Weathering of geomaterials causes degradation of the material, resulting in a surficial collapse in cut slopes. Thus, slope protection methods such as vegetation and shotcrete are empirically applied to retard the weathering. The weathering mechanism has not been fully understood, and the prediction of the progress of the weathering is still challenging. Therefore, we selected 96 cut slopes on Japanese highway routes and have tried to grasp the progress of weathering of the slopes through field surveys such as elastic wave exploration tests. In this study, a cut slope consisting of mudstone on Tomei Expressway in Yoshida, which exhibited a localized collapse in 2016, was studied to investigate the weathering state of the ground. This slope was constructed in 1968 and has experienced more than 50 years after construction. The objectives are to investigate the spatial distribution of the weathering magnitude of the cut slope and to grasp the weathering effect on mechanical properties of rock such as strength and stiffness.

1. Introduction

Since the Meishin Expressway was opened in 1965, Japan's expressways have been developed nationwide. The total extension has reached to about 9000 km, 74 % of which consists of soil structures as embankments and cut slopes. As about half of the expressway has experienced more than 30 years of in-service period, several technical issues have arisen gradually due to the aging of the ground. Thus, to maintain the existing highway assets effectively, it is quite necessary to grasp the long-term weathering behavior of the ground.

In this study, we focus on the long-term behavior of cut slopes. Most of the cut slopes remained stable for several decades after the construction. However, weathering of the ground gradually progresses in some of the slopes and eventually caused instability issues such as surface collapse. Thus, in order to clarify the weathering mechanism of the cut slopes, we carried out field surveys on 96 cut slopes nationwide using elastic wave exploration repeatedly for about forty to fifty years since the opening of the expressways (Fujiwara, 2018). We previously reported that the elastic wave velocity near the surface tends to decrease and the thickness of the weathered layer tends to increase with the progress of weathering (Fig. 1) (Nagata, 2019). In this paper, we discuss the mechanical and chemical consequences of the weathering of the cut slope consisting of mudstone through the results of physical tests and X-ray diffraction analysis on the boring cores collected from one of the target cut slopes.

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