Rock Mass Characterisation and Delineation of Weak Zones in a Foundation Regime Using Integrated Geophysical Methods: A Case Study from Mangalore, India

A refinery complex has been operational since 1988 in north Mangalore which is a part of the Western Ghats. The processing units and pipe racks were installed over the benches developed between the elevation of RL = 12 m to RL = 65 m. During the incessant rains in August 2019, ground destabilisation, minor landslides, slope failures and heavy water discharge were observed within the refinery campus. One such landslide damaged the foundation area of a major piperack leading to disruption in some refinery operations. To plan the rock reinforcement and foundation restoration works, it was extremely important to assess the subsurface rock mass condition and to delineate the damage zone, water saturated areas and destabilised zones. A comprehensive geophysical investigation was taken up using multiple methods to accurately assess the subsurface ground condition and to estimate the in-situ rock mass properties. Four different methods, vis-á-vis, crosshole seismic tomography, electrical resistivity imaging, seismic refraction survey and MASW survey, were used to investigate and accurately deciphering the strata condition, delineating the groundwater flow path and derive the Poisson’s ratio in-situ. Results of the integrated study helped in understanding the characteristics of the lateritic formations and its behaviour under saturated conditions. It showed that changes to the topography had modified the subsurface drainage pattern. The seismic velocity ratio \((\frac{V_{\text{p}} }{V_{\text{s}}})\) in the overburden showed how it is related to soil condition when it changes from dry to wet and finally water saturated. The Poisson’s ratio of lateritic soil in some of the study area was seen to be \(\nu =0.44-0.48\) , indicating the extent in loss of rigidity leading to failure.