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Block caving mining method is one of the most viable and preferred mass underground mining methods for low grade orebody. However, in combined surface and underground mining, the breakthrough of an underground caving under an open pit may result in subsidence at the bottom of the pit. This can pose a serious threat for the stability of the pit and the safety of the mining. In order to ensure the stability of the open pit and guarantee the safety of the mine infrastructures and equipment as well as the continuation of underground mining operations in all serenity, it is extremely important to control pit bottom subsidence. In this paper, the numerical simulation software Phase 2 was used to study the influence of the backfill materials on the stability of an open pit during underground block caving mining. Four undercut levels were considered in the model. The undercuts were located at 250, 500, 750 and 1000 m from the bottom of the pit respectively. Backfilling of subsiding zones of the pit bottom with waste rock was realized each time before moving from a mining level to another. The result obtained show that the pit bottom subsidence increased when moving from the mining level located at 250 m from the pit bottom to the mining level located at 500 m. After that, it begun to decrease progressively with the increasing depth of mining. This is due to the accumulation of waste rock in the stope column. In addition, the tensile stress above the goaf decreased with the increasing mining depth.

Footwall instability is still problematic in many base metal surface mines around the world. The current slope stability analyses by numerical simulation models uses estimated mechanical input parameters to investigate the evolution and failure process of the footwall. However, the estimated mechanical input parameters of the footwall slope in the numerical simulation models may lead to deceptive conclusions. In this article, to cushion the shortfalls of the numerical models, the mechanical input parameters for the footwall slope were adjusted to obtain the numerical simulation results that aligns well with the slope stability monitoring records at the time of footwall slope failure at Nchanga open pit. Then, the geotechnical conditions driving the footwall failure were investigated using the validated numerical simulation model. Thereafter, back analysis was carried according to the field slope displacement monitoring results to determine the mechanical parameters of the rock masses at the time of failure. Then the critical functional relationship between the shear strength parameters of the weak rock unit at the time of failure was established. Study results indicate that the validated numerical model provides more realistic predictions of evolution and failure process of the footwall. The established critical relationship between shear strength parameters of the weak rock unit could be useful to design effective remedial measures against footwall slope stability at Nchanga open pit.

The contributions of the current analytical models on the prediction of the stability of the slope subjected to slide head toppling failure mechanisms, have always focused on the idealized geometry comprising regular blocks dipping into the slope face. Besides, the influence of groundwater and stabilizations from the lowermost block of the slope have been overlooked in the available literature. In this article, the analytical solutions that incorporates the kinematic mechanisms of the jointed rock slope under the influence of groundwater and stabilizing the lowermost block subjected to slide head toppling are derived based on the limit equilibrium. Furthermore, a real slide head toppling failure case history was studied to illustrate the effectiveness of the presented analytical solutions. The investigation results indicate that the presence of groundwater in the jointed rock slope, lowers the distributions of the normal and shear forces thereby inducing slide head toppling. Reinforcing the lowermost block of the slope, enhances the distributions of the normal and shear forces thus improving the stability of the jointed rock slope. The study results depict that the presented analytical solutions can provide an accurate and efficient stability analyses of the jointed rock slope subjected to slide head toppling failure mechanisms considering the presence of groundwater and stabilization effects.

Acid mine drainage is wastewater from a mine having a low pH and an elevated level of dissolved heavy metals. These metals are harmful to aquatic, animal and human life. This paper looks at the removal of copper from acid mine drainage using ion exchange to less than 1 mg/l. A weak acidic cation resin was used. Spectrophotometric determination of copper with sodium diethyldithiocarbamate was used to determine the copper concentrations in the treated water. Using regression analysis, the experimental results gave a correlation coefficient of 0.977 and a coefficient of determination of 99.5%. Results indicated that the higher the flows rate the shorter the period after which the copper concentration in the treated water reaches 1 mg/l. At pH 3.85 and 5.09, the resin performed better and at pH above 6.62 and between pH 3.0 and below the resin’s does not perform well. The higher the resin height the greater is the resin exchange capacity and the longer it takes for the copper concentration to reach 1 mg/l in the treated water. The higher the wastewater copper concentration the shorter the time it takes the resin to reach 1 mg/l. The results for this experiment indicated that acid mine drainage can be treated well by ion exchange resins, but it is also very important to establish suitable operating conditions.

The stability of the slope is essentially controlled by the ratio between available shear strength and acting shear stress, expressed as the factor of safety, along the sliding surface. The slope is deemed stable if the computed factor of safety is larger than unitary and unstable if the computed factor of safety is less than unitary. Although the current analytical method for computing the factor of safety of the jointed rock slope subjected to wedge failure mechanisms allows predictions of the stability of the slope, it tends to oversimplify the key geological features that may influence the resulting factor of safety of the slope. The simplified analytical model was applied to determine the factor of safety of the physical jointed rock slope subjected to the wedge failure mechanisms. Furthermore, numerical model that incorporates key geological feature such as joints and foliations on the same physical case was built to simulate the stability of the jointed rock slope subjected to wedge failure mechanisms. The analytical model results indicate that the computed safety factor is above unitary, suggesting a stable slope while the numerical simulations result of the same physical slope predicted unstable slope. The obtained conservative factor of safety entails unrealistic predictions of the stability of the jointed rock slope subjected to wedge failure mechanisms. A numerical simulation model that incorporates key geological features could provide realistic predictions of the stability of the jointed rock slope subjected to wedge failure mechanisms.

Malundwe open pit has experienced slope failures in the pit’s upper stack during rain-season since mining started in 2008. Laboratory and numerical model investigation methods were performed to investigate the influence of the rainfall pattern on the stability of the upper stack having saprolite domains. The saprolite soil was classified using the British soil classification system as slightly clayey, silty, sand with fine gravel which are of low plasticity. The water retention test using pressure test conducted on saprolite, reveals that the saprolite soil could not dry up completely as it contains about 9% volumetric content of water with the matric suction of 300 kPa during the most dehydrated month. The rainfall patterns at the mine site increase the groundwater level on the saprolite material. The rising of the groundwater table leads to the reduced matric suction and increased volumetric water content in saprolite domain. The reduced matric suction in the saprolite soils reduce the saprolite soil’s shear strength effectively leading to slope instability. Therefore, rainfall infiltration is the facilitator for initiating the failure of the upper slope at Malundwe open pit.

The safety factor of the slope subjected to wedge failure under surcharge load and uplift forces was obtained using an analytical model. A detailed parametric study conducted for the physical slope involved examining the impacts of the surcharge loads and uplift forces acting along the joint planes due to water pressure on the safety factor of the slope subjected to wedge sliding. The study results indicated that the safety factor of the slope susceptible to wedge sliding decreases with an increase in the surcharge loads and the uplift forces acting along the joint planes due to water pressure. The reduction in the safety factor due to an increase in the surcharge load is independent of the uplift forces acting along the joint planes. The rate of deterioration in the stability of the rock slope susceptible to wedge sliding depends on the rate of increase in the surcharge loads and uplift forces acting along the joint planes due to water pressure. The proposed expression can be useful for analytical stability analyses of the rock slopes subjected to wedge sliding induced by the surcharge load and uplift forces.

The current design of hangingwall entails stability throughout the extraction cycle of stopes. However, premature collapses are being experienced during extraction in some stopes especially those mined in the upper ore body. This study analyses the causes and impact of hangingwall instability in open stopes for the case of Chifupu project. Empirical and numerical analysis methods have been adopted. Prior to implementation of analysis techniques, a rock mass classification and geotechnical domain definition was performed based on the discontinuity properties. The classification methods used are Rock Mass Rating and Rock Quality Index. Modified stability graph was then adopted to assess the stability of the stope surfaces. Furthermore, due to the presence of a fault in the hangingwall a two-dimensional numerical analysis model was developed to incorporate rock mass heterogeneity. Instability was then evaluated based on depth of relaxation zone and reduction in strength factor. From the foregoing, it was discovered that when the orebody is overlain by prominent geological features within appreciable depth from the immediate hangingwall surface, empirical analysis must be substantiated with numerical analysis. The identified failure mechanism based on the two analysis methods are mainly due to gravity and sliding which also have the potential to introduce significant unplanned dilution. Therefore, the main conclusions for the cause of hangingwall instability has been poor ground condition in the hangingwall, shallow dipping orebody, presence of a fault, breach of the hangingwall leading to exposure of sandstone. It is therefore recommended that leaving a ‘skin’ of ore about 1 m to avoid exposure of hangingwall rockmass may provide temporary confinement to the immediate hangingwall during extraction.

Based on limit analysis and pseudo-dynamic methods, a proposed method for evaluating slope stability under seismic conditions that considers dynamic changes in the axial force of the anchor cables is presented. The case study shows that compared to the computational conditions that neglect the dynamic changes in axial forces, the dynamic safety factors obtained by the proposed method are greater, whereas the dynamic variation amplitudes of the dynamic safety factors obtained by the proposed method are smaller. The better anti-seismic effect of the anchor cables due to considering the dynamic changes in axial forces is reflected by the proposed method.

In the traditional analytical models for evaluating block toppling failure mechanisms of the slope, a single weak plane angle is assumed, running from upper rock columns and daylighting on the slop...

A broad mathematical technique for evaluating rock slope stability subjected to toppling of rock columns centered on limit equilibrium is presented. In the current analytical models for evaluating toppling of rock columns for the rock slopes, a single weak plane angle is assumed, running from the top surface of the slope and daylights on the toe of the slope. However, in some physical rock slopes, there exists counter-tilted failure surface within the rock masses of the rock slope and the weak plane may not daylight on the predictable point on the face of the slope following the assumed single weak plane angle. Therefore, applying the current analytical techniques in such conditions may not yield realistic results. A searching technique for estimating angles of counter-tilted failure surface is proposed and incorporated into the existing mathematical technique for evaluating toppling of rock columns centering on limit equilibrium principles. The physical slope with counter-tilted failure surface was comprehensively analyzed using the modified mathematical method and results were validated by numerical simulation using 3DEC 4.1 discrete element method. The influences of relative angles of the counter-tilted failure surfaces on the slope stability have been studied and the results show that, progressive increase of the counter-tilted failure surface angles lead to gradual increase in slope instability. The modified analytical method optimizes and combines the advantages of the traditional analytical techniques and could provide precise analytical method for evaluating stability of rock slopes with counter-tilted planar failure surfaces subjected to toppling of rock columns.

In rock slopes where sedimentary rock masses dip into the face of the slope, failure may occur by block toppling. In traditional analytical models, the failure surface is assumed to be a single plane running from the upper columns to the toe of the slopes, which may be inconsistent with the physical situation, where the weak plane has undergone counter-tilting within the rock slope due to variations of lithology and weak plane characteristics. To better reflect the physical situations, the failure surfaces ought to be determined instead of basing it on assumptions and incorporated in the existing analytical methods for stability analyses. Therefore, a searching technique for determining the counter-tilted failure surface angle has been proposed and traditional analytical models for evaluating the stability of rock slopes subjected to block toppling failure mechanisms have been modified by incorporating the counter-tilted weak plane angle. The physical slope with counter-tilted failure surface was comprehensively analyzed using the modified analytical model and the results were validated using numerical simulation models. The simulated failure mode zones are consistent with the failure mode zones obtained by the modified analytical method. The influence of relative angles of the counter-tilted failure surface on the slope stability has been studied and the results show that progressive increase of the counter-tilted failure surface angles lead to a gradual increase in slope instability. The proposed analytical method could provide precise applications to evaluate the slope instability in rock slopes with counter-tilted failure surface.

Numerical investigation on stability of the rock slope with densely distributed fractures is still challenging. To overcome the large computational requirements caused by the fracture representation, material heterogeneity and mathematical nonlinearity, an efficient discrete fracture network (DFN) based modeling scheme is developed in this paper. With DFNs stochastically generated according to the statistical distributions of the fractures for resembling the natural fractured rock slope, finite element method (FEM) based strength reduction analysis is conducted for locating the critical slip surface and assessing the factor of safety, where the rock blocks are discretized as solid elements while discrete fractures are practically modeled by frictional contact interface elements. The effectiveness of the proposed approach is validated by comparison against analytical and several numerical solutions. A case study is further performed to evaluate the effects of DFN realization number and geometrical configuration on the stability of the fractured rock slope. It shows that 20 DFN realizations are sufficient to obtain a statistically convergent mean of the factor of safety. The predicted critical slip surface displays to be non-circular but linear or broken line morphology along the fracture network. The proposed numerical scheme shows to be useful and promising in investigating the deformation response and sliding failure mode in highly fractured rock slope. Massive fractures disorder the displacement distribution of the slope and influence the overall factor of safety, and the estimated factor of safety decreases with the increase ofslope height and angle, indicating the fracture distribution and slope geometric features as the prominent influential factors.

In rock slopes where sedimentary rock masses dip into the face of the slope, failure may occur by block toppling. In traditional analytical models, the failure surface is assumed to be single plane running from the upper columns to the toe of the slope which may be inconsistent with the physical situation where the weak plane has undergone counter-tilting within the rock slope due to variation of lithology and weak plane characteristics. To better reflect the physical situations, the failure surfaces ought to be determined instead of basing it on assumptions and incorporated in the existing analytical methods for stability analyses. Therefore, an analytical technique for determining counter-tilted failure surface angle has been proposed and traditional analytical model for evaluating the stability of rock slopes subjected to block toppling failure mechanisms has been modified by incorporating the counter-tilted weak plane angle. The physical slope with counter-tilted failure surface was comprehensively analysed using the modified analytical model, and results were validated using the numerical simulations models. The simulated failure mode zones are consistent with the failure mode zones obtained by the modified analytical method. The influence of relative angles of counter-tilted failure surface on the slopes’ stability has been studied, and the results show that progressive increase in the counter-tilted failure surface angles leads to gradual increase in slope instability. The proposed analytical method could provide precise applications to evaluate the slope instability in rock slopes with counter-tilted failure surface.

In this paper, a polar slice method is proposed to quantify the energy dissipation in the inner sliding soil mass and evaluate the slope stability in rotational failure mechanism. The polar slice method is compared with the vertical slice method. Despite similar results, the advantages of the polar slice method are explained. Besides, the comparison between the polar slice method and Chen’s method shows that the polar slice method would lead to more accurate results than Chen’s method, while Chen’s method which neglects the energy dissipation in the inner sliding soil mass is relatively conservative.

Mufulira underground mine has been facing great geotechnical challenges due to crown pillars left after mining which act similar to stress windows and these bring about high stress build up in the footwall drifts. A numerical analysis was performed with the objective to investigate how a change in mining method would affect the high stress levels. The numerical analysis was achieved using the software Flac 3D based on data corrected at the mine. One of the major aims of the analysis was to determine the change in mining height and its implications. Results from the numerical analysis showed that altering the mining technique would decrease the stress levels in the footwall.

In the current numerical models for evaluating block toppling of the jointed rock slopes under dynamic conditions, a single weak plane angle is assumed, running from the top surface and daylighting on the slope's toe. The potential source of shortfalls in the current numerical models are related to the assumptions on the distributions of the weak planes in the jointed rock slope. In this article, the relationships between earthquake event and toppling failure of the jointed rock slopes with varying counter-tilted weak plane angles have been studied using 3DEC software. The study results indicate that the counter-tilted weak planes play a major role in adjusting the earthquake ground motions thereby influencing the toppling displacement response behaviours, failure mechanisms and the rate of displacements for the jointed rock slope. The study could provide precise applications for predictive scenarios of dynamic toppling of the jointed rock slopes with counter-tilted weak planes.