Deformation of cut slopes of Japan's highway is taking place due to weathering over the passage of a substantial time after excavation. However, the mechanism of progression of the weathering and its effect on the stability of the cut slopes have not been fully understood. In this study, a cut slope of the Tomei expressway was analyzed to understand the mechanism of the weathering. This slope consists of a relatively uniform mudstone and experienced more than 50 years of the in-service period after excavation. The state of the weathering of the ground was investigated by checking the basic soil properties and mineralogy from field surveys and laboratory tests. Seismic wave velocity logging tests and elastic wave velocity tests have been conducted periodically throughout 50 years after the construction. The results are used to grasp the profile of gradual variation of stiffness of the ground with time, and the time-dependent reduction in the strength of the ground is modeled. Finally, the variation of the safety factor and change in the failure mechanism of the slope are investigated.
Japan being a mountainous region, excavations for highway constructions are usually carried out through sedimentary rocks. Hence, a great number of cut slopes have been excavated in soft rocks as mudstone. Weathering in newly exposed mudstone can start within a few days, weeks or months (Taylor, 1988) due to stress release along with rehydration. Reduction of strength is expected to occur with the degradation of the soft rock due to weathering. This phenomenon may weaken the slope with time eventually leading to the failure.
Weathering of mudstone in natural mountain and embankment slopes has been studied by several researchers such as Pye and Miller (1990), Chigira (1990), and Bhattarai et al. (2007). However, the progression of weathering of weak rocks has not yet been understood well. Hence, in this study, a mudstone cut slope (Fig. 1) at Tomei Expressway in Yoshida district was studied to investigate the weathering profile and its effects on slope stability. This slope was constructed in 1968 which now has aged more than 50 years after construction. The slope mainly consists of uniform mudstone with intermittent layers of sandstone (Fig. 2). The primary objective of this study was to discover the extent of physical and chemical weathering and the reduction in the strength of the rock due to weathering. Seismic wave velocity (PS) logging and elastic wave exploration have been conducted periodically throughout 50 years after the construction, which provide a profile of gradual stiffness reduction of the slope with time starting after the excavation. Combining the results, the timedependent reduction in the strength of the ground was modeled. Finally, the variation of the factor of safety (FS) and change in the failure mechanism of the slope were investigated using rigid-plastic finite element simulation.