Investigation of Surface Rupture for Phuket Landslide Using Integrated Electrical Resistivity Imaging and Slope Modeling

Abstract

Landslides are worldwide natural disasters and they usually occur in the mountainside areas. In this phenomenon, the resistance force of the slope is decreased due to the side effects (geological condition, morphology, heavy and prolonged rainfall, and anthropogenic effect) then the slope is collapsed. Life losses and economic losses are occurred because of this hazard. The purpose of this paper is to delineate the structural deformation of the slope in Phuket, Thailand, using electrical resistivity imaging (ERI) and coupled slope modeling of SEEP/W-SLOPE/W under rainfall- induced conditions. The landslides' geometries are defined using ERI method then coupled slope modeling (using/without using ERI geometry reconstruction) under rainfall-induced condition are used to check the stability of the slope. Moreover, both methods can also estimate the maximum thickness of the surface rupture. Slope modeling results between using ERI geometry and without using ERI geometry are compared. Therefore, the thicknesses of the landslide body are compared and identified from these two results. ERI surveys (parallel to the slope) are done in three locations: Location 1 at Kamala, and Locations 2 and 3 at Chalong. Locations are selected based on the past event landslide area, and the landslide hazard high-risk areas of previous researches. Direct boreholes cannot make on the study locations. So, the secondary hydro-mechanical and geological parameters are applied in coupled modeling. The 24- hr rainfall data are used to simulate the transient seepage analyses. Slope geometries of SEEP/W-SLOPE/W modeling are constructed using the sharp evident near study areas for first modeling and ERI geometries for second modeling. Theoretically increasing the positive pore water pressure (PWP) makes the slope failure because of its degree of saturation increase. Both model simulations show in Location 1 for 16th-29th June,2018 rainfall data, the peak PWP occurs on 26th June with the factor of safety 3.884 for "without using ERI geometry reconstruction" and 3.484 for "using ERI geometry reconstruction". However, the variations of PWP in Location 2 for 10th-29th May, 2018 rainfall data are different. The maximum PWP appears for "without using ERI geometry reconstruction" on 29th May with the factor of safety 1.554 and "using ERI geometry reconstruction" on 18th May with the factor of safety 1.175. The F.S. values for both locations do not reach under 1 which means the slopes are stable. But, Location 3 is governed by homogeneous fresh granite bedrock and SEEP/W-SLOPE/W coupled modeling is not considered because this place is already stabled with bedrock. However, its ERI result supports to determine the resistivity range of fresh granite bedrock. Location 1 shows the surface rupture is at the top layer of clayey gravel and Location 2 is in the fractured granite. As the final results, the maximum thickness of the sliding mass getting from ERI and using ERI geometry reconstruction are good in correlation at a slope distance. The factor of safety between using and without using ERI geometry are found quite different. Moreover, the disparity of the thickness of sliding mass given from without using ERI geometry for Location 2 is very large in comparing to those of ERI result and using ERI geometry reconstruction. Finally, it can confirm that the maximum thickness of surface rupture for Location 1 is about 5.5 m at a slope distance 13.5 m and Location 2 is about 2.5 m at a slope distance 5.5 m.