Effective Stress v/s Total Stress Ground Response Analyses for a Typical Site in Chennai (India)

This paper presents the results of ground response analyses carried out for a typical sandy site in Chennai city by equivalent linear and nonlinear total and effective stress approaches. The soil profile at the site consists of 26m thick sandy layer with SPT blow count increases from 16 to above 50 with depth. The shear wave velocity profile measured using field Multichannel Analysis of Surface Wave (MASW) test is found to increase from 170 m/s to 400 m/s at 26m depth. The equivalent linear ground response analysis was carried out using SHAKE2000. The nonlinear total and effective stress analyses were performed using D-MOD2000. In the nonlinear total stress analysis, the Modified Kondner and Zelasko (MKZ) constitutive model was used. In the case of nonlinear effective stress analysis, modulus degradation and stress degradation models of Matasovic and Vucetic (1993) were used to incorporate the pore pressure parameters. The analyses were carried out for a time history of bedrock acceleration with PGA of 0.16g obtained from the seismic hazard analysis. The results of the analyses are presented in terms of ground acceleration, shear stress and shear strain. The results of the equivalent linear, nonlinear total and effective stress analyses show similar ground response characteristics except marginal variation in the period corresponding to the peak spectral acceleration due to low intensity of input motion

Reliability analysis of slopes using fuzzy sets theory

The stability assessment of slopes is difficult because of many uncertainties. Possibilistic approach using fuzzy sets allows for a logical and systematic analysis of the uncertainties. In this paper an attempt has been made to present a new approach for the stability analysis of slopes incorporating fuzzy uncertainty. Uncertain parameters are expressed as fuzzy sets. A methodology has been presented in the study to process the fuzzy uncertainties in a slope reliability analysis. Fuzzy uncertainty is incorporated in the estimated probability of failure. A numerical example of the finite earth slope problem illustrates the methodology. The approach allows assessment of the likelihood that a particular slope section will have a higher failure probability than the failure probability of the 'critical' deterministic failure surface. (.

Centrifuge modelling of contaminant migration through composite liners

The transport, fate and remediation of contaminants in unsaturated porous media are not readily amenable to numerical simulation and bench testing is limited. The centrifugal technique is advantageous for modelling contaminant migration in fine-grained soils since the method is relatively quick and is performed under bulk densities and confining stresses encountered in natural soil deposits. In this paper, the results of centrifuge experiments of progression of a contaminant pulse through composite liners are reported. Centrifuge experiments have been performed with compacted clay liner having a model thickness of 10 mm and polyethylene (representing geomembrane) having a thickness of 0.05 min at different acceleration levels. In all the experiments, the polyethylene was placed above the clay liner. The results are presented in the form of solute breakthrough curves. It is concluded that the centrifuge test results yield more realistic data on contaminant migration through composite liners

Seismic input for chennai using adaptive kernel density estimation technique

Seismic input at a particular site can be estimated quantitatively using probabilistic or deterministic approach. Probabilistic seismic hazard analysis (PSHA) provides a framework in which uncertainties in the size, location, rate of recurrence and effects of earthquakes are explicitly considered in the evaluation of seismic hazard. The probabilistic way of analyzing the seismic hazard was developed conventionally by introducing zones in the seismogenic regions based on regional seismotectonic and geologic setting. The seismic uniformity is assumed within these source zones. Later, many researchers found that the conventional approach has many drawbacks viz., difficulty in delineating seismic sources into various zones, difficulty in applying Gutenberg-Richter (G-R) recurrence relationship to characterize the seismic source for low seismicity regions and distributed seismicity, and the consideration of uniform seismicity within the zone is also questionable. Because of these issues, several alternative methods to hazard estimation have been proposed in the literature. In the present study, zone free approach is proposed to evaluate the spatial distribution of seismicity based on kernel density estimation technique. The kernel technique provides a spatial variation of the seismic activity rate unlike the conventional approach where it is constant for a seismic source zone. The fixed bandwidth kernel poorly evaluates the earthquake distributions since the earthquake catalogue has several areas of high activity clusters and low background seismicity. Therefore in this study, clustering based adaptive kernel technique is proposed to find the spatial activity rate and integrated with other forms of uncertainty in magnitude and distance to determine the probability of exceedance of the selected ground motion parameter. The proposed methodology of seismic hazard analysis has been used for Chennai, southern India and the seismic input is provided in the form of Peak Ground Acceleration (PGA) and Uniform Hazard Spectra (UHS) for return periods of 475 and 975 years. The UHS obtained are compared with the Cornell-McGuire approach and IS 1893: 2002

Piles in rock: Socket resistance estimation using JRC and fractal dimension

Socket resistance contributes significantly to pile capacity. In the present work, an approach is presented for estimating the capacity of side-resistance-only piles socketed in rock by considering the effect of interlocking in terms of JRC and Fractal Dimension. Unlike available methods of quantification of roughness of surfaces, Fractal geometry allows the description of irregular shapes. A correlation is established between JRC and Fractal Dimension (D). This is used to estimate socket contribution and predict pile capacity based on an iterative procedure. Here the axial load is calculated by incrementing vertical displacement along the interface and estimating the shear strength mobilised at the interface using above correlation. Pile socket data reported in literature is used for validating the approach. The study is then extended to piles socketed in weathered rock in Mumbai region. The results show that it is feasible to make a reliable estimate of the socket strength contribution

Landslide modelling using remote sensing and GIS

One of the forms of natural disasters given much attention by researchers all over the world during the International Decade for Natural Disaster Reduction (1990-2000) is the occurrence of landslides. Landslides take place in various regions of India, notably in the Himalayas and the Western Ghats, especially during monsoon. This paper describes some of the on-going studies related to modelling and risk evaluation of landslides at IIT Bombay, India. The studies described are: (i) rainfall induced landslide evaluation using DEM and GIS, (ii) fractal modelling of mass movement including critical failure surface determination using shortest path concept of GIS and (iii) evaluation of risk due to landslide. The unconventional methods used here demonstrate the usefulness of a GIS based technique in identifying potential slide regions in a long stretch of a hill slope and the corresponding risk, which could serve as supports for landslide hazard management.© IEE

Spatial Variability of Soil-Rock Interface in Chennai using Geophysical and Geotechnical data

Mapping surficial and subsurface conditions play an important role in analysis and design of geotechnical structures and facilities. Spatial distribution of depth to soil-bedrock interface has a prime significance in developing input ground motion at the surface and estimating damage potential of an earthquake. This paper deals with the evaluation of spatial variability of soil-bedrock interface in Chennai, south India using Multichannel Analysis of Surface Wave (MASW) test and Standard Penetration Test (SPT) data. The MASW tests have been carried out at selected locations in Chennai city using 24 channel Geode and shear-wave velocity profiles are developed. Based on the MASW test results, a shear wave velocity (Vs) of around 300 m/s is obtained for weathered/soft rock, and a value of 700 m/s for hard rock in the present study. Depths corresponding to these velocity ranges are identified to estimate the position of soil-bedrock interface in the study area consisting of IIT Madras campus and its surrounding area. In addition, nearly 85 borelogs have been used to identify the main soil types and variation in depths to bedrock in the study area. A comprehensive geo-database containing all subsoil information has been designed to create three-dimensional borelog information and depth to bedrock interface. The spatial variability of depth to soil-bedrock interface at the locations where field measurements are not available has been mapped using spatial interpolation technique called kriging. The developed bedrock map indicates that the depth to soil-rock interface varies from 2 to 18 m in the study area. The evaluated depths to soil-bedrock interfaces have been compared and validated with the subsurface profile information obtained from a few selected borehole data and MASW test results. Further, the available SPT data can effectively be utilized for developing shear wave velocity distribution maps at different depths along the soil profile using the correlation between Vs and SPT-N values for different categories of soils of the study area. These maps will be of immense use for ground response studies and development of microzonation maps

Dynamic Behavior of the Transition Zone of an Integral Abutment Bridge

Weaker sections of a railway track, such as the approach sections, are prone to differential settlement under the action of repeated train loads. The railway tracks degrade more quickly at a critical section adjacent to a traditional rail bridge because of progressive deterioration. Opting for an integral abutment instead of a traditional bridge is gaining importance due to its improved performance in terms of track stiffness and reduced settlement. It is essential to understand such issues with the appropriate methodologies. This study investigates the behavior of an integral abutment bridge with the transition zone subjected to train loading. Generally, the transition zone is a two-part wedge section consisting of unbound granular material and cement bound mixture. A field monitored traditional abutment bridge system is used to validate the developed two-dimensional Finite Element (FE) model. The model is further developed to simulate the dynamic behavior of the transition zone against the varying speeds of the train. The parametric study is performed on the transition zone by varying its geometric configurations and considering different materials for the backfill. The results indicate that the trapezoidal approach slab influences the track displacement significantly. The transition zone thickness and material properties of the backfill have a greater effect on the overall track response. Based on the results, the stable transition zones are identified to cater to the gradual stiffness variation during train–track interaction