| Abstract | A conventional investigation method for landslide in mountainous area is usually hard due to the difficult terrain condition and expensive cost of drilling and sampling. Electrical Imaging (EI) is a non-invasive testing method and relatively easy to be used. Besides that EI method can provide a better coverage of the investigated subsurface. This research study presents the results of an Electrical Imaging (EI) survey and Geotechnical investigation at a landslide area that the displacement of the road crossing was of more than 2m in vertical direction and from 6 to 8m in horizontal direction , in Lao Cai Province, Vietnam. The investigation work included 6 EI lines using Dipole – Dipole array and 4 boreholes that located along the EI Line 2, the depth of the boreholes is from 18.5m to 44.0m. For EI survey, data acquisition was done using the SuperSting R8 resistivity meter and recorded data were corrected based on the measured coordinates of each electrode and converted to “general array” format. For inversion, the EI results after inverting the data by RES2DINV software are 6 electrical imaging profiles that modeled exactly electrode locations at study site and recalculated resistivity values matching with above mentioned electrodes. Preliminary layering according to the resistivity values, rock weathering profile and typical resistivity values of some soil and rock in previous studies also conducted to build a geo-electrical model. The borehole investigation work was conducted at 4 boreholes along the EI survey Line 2 to take the sample for laboratory testing. Standard penetration test was also tested in the boreholes to determine the soil hardness. A geological profile was made base on soil properties, SPT values and site descriptions; table of soil properties for each layer also summarized corresponding to the layers from geological profile. At site, the drilling and EI acquisition works were operated at different time and by different contractors; the samples taken from drilling work were not measured the resistivity values. Therefore, to confirm the resistivity value of soil and rock in the electrical profile, some soil and rock samples were collected at some predefined locations. The soil and rock samples taken were tested by using TD2000 resistivity meter and recorded data were corrected using the method that proposed by Morris et al., 1996. The resistivity values of soil and rock samples taken after correction varied from 60.8 to 92.1 m and 1147 to 4398.2 m, respectively. By borehole investigation and testing method, a thin elastic silt, soft layer was discovered and proposed as a sliding surface at the depth of 4.0 to 7.2m and the thickness of 1.2 to 2m, but the resistivity imaging method did not recognize due to the resolution of electrical imaging method conducted was not high enough and the variety of resistivity values between the discovered layer and the adjacent layers in very small range. A method based on resistivity gradient versus depth was proposed to find the appearance of this soft layer. The graphs of resistivity gradient versus depth was made and found that a different trend at the vi depth from around 4.0 to 10.0m. This could be said that there must be the correlation between resistivity gradients and the changing of soil layer existing here. Usually, the soil hardness and other engineering properties are determined by destructive method with a lot of time and budget needed for conducting those tests. With the aim of reducing the cost and time for the same soil types in similar site conditions, the relationship between SPT – N and resistivity versus depth as well as SPT – N versus resistivity were also taken into account, but no correlation had been found. In order to confirm the soft elastic silt layer that proposed as a sliding surface and the advance of combining geophysical and geotechnical investigation methods, slope stability was analyzed by modeling both geological and geo-electrical profiles. From analysis results, it was found that combining geophysical and geotechnical investigation could be very useful and using borehole logs model without geo-electrical model consideration might require more expensive or unnecessary against sliding solutions. |