A large number of shotcrete slopes that were constructed in Japan during the 1970's are now more than 30 years old, and there has been a significant degree of deterioration. In Japan, many of the slopes covered with shotcrete have aged considerably. Therefore, there is a risk that slope failures may occur due to the effects of factors such as seasonal weather patterns, natural disasters, climate change, heavy rainfall, and earth quakes. Therefore, it is important to develop a method for monitoring the stability and the durability of these slopes. In this paper, we propose a technique that converts seismic velocity and electric resistivity data to porosity and saturation, which is then used to monitor weathering and groundwater fluctuation behind the slope. The evaluation results of this methodology confirm that the distribution of porosity and saturation of rock mass around the evaluated slope arrived at by this conversion system agree with those of the actual rock mass conditions evaluated using boring samples. In addition, it was possible to monitor the signs of seasonal variation and weathering in the ground by performing monitoring using this methodology over a period of multiple years.
Slope stabilization structures made with shotcrete were constructed in large numbers in Japan in the 1970's. As more than 30 years have passed since their original construction, a variety of problems have come to light, such as cavitations behind the concrete lining, and slope failures caused by weathering of the natural ground. Currently slope management is conducted primarily by road patrols and road facility inspections to prepare disaster prevention records regarding aged slopes. If further investigation of a particular slope is found to be required, a geophysical exploration is conducted to review countermeasures to be taken against defects. For these reasons, Kusumi and Nakamura [1] devised a technique that can convert in-situ seismic velocity and resistivity to porosity and water saturation distributions (hereafter referred to as "conversion analysis"). In this study, using this conversion analysis methodology we tried to monitor the thickness of weathered rock and the degree of moisture build up in a more accurate manner, with a focus on seismic and electrical explorations that are considered to be effective in assessing the weathering of the natural ground behind the slope and variations in ground water levels.
The geophysical data for the analysis in this study was acquired at a shotcrete slope (A-district) and a non-supported slope (B-district) of a national highway as shown in Figure 1. In geological terms, the overall slope consists of a sandstone layer, alternating sandstone-shale strata, and a basalt lava layer, which are located in the Tanba strata formed during the Triassic and Jurassic periods of the Mesozoic era. Figures 2 and 3 show the locations of the investigation, which are near the south side of the national road. The overall slope is comparatively large-scale, with dimensions of about 200m in length and about 50m in height.
