Abstract:

Loose deposits are extensively distributed in natural environments and are frequently destabilized by external disturbances such as rainfall, which can trigger debris flow disasters. During rainfall, fine particles in the loose deposits combines with rainwater, forming a mud that induces instability. In order to investigation the change of shear strength, large-scale shear tests of loose deposits saturated with different water contents of mud are conducted for the first time by a self-developed large-scale shear testing apparatus capable of conducting shear tests under saturated conditions. There are some findings. (1) Relative density range of loose deposits is 0.35–0.45. Below a relative density of 0.35, loose deposits tend to settle under gravity, leading to an increase in relative density. Conversely, above 0.45, the deposits no longer exhibit loose characteristics. (2) Mud properties correlate with water content. As mud water content increases, cementation weakens and lubrication strengthens. The minimum water content required to form mobile mud is 66.6%. Below this threshold, the mud loses its fluidity. At 90.9% water content, the mud’s fluidity approaches that of water. (3) Mud weakens the shear strength of loose deposits. As mud water content increases, the internal friction angle (φ) and cohesion (c) of loose deposits decrease. The looser the deposits, the greater the weakening effect of the mud, and the more prone they are to instability. The variation in shear strength of loose deposits saturated with different water contents of mud explains the mechanism of debris flow after short-term heavy rainfall.

Key words: loose deposit, shear test, shear strength, relative density, mud medium, rainfall