GEOTECHNICAL SITE CHARACTERIZATION THROUGH GEOELECTRICAL IMAGING

Abstract

Geophysical methods are increasingly being used for civil engineering related site investigation studies. They are non-invasive, cost-effective with proven spatial resolutions of different scales. While geotechnical tests provide geo-mechanical information on a very refined depth scales but their advantage is offset by their inherent point-wise information attended by drilling, which is costly; Further, in a complex geological settings, the problems get compounded due to excessive drilling budget to fulfill the geotechnical site investigation needs. While geophysical methods are quite effective in subsurface exploration, yet their results can not be directly translated to geotechnical knowledge due to absence of suitable transforms. So, a leveraged approach is needed to yield better quality subsurface information at a much lesser cost. The key problem which still remains to be addressed concerns with the prediction of different formation and geotechnical parameter images of subsurface on the basis of few geotechnical investigations and ample number of geophysical measurements. A very good coverage of SASW & MASW methods (Park et al., 2005) is available in literature and these methods are routinely used in site testing. However, similar progress has not been made in the application of geo-electrical imaging to geotechnical site characterization. As evident from literature, very meager attention is paid to this important aspect. Recent literature (Gautam et al., 2007; Sudha et al., 2008; Cosenza et al., 2006) shows that efforts are limited to site-specific qualitative correlations between geo-electrical and geotechnical data. However, a preliminary attempt has been initiated by Gautam et al. (2007) to predict the SPT profile using the correlations of geo-electric and geotechnical data (SPT). Even though qualitative correlations are available for shear wave / elastic modulii and SPT (Morgan et al., 2005; Ulugergerli and Uyanik, 2007; Iyisan, 1996), no worthwhile predictive effort has been made to complement the geotechnical tests at a site. Here, 2-D resistivity and IP image profile data along with projected geotechnical data (Standard Penetration Test, SPT / Direct Cone Penetration Test, DCPT / Static Cone Penetration Test, SCPT) from nearby boreholes have been used for predicting different 2-D in formation and geotechnical parameter sections along the same profile. This prediction method is based on site-specific regression equations describing observed correlations of geo-electrical and geotechnical data and site-independent well established empirical relations of SPT 'N' with different formation and geotechnical parameters. The designed methodology is applied to geo-electrical and geotechnical data gathered from four sites. Three sites belong to Indo-Gangetic Alluvial Plains of Uttarakhand and fourth site belongs to Delhi Group of rocks in the vicinity of Delhi, India. The achieved results in four case studies demonstrate that the inferred 2-D formation, geotechnical test results and parameter sections along a chosen geo-electric profile describe near surface soil structure in a vivid manner. These may prove to be quite useful to a geotechnical engineer for site investigation studies. Even though the reported results are SPT based, the outlined methodology is quite general enough to deal with any other relevant geotechnical data sets for a comprehensive geotechnical assessment of a site. In this context, the following remarks need to be mentioned: a) The bearing capacity factors based on CPT method is not attempted here. However, earlier outlined procedure for SPT 'N' can easily be extended to CPT method also. b) The present procedure is applicable to soil strata only, where conventional geotechnical tests are valid. c) The quality of input geo-electric sections affect inferred lithology, formation and geotechnical sections. So, basic data quality of geo-electrical data and attendant processing and inversion schemes has to be very high. d) The regression equations that were employed for angle of internal friction, unit weights of soil (dry & saturated), unconfined compressive strength and different bearing capacity factors, have remained constant throughout the study and they are based on current geotechnical literature. e) The regression equations for prediction of geotechnically derived sand, clay / shale, lithology, SPT, DCPT, SCPT, porosity and water saturation are site-specific and geo-electrical profile specific. IV f) Here, a pair of boreholes in the vicinity of geo-electric profile is used for arriving at a regression equation. But the procedure remains unaltered if more boreholes are available for this analysis and in such an event, prediction quality improves. g) The designed methodology is applied in four case study sites and the results support the efficacy and cost-effectiveness of the approach. h) By a careful scrutiny of achieved results suggest that geo-electric imaging could be implemented at pre-investigation stage leading to better location of requisite number of boreholes for carrying out conventional geotechnical field tests. Further, it can be utilized in the next stage to infer 2-D image sections of lithology, formation and geotechnical parameters. Such a scheme optimizes the entire site investigation procedures, minimizing both cost and time and provides quality information to a geotechnical engineer to refine his models. i) The designed methodology is very general and it can rope in other scientific inputs also, so that new methodologies can be devised to meet the emerging challenges before a site geotechnical engineer. j) It is envisaged that densification of geoelectrical profiles could lead to a 3-D reconstruction of various formation and geotechnical parameter sections in the study region. But traditional geotechnical tests are still needed for better control. k) All inferred 2-D sections clearly show that 1-D models often resorted to by geotechnical engineers are far from reality and efforts need to be made to refine their quality or new procedures need to be evolved.