Authors: Pipatpongsa, T; Khosravi, MH; Takemura, J; Leelasukseree, C; Doncommul, P
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The classical textbook “Rock Slope Engineering” by Hoek and Bray (1977, p. 166) states that, “It is not usual for the toe of a slope to be undercut ...”. Therefore, it is of great importance to evaluate the maximum undercut width. This is because undercutting at the toe of a slope is considered as a destabilising mechanism. For a problem limiting an undercut slope lying on a bedding plane, a practical technique for surface mining was established at the Mae Moh mine located in Northern Thailand under collaborative research with the Electricity Generating Authority of Thailand. This research successfully resulted in a fundamental study on the failure mechanism of undercut slopes (Pipatpongsa et al. 2009–2013; Khosravi et al. 2009–2012, 2016). The possible modes of slope failure, due to an excavation at the toe of a slope, were examined through a series of simple physical model tests using humid sand placed on an inclined rigid plate. While ensuring the removal of propped portions of sand at the toe of the slope did not cause the collapse of the sand mass, due to arch action over the abutments. However, the effect of the arch action could not be maintained once the excessive removal of propped portions had been made. Different failure modes were observed in accordance with various boundary/material conditions. The arch action and its mechanism of load transfer were confirmed through a number of physical and numerical models. The failure mechanism along the inclined plane of weakness was seen to involve the passive condition, whereby the major principal stresses dominate the force supporting the arches in the circumferential direction, and the minor principal stresses represent the force confining the stacks of arches. This paper summarises the concepts used in setting up 1 g physical models, the required equipment and the measurement techniques. The development of the physical models in the preliminary study, the 1 g physical modelling and the geotechnical centrifuge modelling conducted from 2010–2015 are also reviewed. Based on the results of field tests, the new design concept for undercut slopes proposed in the previous study (Pipatpongsa et al. 2013) is correctly revised in this paper.

Keywords: undercut slope, physical modelling, arching effect, excavation, coal mining

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