INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 3, No 1, 2012

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1 INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 3, No 1, 2012 Copyright by the authors Licensee IPA Under Creative Commons license 3.0 Research article ISSN Safety check of existing dam against altered seismic hazard conditions Gaurva Verma 1, Verma M.K 2, Tripathi R.K 3 1 M.Tech (Water Resources), Civil Engineering Dept., National Institute of Technology, Raipur, Chhattisgarh, India 2 Professor, Civil Engineering Dept.,National Institute of Technology, Raipur, Chhattisgarh, India 3 Associate Professor, National Institute of Technology, Raipur, Chhattisgarh, India gaurva.verma@gmail.com doi: /ijcser ABSTRACT The paper presents seismic hazard analysis of Gangrel dam situated in Chhattisgarh, India using CADAM. The static and seismic safety of existing concrete gravity dams is a continuous concern owing to the dynamic seismic activities due to the tectonic movement of the earth plates. These tectonic movements results in earthquakes and may alter the important seismic parameters like Peak Ground Acceleration (PGA). Earthquake actions are taken into account pseudo statically through inertia force characterized by a seismic coefficient. The design check of existing dam must be performed to assess whether seismic upgrading of a particular place is necessary from seismic safety point of view. According to NRC (1985) Dam safety must take precedence over all other considerations. CADAM software has been used for design check. Seismic analysis is done using the pseudo static method. Gangrel Dam, a major dam in CG state was constructed in the year Revised Seismic parameter, PGA for this site had been reported in the year With reference to altered value of PGA, seismic hazard analysis for Gangrel dam has been performed and presented through this paper. The Dam stability is checked for altered value of PGA for various loading conditions and it was found to be within safe limits presently. Keywords: Deterministic Seismic hazard Analysis, MCE (Maximum Credible Earthquake), OBE (Operating Basis Earthquake), PGA (Peak Ground Acceleration), Pseudo static Seismic analysis, Seismic Hazard. 1. Introduction Earthquakes are vibrations caused by movement of base rocks along fault surfaces. Most earthquakes occur when the energy stored by elastic deformation in the rocks on both sides of a fault is enough to rupture the rocks or to overcome the friction on an existing fault plane. The deformation is understood as being caused by internal forces such as of convectional, gravitational and magnetic origins. The energy of the earthquake, generated at the fault is radiated outwards by means of elastic waves. As these waves travel through and along the crust of the earth they shake the earth in all directions with varying degree of intensity and the pattern of oscillation changes by refraction, reflection and superposition of one type of wave on others. Generally the magnitude of these waves decrease with distance. The size of an earthquake depends on the amount of energy released. This can be measured by earthquake magnitude. The amount of energy released in turn can be related to the size of the geologic offset, fault parameters and to the consequences of the seismic hazard on people and their environment. These fault movements, ground shaking and landslide can induce seismic Received on June, 2012 Published on August

2 hazard on dams. These in turn result in deformation, liquefaction, slope instability and overtopping of the water of the dam. Earthquakes present a threat to people and the facilities they design and build. Seismic hazard Analysis (SHA) is the evaluation of potentially damaging earthquake related phenomenon to which a facility may be subjected during its useful lifetime. SHA is done for some practical purpose, typically seismic resistant design or retrofitting. SHA involves the quantitative estimation of ground shaking hazards at a particular site. Kramer (1996) considered that the seismic hazards may be analyzed deterministically when a particular earthquake scenario is assumed, or probabilistically, in which uncertainties in earthquake magnitude, locations and time of occurrences are explicitly considered. PGA and Response Spectrum are the main Strong ground motion parameters altered during an earthquake event. Seismic safety assessment consists basically of 1. Seismic Hazard assessment, which includes the seismotectonic features (i.e. fault movements), and the ground shaking (acceleration time histories) for different type of design earthquakes; 2. (Seismic response analysis, which combines the model of the damreservoir foundation system, the material properties and the method of analysis; 3. Performance assessment which includes possible damage assessment. Similar analysis was performed for Hoover Dam using two different approaches. Chopra A.K. and Hanchen T. (1996)applied a 3D linear elastic analyses using EACD3D96 incorporating foundation structure interaction with mass in the foundation, impedance contrast between the dam and the foundation. Payne (1998) and Chopra (2001) studied. hydrodynamic interaction using compressible fluid. Koltuniuk (1997) used a second approach of a nonlinear three dimensional dynamic finite element analysis incorporating concrete cracking and contraction interaction using smeared crack techniques. Mills(1997) studied the kinematic stability analysis of the top of the dam. Gupta, (2002) stated that the seismic hazard analysis is concerned with getting an estimate of the strong motion parameters at a site for the purpose of earthquake resistant design or seismic safety Assessment. CADAM is based on the gravity method (rigid body equilibrium and beam theory). It performs stability analyses for hydrostatic loads and seismic loads. 2. Study area Gangrel is a multipurpose concrete gravity dam constructed across river Mahanadi at N latitude and E longitude in Chhattisgarh, India. The dam was completed in 1979, has a crest length of 455m, a crest thickness of 7m and a maximum base width of 14m. According to Global Seismic Hazard Assessment Program (GSHAP) data, the state of Chhattisgarh falls in region of low seismic hazard with the exception being moderate hazard in areas along Maharashtra and AP state borders. The dam site is considered in seismic zone II as per I.S. 1893(partI): Geology The dam is a part of the Mahanadi Project. Mahanadi project area is located within the Chhattisgarh sedimentary basin. This is formed by an ancient sequence of sand stones, carbonate, rocks (lime stones and dolomites) and shales which are preserved with a down faulted block on the crystalline basement. Stable land surface have developed a thick laterite International Journal of Civil and Structural Engineering 240

3 cover. Sandy alluvium occurs along some reaches of the major river courses and there is a relatively extensive area of coarse alluvium in the Southern part of the command. At the dam site, Chandrapur sand stone (quartzite sand stone) are exposed in the left flank and are available in depth of about 7 to 10 metres for about half the river width. In the remaining width the rock is totally scoured out and is not available even at a great depth. Clay and boulders are available below sand in this portion of the river and on the right flank Methodology The pseudostatic slope stability analysis is done with the conventional limit equilibrium method by using the design ground motion as an input. The analysis is performed for the upstream and down stream slopes of the dam by varying the possible critical cases. The evaluation of the structural stability of the dam against sliding, overturning and uplifting is performed considering two distinct analyses, a stress analysis to determine eventual crack length and compressive stresses, a stability analysis to determine the (i) safety margins against sliding along the considered, and (ii) the position of the resultant of all forces acting on the. The use of the gravity method requires several simplifying assumptions regarding the structural behavior of the dam and the application of the loads 1. The dam body is divided into lift s of homogeneous properties along their length, the mass concrete and lift s are uniformly elastic, 2. All applied loads are transferred to the foundation by the cantilever action of the dam without interactions with adjacent monoliths, 3. There is no interaction between the s, that is each is analyzed independently from the others, 4. Normal stresses are linearly distributed along horizontal planes, 5. stresses follow a parabolic distribution along horizontal plane in the uncracked condition.usbr (1976) Geometric parameters considered The dam parameters and reservoir levels have been reproduced from the software. L4 G L2 L3 F UPSTREAM E DOWNSTREAM H A C L1 D B I Figure 1: Dam geometry International Journal of Civil and Structural Engineering 241

4 The dam section had been highlighted in Fig.1.Considering the geometry the dam base width(l1) is m.The crest width of the dam(l3,l4) is 7.5m.The elevation of point F from Ground Level is 20.5 and the height of dam is 30.5m. The weight of the concrete is 2630Kg/m 3.The Poisson s coefficient was 0.2. The dynamic flexibility of the structure is modeled with the dynamic concrete Young s modulus (Es) 27400MPa. The dam damping on rigid foundation without reservoir interaction is considered to be Any change to these basic parameters affects the fundamental period of vibration and the damping of the damfoundationreservoir system. Thus the spectral accelerations are evaluated. The wave reflection coefficient (α) is the ratio of the amplitude of the reflected hydrodynamic pressure wave to the amplitude of a vertical propagating pressure wave incident on the reservoir bottom. A value of α = 1 indicates that pressure waves are completely reflected, and smaller values of α indicate increasingly absorptive materials.the value of α is considered to be 0.5. The velocity of pressure waves in water is in fact the speed of sound in water. Generally it is assumed at 1440 m/sec (4720 ft/sec).as considering the reservoir levels the Normal Operating Level is considered as 26.2 m and 3.00 m. Gallery is at an elevation of 2.00m at 3.00 m from the heel of the dam. Drain efficiency is Seismic parameters considered International committee on Large Dams,ICOLD(1989) recommendations are followed while evaluating the seismic parameters ;therefore an Operating Basis Earthquake (OBE) and a Maximum Credible Earthquake (MCE) are considered. The OBE is defined as the ground motion with a 50 percent probability of being exceeded in 100 years. The dam, its appurtenant structures, and equipment should remain fully operational with minor or no damage when subjected to earthquake ground motions not exceeding the OBE. The Maximum Design Earthquake (MDE) is the maximum level of ground motion for which the concrete dam should be analyzed. The MDE is usually equated to the MCE which, by definition, is the largest reasonably possible earthquake that could occur along a recognized fault or within a particular seismic source zone. In cases where the dam failure poses no danger to life or would not have severe economic consequences, an MDE less than the MCE may be used for economic reasons. Krinitzsky (2005) highlights that a Deterministic Seismic Hazard Analysis (DSHA) uses geology and seismic history to identify earthquake sources and to interpret the strongest earthquake each source is capable of producing regardless of time, because that earthquake might happen tomorrow. According to USCOLD (1995), the MCE is the largest earthquake that appears possible along a recognized fault under the presently known or presumed tectonic activity, which will cause the most severe consequences to the site. An MDE event should be considered as an extreme loading condition for which significant damage is acceptable, but without a catastrophic failure causing loss of life or severe economic loss. The seismic input is defined in terms of maximum horizontal accelerations and unified response spectra. Sahu T. (2006) evaluated that Regional Recurrence relationship between magnitude, distance and ground acceleration is used when evaluating the maximum horizontal acceleration which is based on the Assessment of Seismic Hazard for Gangrel Dam.For the OBE, a return period of 200 years is selected with a minimum value of 0.5 m/s². For the MCE, not only the results of extremevalue statistics are considered, but also the global geology and longterm tectonic processes are taken into account. The resulting ground accelerations could be considered as approximate values only and, in general, more detailed studies including the local geological International Journal of Civil and Structural Engineering 242

5 situation are necessary for a specific site. The maximum acceleration of the vertical excitation is defined as 2/3 of the respective maximum horizontal acceleration. The first step in the seismicity study of a dam site is to define whether seismic loading of the structures must be incorporated in to the design or not. The usual basis for this initial assessment is the map of seismic activity. Sarma (1975) stated that the absence of any record of an earthquake within 400km of the proposed site is regarded as sufficient justification for regarding it as aseismic. The presence of earthquake record with in limited distance indicates that the Gangrel Dam site is aseismic. The seismic parameters are reproduced from the software and given in Table 1. Table 1: Seismic Coefficients Horizontal Peak Ground Acceleration(HPGA) Vertical Peak Ground Acceleration(HPGA) Horizontal sustained acceleration(hsa) Vertical sustained acceleration(vsa) Pseudostatic (seismic coefficient) 0.100g Earthquake return period 200years g Earthquake accelrogram period(te) g Depth where pressure remains constant g Westergaard correction for Inclined surface 1sec Generalized Corns et al Material and design parameters considered The clay material available in the vicinity of the dam will provide a highly impermeable fill for construction of the dam core. The clay core is of low shear strength and of highly plastic consistency. Due to the high clay content of the proposed clay core fill, several filter zones or a multi stable filter will be required to prevent piping in to the rock fill shells. Different laboratory and field tests have been carried out to estimate shear strength parameters as described in Table 2. Table 2: Material properties Lift Material Properties Material Name Concrete Strength Peak Friction Residual Friction Minimal compressive stress for cohesion(kpa) Fc (kpa) Ft (kpa) Cohesion (kpa) Angle (deg) Cohesion (kpa) Angle (deg) Base Pseudo static seismic analysis (Seismic coefficient method) Pseudo static analysis is similar to the static limit equilibrium analysis routinely conducted by geotechnical engineers. It produces a scalar index of stability (the factor of safety) that is analogous to that produced by static stability analyses. The inertia forces induced by the earthquake are computed from the product of the mass and the acceleration. The dynamic amplification of inertia forces along the height of the dam due to its flexibility is neglected. In International Journal of Civil and Structural Engineering 243

6 the pseudostatic method of seismic stability analysis, some empirical values are adopted for the design seismic coefficient; typically this lies in the range of Sahu, T(1996) calculated the PGA value for Gangrel dam as.05g which has been used for the analysis. Westergaard H.M., (1933) method of Hydrodynamic analysis is considered. 4. Analysis and results Stability Analysis of the dam section has been performed using CADAM with the parameters of the dam as input. The dam section has been checked for various load combinations.the result of stress and stability analysis for usual combination had been presented through Table 3 and Table 4 respectively whereas Table 5 and Table 6 depicts the results of stress and stability analysis for flood combination. It is evident from the results that the stress are within the permissible limits on all the s and the Factor of safety for Overturning and Sliding is quite higher than the desired/safe values as per the code. The results of stress and stability analysis for peak acceleration values and sustained acceleration values for Seismic 1(OBE) has been presented through Table 710 and seismic 2 combinations has been figured in Table It is observed that the dam section is safe for all seismic combinations and the dam is safe against stresses, sliding and overturning at all the s considered. The overall results can be summarized as follows. The dam section is found to be safe for the present PGA values of 0.1g and no further retrofitting measures are required for the section presently. The FOS for sliding and overturning are observed as for usual combination where as required is For flood combination the FOS is observed as whereas required is 1.1. For seismic 1 combination FOS is when required is 1.1 and for seismic 2 combinations it comes to be Table 3: Usual combination (stress analysis) Normal stresses Allowable stresses Upstream Tension compression u/s maximum Elevation (kpa) BASE Table 4: Usual Combination (stability Analysis) Safety Factors Resultants Uplift Upstream Sliding Overturning Uplifting Normal Moment Elevation Peak residual toward KN KN u/s d/s >100 >100 >100 >100 > BASE International Journal of Civil and Structural Engineering 244

7 Upstream Elevation Safety check of existing dam against altered seismic hazard conditions Table 5: Flood combination (Stress analysis) Normal stresses Allowable stresses Tension Compression maximum (kpa) BASE Table 6: Flood combination (Stability analysis) Safety Factors Resultants Uplift Upstream Sliding Overturning Uplifting Normal Moment Elevation Peak residual toward KN KN >100 >100 >100 >100 > BASE Table 7: Seismic #1 combinationpeak accelerations (Stress analysis) Upstream Elevation Normal stresses Allowable stresses Tension compression maximum (kpa) BASE Table 8: Seismic #1 combinationpeak accelerations (Stability Analysis) Safety Factors Resultants Uplift Upstream Sliding Overturning Uplifting Normal Moment Elevation Peak residual toward KN KN > > 100 > BASE Required > >1.000 >1.100 >1.100 >1.100 International Journal of Civil and Structural Engineering 245

8 Table 9: Seismic #1 combinationsustained accelerations (Stress Analysis) Upstream Elevation Normal stresses Allowable stresses Tension Compression maximum (kpa) BASE Table 10: Seismic #1 combinationsustained accelerations (StabilityAnalysis) Safety Factors Resultants Uplift Upstream Sliding Overturning Uplifting Normal Moment Elevation Peak residual toward KN KN > 100 > 100 > BASE Required > >1.000 >1.100 >1.100 >1.100 Table 11: Seismic #2 combinationpeak accelerations (Stress Analysis) Upstream Elevation Normal stresses Allowable stresses Tension compression maximum (kpa) BASE Table 12: Seismic #2 combinationpeak accelerations(stability Analysis) Safety Factors Resultants Uplift Upstream Sliding Overturning Uplifting Normal Moment Elevation Peak residual toward KN KN > 100 > 100 > BASE International Journal of Civil and Structural Engineering 246

9 Table 13: Seismic #2 combinationsustained accelerations (Stress Analysis) Upstream Elevation Normal stresses Allowable stresses Tension compression maximum (kpa) BASE Table 14: Seismic #2 combinationsustained accelerations (StabilityAnalysis) Safety Factors Resultants Uplift Upstream Sliding Overturning Uplifting Normal Moment Elevation Peak residual KN KN toward > 100 > 100 > BASE Required > >1.000 >1.100 >1.100 > Conclusions and recommendations Results presented in this paper demonstrate that the response of concrete gravity damreservoir systems is significantly affected by various static and dynamic loading parameters. The design check of existing dam is performed, for the present PGA value of 0.1g, to assess whether seismic upgrading of Gangrel Dam is necessary from seismic safety point of view. It can be concluded from the present study that the dam section is safe for all possible load combinations and no further retrofitting measures are required for the section. 6. References 1. Chopra A.K, (2001), Dynamics of Structures Theory and Application to Earthquake Engineering. Prentice Hall, Chopra A.K. and Hanchen T., EACD3D96: A Computer program for 3 dimensional analysis of concrete Dam, University of California, Berkeley, California, Report No.UCB/SEMM96/06,October Gupta I.D, (2002), The state of the art in Seismic Hazard Analysis, ISET Journal of Earthquake Technology, 39(4), pp International Journal of Civil and Structural Engineering 247

10 4. ICOLD, (1989), Selecting Parameters for Large damsguidelines and Recommendations, ICOLD Committee on Seismic Aspects of Large dams, Bulletin Koltuniuk, R.M., HVD8110MDA974,Nonlinear Dynamic Structural Analysis of Hoover Dam Including Modelling of Contraction Opening and Concrete Cracking, Bureau of Reclamation, September Kramer, Steven L.1996, Geotechnical Earthquake Engineering, Prentice Hall, pp Krinitzsky, E. L, (1995), Deterministic versus probabilistic seismic hazard analysis for critical structures, Eng Geology, 40, pp Mills,B.L.,HVD8110MDA975, Kinematic studies to determine the stability of postulated independent concrete blocks indicated by the nonlinear analysis of Hoover Dam during Seismic Loading, Bureau of Reclamation, December National Research Council (NRC), (1985), Safety of Dams; Flood and Earthquake Criteria, Washington D.C; National Academy Press. 10. Payne, T.L.HVD8110MDA972, Linear Elastic Dynamic Structural Analysis including mass in the foundation for Hoover Dam, Bureau of Reclamation, April, Sahu, Tejram , NIT, Raipur, Chhattisgarh, Seismic Hazard Analysis of Gangrel and Sondur Dam Sites, M.Tech.desertation. 12. Sarma S. K. (1975), Seismic stability of earth dams and embankments, Geotechnique, 25, pp United States Committee on Large Dams, (1995), U.S. and World Dam, Hydropower and Reservoir Statistics, USCOLD Committee on Register of Dams. 14. USBR, Design of Gravity Dams, (1976), Denver: United States Department of the Interior Bureau of Reclamation. 15. Westergaard H. M., (1933), Water pressure on dams during earthquakes, Transactions ASCE, 98(1835), pp International Journal of Civil and Structural Engineering 248

SAFETY CHECK OF SONDUR DAM FOR CHANGED SEISMIC CONDITION Aryak shori 1, R.K.Tripthi 2 and M. K. Verma 3

ABSTRACT SAFETY CHECK OF SONDUR DAM FOR CHANGED SEISMIC CONDITION Aryak shori 1, R.K.Tripthi 2 and M. K. Verma 3 The paper presents Seismic Hazard Analysis (SHA) of Sondur dam situated in Chhattisgarh