GEOELECTRIC AND GEOTECHNICAL SITE INVESTIGATIONS FOR SOIL LIQUEFACTION STUDIES

Abstract

Indo-Gangetic Alluvial Plains (IGP) is highly fertile and populated sedimentary foreland basins of the world. At places the sediments reach a thickness of 4-5 km even. Due to Himalayan orogeny, a number of deep-seated faults transverse up to Siwaliks and divide IGP into several basins. Besides, faults parallel to extension of Himalaya also affect the structure of these basins. The vast region of IGP is prone to earthquake hazard. Even though earthquakes do not kill humans but the buildings do. Added to this threat, major earthquakes (Mw≥7.5) induce soil liquefaction in sedimentary tracts and as a result the near surface soil behaves like a fluid and it can’t support any civil structures. So, a strong scientific approach based soil liquefaction hazard assessment of major metropolitan cities, industrial sites, vital and strategic installation sites and heavily populated regions of IGP and rest part of India is needed. Such scientific efforts need non-invasive and cost-effective methods. Accordingly, our present study involving Electrical Resistivity Tomography (ERT) with minimal invasive SPT tests seem to be quite attractive with predictive results either comparable or better than the existing practices. The present thesis is aimed at this objective and it uses electrical resistivity tomography (ERT) for this purpose with various transforms leading to assessing the vulnerability of a site for soil liquefaction. The theory involves use of transformations (regression equations) for conversion of resistivity to either Young’s modulus or shear wave velocity, Vs and another regression equation converts Vs to synthetic SPT-N (blows per 0.30m) values, which characterize the mechanical strength of near surface soils. Irrespective of nature of SPT-N (synthetic or results of SPT site investigation measurements), the next set of transforms use a standard geotechnical practice supported by an appropriate theory result in Factor of Safety (FOS) computations. The conversion of SPT-N data into FOS sections involves standard cyclic stress and cyclic resistance ratio computations. Our methodology is illustrated with the help of a field study involving three sites on our IIT Roorkee Campus (three ERT and MASW profiles), India besides Noida (Seven ERT profiles and 45 boreholes for SPT-N data acquisition) and Lucknow (three ERT profiles and 16 boreholes for SPT-N data acquisition) sites. The acquired data sets include ERT and MASW. The achieved results including two different regression equations indicate that our ERT-based ii soil liquefaction study is much broader and seems to be more refined than the popular MASWbased one .From practical considerations, if FOS ≥1.3, the site is free from soil liquefaction hazard for major earthquakes, Mw ≥ 7.5 else the site is prone to soil liquefaction hazard. On our campus, one of the reported sites (Earthquake Engineering) has liquefiable sands at a shallow depth range of 5–15 m. Further, the similarity of our results to that of the MASWbased method is corroborated by different Normalized Cross-correlation (NCC) plots. However, the relative resolution aspects need a separate, thorough study. We have tried our methodology with two different regression equations to ascertain our findings independently. The achieved results are similar, thereby validating our approach. Similarly, We have demonstrated our methodology successfully in other two study regions(Lucknow site and Noida site) with both implicit and explicit use of SPT-N data. At Lucknow site the estimated factor of safety lies in range (0-26) indicate liquefiable sands at a shallow depth range of 18-24 m. We have tried our methodology with two different regression equations to ascertain our findings independently with actual data sets. Accordingly, we prepared depth-wise stacks for different regression equation based SPT-N and FOS for two sites (Lucknow and Noida). For Lucknow site, the deviation of computed FOS logs using Srivastava (2015) from FOS logs derived actual SPT-N is assessed in terms of mminimum and maximum RMS errors, which lie in the range (13.40-33.57) and (5.74-9.84) respectively. Similarly, RMS errors for Noida site are (4.85-14.06) and (0.468-12.3) respectively. Thus, FOS log analysis confirm that ERT can be safely be used in place of point-based, invasive and costly boreholes for SPT-N data acquisition. ERT-based method for soilliquefaction hazard assessment is novel, and it is alternate and cost effective to the popular MASW-based one. However, it is advisable to have few direct SPT-N measurements to check the quality of FOS logs derived from ERT vis-à-vis direct SPT-N derived FOS logs. Based on our detailed analyses in reported case studies, it seems that our ERT-based method offers better spatial resolution in Vs, SPT, and ultimately FOS sections than MASW. As per the current study, Lucknow site faces liquefaction hazard while two profiles at Noida site show liquefaction hazard and IIT Roorkee site is free from soil liquefaction hazard for any future earthquakes of magnitude Mw ≤ 7.5. However MASW studies at Saraswati temple site on IIT Roorkee campus shows Liquefaction hazard