Deep-seated gravitational slope deformation in Lushan, Taiwan: Transformation from cleavage-controlled to weakened rockmass-controlled deformation

Abstract

The northern slope in Lushan (NSL) in central Taiwan is recognized as a large deep-seated gravitational slope deformation (DSGSD) in anisotropic slate and is often activated by typhoons and heavy rainfall. However, a recent southward movement of the DSGSD was apparently not caused by translational sliding along the eastward-dipping cleavage in the NSL. Because the regional attitudes of the cleavages in the NSL moderately vary and no major fault has been observed in the field, the deformed cleavages with their random dips indicate that the deformation of the rockmass was caused by older mass movements. In addition, ancient progressive erosion of the NSL toe by the Talowan River, which passes through the eastern side of the slope, induced deep, large-scale mass movement toward the east. Based on detailed core recognition measurements including the cleavage dips, log Q values, and inclinometer measurements, we identified five geometrical styles and geotechnical zones of the deformed rockmasses for characterization of the evolution of the NSL DSGSD with regard to the spatial distributions of old mass movements and recent transformations. Curves obtained from recent inclinometer measurements suggested two deformation behaviors, namely, specific sliding and progressive deformation. The underlying rockmass with overturned and flexural folding in cleavages at the slope toe have baffled a recent southward advancement of the DSGSD. Consequently, the deep specific sliding surface close to the basal disintegrated rockmass at the center of the NSL was transformed into progressive deformation, which mostly occurred in the overturned rockmass at the toe. Based on our observations, we propose the following relationship between the P-wave velocity (Vp, km/s) and log Qc (where Qc is a normalized Q) as an empirical model for slate in Taiwan: Vp=0.6 × log Qc + 4.0. This empirical model might be suitable for the low-uniaxial compressive strength slate in the Lushan area and could be used for the quantitative evaluation of the geotechnical zone of the DSGSD of the NSL.