An underground copper ore deposit below an open pit mine is to be extracted using large diameter blast hole (LDBH) method. This paper is focused on static and dynamic responses of LDBH stopes of dimension 60 m height, 20 m width and varying length. In a single round, about 3 tonne of explosive will be blasted for a burden of 3.5 m having hole diameter ranging from 57 mm to 165 mm. Both static and dynamic loads along with mass extraction of ore adversely affect the stability of underground structures. A 3D finite element analysis is carried out to examine the stability of transverse stopes with static load considering anisotropic condition of jointed rock mass. Numerical simulations are carried out using transient dynamic finite element method considering a production stope and a neighboring mined out stope. Stability indicators are assessed in terms of displacement, stresses, and yield zones around underground.
Whenever any excavation is done below the surface, the stress field is locally disrupted and the stresses redistributes in the rock strata surrounding the opening. The estimate of these redistributed stresses in terms of magnitude and direction plays a vital role in design of the excavation. Alleviating the stress concentration is of utmost importance to ensure safety of existing underground structures such as main drive, crosscut, and decline and also to maintain stable production in underground mines. Thus, design and selection of appropriate mining method and mining sequence can mitigate the risk of failure of existing underground structures nearby.
Numerical methods are an important tool to analyze the stability of large underground mine structures to estimate the geotechnical risks and hazards. It provides a better understanding to engineers and researchers to solve complex mining problems. Zhang & Mitri (2008) performed 2D nonlinear elastoplastic FEM analysis to study the stability of haulage drift considering the effect of stope mining sequence, mining depth and distance between the stope and the haulage drift. The 2D analysis provided an insight to understand the effect of sublevel stoping on haulage drift; however 3D analysis was considered necessary to evaluate stress distribution as stopes are mined and backfilled along the orebody strike. Ferrero et al. (2010) analyzed the stability conditions of old abandoned quarry of Varese, Italy by conducting insitu measurement tests and laboratory experiments. The rock mass mechanical properties were determined from the tests and a 3D FEM model of the entire rock mass was developed to analyze the stability of underground workings. The constitutive behavior of the rock mass was modeled with Mohr-Coulomb plasticity model. The result indicated rock block instabilities at the roof and sides of the pillars. Suggestive support measures such as rock bolting and shotcrete were proposed for the rock mass at such locations. Wang et al. (2013) simulated the optimum mining sequence for the mining area in the Jinshandian iron mine with 3D model using FEM based software ANSYS. The simulations showed that a reasonable stoping sequence can minimize the high compressive stress concentration, thereby ensuring safety for mining out the stopes from the orebody. Abdellah et al. (2014) performed 3D analysis to study the stability of mine development intersections at Garson mine of Vale in Sudbury, Canada using FLAC3D software. It was observed that the stability of intersection deteriorates with mining advance and the roof of intersection required secondary support at later stages of mining. However, in field, it is not just the static load but also the dynamic load due to blasting which affects the stability of the underground structures. Stability of the underground structures, thus, is a major concern, especially when dealing with huge scale production blast in large underground mines. Numerical studies on rock blasting using various numerical codes and models have been implemented to simulate the process of rock fracture and fragmentation in blasting. Zhu (2009) studied the 2D simulation of rock fracture and fragmentation in crater blasting and bench blasting, so as to obtain a better understanding of the controlling parameters.