57 / 2023-03-31 03:10:31

Effect of open-pit wall fracturing on the stability of underground excavations in the under-pit rock mass

fracturing, quarry wall stability, rock mass, stress-strain state, failure

The development of ore deposits is a complex process, the success of which is largely due to the ability of the exposed rocks to retain their shape and size under a certain range of technological and geological conditions. The problem of maintaining excavations in a stable state is relevant for the Akzhal deposit developing polymetallic ores in the central part of Kazakhstan both by open pit and underground mining. A reliable assessment of stability is relevant both for the quarry sides and underground galleries in the under-pit part of the rock mass.

Mining operations on the northern side of the Akzhal Сentral quarry revealed a tectonic fault at a horizon of 250-300 m.  Surveying observations with quadrocopters have detected a zone of intense fracturing formed as a result of undermining the tectonic fault. A system of joints is traced parallel to the identified fault. The risk of a sliding surface occurring in the area of intense fracturing should be assessed in order to develop the correct strategy for the quarry operation.

The most effective means to meet this purpose is mathematical modeling.  In particular, a simulation based on numerical methods of solid mechanics has confirmed its effectiveness in rock mechanics. The finite element model of the rock mass area including both the surface and underground excavations was created in the RS2 code (Rocscience) based on the real contours of the Akzhal Сentral quarry. The numerical model was originally rebuilt using the Micromine software and then exported to the RS2 calculation module.

The effect of the fracture zone on the rock stress-strain state near the quarry walls and around underground galleries in the under-pit area was studied for this case. Fracturing was simulated directly by creating a network of joints using special contact elements provided by the functionality of the RS2 codes. Such fracturing parameters as normal and tangential stiffness of the joint filler were set depending on the filler type and the degree of the joint openness. The quarry wall safety factor was determined in accordance with the shear strength reduction procedure based on the Hoek-Brown failure criterion.

FEM analysis displayed that the occurrence of the zone of intense fracturing in the fault vicinity reduces the safety factor by 15-27%  depending on the density of the simulated joints, as well as on the mentioned joint filler stiffness components.

The zone of intense fracturing on the northern side of the quarry has an impact on underground galleries, excavated in the under-pit area in hard massive limestones. According to the original design, the potential slope sliding surface does not affect the rock mass around roadways. However, the undermined fault and the accompanying fracture zone change the distribution of normal and shear stresses in the under-pit rock mass, causing intensive development of shear strains. As a result, the potential sliding surface relocates and becomes closer to the underground transport roadways. They get involved in additional deformation processes indicated by shear strains.

The maximum contour displacement of the roadway located closest to the surface in the bottom part of the quarry, as well as the dimension of the failure zone around it, are considered as a target function in a computational multivariate experiment. Variable parameters are the size of the fracture zone, the distance between the joints, the degree of joint openness, as well as the distance between the transport roadway and the surface of the quarry bottom.

The computational procedure gives a multifactorial prognostic model of the underground excavation stability in the area of rock mass under the impact of the potential sliding surface in the quarry wall. The fracture zone occurring in the quarry wall is a risk factor that should be taken into account when designing underground excavation in the under-pit area. Based on the proposed prognostic model the support design has been developed considering various types of rock bolts and polymer resin injection.