Interpreting the Slaking Mechanism of Soft Rock Using a Coupled Hydraulic–Mechanical Model

Summary

Abstract

The long-term deterioration of soft rocks such as mudstone due to slaking—a weathering process induced by repeated wetting and drying—has become a serious geotechnical and social issue in Japan, as it can eventually lead to the failure of embankments composed of soft rock fragments over several decades. In this study, a new numerical model was developed to simulate the slaking behavior of soft rock, aiming to predict the long-term degree and rate of weathering. Swelling of clay minerals has been identified as one of the potential causes of slaking. To examine this mechanism, the present work assumes that water infiltrating into the rock through unsaturated flow locally induces the swelling of clay minerals, generating stress that results in expansion or shear failure. Before investigating the overall compression behavior of an embankment, the slaking behavior of individual rock fragments composing the embankment is analyzed. For simplicity, the rock was idealized as a porous sphere, and the governing equations for water continuity and equilibrium of forces were derived in one-dimensional radial form. The Bishop’s effective stress equation for unsaturated soils was extended by introducing a swelling pressure assumed to vary linearly with the degree of saturation, and the continuity and equilibrium equations were solved in a coupled manner. Using the proposed model, the water absorption behavior of uniformly unsaturated rock was analyzed through finite element simulation. The results demonstrated that as the degree of saturation increased from the surface inward, tensile effective stresses developed, inducing outward displacement corresponding to rock expansion. These findings indicate that the proposed model successfully reproduces the deformation of rock caused by water migration.