Recent studies of undercut slope using physical models have indicated that the failure of undercut slope during the excavation of the base part involves the stress transition from active condition to passive condition. This study aims to further investigate the factor by which the stress condition and its transition in undercut slope are influenced. The stability of undercut slopes under the groundwater seepage was studied by geotechnical centrifuge modelling. Edosaki sand with 10% water content was selected as the modeling material. The humid sand was compacted layer by layer using rigid wooden frames on the inclined plane covered by a low-friction Teflon sheet. A set of pressure gauges and pore pressure transducers were embedded at the designed locations in order to observe the changes of earth pressure and pore water pressure during centrifugal acceleration and seepage flow. In addition, slope movements were monitored by laser sensors installed along the top of slope model. The seepage water was supplied by setting up a water tank on the side of the soil chamber with a remote controller of inlet valve. Influences of seepage pressure and groundwater flow to the critical undercut width of slope models were examined. The experimental results confirmed that the higher the seepage, the lower the undercut width. Nevertheless, it was found that destabilization due to seepage water causes the subsequent arching failure instead of the collapse of a whole slope.


Slope instability in surface coal mining is a relatively common occurrence. Slopes in these areas are often excavated along bedding plane. The unsupported slope width increases with time when mining process along the toe part starts, leading to instability problems in the slope part where potential failure planes are likely to trigger. Based on the report of Electricity Generating Authority of Thailand (1985), after a certain excavation in Mae-Moh mine, some failures on rock slope along weak planes filled with clay seams were recorded. Over a few years later, undercut slopes in these regions affected by in-filled clay seams raise concern. Figure 1 shows a view of undercut slope in Mae-Moh mine. The failure mechanism related to undercut slope are attributed to collapse of arch action that keeps arch-shaped scarp stable (Pipatpongsa et al., 2013, 2016).

Arching effect in soils, which has been defined firstly by Terzaghi (1936) as the transfer of pressure from a yielding mass of soil onto adjoining stationary parts, are widely studied in various fields in geotechnical engineering, such as underground tunnel, retaining wall and piled embankments (Low et al., 1994; Galli et al., 2004; Handy, 1985). In terms of excavation process in slope, Khosravi et al. (2011, 2012, 2016 and 2017) confirmed the existence of passive arch action via physical models.

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