Relationships between geophysical and geotechnical parameters focusing on a site specific results of a landslide risk area

Abstract

The population growth and extension of a settlement on a risky area have increased the impact of natural disaster. Slope failures, landslides and subsidence of foundation have been identified as the most commonly occurring natural disaster if such sensitive areas are not well monitored. On the other hand, a detailed analysis of the triggering factors is often hindered by the lack of information gathered from the field measurements. A survey investigation was performed in a possible landslide risk area, using the geotechnical, geophysical and geological mapping approaches. The geotechnical investigations included coring in order to obtain the lithological sequence and for sampling purposes. And standard penetration test (SPT) in-situ field tests of soil strength. Electrical resistivity tomography (ERT) and seismic refraction tomography investigations were executed in order to determine the hydrogeological characteristics and delineate the regions of weak and hard subsurface materials. The 2D inversion results of resistivity technique suggested the presence of a two-layer structure model. Moreover, the 'break' in the unit was apparent, indicative of the presence of weak zones, fractured zone and cracks. As also demonstrated clearly by the seismic refraction data, the depth to bedrock (a sharp boundary interface approximately at a depth of 15 m) varies, and such variation is mainly attributed due to the thickness of the overlying backfill material.

Additionally, we examined the possible correlation (if any) between geophysical parameters and geotechnical parameters to establish the quantitative estimates of a particular geotechnical parameter (e.g., soil strength) from geophysical surveys. From this study, a good relationship between electrical property (resistivity) and geotechnical property (soil strength) with the empirical equation RS= 31.733 (N60) -165.88 and regression coefficient R2=0.77 was observed. Based on the correlation between elastic property and weathering profile, we divided the subsurface materials into three zones: the first zone is classified as Residual soil with p-wave velocity (300 – 900 ms-1), the second zone is classified as highly weathered granite with p-wave velocity (900 – 1800 ms-1), and the third zone is classified as moderately weathered granite with p-wave velocity (1800 – 3000 ms-1). All geophysical and geotechnical data suggest that a fairly weak/uncompact backfill materials underlying the bedrock are likely to provide a planar surface where the landslide mass would move/be triggered.