Assessment of seismic vulnerability of Agartala town based on site response analysis

Transcription

1 INDIAN GEOTECHNICAL CONFERENCE Assessment of seismic vulnerability of Agartala town based on site response analysis Vedatri Acharya, MTech, NIT Agartala, Dr. Rajib Saha, Assistant Professor, NIT Agartala, Animesh Pandey, Post Graduate student, NIT Agartala, ABSTRACT: Assessment of local site effect on seismic ground motion is of great importance in geotechnical earthquake engineering practice. The influence of local soil conditions on the nature of earthquake damage has been recognized for many years. Local site conditions profoundly influence most of the important characteristics mainly the acceleration amplitude and frequency characteristics of ground motion during an earthquake. The extent of this modification depends on the geometry of the soil profile, thickness and properties of the soil profile and characteristics of the input motion. The local site effects on the ground motion are commonly evaluated by carrying out one-dimensional ground response analysis. According to seismic zonation map of India (BIS 2002), the study area Agartala town, capital of Tripura is situated in north-eastern part of India which is categorized as zone V, the highest level of seismic hazard potential zone. The Global Seismic Hazard Assessment Programme (GSHAP) also classifies the region in the zone of high seismic risk attributing peak ground acceleration (PGA) to the tune of g. Hence, site response analysis is felt to be important for evaluating the seismic risk of this particular area. The primary backdrop for carrying out site response analysis in this study area is the lack of recorded strong motion data. Hence, this paper is a humble attempt to carry out site specific study where limited strong motion data is available. A wavelet based spectrum compatibility approach is considered to generate synthetic ground motions consistent with BIS (Bureau of Indian standard) spectrum curve. Such synthetic ground motions are used to carry out site response analysis and subsequent prediction of seismic response of structure in order to assess the seismic vulnerability of structure present in that area. As a first step, several earthquake motions of fault normal and parallel available in PEER database were selected attributing similar nature of ground motion parameters and fault mechanisms of past earthquakes occurred in north-east India. The generated synthetic motion is validated with codal spectrum curve to verify the sanctity of the technique. One-dimensional site response analysis is performed for limited site locations at Agartala municipal area based on subsoil parameters investigated upto 20m depth with an input of generated synthetic motions. A finite element based code OPENSeeS is used for modeling and to carry out the analysis. However, an equivalent linear approach based code, such as DEEPSOIL, is also used for validation of OPENSEES results. The validation shows a good agreement of results obtained from OPENSeeS and DEEPSOIL. This study gives significant input in order to consider the site effect for assessment of vulnerability of structures which may help to modify the conventional design practice. Keyword: Synthetic ground motion, Response spectra, one-dimensional site response analysis, de-convolution.

2 INDIAN GEOTECHNICAL CONFERENCE INTRODUCTION The term seismic vulnerability is defined as the susceptibility of ground shaking at a particular area. The strength and duration of ground shaking at a particular site depends upon the location of earthquake, intensity of ground motion and on the geological and seismological characteristics of that site. As a result, the performance of structures depends upon the ground condition of the site and the ground motion experienced by this foundation. Hence, the response of structures is essentially a function of the regional seismicity, the nature of source mechanism, geology and local soil conditions which is called site effect. Local site conditions such as soft sediments of considerable depth strongly affect the amplitude of ground motions as reported in many studies. The groundmotion amplification can cause subsequent ground failures such as liquefaction, structural failures or landslides due to excessive ground shaking. It is observed that different experimental, analytical and numerical approaches were adopted to carry out site response analysis and significant observations were also addressed in past literatures [1, 2, 3, 4, 5, 6]. These studies indicate that evaluation of seismic hazards and microzonation of different urban areas is one of the most important steps to characterize the potential seismic areas which have similar exposures to hazards of earthquakes and these results can be used for designing new structures or retrofitting the existing ones. Naik et al. [7] has recently presented 2D ground response analysis and liquefaction hazard analysis of alluvial soil deposits from Kanpur region along Indo-Gangatic plains with some significant inputs for designing the future structure. Few researchers [3,8] has carried out microzonation mapping based on shear wave velocity and site response analysis of Kolkata Municipal area. However, Chowdhuri et al [9] has carried out a site response study by covering 76 sites in and around the study are Agartala using digital micro earthquake recorders with short period SS-1 seismometers. The study presents that in the northern part of this study area, SR (site response) varies from 1.15 to 1.85 corresponding to peak frequency 0.76 to 0.93 Hz where soil is mostly semi-consolidated and stiffer than recent Quaternary deposits (Haora river formation). In the southern part of this study area, SR varies from 1.12 to 2.42 corresponding to peak frequency from 0.71 to 0.85 Hz within the Dupitila formation (early Quaternary). This reflects that site response by H/V method is influenced by impedance contrast, whereas computed amplification from 1-D model shows opposite trend. Sil et al. [10] has performed a probabilistic seismic hazard analysis considering the available earthquake catalogs for the states of Tripura and Mizoram in North - East India where it has observed that Tripura is more vulnerable than Mizoram where PGA value varies from 0.06 to 0.5g (2% probability exceedance) and g (10% probability exceedance). The previous studies gives crucial experimental and probabilistic findings on Agartala municipal area in limited form. From this viewpoint, present study is an attempt to carry out detailed site response analysis theoretically based on bore hole data in order to verify the earlier results and to develop local site spectra along with other important seismic design inputs, such as, ground shaking intensity, amplification, resonating frequency and liquefaction susceptibility, which may be useful for the city planners/designers. This study may be considered as one of the important steps of seismic microzonation of Agartala town. SEISMOLOGICAL AND TECTONIC DETAILS OF NORTH-EAST INDIA The entire north-eastern India and adjoining regions comprises of one of the seismically highly active belts of the global surface. These two mobile belts meet at the eastern syntaxis zone which was the source zone of the 1950 Assam earthquake (Mw 8.7) [9], 1897 Shillong earthquake of (Mw 8.7) and two large earthquakes (Mw > 7.0). Fig 1 represents the tectonic map showing the location of the proposed study area within north-eastern region of India. Agartala, the capital town of one of the North Eastern state Tripura is located between

3 INDIAN GEOTECHNICAL CONFERENCE latitudes N and longitudes E of world map covering an approximate area of sq-km. The earthquake fault line which is surrounded by the study area and its adjoining areas is shown in Fig 2. It is observed that there are three active faults near Tulashekhar, Chamanu which are relatively near to the study area. GEOTECHNICAL DATA The geotechnical characteristics of geological layer up to the depth of first m is very important in amplification of earthquake shaking. Ten numbers of bore-hole (BH) locations spread over four different zones within Agartala municipal area are selected. Fig 3 shows these BH locations along with their GPS directions in the map. Extensive geotechnical site investigation data of these locations are collected and analyzed. Present paper is limited to the analysis of BH01, BH07, BH08, BH09 and BH10 which is categorized as Type 1 soil profile based on similarity in geological layers. Type 1 profile consists of four distinct layers which contains dark brownish silty clay as well as light medium sand. The surface geology in and around Fig. 2 Fault map of Tripura [11] agartala is rather uniform characterized by the presenae of dark brownish sandy clayey sand overlying a relatively coarser sediments consisting of either fine to light medium sand or coarse sand. The maximum depth of these selected soil deposits are 20m from the existing ground level. The watertable is located maximum up to 1.5m below the ground level. The shear wave velocities of each soil layer are obtained by using standard Fig. 1 Tectonic map showing the location of the study area within north-eastern part of India [9] Fig. 3 Location of bore-holes in the map of study area (Reproduced)

4 INDIAN GEOTECHNICAL CONFERENCE Fig. 4 Bore-hole deposit of Type1, variation of standard penetration (N) value and shear wave velocity with the layers of this bore-hole deposit. penetration data from the empirical relation suggested by Japanese Road Association [12]. Fig 4 shows the soil deposit of location Type 1, variation of standard penetration data (N) value and shear-wave velocity with depth which is one of the most important parameter in site-response analysis. GENERATION OF SYNTHETIC EARTHQUAKE MOTION According to seismic zoning map of India, Bureau of Indian Standards [13] Tripura, lies in moderate seismic zone (Zone v) with a zone factor of 0.36 g and expected earthquake of magnitude is in the range between according to past recorded earthquakes of Tripura and its nearby north-eastern region. The Global Seismic Hazard Assessment Programme (GSHAP) which is based on 10% probability of exceedance in 50 years also classifies this area in the zone of high seismic risk with peak ground acceleration rising to the tune of g. Sarangi et al. [14] found that the PGA value of Tripura and its adjoining northeastern region ranges from 0.05g to 0.6g for 10% exceedance while the PGA value ranges from 0.01g to 0.4g for the 20% exceedance in 50 years. Hence, in this present study, the maximum peak ground acceleration (PGA) of g is selected for Agartala town which is the capital of Tripura to study the effect of local soil conditions due to seismic excitation. In seismically active regions, large amount of ground motion data are available and by analyzing those ground motion records a specific precaution can be taken to reduce the earthquake damage. But especially the regions where ground motion records for taking prediction from various earthquake hazard and saving the structures of that particular area from damage, are not clearly available then the approach of generation of synthetic ground motion based on the knowledge of earthquake source, mechanism of fault, path-effects and previous recorded ground motions of that particular region is of great importance As this study area have very few strong ground motion records are available so it is very necessary to develop synthetic ground motions in this region. A wavelet-based method [15] has been used for the generation of spectrum-compatible time history of earthquake for Agartala town. Other programs such as RSPMatch, SEISMOMATCH can equally be used. As a first step in this methodology is to use the strong ground motion records which are available from past historical earthquakes. Various important factors, such as, similar type of faulting, peak acceleration close to the target value, and similar range of magnitudes and similar site conditions are considered for matching selected suitable strong motion with the generated synthetic motion. Various major earthquakes that had occurred in Tripura and its nearby region at past like 1897 Shillong earthquake (M 8.7), 1918 Srimangal Earthquake (M 7.6), 1950 Assam earthquake (M 8.7) caused much damage are from Himalayan frontier and the Arakan-Yoma ranges and the occurrence of earthquakes in this region may be considered due to thrust fault and strike-slip fault mechanism on the basis of tectonic map of northeast India. Based on these factors, three earthquakes of magnitude (M) in the range of recorded at rock sites (site class A as per USGS classification for which shear wave velocity, Vs [>750 m/s or response spectra for rock/hard

5 INDIAN GEOTECHNICAL CONFERENCE soil] as per IS1893 (Part I) [13] were selected from the pacific Earthquake Engineering Research Centre (PEER) database, University of California at Berkeley. According to International Building Code [16] based on the shear wave velocities for the top 30 m depth of soil deposit, the soil sites in this region can be classified as class D. Further, as per Indian Standard (IS) code of practice [13], the soil sites are classified as rock or hard soil sites from IS code and spectrum-compatible time history of accelerations (synthetic ground motions) were developed. The response spectrum of these modified synthetic ground motions has been verified with the target spectra of IS-1893: (Part I): After matching these generated spectra with the target spectra synthetic response spectrum has been developed. MODELLING AND METHOD OF ANALYSIS 1-D site response analysis is performed for layered soil deposits underlain by an elastic half-space which represents the finite rigidity of an underlying medium such as bedrock. The groundwater table is located at particular depth, therefore, saturated unit weights are used for the soil below this point and effective stress analysis is considered through the use of nine-node quadrilateral elements which are able to simulate fluid-solid coupling. These elements are rectangles with nine nodes with corner nodes in blue having 3 degrees of freedom (i.e. two for translational, one for pore pressure degree of freedom) and the interior nodes in red having two translational degrees of freedom. Fig 5 shows the site response analysis model using OPENSeeS code. Lysmer-Kuhlemeyer dashpot is used at the base of soil column. The nodes at the base of the soil column are fixed against vertical translation and the pore pressure nodes above the groundwater table are fixed in the pore pressure dof (representing an open drainage condition). Element nodes in the horizontal direction are connected by equal-degrees of freedom. The base of the soil profile is considered as fixed. Dynamic excitation is applied as a force time history to the base of the soil column, at the node which shares equal-degrees of freedom with Lysmer-kuhlemeyer dashpot. Nonlinear time history analysis is performed to obtain the dynamic response of the finite element model of soil-piled raft-structure system and fixed base structural system. Newmark s method considering a small time step of T/100 is used to solve the dynamic motion of equation, where T is the fundamental time period of the structure. Thus acceleration, displacement at these nodes can be captured in recorder files which can be used for the calculation of maximum acceleration and maximum displacement and amplification of ground motion between layer by layer of soil deposit. The following assumptions have been taken in this 1-D analysis. 1. Newmark displacement parameters gamma and beta are assumed as 0.5 and 0.25 respectively. 2. The element nodes at the same level are tied together by equal degrees of freedom, causing them to translate by the same amount in vertical and horizontal directions. 3. Dashpot coefficient is assumed as the product of the mass density and shear wave velocity of the underlying layer with the area of the base column. (This is modeled as zero length elements in OpenSees). 4. Force time history applied along the dashpot is assumed as product of velocity time history with the base area of the soil column. 5. Variation of Poisson's ratio along with the depth is not considered, rather it is considered to be constant. 6. The soil constitutive behavior is modeled using pressure dependent multi-yield material and pressure independent multi-yield material. For sand layer pressure dependent multi-yield material and for clay layer pressure independent multi-yield material is used. 7. The recorders are separated in to two groups based on the analysis stages. The information recorded by the gravity recorders is useful to confirm that the model has been generated properly and that the appropriate conditions

6 INDIAN GEOTECHNICAL CONFERENCE exist prior to the application of the horizontal excitation and information recorded by dynamic recorders are for further analysis purpose. In this analysis one-dimensional equivalent linear approach is used where the results such as acceleration, displacement are obtained in x and y direction and other parameters such as stress, strain are recorded in x, y and x-y direction. concluded that OPENSeeS model may give reasonably accurate prediction of response. The site response analysis results using OPENSeeS are presented considering various parameters such as peak ground acceleration (PGA), peak ground displacement (PGD), amplification ratio, shear stress strain relationship and generation of response spectra considering present site location. Fig. 6 Acceleration time history response at the ground surface of the soil column for LomaPrieta ground motion. Fig. 4 Site response analysis model RESULTS AND DISCUSSION A study is attempted to validate the results obtained from 1D finite element model developed in OPENSeeS using well accepted onedimensional, equivalent linear site response analysis program DEEPSOIL. The validation is performed for a particular subsoil profile (Type I) with the application of same deconvoluted synthetic motions as considered in present study. Fig 6 presents the acceleration response history obtained from OPENSeeS and DEEPSOIL for Type I soil profile recorded at central node of top element near to ground surface under LomaPrieta ground motions. Similarly, Fig 7 presents shear strain history for the similar motions and same nodes of soil column. Results indicate a concordant trend in response parameters obtained from OPENSeeS and DEEPSOIL. Further, it may be Fig. 7 Shear strain time history response at the ground surface of the soil column for LomaPrieta ground motion. PEAK GROUND ACCELERATION (PGA) PEAK GROUND DISPLACEMENT (PGD) The PGA of input spectrum-compatible deconvolved motions used in present study varies within a range of ( g). While, the maximum acceleration (PGA) at the ground surface varies over a range from 0.75g to 1.92g for Abhoynagar area of soil profile (Type 1) which is around 2.3 to 3.8 times of the PGA of input motion

7 INDIAN GEOTECHNICAL CONFERENCE as presented in Fig 8. It is to be noted that the time for maximum acceleration at the ground surface for site locations having category of Type 1 is not the same for all the ten events. The results obtained from this limited study with a simplified modelling clearly show a possibility of significant increase in PGA which may lead to a detrimental consequences of the structure and may be dealt with priority. Similar observation on amplification of motion for Agartala was given by earlier researchers [9]. The amplification of ground motion is defined as the ratio of any intensity measure of the motion measured at the soil surface to the counterpart value at the bedrock level. In present study, the ground motion amplification for each frequency is obtained. For instance, the maximum amplification of ground-motion parameters considering ten de-convolved spectrum compatible input motions for Type 1 location (Abhoynagar) is shown in Fig. 9. Fig. 9 indicates that the maximum amplification ratio (A) varies over a range of for the corresponding frequency (f) range of Hz and Hz respectively. Fig. 8 PGA versus Depth for Type 1 STRESS-STRAIN Figs. 10 presents induced shear stress with respect to shear strain recorded at the top element, i.e., near ground surface for Type1 soil profile. A significant increase in shear stress for low strain level is observed for Type 1 which may result to tension failure at the ground surface during a strong earthquake at Agartala Municipal area. However, these curves will be helpful to obtain the percentage of critical damping for each site and may be verified with the experimental results leading to a future scope of study. Fig. 9 Amplification ratio for Type 1

8 INDIAN GEOTECHNICAL CONFERENCE SUMMARY AND CONCLUSIONS Fig. 10 Shear-stress, shear-strain for Type1 RESPONSE SPECTRA Fig 11 presents the variation of spectral acceleration (Sa) with respect to period of structure corresponding to 5% damping for Type1 (Abhoynagar) location in Agartala town based on ten earthquake input motions. The maximum spectral acceleration varies from 0.20 to 0.40g. However, based on the average of all ten earthquakes, the maximum spectral accelerations of 0.3g may be obtained for respective locations. It may be observed that the normalized spectral values exceed the code specified values at some periods which may be a significant input for designing structures in this particular area and indicates the importance of consideration of local site effect in seismic design. Spectral Acceleration (g) Northridge (St-Arleta) Lomaprieta (St-Cap 090) Sanfernando Lomaprieta (St-Cor 090) Lomaprieta (St-Cor 091) Lomaprieta (St-Coyoto dam) Fruili,Italy Lomaprieta (St-Santa-cruz) Northridge (St-Pacoima-dam) Tabas,iran Period (s) Fig. 11 Spectral accelerations for Type1 location Present study is a limited attempt to carry out sitespecific seismic response analysis of soil deposits for Agartala town which is located in a high seismic potential zone as reported in BIS [13]. Physiographic and geologic setting indicates that the central area of the town consists of river transported alluvial deposits and extended outskirt area is of dupitilla land deposits. Seismicity of the study area was outlined. The relevant data of subsoil layers at different locations were collected from geotechnical investigation reports in order to study the effect of local soil conditions on the earthquake ground-motion parameters. The peak ground acceleration at rock level sites for the study area is obtained from Global Seismic Hazard Assessment Program (GSHAP) map and IS Part I (2002) code. Synthetic ground motions compatible to codal spectra were generated from wavelet based target spectrum matching code WAVGEN. The obtained response parameters are analyzed and inferred in the light of seismic vulnerability of that area which may give significant input to the designers/planners for designing structures considering available seismic code. REFERENCES 1. Mukhopadyay, S., Bormann, P. (2004), Low cost seismic Microzonation using Microtremor data, Journal of Asian Earth sciences, Rao, Purnachandra, M, Ravi.,T. Seshunarayana and Shukla, A.K,(2011), Site amplification studies towards seismic microzonation in Jabalpur urban area, central India, Physics and Chemistry of the Earth,36 (2011) Govindaraju, L. and Bhattacharya.S, Site Response studies for seismic Hazard Analysis of Kolkata city, Jl. of national hazards,v-61, P, Anbazhagan, K.K.S. Thingbaijam, S.K. Nath, J.N. Narendara Kumar, T.G. Sitharam (2010),,A Multi-criteria seismic hazard

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