ABSTRACT: Footwall slopes refer to bedding rock slopes in which the slope face is parallel to a set of persistent discontinuities, which exist extensively in opencast mine. Stability evaluation of the footwall slopes is significant for theoretical study and engineering application. In this paper, taking a certain footwall slope for the example, three calculation methods were compared for stability analysis of the footwall slopes. Firstly, calculation parameters of the rock masses and discontinuities were obtained through site tests and laboratory tests. Secondly, a strength reduction program was written using fish language and applied to the Ubiquitous-Joint Model in the finite difference method. Thirdly, a limit equilibrium method to calculate safety factor of the footwall slopes was proposed. In this case, the persistent discontinuities in footwall slopes were simulated by lamellar materials. Finally, comparison of the results, respectively, obtained by the finite difference method, the discrete element method and the limit equilibrium method were conducted. The results show that the safety factors determined by the three methods are consistent.
1. INTRODUCTION
In the opencast mining, in which the strata have subjected to tectonic folds, slopes are often excavated parallel to the strata dip in synclinal and anticlinal structures forming extensive, high and unbenched footwall slopes (Stead and Eberhardt, 1997). Footwall slopes, also referred to as bedding slopes, are encountered in other engineering rock slopes (e.g., hydropower and highway) where instability is structurally controlled, typically by one joint set, fault or weak zone parallel or subparallel to the slope (Konietzky, 2004; Fisher, 2009; Alejano et al., 2011). How to establish a reasonable description of this instability has aroused the great concern of many scholars, and different numerical methods have been put forward, such as finite element method, finite difference method, discrete element method and limit equilibrium method.
Zheng et al. (2013) considered the dominated structural plane of the steep rock as a macro-crack, and the extended finite element method (XFEM) was proposed to investigate the influence of the dip angle and position of the dominant structural plane on the deformation and fracture of the steep rock. Lu et al. (2011) used the software of the finite difference method (FLAC) to establish a three-dimensional calculation model of bedding rock slope, and the variation law of slope safety factor was obtained when the cohesion and friction angle of structural surface changed. Lei and Wang (2006) combined the software of Universal Distinct Element Code (UDEC) with the strength reduction method to analyze the stability of rock slopes with multi-structural planes. And compared with the results of traditional slice method, it shows that UDEC-based strength reduction method is a reliable and effective method. Fisher and Eberhardt (2007) carried out discrete element simulation together with limit equilibrium analyses to provide practical recommendations for the stability evaluation of footwall slopes.
