Applications and challenges of near surface geophysics in geotechnical engineering

Abstract

Geophysical exploration methods have been applied to geotechnical engineering problem since their early developments. However, the results often do not live up to engineers\' expectations. Works still need be done before we see the widespread use of geophysical methods in engineering practice. This study provides an overview of newer developments and applications of near surface geophysical techniques in geotechnical problems. More importantly, the limitations and challenges of current geophysical methods in this context are identified and possible countermeasures are proposed. Near surface geophysical techniques, such as travel time velocity tomography, electrical resistivity tomography (ERT), and multi-channel analysis of surface wave (MASW), have advanced significantly in the last couple of decades within the scientific community. The applications of these methods in Taiwan\' s geotechnical problems are first examined, including assessment of liquefaction potential, evaluation of dam safety, investigation of soil and groundwater contamination, and quality control and assurance of ground improvements. The seismic travel time tomography was selected to examine the integrity of a concrete dam in terms of P-wave velocity. ERT was used to investigate abnormal seepage in earth dams and soil and groundwater contamination. Shear-wave velocity profiles non-destructively obtained by MASW are relevant to many traditional geotechnical problems, in which the quantitative assessment of liquefaction potential and ground improvements were particularly presented. The effectiveness of these applications is discussed from an engineer\'s perspective, and the associated challenges and practical countermeasures are systematically addressed. The velocity imaging of the concrete dam was quite successful and promising, allowing the engineer non-destructively "CT scan" the strength of the dam body. ERT works in a similar fashion for water-related problems. However, the results on abnormal dam seepage and groundwater contamination were less conclusive since the resistivity depends both on pore-water properties and geological factors. So it\'s important to integrate geological background and results from geotechnical investigation or monitoring. In addition, time-lapse geophysical measurements together with geotechnical monitoring reveal additional information and are valuable for geotechnical process control, such as groundwater remediation and ground improvement. Shear-wave velocity, which has a stronger link to geotechnical stiffness property, is now readily measured by MASW. Its applications on assessment of liquefaction potential and ground improvements were quite effective, at least qualitatively. However, MASW is basically a 1-D method and does not provide S-wave velocity image with high spatial resolution. Many limitations and potential pitfalls of geophysical methods exist but are not apparent to end users. They are systematically discussed from an engineer\' s perspective. The non-uniqueness nature and weak link to engineering parameters are common problems of geophysical methods. Reasonable inversion results should be obtained with sufficient a priori information and proper initial models. More conclusive or quantitative engineering interpretation can be achieved by data fusion, time-lapse measurements, and physics-based quantitative modeling. Different assumptions and limitations of investigation depth and spatial resolution are inherent in each geophysical method. They are summarized and made clear to avoid overpromise and over-interpret geophysical results. Some examples of practical countermeasures are illustrated. Finally, researches towards the standardization of geophysical methods are suggested to ultimately promote their widespread use in engineering community. Although successful case studies and innovative applications have strengthened the contribution of new geophysical developments to geotechnical problems, several challenges are identified for more common practice of geophysical surveys in engineering applications from an engineer\'s perspective. These include the lack of standard in data reduction, non-uniqueness of data inversion, limitations of exploration depth and resolution, field conditions violating model assumptions, and the weak link between geophysical parameters and engineering parameters. Relevant researches and practical countermeasures regarding these issues are partially discussed herein. More rational and widespread use of geophysics may be realized through the understanding of the limitations and potential pitfalls of geophysical techniques and researches to overcome them.