The phenomenon of time-dependent deformation is commonly observed in underground openings where mining excavations are the main sources of mechanical loads. To predict the long-term deformations in soft rocks, a viscoplastic law has been implemented in a two-dimensional finite element program. A special time marching scheme is adopted to avoid numerical instability. A case study of a potash mine in Canada has been conducted to predict the long-term closures, as well as to analyze the stress distributions around the openings. The simulation results of displacement histories are closely matched with the field data. Comparing the deformations in average, the predicted values are higher than the field values. The maximum discrepancy is less than 20%. From a practical point of view, the predicted values are considered to be acceptable as heterogeneities and unknown discontinuities always exist in the mine. From overall evaluation, it suggests that model can be used as a practical tool for underground mine design of potash deposits.
1. INTRODUCTION
The time-dependent behavior such as creep and relaxation of geomaterials is of great importance for the design of deep underground mines in soft rocks. For example, the deformation process of a mined-out room in potash mine may continue for several months and even years after excavation to the extent of complete closure of the openings. The engineer is faced with the problem of adequately predicting the ultimate status of openings based upon evaluations of time-dependent analysis and observations made during the excavation period. A finite element program VISROCK, which implements the elastic viscoplastic model [1], has been developed in CANMET-MMSL, Natural Resources Canada to serve this purpose for salt mines and other soft rock structures.
Potash and rock salts are rate sensitive materials, which creep under sustained load, and they can yield excessively if the stress state is beyond a certain threshold. The general time-dependent behavior in creep of a rock sample can be divided into three stages, i.e., primary creep at an exponentially decaying rate, secondary creep at a constant rate, and tertiary creep at an accelerating rate leading to failure.
A simple rheological model capable of considering the primary, secondary and tertiary creeps can be represented by a combination of ideal elements [2]. The basic deformation elements include a spring accounting for elastic behavior, a dashpot and a slider accounting for viscous and plastic behavior. Tertiary creep is modeled by assuming that strength (cohesion and friction angle for geomaterials) degrades with increasing viscoplastic strain rate and is herein regarded as a drop in the strength of the slider.
A case study of a potash mine in Saskatchewan, Canada has been conducted to predict the long-term closures, as well as to analyze the stress distributions around the openings. Mining sequence of five openings at a depth of 960 metres is simulated. The deformation histories from the analysis are verified by those of the field data over a period of 2820 days after the mine was excavated.
