1, 2012, pp. 99 - 110, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316.  Worakanchana Kawin, Kimiro Meguro, “Failure Mechanism of Shih-Kang Dam by Applied Element Method”, New Technologies for Urban Safety of Mega Cities in Asia, pp.119-128, October 2005. 115