SEISMIC GROUND RESPONSE ANALYSIS AND MICROZONATION STUDIES FOR AHMEDABAD REGION

Transcription

1 SEISMIC GROUND RESPONSE ANALYSIS AND MICROZONATION STUDIES FOR AHMEDABAD REGION by SANDIP S. TRIVEDI Department of Civil Engineering Submitted in fulfillment of the requirements of the degree of DOCTOR OF PHILOSOPHY to the INDIAN INSTITUTE OF TECHNOLOGY DELHI HAUZ KHAS, NEW DELHI , INDIA MARCH 2010

2 D LIBRARY Acc. No. V

3 Dedicated to My loving Parents

4 CERTIFICATE This is to certify that the thesis entitled -Seismic Ground Response Analysis and Microzonation Studies for Ahmedabad Region" being submitted by Mr. Sandip S. Trivedi to the Indian Institute of Technology Delhi for the award of the degree of DOCTOR OF PHILOSOPHY is a record of the bonafide research work carried out by him. Mr. Sandip S. Trivedi has worked under our guidance for the submission of this thesis, which to our knowledge has reached the requisite standard. The thesis or any part thereof has not been presented or submitted to any other University or Institute for any degree or diploma. Dr. K.K. Gupta Associate Professor Prof. K. Seshagiri Rao Professor Department of Civil Engineering Indian Institute of Technology Delhi Hauz Khas, New Delhi INDIA

5 ACKNOWLEDGEMENTS The research work presented in this thesis was carried out from January 2003 to December 2010 at the research group of Geotechnical Engineering, Department of Civil Engineering, Indian Institute of Technology Delhi, India. During this period many people have contributed to this work. I would like to express my utmost gratitude to all of them. First of all, I wish to express my deep sense of gratitude and indebtedness to my supervisors Prof. K.Seshagiri Rao, and Dr. K. K. Gupta, of the Department of Civil Engineering, I.1.T.Delhi for their inspiring guidance, unfailing support, ever extending cooperation and constant encouragement through out the research work. It is because of their great care that this thesis is brought to the present form. I am also thankful to the faculty members of the Geotechnical section, Department of Civil Engineering, I.I.T. Delhi for the cooperation extended throughout the PhD studies. I express my deep appreciation and sincere thanks to my fellow researchers Dr. R. P. Tiwari, Dr. D. Neelima Satyam, Dr. Hossein Noferest, Dr. Sandeep Chaudhary, Dr. Umesh Pendharkar, Dr. B. K. Singh, Mr. Ganesh.W. Rathod. Mr. Tejas Thaker and Mr. Nilesh Chowdary for their discussions and cooperation through out this programme. Thanks are due to the staff of the Geotechnical, Engineering Geology and Computational Laboratories especially, Dr. M. M. Rao, Mr. D. S. Gossain, Mr. D. Biswas, Mr. Bikram Chand, Mr.Amit, Mr. Gehlot and others. I am also thankful to the soil testing laboratories in Ahmedabad. I am extremely grateful to the management of Nirma University, Dr. K Kotecha, Director, Institute of Technology, Nirma University, and Dr. P. H. Shah. Head, Department of Civil Engineering and all faculty members of Civil Engineering in general and Shri Nirzar Lakhiya, Shri Rajendra D. Shah and Shri Kaliprasad Mourya in particular. iii

6 No words are adequate to express my gratitude to my parents and my family for all their pains and sufferings to bring me up to this stage. My special thanks are due to my cousins Elaben and Vandanaben for kind hearted support and motivation. The constant encouragement and inspiration provided by my wife Madhu, and my son Siddharth during the difficult time of work is forever indebted. Date: Sandip S. Trivedi (2002RCE019) iv

7 ABSTRACT Microzonation is the subdivision of a seismic zone into smaller zones according to a certain criterion to facilitate the implementation of seismic mitigation measures. Microzonation has been carried out for major cities in Japan, the USA and other advanced countries in the recent past, based on extensive site-specific studies. However, the microzonation efforts are in a primitive stage in India. Safety against earthquake hazard has two aspects: firstly, structural safety against potentially destructive dynamic forces and secondly the safety of a site itself related with geotechnical phenomena such as amplification, subsidence, land sliding and liquefaction. It is evident that microzonation requires extensive inputs related to seismicity, attenuation of ground motion intensity, geology, geotechnical characteristics, local site effects and susceptibility of local soils to liquefaction. The state of Gujarat, has areas with high seismic activity, with seismic zones varying from III to V. The Kutch region in Gujarat falls in seismic zone V, and has been subjected to high magnitude earthquakes. The Bhuj earthquake of January 26, 2001, in the state of Gujarat in India, struck at 8:46 am Indian Standard time and had a Richter magnitude of 7.7. The epicentre was located at N and E. The city of Ahmedabad, in Gujarat, being in seismic zone III and approximately at 250 Km east from the epicentre, located on the bank of Sabarmati River, received an unexpectedly high damage. There are number of causes put forward, to be responsible for such high damage. One of the major causes envisaged for the extent of damages, is a strong possibility that the recently deposited sediments and loose soils could be a factor contributing to the damage pattern in the city. This region has experienced many earthquakes, in past and recent times, and it also faces the danger of severe seismic threat from the Kutch region. According to seismic

8 zonation map of India, Ahmedabad falls in Seismic zone III. Ahmedabad is having a typical geological set up, which can sustain large amplified shaking not only due to earthquakes in and around Ahmedabad, but also due to strong earthquakes in the Kutch region of Saurashtra. This imposes a very high risk of an earthquake disaster in Ahmedabad in future again, resulting into high casualties and great damage to properties. To determine the potential hazard, appropriate site characterization and determination of the soil properties are very essential in order to design a structure with safety and stability. This establishes the great need of doing seismic microzonation of Ahmedabad region. In the present thesis detailed seismological, geological, geotechnical and geophysical studies have been carried out for the seismic microzonation of the Ahmedabad region. The Ahmedabad region is largely covered by Quaternary alluvium and forms a part of the petroliferous Tertiary Cambay basin. The Cambay basin is one of the three major marginal rift basins of the Indian craton which developed subsequently during India's drift after the break up of Gondwanaland. The architecture of the basin is controlled by three Precambrian orogenic trends viz, the ENE-WSW Satpura trend paralleling the Narmada-Son lineament, the NE-SW Aravalli trend and the NNW-SSE Dharwar trend, Within the Cambay basin, the Deccan basalt forms the basin floor over which Tertiary and Quaternary sediments have been deposited. Combined thickness of these sediments in most parts of the basin varies from 3000 to 5500 m. The steep en-echelon boundary faults delimiting the basin width, simultaneous shoulder uplift of the basin with concomitant sedimentation etc. are some of the major features of the basin. Based on structural styles, such as fault pattern, symmetry, size and orientation of the depression, the Cambay basin is divided into five tectonic blocks, namely, (1) Tharad-Sanchor block, (2) Ahmedabad- Mehsana block, (3) Cambay-Tarapur block, (4) Broach-Jambusar block and (5) Narmada Ankleshwar block. vi

9 Several soil profiles are made covering almost the entire Ahmedabad region based on the large geotechnical borehole data collected from several organizations, and also from confirmatory drilling of boreholes to study the sub soil heterogeneity. Using the data so collected, grain size distribution (GSD) curves are drawn at different depths for different locations of Ahmedabad. These GSD curves are also used in the preliminary assessment of liquefaction potential. The bedrock is not encountered in Ahmedabad up to great depth, hence, for canying out ground response analysis in seismic microzonation, the hypothetical engineering depth to bedrock, on the basis of shear wave velocity has been used. This depth is taken equal to 30m. The soil layers observed in the region at different depths mainly include clayey silt, silty sand, clayey sand, sandy silt, besides the filled up soils at many locations. Also, the data regarding levels of ground water table, has been collected. The X-ray diffraction studies on Ahmedabad soil samples have been conducted. Quartz was the predominant mineral with lesser amounts of kaolinite, illite, mica, chlorite and others. The detailed site characterization of Ahmedabad region is carried out by conducting the geophysical methods i.e., seismic refraction and Multi channel Analysis of Surface Wave (MASW) tests at 54 different locations in the region. The latitude and longitude of the test locations are measured using the GPS system. The 48 channel digital Engineering Seismograph (McSeis SX 48) with a frequency band of 4.5 to 4600Hz is used. Two varieties of geophones i.e., 28 Hz (for refraction tests) and 4.5 Hz (for MASW tests) were used to acquire the seismic data. In the refraction test, 24 geophones were used with a spacing of 3m and seismic energy is generated using propelled energy generator (PEG). And in the MASW testing, a wooden hammer of II kg weight is used as a source generator and 24 geophones with 3 m spacing are adopted. The seismic wave data so generated, has been processed using SeisImager/2D (refraction) and Seislmager/SW vii

10 (MASW) software for getting two-dimensional P and S wave velocity models respectively. The two dimensional P and S wave velocity models for all the 54 locations are made. Also, the 2D contour maps at every 5m interval from the ground surface are also generated for both Vp and Vs. The average shear wave velocity at 30m depth i.e., Vs30 is also calculated and is ranging from 209 to 483 m/s. The detailed site characterization based on Vs30 is done by dividing the area into three zones SA, SB and SC. Several correlations among Vs, Vp, SPT value 'N' and depth are developed as given in Table 1. Table 2 Proposed Correlations for All the Three Zones ZONE: SA ZONE: SB ZONE: SC Vs30 > 370 m/s 270 < Vs30 < 370 m/s VS30 < 270 m/s Vs = D -I2 Vs = D.15 Vs = 116 D -3 Vs = 72 N i5 Vs = 55 N 0 34 Vs = 29.2N.46 Vp = 1.55 Vs Vp Vs Vp = 0.82 Vs + 92 Since Ahmedabad is seismically very active, there is a great need to estimate the peak ground acceleration, which is essential in any seismic hazard analysis. The computer code FINSIM, a finite fault simulation technique is used in this study and based on this analysis three different PGA maps at bedrock level, for three source epicenters from different faults, are generated. The actual instrumented time history of Bhuj Earthquake of 2001, is also used. and the PGA values are also generated using the ProShake software, and other measured input values of shear wave velocity, density, plasticity index, type of soil etc. The final PGA map is generated considering these four sources. The peak ground acceleration at the surface is calculated with help of ProShake software. viii

11 A qualitative and quantitative estimation of site effects is often expressed by the resonance/fundamental frequency, which depends on soil condition and bedrock depth. The Nakamura (1996) method has proved to be the most convenient and reliable technique to estimate fundamental frequencies of soft deposits. The microtremor measurements at 118 sites were performed in Ahmedabad region using MR2002 CE equipment, inclusive of same 54 locations where the seismic refraction and MASW tests were carried out to find the site response. The 30 minutes data is recorded at each test location and analysis was done for getting the average H/V resonance spectra. Based on the analysis, a classification is proposed with three categories (ZA, ZB, and ZC) based on the shape of the HN spectra, predominant frequency, vulnerability index and soil characteristics. Table 2 gives the predominant frequency, vulnerability index value and soil type for each category. Table 2 Range of Fundamental Frequency, Vulnerability Index, Kg and Zones Proposed along with Soil Type for each Classification Type Proposed Classification ZA ZB ZC Soil Type Stiff to hard clayey silt, Dense silty sand/ silty sand with high `N. value Clayey silt, sandy silt and silty sand with seams of clay, relatively higher "N" value, (Older alluvium: Pleistocene) Clayey silt and silty sand with low -N- value, lower density, soft/ loose soils (Newer alluvium: Holocene) Fundamental Frequency Vulnerability Index (Kg) Zone > 3.0 Hz 2.0 to 4.0 SA Hz 4.0 to 8.0 SB < 1_0 Hz > 8.0 SC ix

12 The resonance frequency depends inversely on the soil thickness. This can be used to identify possible areas or structures that may be damaged in the future by earthquakes. The correlation between fundamental frequency and the average shear wave velocity at 30m is developed for Ahmedabad region as given below: \Ism' = * f 0.12 (2) Site specific ground response analysis is required to determine the response of a soil deposit to the motion of the bedrock immediate below the soil and also determining the effect of local soil conditions on amplification of seismic waves and hence estimating the ground response spectra for future design purposes. Soft deposits of soil amplify (increase) certain frequencies of ground motion thereby increasing earthquake damage. Thus local soil conditions have significant role to play, on amplification of seismic waves. The one dimensional equivalent linear ground response analysis for 54 sites is carried out, using ProSHAKE software, and site specific response spectra have been developed. The ground motion data (time history), which was recorded at Passport Office during 26th January 2001 Bhuj Earthquake, is considered. This earthquake time history data, along with other actual site-specific measured data for grading, plasticity, density, shear wave velocity from MASW tests etc., is provided as input data. The graphs for response spectrum for 54 locations of Ahmedabad region have been obtained and are compared with the normalized response spectra available from IS: The zonation map for peak normalized spectral acceleration, is made for Ahmedabad region. Since Ahmedabad falls in the high seismic risk zone, there is a great need for the assessment of liquefaction potential also. With the collected borehole data and PGA values so generated, analysis for liquefaction is attempted using SPT based two methods e.g. Seed and Idriss (1971), and Idriss and Boulanger (2004) and the liquefaction potential map is prepared. Also, using the estimated shear wave velocities liquefaction analysis

13 based on Andrus and Stokoe (2000) method is done. It is clear that the liquefaction is not occurring in places with the Vs greater than or equal to 190m/sec. The liquefaction hazard map with respect to factor of safety is also prepared. These maps will help in selecting a suitable ground improvement technique and a foundation system for future constructions in the region. The microzonation maps generated are very useful for the pre and post disaster mitigation in the event of future earthquakes. xi

14 CONTENTS Page Certificate Acknowledgements Abstract Contents List of Figures List of Tables List of Notations iii xiii xxiii xxxiii xxxvii CHAPTER 1 INTRODUCTION 1.0 General Scope of the Thesis Organization of the Thesis 7 CHAPTER 2 LITERATURE REVIEW 2:0 General Seismic Microzonation Scales of Mapping and Methodology Earthquake and Source Parameters Seismic Waves and Propagation Plate Tectonics Faulting of Rocks Earthquakes: Elastic Rebound Theory Source Parameters Size of Earthquake Magnitude Local Magnitude, ML Surface Wave Magnitude, Ms 20

15 Body Wave Magnitude, Mb Moment Magnitude, Mw Intensity Ground Motion Parameters Amplitude Parameters Peak acceleration Peak horizontal velocity Peak displacement Frequency Parameters Duration Parameters Estimation of Ground Motion Parameters Site Characterization Geological Details Geotechnical Investigations Geophysical Investigations Local Site Effects Methods for Estimating Local Site Effects Empirical Methods Based on Geology and Intensity Based on Geology and Amplification Based on Geotechnical Parameters and Amplification Based on Surface Geology and Response Spectrum Based on Surface Topography Experimental Methods Microtremor Data Weak Motion Data Strong Motion Data Numerical Methods 73 xiv

16 2.6 Ground Response Analysis General Basic concepts Site-specific site response analyses Simplified seismic site response analyses Equivalent-linear one-dimensional site response Analyses Advanced one and two-dimensional site response Analyses General One-dimensional non-linear site response analyses Two-dimensional site response analyses Linear approach Selection of representative time histories Soil Liquefaction Mechanism of Soil Liquefaction Stress Condition at Liquefaction Liquefaction Caused by Seepage Pressure Only: Sand Boils Liquefactioh Caused by Monotonous Loading or Shearing: Flow Slide Liquefaction Caused by Cyclic Loading or Shearing: Cyclic Mobility Factors Affecting Soil Liquefaction Soil Type Relative Density or Void Ratio Initial Confining Pressure Age of the Deposits Entrapped Air Previous Strain History Drainage Condition 97 xv -

17 Intensity of Ground Shaking Duration of Ground Shaking Evaluation of Liquefaction Potential Field Methods SPT Based Methods Factors Affecting Test Results Corrections Applied in SPT Seed and Idriss (1971) Method Seed and Peacock (1971) Method Iwasaki et al. (1982) Method Idriss and Boulanger (2004) Method _1.2 CPT Based Methods L2.1 Robertson and Wride (1998) Method Shear Wave Velocity (Vs) Based Methods Andrus and Stokoe (1997) Method Hatanka et al. (1997) Method Tokomatsu et al. (1986) Method Laboratory Methods Cyclic Triaxial Test Cyclic Direct Simple Shear Test Cyclic Torsional Shear Test Shake Table Test Magnitude scaling factor Seed and Idriss (1982) scaling factor Ambraseys (1988) scaling factor Andrus and Stokoe (1997) scaling factor Youd and Noble (1997) scaling factor Efforts for Seismic Microzonation in India Conclusions and Scope of Present Studies 124 xvi

18 CHAPTER 3 GEOLOGICAL AND GEOTECHNICAL ASPECTS OF AHMEDABAD REGION 3.0 Introduction Geology of Ahmedabad Region Regional Geology Geotechnical Characteristics of Ahmedabad Region Collection and organization of data Detailed Soil Profiles Grain size distribution (GSD) curves X-Ray diffraction analysis Ground Water Levels Bedrock Depths Conclusions 177 CHAPTER 4 SITE CHARACTERIZATION THROUGH SEISMIC REFRACTION AND MASW METHODS FOR AHMEDABAD REGION 4.0 General Geophysical Methods Seismic Reflection Seismic Refraction Method Spectral Analysis of Surface Waves (SASW) Method Multi Channel Analysis of Surface Waves (MASW) Method Details of Equipment Channel Engineering Seismogram Geophones Source Generators 194 xvii

19 Sledge Hammer PEG Seismic Refraction Test Field Testing Program Data Acquisition Analysis of the Data Multi Channel Analysis of Surface Wave Testing Field Testing Program Data Acquisition Analysis of the Data Average Shear Wave Velocity up to 30m Depth Results and Discussion Conclusions 245 CHAPTER 5 SEISMICITY AND GROUND MOTION STUDIES FOR AHMEDABAD REGION 5.0 General Seismicity of Gujarat and Ahmedabad Historical Seismicity Tectonic Features of the Area Estimation of Peak Ground Acceleration A Stochastic Model Earthquake Sources Considered FINS1M Approach Results Discussions and Conclusions 270 xviii

20 CHAPTER 6 LOCAL SITE EFFECTS FROM MICROTREMOR STUDIES 6.0 General Nakamura H/V Ratio Method Field Testing Program Data Acquisition Analysis of the Data Vulnerability Index Results and Discussion Proposed Classification 319 CHAPTER 7 GROUND RESPONSE ANALYSIS FOR AHMEDABAD REGION 7.0 General One Dimensional Ground Response Analysis Linear approach Computing ground surface motion from the bedrock motion Equivalent Linear Approximation of Nonlinear Response Deconvolution Selection of Rock Motions Response Spectrum Analysis with ProSHAKE Software Computation of peak ground acceleration (PGA) Development of Site-specific Response Spectra Soil Amplification Factor 338 xix

21 7.5 Comparison of Response Spectra of IS-1893: 2002 with the Response Spectra obtained for Ahmedabad Sites Results and Discussion Conclusions 359 CHAPTER 8 LIQUEFACTION HAZARD ASSESSMENT FOR AHMEDABAD REGION 8.0 General Liquefaction Potential and Analysis SPT Based Methods Seed and Idriss (1971) Method Idriss and Boulanger (2004) Method Velocity Based Methods Andrus and Stokoe (1997) Method Results and Discussion 401 CHAPTER 9 SUMMARY AND CONCLUSIONS 9.0 General Geological and Geotechnical Characteristics Site Characterization Through Geophysical Testing Seismic Refraction Tests MASW Tests Seismicity and Ground Motion Studies Seismicity of Gujarat and Ahmedabad Region Ground Motion Studies Local Site Effects from Microtremor Studies 416 xx

22 9.5 Ground Response Analysis using ProSHAKE software Soil Amplification factor Liquefaction Hazard Assessment of Liquefaction Potential Preparation of Microzonation maps Limitations and Suggestions 422 REFERENCES 425 APPENDIX A 1D Velocity Models 459 APPENDIX B 2D Vp Models 469 APPENDIX C 2D Vs Models 489 APPENDIX D Liquefaction Analysis Using SPT Methods 509 APPENDIX E Liquefaction Analysis Using Vs Method 547 VITAE 567 xxi