Abstract:
Forestry reclamation approach (FRA) is a technique used in surface mining to reclaim already mined areas, which recommends that upper 1.3 m (˜ 4 ft) of reclaimed slope should be left as uncompacted as possible to facilitate successful tree root growth. However, the consequences of leaving upper 1.3 m of uncompacted material may lead to instability of reclaimed slopes that is not accurately simulated using traditional equilibrium limit methods. A numerical model for slope stability analysis based on the fracture mechanics approach is developed in this research. The interface slope region between the lower compacted and upper uncompacted material is assumed as a joint surface. Two-dimensional constant strength displacement discontinuity (DD) analysis of the joint surfaces is carried out for the dry and hydrostatic pressure conditions. The weight of the overlying material acts as boundary conditions on the joint surfaces. Once, the DDs over the joint surface(s) are estimated, then the stresses at the prescribed points on the slope are estimated and checked using Mohr-Coulomb criterion to locate potential failure surfaces. The advantage of applying the DD analysis is that it reduces the problem dimension by one degree and eliminates discretization of whole slope area. The fracture mechanics based slope analysis gives better estimate of stability, because it accounts for the local stress concentration in vicinity of the cracks. The analytical slope stability results using the DD model, GeoSlope/W model and field slope monitoring investigation results demonstrated that overall slope is stable. Hence, the FRA is applicable to steep-slopes provided the slope gradient is maintained through careful final grading.
Introduction
The stability of natural or man-made slopes are of the great concern in Civil and Mining engineering. The most commonly used approach for the slope stability analysis is the limit equilibrium method (LEM). The LEM assumes slope as a rigid body and the rigid body motion or rotation causes the slope failure. The factor of safety in the LEM approach is determined by the ratio of shear resistance to average shear stress on a pre-defined failure surface. However, the LEM approach neglects the local development of stress concentration in the discontinuities or crack/joints in slope. The most suitable approach in jointed rock slopes appears to be that which considers a failure plane consisting of discontinuous joints separated by intact rocks. Jennings (1970) first suggested the LEM analysis of the jointed rocks and assuming that the intact rock fails in shear mode as a Mohr-Coulomb material and the strength of rock mass is estimated by using the joint persistence, which is defined as the ratio of total length of joints to the length of the failure plane. Tharp and Coffin (1985) applied fracture mechanics approach to the stability of small slope containing single crack. The fracture mechanics based slope stability analysis accounts for the crack-tip stress concentration characterized by the stress intensity factors. This approach is based on the principle that jointed rock slope stability is governed by the stresses intensity factors at the joint tips rather by the frictional resistance along the joint surfaces. The crack initiation starts when the tip stress intensity factor exceeds the fracture toughness of the jointed rocks. The factor of safety in this approach is defined in terms of stress intensity factor required to cause joint initiation compared to the fracture toughness. Scavia (1990 a&b) adopted the displacement discontinuity method (DDM) to deal with system of discontinuities in the slope and analyzed the slope stability using combination of fictitious stress method (FSM) and the displacement discontinuity method (DDM) and suggested to use stress discontinuity (SD) elements for the outer boundary discretization, whereas the displacement discontinuity (DD) elements are used to discretize the joint surfaces. Recently, Zhang et al. (2013 & 2014) presented slope stability studies based on the displacement discontinuity method (DDM) and used only the DD elements to discretized both the slope boundaries and joint surfaces. This approach is adopted in this study.
