The mechanism of long-time displacement in a colluvium slope underlain by mudstone

Abstract

This paper aims to investigate the mechanism of an unstable slope with a long-time displacement. The unstable slope is located along the coast and covered with colluvium underlain by mudstone. Rainfall is considered the main source for triggering the slope displacement. Slope inclinometers are installed at the site, and sensors for measuring soil water content are installed in the colluvium and in the mudstone deposits. The water content in the colluvium with a thickness of 0.5~1 m above the mudstone increases considerably during rainfall, and the mudstone deposit is also highly saturated. These data imply that discontinuities or thin permeable deposits, e.g. sand layer, may exist in the mudstone and provide channels for rainfall-induced subsurface runoff to reach the inside of the mudstone. The mudstone deposit underneath the colluvium undergoes displacements during high-intensity rainfall based on data of slope inclinometers. Creep behavior in the mudstone deposit may take place due to its high saturation. The high saturation with a thickness of 1 m above the colluvium-mudstone interface during rainfall and high saturation in the mudstone deposit are major reasons for the long-time displacement in the colluvium slope.