Determination of mechanisms and hydrogeological environments of Gangxianlane landslides using geoelectrical and geological data in central Taiwan

Electrical resistivity tomography (ERT), single-point-resistance logging with time-lapse, and geological and groundwater level data, are utilized to determine the mechanism of, and a hydrogeological model of, the Gangxianlane landslide in the west-central part of Taiwan. The rock surface and geometry of the colluviums were found by integrating ERT and borehole data. The iso-resistivity line in the bedrock broadly followed the bedding plane, indicating that the contribution of lithology to resistivity exceeds that of groundwater. The results of single-point-resistance logging with time-lapse and borehole data reveal that the bedrock has low permeability, which is associated with poorly developed joints and fractures. The fine grain content part (including the clayey layer), almost parallel to the bedding plane of the bedrock and between the colluvium and the bedrock in the upper part of the landslide, served as the slip surface and is believed to have been recently produced. It can block and retain water in the colluviums. Accordingly, this slip surface has the potential to re-slip in the future. Another slope failure involved rock collapse by a well-developed open release joint close to the Huoshaoping Terrace. It is also related to groundwater flow over a large gradient from the Huoshaoping Terrace. It is a key to determining whether this landslide will expand in the future.