Subsurface soil characterization using geoelectrical and geotechnical investigations at a bridge site in Uttarakhand Himalayan region

Abstract

Geoelectrical characterization of subsurface soil has been done at a bridge foundation site on the banks of Bhagirathi River at Tehri reservoir site, Uttarakhand, India. For this purpose, the Electrical Resistivity Tomography (ERT) and the Standard Penetration Test (SPT) data, recorded at both banks of Bhagirathi River are analyzed. A total of six ERT profiles, recorded on both the West and East banks, were interpreted to determine an electrical resistivity image showing the resistivity variations with depth. The borehole data and geological inputs were used for lithological correlation and calibration of the resistivity values to the subsurface formation. Subsequently the electrical parameter (resistivity) for different subsurface lithological units has been inferred. Further, at selected points, the electrical resistivity sounding data, derived from the ERT, have been correlated with the Standard Penetration Test (SPT) data. This correlation results from the fact that in the subsurface soil both the electrical resistivity variations and the soil strength measured by SPT are controlled by the soil properties: grain size distribution, compactness, porosity and water saturation. It has been observed that the N-values smaller than 16 are unreliable and inconsistent. In the River Borne Material (RBM) on the West Bank it is due to the presence of coarse gravels while on the East Bank it is due to the boulders. The N-values greater than 16 mainly correspond to the weathered rock formation. For these values, there exists a linear relationship between N-values and resistivity with a correlation coefficient greater than 0.80. The coefficients of linear relationship at the two banks vary due to varying amount of clay content. Such a relationship is important for any site in tough Himalayan terrain because it can be used as an alternative to the SPT for determining soil strength parameters from ERT.