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