Showing papers by "T. G. Sitharam published in 2009"

Probabilistic seismic hazard analysis for Bangalore

Abstract: This article presents the results of probabilistic seismic hazard analysis (PSHA) for Bangalore, South India. Analyses have been carried out considering the seismotectonic parameters of the region covering a radius of 350 km keeping Bangalore as the center. Seismic hazard parameter ‘b’ has been evaluated considering the available earthquake data using (1) Gutenberg–Richter (G–R) relationship and (2) Kijko and Sellevoll (1989, 1992) method utilizing extreme and complete catalogs. The ‘b’ parameter was estimated to be 0.62 to 0.98 from G–R relation and 0.87 ± 0.03 from Kijko and Sellevoll method. The results obtained are a little higher than the ‘b’ values published earlier for southern India. Further, probabilistic seismic hazard analysis for Bangalore region has been carried out considering six seismogenic sources. From the analysis, mean annual rate of exceedance and cumulative probability hazard curve for peak ground acceleration (PGA) and spectral acceleration (Sa) have been generated. The quantified hazard values in terms of the rock level peak ground acceleration (PGA) are mapped for 10% probability of exceedance in 50 years on a grid size of 0.5 km × 0.5 km. In addition, Uniform Hazard Response Spectrum (UHRS) at rock level is also developed for the 5% damping corresponding to 10% probability of exceedance in 50 years. The peak ground acceleration (PGA) value of 0.121 g obtained from the present investigation is slightly lower (but comparable) than the PGA values obtained from the deterministic seismic hazard analysis (DSHA) for the same area. However, the PGA value obtained in the current investigation is higher than PGA values reported in the global seismic hazard assessment program (GSHAP) maps of Bhatia et al. (1999) for the shield area.

Post-liquefaction undrained monotonic behaviour of sands: experiments and DEM simulations

Abstract: This paper presents the results of a series of servo-controlled cyclic triaxial tests and numerical simulations using the three- dimensional discrete element method (DEM) on post-liquefaction undrained monotonic strength of granular materials. In a first test series,undrained monotonic tests were carried out after dissipating the excess pore water pressure developed during liquefaction. The influence of different parameters such as amplitude of axial strain,relative density and confining pressure prior to liquefaction on the post-liquefaction undrained response have been investigated.The results obtained highlight an insignificant influence of amplitude of axial strain, confining pressure and a significant influence of relative density on the post-liquefaction undrained monotonic stress-strain response.In the second series, undrained monotonic tests were carried out on similar triaxial samples without dissipating the excess pore water pressure developed during liquefaction. The results highlight that the amplitude of axial strain prior to liquefaction has a significant influence on the post-liquefaction undrained monotonic response.In addition,DEM simulations have been carried out on an assembly of spheres to simulate post-liquefaction behaviour.The simulations were very similar to the experiments with an objective to understand the behaviour of monotonic strength of liquefied samples from the grain scale. The numerical simulations using DEM have captured qualitatively all the features of the post-liquefaction undrained monotonic response in a manner similar to that of the experiments.In addition,a detailed study on the evolution of micromechanical parameters such as the average coordination number and induced anisotropic coefficients has been reported during the post-liquefaction undrained monotonic loading.

Estimation of peak ground acceleration and spectral acceleration for South India with local site effects: probabilistic approach

Abstract: . In this work an attempt has been made to evaluate the seismic hazard of South India (8.0° N–20° N; 72° E–88° E) based on the probabilistic seismic hazard analysis (PSHA). The earthquake data obtained from different sources were declustered to remove the dependent events. A total of 598 earthquakes of moment magnitude 4 and above were obtained from the study area after declustering, and were considered for further hazard analysis. The seismotectonic map of the study area was prepared by considering the faults, lineaments and the shear zones in the study area which are associated with earthquakes of magnitude 4 and above. For assessing the seismic hazard, the study area was divided into small grids of size 0.1°×0.1°, and the hazard parameters were calculated at the centre of each of these grid cells by considering all the seismic sources with in a radius of 300 km. Rock level peak horizontal acceleration (PHA) and spectral acceleration (SA) values at 1 s corresponding to 10% and 2% probability of exceedance in 50 years have been calculated for all the grid points. The contour maps showing the spatial variation of these values are presented here. Uniform hazard response spectrum (UHRS) at rock level for 5% damping and 10% and 2% probability of exceedance in 50 years were also developed for all the grid points. The peak ground acceleration (PGA) at surface level was calculated for the entire South India for four different site classes. These values can be used to find the PGA values at any site in South India based on site class at that location. Thus, this method can be viewed as a simplified method to evaluate the PGA values at any site in the study area.

Spatial Variability of the Depth of Weathered and Engineering Bedrock using Multichannel Analysis of Surface Wave Method

Abstract: In this paper an attempt has been made to evaluate the spatial variability of the depth of weathered and engineering bedrock in Bangalore, south India using Multichannel Analysis of Surface Wave (MASW) survey. One-dimensional MASW survey has been carried out at 58 locations and shear-wave velocities are measured. Using velocity profiles, the depth of weathered rock and engineering rock surface levels has been determined. Based on the literature, shear-wave velocity of 330 ± 30 m/s for weathered rock or soft rock and 760 ± 60 m/s for engineering rock or hard rock has been considered. Depths corresponding to these velocity ranges are evaluated with respect to ground contour levels and top surface levels have been mapped with an interpolation technique using natural neighborhood. The depth of weathered rock varies from 1 m to about 21 m. In 58 testing locations, only 42 locations reached the depths which have a shear-wave velocity of more than 760 ± 60 m/s. The depth of engineering rock is evaluated from these data and it varies from 1 m to about 50 m. Further, these rock depths have been compared with a subsurface profile obtained from a two-dimensional (2-D) MASW survey at 20 locations and a few selected available bore logs from the deep geotechnical boreholes.

Critical state behaviour of granular materials from isotropic and rebounded paths: DEM simulations

Abstract: This paper presents the results of numerical simulations using the three-dimensional discrete element method (DEM) on the critical state behaviour of isotropically compressed and rebounded assemblies of granular materials. Drained and undrained (constant volume) numerical simulations were carried out. From these numerical simulations of drained and undrained tests, it has been shown that the steady state is same as the critical state. Critical state for both isotropically compressed and rebounded assemblies form unique curved line that can be approximated by a bilinear line as proposed by Been et al. [G,otechnique 41(3): 365-381, 1991]. Further more, evolution of the internal variables such as average coordination number and induced anisotropy coefficients during shear deformation has been studied.

Undrained Cyclic Pore Pressure Response of Sand-Silt Mixtures: Effect of Nonplastic Fines and Other Parameters

Abstract: Literature regarding the pore pressure generation characteristics and in turn the cyclic resistance behaviour of silty sand deposits is confusing. In an attempt to clarify the effect of nonplastic fines on undrained cyclic pore pressure response of sand–silt mixtures, an experimental programme utilising around 289 stress-controlled cyclic triaxial tests on specimens of size 50 mm diameter and 100 mm height was carried out at a frequency of 0.1 Hz. Specimens were prepared to various measures of density through constant gross void ratio approach, constant relative density approach, constant sand skeleton void ratio approach, and constant interfine void ratio approach to study the effect of nonplastic fines on pore pressure response of sand–silt mixtures. The effect of relative density, confining pressure as well as the frequency and magnitude of cyclic loading was also studied. It was observed that the pore pressure response is greatly influenced by the limiting silt content and the relative density of a specimen corresponding to any approach. The influence of other parameters such as relative density, confining pressure and magnitude of cyclic loading was as usual but an increase in frequency of cyclic loading was seen to generate excess pore pressure at a higher rate indicating an impact load type of behaviour at higher frequency. Utilising the entire test results over a wide range of parameters a new pore pressure band for sand–silt mixtures in line with Lee and Albaisa (1974) has been proposed. Similarly another pore pressure band corresponding to 10th cycle of loading as suggested by Dobry (1985) and up to a shear strain of around 25% has been proposed. These two bands can readily be used by researchers and field engineers to readily assess the pore pressure response of sand–silt mixtures.

Numerical simulation of geocell-reinforced sand and clay

Abstract: A numerical simulation of laboratory model tests was carried out to develop an understanding of the behaviour of geocell-reinforced sand, and soft clay foundation beds under a circular footing. The influence of the geometrical parameters of the geocell (width, b and height, h) on the overall performance of the footing was investigated and the pressure–settlement responses of geocell-reinforced foundation beds were predicted and compared with the unreinforced test results. Simulations were also carried out on conventional-type planar geogrid reinforcement to study the relative performance of both reinforcement forms. The influence of the boundary constrains on the behaviour of the foundations was also studied. In all the cases, good agreement between the results obtained from numerical simulations and laboratory experiments was observed. The results demonstrated that the geocell mattress redistributed the footing pressure over a wider area thereby improving the performance of the footing. The pressure–sett...

Application of Soft Computing Techniques to Expansive Soil Characterization

Abstract: Very often it is difficult to develop mechanistic models for pavement geotechnical engineering problems due to its complex nature and uncertainty in material parameters. The difficulty in mechanistic analysis has forced the engineers to follows certain empirical correlations. The artificial neural network (ANN) is being as an alternate statistical method, mapping in higher-order spaces, such models can go beyond the existing univariate relationships. The applications of ANNs in pavement geotechnical engineering problems is mostly limited to constitutive modeling, with few applications on prediction of soil layer properties using Falling Weight Deflectometer (FWD), prediction of swelling potential and compute the remaining life of flexible pavements. However, ANN is considered as a ‘Black box’ system being unable to explain interrelation between inputs and output. The ANNs also have inherent drawbacks such as slow convergence speed, less generalizing performance, arriving at local minimum and over-fitting problems. Recently support vector machine (SVM) is being used due to its, better generalization as prediction error and model complexity are simultaneously minimized. SVM is based on statistical learning theory unlike ANNs (biological learning theory). The application of SVM in pavement geotechnical engineering is very much limited and to best of the knowledge such methods have not been applied to pavement geotechnical engineering. However, engineering application of numerical methods is a science as well as an art. This juxtaposition is based on the fact that even though the developed algorithms are based on scientific logic and belong to the special branch of applied mathematics, their successful application to new problems is problem oriented and is an art. As no method can be the panacea to solve all problems to the last details, their application to new areas needs critical evaluation. With above in view, an attempt has been made to develop the art of applying the above artificial intelligence techniques (ANN and SVM) to different pavement engineering problems such as prediction of compaction characteristics, permeability, swelling potential, coefficient of subgrade reaction etc. The parameters associated with the model developments are discussed in terms of guide line for its future

Site characterization model using least-square support vector machine and relevance vector machine based on corrected SPT data (Nc)

Abstract: Statistical learning algorithms provide a viable framework for geotechnical engineering modeling. This paper describes two statistical learning algorithms applied for site characterization modeling based on standard penetration test (SPT) data. More than 2700 field SPT values (N) have been collected from 766 boreholes spread over an area of 220 sqkm area in Bangalore. To get N corrected value (N,), N values have been corrected (Ne) for different parameters such as overburden stress, size of borehole, type of sampler, length of connecting rod, etc. In three-dimensional site characterization model, the function N-c=N-c (X, Y, Z), where X, Y and Z are the coordinates of a point corresponding to N, value, is to be approximated in which N, value at any half-space point in Bangalore can be determined. The first algorithm uses least-square support vector machine (LSSVM), which is related to aridge regression type of support vector machine. The second algorithm uses relevance vector machine (RVM), which combines the strengths of kernel-based methods and Bayesian theory to establish the relationships between a set of input vectors and a desired output. The paper also presents the comparative study between the developed LSSVM and RVM model for site characterization. Copyright (C) 2009 John Wiley & Sons,Ltd.

Effect of initial gross void ratio on cyclic response of sand and silt mixtures

Abstract: This paper presents the influence of non-plastic fines on cyclic response of sand-silt mixtures. A series of undrained stress controlled cyclic triaxial tests were conducted on Ahemadabad sand containing 0 to 100% silt with an interval of 5% using cylindrical specimens of size 50mm diameter and height 100mm with different cyclic stress ratios to compare the cyclic resistance of clean sand to that of sand with various silt contents at a constant gross void ratio. The gross void ratio of a soil specimen considers the voids formed by sand particles and fines. It is the ratio of the volume of voids in the specimen to the volume of the soil solids. The effect of gross void ratio with and without fines on pore water pressure build up and liquefaction potential of sandy soils in stress controlled tests are presented. The results obtained from this study provide direct evidence that the limiting silt content plays an important role in the cyclic resistance of sandy soils. Below limiting silt content, the ...

Pullout capacity of small ground anchors: a relevance vector machine approach

Abstract: This paper examines the potential of relevance vector machine (RVM) in prediction of pullout capacity of small ground anchors. RVM is based on a Bayesian formulation of a linear model with an appropriate prior that results in a sparse representation. The results are compared with a widely used artificial neural network (ANN) model. Overall, the RVM showed good performance and is proven to be better than ANN model. It also estimates the prediction variance. The plausibility of RVM technique is shown by its superior performance in forecasting pullout capacity of small ground anchors providing exogenous knowledge.

Evaluation of Liquefaction Return Period for Bangalore Based on Standard Penetration Test Data: Performance Based Approach

Abstract: Problem statement: The conventional liquefaction evaluation is based on a deterministic approach. However in this method the uncertainty in the earthquake loading is not properly taken into account. However recent research in this field indi cates that this uncertainty in the earthquake loadi ng has to be considered in the liquefaction potential evaluation. Moreover the evaluation of liquefaction return period is not possible in the conventional d eterministic methods. This study explained the methods for evaluating the probability of liquefact ion and the return period of liquefaction based on probabilistic approach. Approach: In this study the geotechnical data was collected from 450 bore holes in Bangalore, India, covering an area of 220 km 2 . The seismic hazard analysis for the study area is carried out using Probabilistic Seismic Hazard A nalysis (PSHA) and the peak acceleration at ground surface was evaluated for site class-D after considering local site effects. For assessment of site class, shear wave velocity profiles in the cit y had been obtained using Multichannel Analysis of Surface Waves (MASW) survey. Based on this data the probabilistic liquefaction analysis was done to evaluate the probability of liquefaction in the study area. Based on the performance based approach the liquefaction return period for the study area w as also evaluated. Results: The results showed the variation of liquefaction susceptibility for the st udy area. The corrected standard penetration values required to prevent the liquefaction for return per iods of 475 and 2500 years were also presented here . Conclusion/Recommendations: The spatial variation of probability of liquefacti on and the factor of safety against liquefaction evaluated using the two methods match well for the study area.