Shear strength characteristics (fully-softened and residual) and the recovery of the shear strength by re-consolidation following the large displacement shear were investigated for the soil sample collected from the moving mass of a reactivated and large-scale landslide which occurred at the middle part of Taiwan in 1994 triggered off by the heavy rainfall after road construction. Geology of the landslide area is the Miocene formation composed mainly of shale. The soil sample was characterized by the low clay content and the low plasticity and dominated by quartz accompanied by mica, chlorite, and feldspars.
Slurried subsamples passed through a 420-μm sieve were normally consolidated at the respective normal stresses ranging from 50 to 300kPa. The subsamples were sheared using a ring shear apparatus under drained condition with a slow shearing rate until about 700mm of displacement. They were then re-consolidated at the corresponding normal stresses for 2 days and re-sheared.
The fully-softened strength was C_sf=4.6 kPa and φ_sf=35.3°. The relationship between residual strength and effective normal stress tended to be curve, showing the dependence of the residual strength on the effective normal stress. The residual strength was calculated to be φ_γ=31.2° under σ_n'of 50-150 kPa and φ_γ=25.4°underσ_n'of 200-300 kPa with a assumption of cγ=0 kPa.The recovery of the shear strength ofthe soil subjected to large displacement shear by re-consolidation was not detected under σ_n'of 200-300 kPa but clearly observed under σ_n' of 50-150kPa. The recovery rate in strength increased with the decreasing normal stress but the strength did not recover to the level of the fully-softened strength. The above results are important to clarify the mechanism of the recurrence of the reactivated landslide.