Practical Design, Numerical Analysis, and Site Monitoring for Huge Arching Effect during Massive Excavation of Undercut Slope in Open-Pit Mine

Abstract

This paper reviews a successful case history where applications of numerical predictions for a massive excavation were undertaken for both Class A and Class B predictions. Forming a stable arch across the pit of an open-pit mine is beneficial to the design of an undercut slope. However, predicting the maximum stable undercut width under which the slope does not collapse is a challenging problem in geotechnical engineering. Slope undercutting techniques originally developed for 1g physical and centrifuge models were realized in practice for a full-scale design at the Mae Moh mine, one of the large coal open-pit mines in Southeast Asia, during the period of 2011 - 2017. Numerical analyses, along with in-situ measurements for data assimilation, were applied to test the novel design concept before and during the surface excavation. At the initial stage of excavating the undercut slope, the mine was studied in terms of forming an arch, by conducting a field experiment of a partial undercutting in Area 4.1, which was of monumental size, namely, 300 m in width and 80 m in height. It was planned that, during the first stage, from 2011 - 2014, Area 4.1 would be undercut 150 m and then back-filled using well-equipped surface and subsurface monitoring systems. After the completion of the first stage of the cut-and-fill, the undercutting and back-filling continued and the field experiment was successfully completed in 2017. A series of three-dimensional numerical models, using 3DEC, was generated and used in the preliminary studies. The numerical models favorably showed that the undercut slope was stable with large arching. The magnitudes of slope displacement from the numerical models and the field data were incomparable, but their directions and tendencies seemed to be quite well matched. Conclusively, the field monitoring data and the numerical model results of the undercut slope of Area 4.1 provided further understanding of undercut slope behavior. Therefore, the design of the undercutting method proposed in this study confirms that temporary cutting/excavating undertaken during construction can be done safely and economically.