Influence of Geotechnical Factors on Slope Stability in the Western Bielawy Limestone Open-Pit Mine (Poland)

The stability of internal dumps in open-pit coal mines is critical for the safe production and economic performance of the entire mine. To further enhance slope stability and ensure safe production, a new method for constructing trenches (referred to as an anti-sliding trench) on the sloped basal bed of the dump slope in open-pit mines was proposed to improve slope stability. The internal dump slope at the Luzigou anticline of the Anjialing Open-Pit Mine was studied. The slope failure modes of the dumping steps were studied experimentally and by numerical simulations at different widths of anti-slide trenches at the slope’s toe in a staged loading state. Without anti-slide trenches, shear-layer and along-layer failure modes occurred, while the failure modes with anti-slide trenches included shear-layer, along-layer, and squeeze-out failure. Based on the limit equilibrium theory and the determined failure modes, the preset anti-slide trenches at the toe of the dumping steps were theoretically analyzed. The relationships between the slope stability coefficient and the width and depth of anti-slide trenches, as well as the physical and mechanical parameters of the slope body, were derived. Given the physical and mechanical parameters of the slope body and targeted improvement in the slope stability coefficient, the size parameters of anti-slide trenches were designed and optimized through the derived relationships. At the Anjialing Coal Mine, presetting anti-slide trenches with a depth of 1.5 m and a width of 22.68 m at the toe of the dumping steps increased the slope stability coefficient from 1.3095 to 1.6. The proposed method provides a guiding reference for designing similar internal dump slopes in open-pit coal mines and for disaster prevention.

Open-pit mining often exposes weak rock layers, the strength of which significantly affects the stability of slopes. If these rock layers are also prone to disintegration and expansion, cyclic rainfall can exacerbate instability. Rainfall-induced changes in the seepage field also indirectly threaten the stability of slopes. Therefore, investigating the characteristics of mudstone limestone and the impact of the seepage field on slope instability under different wet–dry cycles is of great significance for the safe mining of open-pit mines. This paper takes the mudstone limestone slope of a certain open-pit mine in the southwest as the starting point and conducts experiments on saturated density, water absorption rate, permeability coefficient, compressive strength, and variable angle shear strength. Combined with scanning electron microscopy and phase analysis of X-ray diffraction analysis, the macroscopic and microscopic characteristics of the samples are comprehensively analyzed. FLAC3D software is used to explore the changes in the seepage field and the mechanism of instability. Our research found that for the preparation of mudstone limestone samples, a particle size of less than 1 mm and a drying temperature of 50 °C are optimal, with specific values for initial natural and saturated density, and natural water content. As the number of wet–dry cycles increases, the saturated density of mudstone limestone increases; the water absorption rate first rises sharply and then rises slowly; the permeability coefficient first rises sharply and then stabilizes, finally dropping sharply; the compressive and shear strength decreases slowly, and the internal friction angle changes little; frequent cycles also lead to mudification and seepage filtration. At the microscopic level, pores become larger and more regular, and the distribution is more concentrated; changes in mineral content weaken the strength. Combined with numerical simulation, the changes in the seepage field at the bottom of the slope exceed those at the slope surface and top, the transient saturated area expands, and the overall and local slope stability coefficients gradually decrease. During the third cycle, the local stability is lower than the overall stability, and the landslide trend shifts. In conclusion, wet–dry cycles change the pores and mineral content, affecting the physical and mechanical properties, leading to the deterioration of the transient saturated area, a decrease in matrix suction, and an increase in surface gravity, eventually causing slope instability.

The stability of slopes in open pit mines is an issue of great concern because of the significant detrimental consequences instabilities can have. The stability of open pit is controlled by several geotechnical and geological factors, the most important cracks in the rocks as well as various and homogeneous of layers. In the case of study presence of a tensile crack is the most critical case in the calculation of factor of safety because of the possibility of the presence of waters in this crack that influence negatively on the stability of the open pit with traction crack, Then the mechanical characteristics of the crack filling material are more degraded than those of the marl layer between the layer of limestone open pit LAFARGE. With the differences both methods possess, sensitivity analyses and comparisons of result were done using Slide, Flac and Plaxis software for the analysis of sliding and stability of open pit. These evaluations have been performed by the limit equilibrium method through the SLIDE software, the finite element, finite difference methods through PLAXIS and FLAC respectively.

ABSTRACT: The slope stability analysis usually requires consideration of both geological and geomechanical factors. However, due to the limitation in geological model, current slope stability analysis is typically based upon simplified geological frameworks and does not integrate the key geological complexities required to accurately model the geomechanical response to slope excavation. This study presents an integrated geology and geomechanics approach that can readily assess the geomechanical risk of open pit mining while preserving the geology's integrity. The geological model adopts an implicit modeling of radial basis functions to represent the geology generated from mining survey and borehole drill samples. The 3D geomechanical model is based on a non-linear quasi-static finite element method and the mesh is generated directly from the underlying geological framework. The geomechanics simulation adopts the strength reduction method to evaluate the factor of safety (FOS). The baseline simulation results show a FOS of 1.94, which indicates no shear failure on the open pit mine slope. We also investigate the impact of rock mass geomechanical parameters. The proposed workflow permits us to combine a reliable geological representation with a state-of-the-art rock mechanics finite element solver, which can be used to yield a high fidelity slope stability analysis. 1. INTRODUCTION Open pit mining refers to mining directly on the ground surface and is a critical mining type around the globe. Slope failures of an open pit mine may lead to catastrophic consequences such as economic losses and fatalities. As the mining depths of open pits continue to increase with the decrease in shallow mineral resources, slope stability analysis is becoming more and more important (Du et al., 2022; Zhang et al., 2022). Slope stability analysis is one of the most classical areas in geotechnical engineering (Christian et al., 1994; Duncan et al., 2014; Sjöberg, 1996). In spite of the slope analysis using conventional techniques, open-pit mines are still experiencing slope failures, which indicates the need to develop better models which improve the prediction and understanding of slope behavior (Kolapo et al., 2022).

During the production of open-pit mines, the stability of slopes can be affected by various factors such as structural surfaces, production blasting vibrations, and mining areas. In this study, the researchers focused on the slope of the open-pit mine at Yinshan and employed UAV mapping technology to conduct an on-site geological engineering investigation. Information on the yield, trace length, spacing, and density of the structural surface of the south slope was obtained. The researchers also carried out vibration blasting tests in combination with the production blasting activities in the mine to determine the blasting vibration attenuation law and whether the blasting vibration speed met safety specifications. Additionally, numerical simulation methods were used to examine the influence of the mining area on the stability of the current slope and the designed excavation slope. The slope stability was evaluated using the limit equilibrium method, and the researchers separately discussed the influence of self-weight load and self-weight load plus blasting vibration force on the stability of the high slope of the open pit. The results showed the following: (1) The rock mass structural plane in the south slope of the mining area was mainly dominated by a medium-large dip structural plane, and three faults and joint fissures in the investigation area combined to form cutting and sliding surfaces in the rock mass that were prone to collapse and sliding. (2) The maximum blasting vibration speed met safety requirements. (3) There was no large range of plastic zone damage in the entire slope, and the overall stability of the slope was good. (4) The present slope was relatively stable when considering only self-weight stress and the blasting vibration force. However, there was a certain risk of instability in the design of the excavation slope.

Mineral resources play a crucial role in modern construction, and open-pit mining is the primary method for resource extraction. Ensuring slope stability is vital for the normal operation of open-pit mining. This study focuses on an open-pit coal mine in Xilin Gol League, Inner Mongolia, China. Through comprehensive methods such as geological data collection and analysis, field investigation, and UAV mapping, the Slope/W module in GeoStudio software was employed, and the Bishop limit equilibrium method was used to perform a stability analysis on two cross-sections of the composite slope between the mining pit's East Slope and the external dumping site. The results showed that the safety factors for these two sections of the composite slope do not meet the required reserve safety factor. After applying a toe-weighting optimization design, a subsequent stability analysis was conducted, ensuring the stability requirements were met. The main factors affecting the stability of the composite slope were analyzed, including the geotechnical properties, slope rock mass structure, the design distance between the external dumping site and the mining slope, and surface water erosion and seepage. Safety and mitigation recommendations were proposed for other open-pit mines with composite slopes involving external dumping sites and mining slopes.

Three-dimensional (3D), stress-deformation models are useful for analysing the stability of complex 3D problems in open pit mine slopes. Stress-deformation models are very powerful because in addition to the Factor of Safety, the states of stress and strain throughout the pit are computed in a single model where a large number of potential failure mechanisms may be considered simultaneously. However, the massive quantities of information provided by these models make them difficult to interpret. This paper presents and discusses a FLAC3D, version 4.0 (Itasca, 2009) numerical model that was developed to assess the stability of the proposed Oyu Tolgoi open pit, which will be approximately 2,900 × 2,100 m wide, and 600 m deep once it is completed. This large pit will include two distinct sub-pits that will have numerous fault intersections within the pit walls that cannot be adequately represented and assessed by 2D slope stability analysis models. The FLAC3D model of the proposed Oyu Tolgoi open pit focused on assessing 3D complex failure mechanisms that involve one or more of the steeply dipping faults within the pit walls at two critical stages over the life-of-mine. This model allowed consideration of the pit stability from a stress-deformation perspective. While this approach can be used to assess pit stability in cases where very large displacements associated with well-defined failure mechanisms are computed (e.g. catastrophic failure), assessing the large scale structurally controlled stability is more challenging if a catastrophic failure is not predicted. In this analysis, a number of methods were considered to determine the overall and local stability of the pit slopes. The following characteristics were considered: i) displacements, ii) strains, iii) velocities, iv) the yielding state of the model elements, and v) FLAC3D’s internal Factor of Safety calculation. The automated Factor of Safety (FS) calculation within FLAC3D provides only the single most critical failure in the model. In the case of the Oyu Tolgoi open pit, the computed FS was relatively large and not related to 3D complex failure mechanisms involving the faults within the pit walls. Had the model been be set up to focus only in specific mechanisms of interest, the computed FS would have been larger, but nothing more than another number. From this perspective, the FS calculation was found of limited use in this case and hence, other information from the stress-deformation analysis was used to assess the stability of the pit slopes beyond the FS calculation. Examination of the patterns of displacement, strain, velocity, and yielding elements were found to be far more useful in determining the stability state of the pit. These were used to identify areas of the pit which showed the highest potential for movement or failure.

Deep open-pit mines and large rock slopes expose many diverse rock lithologies and geological structures (e.g., faults, bedding planes) that may reduce the integrity of slopes. Numerical modeling is a powerful tool for simulating these structures; however, there are few guidelines and methods for calibrating/validating and implementing faults in a numerical model. This paper presents a novel laboratory method to calibrate numerical models and highlights the challenges in simulating faults. One of the main issues in reliable modeling of faulted rock structure is the scarcity of experimental analyses in the laboratory under the controlled conditions. Moreover, a comprehensive evaluation of the effect of using the conventional fault modeling methods on the stability of rock structures is required, as well as a benchmarking between theoretical and experimental results. This research combines theory and experiment, to fill the existing gaps, using numerical simulation and laboratory measurements. Using FLAC3D software, the sensitivity and comparative analyses are carried out for the numerical simulations to investigate the stability of rock slopes on large and small scales (overall open-pit slope and bench slope), and the fault zones. The weak zone (WZ), ubiquitous-joint (UJ), and interface (IF) techniques are the widely used methods in the modeling to capture fault slip mechanisms. The factor of safety (FOS) of the slope is monitored upon variation of the design parameters, such as fault and rock mass mechanical properties, fault types, and modeling framework (e.g., mesh density, convergence ratio). In addition, parameters such as shear displacement and shear stress are investigated to deduce the failure mechanism of the studied models. Finally, laboratory tests were performed to calibrate the modeling results and approximate the agreement between theoretical and experimental results. The results of sensitivity analysis showed that choosing an adequately low convergence ratio is critical for estimating FOS. However, beyond a certain convergence ratio, below 10−7, this change is negligible (less than 5%). The results of mesh density sensitivity analysis indicate that the FOS values are insensitive to the mesh density in the WZ method (less than 5% change in FOS), the IF method shows the median sensitivity (5–12% change in FOS), and the UJ method is the most sensitive (FOS values improves by ~ 31%). Comparison between laboratory test and numerical modeling (FOSlab = 1.71, FOSWZ = 1.51, FOSIF = 1.62, and FOSUJ = 1.76) indicates a good agreement between the UJ and IF methods and the laboratory model (~ 3–5% discrepancy). It needs to be mentioned that these analyses/tests are not to favor one method over the other, but rather to emphasize the pros and cons of each within the assumptions of this study.

The stability of high and steep slopes in open-pit mines is closely related to the mine operations and the lives of the surrounding residents, so it is important to ensure the safety and stability of the slopes. Hazard classification and stability analysis of high and steep slopes under different working conditions are studied using the Shizhuyuan non-ferrous metal mine from underground to open-pit mining as a typical example. Firstly, data on rock mechanics parameters were obtained through site investigation and sampling. Then, the slope model of the open-pit mine was established and some slopes were selected in the model for qualitative and quantitative analysis. The strength reduction method and the limit equilibrium method were used to calculate the safety factor under each working condition and point out the potential instability areas. The results show that the selected slopes are safe and stable under all working conditions. Finally, on the premise of maintaining the safety and stability of the mine, the final slope angle was optimized from the original 45°21′35″ to 55°30′41″ to reduce production costs and increase mining efficiency. The final open-pit boundary that meets the stability requirements was eventually obtained.

The slopes of open-pit mines are often at risk of failure. To identify this hazard, stability analyses are performed. An important element of these stability analyses is the reliable selection of input parameter values for the calculations. This selection is difficult because the slopes of the open pit are disturbed by mining activities. In such conditions, rheological processes, intensified by weathering, develop in open-pit slopes. This study is aimed at setting the strength parameters for the stability analysis of open-pit slopes with a developed slide process, using the random set method. The study was performed on the example of the open pit of the Bełchatów lignite mine, central Poland. A four-stage methodology, according to the random set method, was proposed. The methodology covered the following: site investigation, sensitivity analyses, shear strength reduction (SSR) analyses using numerical calculations, and probability analyses of the factor of safety (FoS) calculation results. The setting of the input parameters took into account the peak and residual strength parameters for each lithological unit in the physical model of the open-pit slope. Samples for laboratory tests were taken from the cores of nine test boreholes. The sensitivity analysis included all peak and residual strength parameters for each lithological unit in the body. As a result of the sensitivity analysis, specific strength parameters were adopted that would have a great impact upon the results of the calculations. Selected sets of parameter values were then used for the FoS calculations. The resultant FoS values revealed the probable slide planes. The positions of the slide planes were consistent with the interpreted slide surfaces based on the control boreholes and terrain observations. Knowledge of the slide planes positions and the values of the strength parameters enabled the designing of a securing approach for this landslide, and the taking of preventive measures to reduce this risk.

Northwest slope of An Taibao open-pit mine in Shanxi province is under the condition of open-pit and underground combined mining, based on this slope project, the computation model is established for research and analysis using MIDAS / GTS and FLAC3D software, the deformation response characteristics of steep slope are researched by the method of numerical simulation. It is demonstrated that the steep slope formed by open-pit mining has a certain deformation response after the underground mining of No.4 coal, which is little effected, with the continuing advance of No.9 coal mining working face, the deformation of steep slope is increasing, it is expected that the maximum subsidence will be up to 3.12m, and the maximum displacement toward free face will be up to 0.48m after the mining of 9005 working face. According to the above study, it is suggested that monitoring and reinforcement measures should be taken in the study area, in order to realize sustainable development and utilization of coal resources.

The creep failure of open-pit mine slopes with weak interlayers is one of the main types of slope instability in open-pit mines. The scientific and reasonable treatment of this type of landslide is of great significance for improving the quality of open-pit mining. In this study, we study a gently inclined and creep-type slope with weak interlayers in an open-pit mine in Inner Mongolia, China, and conduct systematic on-site engineering geological investigations, laboratory tests, and numerical simulations. The particle swarm optimization algorithm is introduced, and the creep model combining Burgers and Mohr–Coulomb is selected. Combined with triaxial compression creep test data, the creep model parameters of the weak interlayer soil are intelligently inverted. A typical profile is selected to analyze the stability of the slope. The results show that the creep of the weak interlayer is the main controlling factor for the deformation and failure of the slope. Under natural conditions, a clear continuous plastic zone appears at the front edge of the weak interlayer and the rear edge of the sliding body, resulting in slope instability and large deformation. Our results are in good agreement with the reality of engineering. Furthermore, we study the effectiveness of the local reinforcement treatment method for the weak interlayer. This study shows that local reinforcement of the weak interlayer is one of the most economical and effective means of preventing and controlling landslides. After reinforcement, the plastic zone of the slope only appears near the rear edge of the sliding body and the reinforced rock mass, with a poor connection, and the stability of the slope is good. Our results provide effective technical support for the treatment of this slope and offer a reference for the disaster prevention and mitigation of gently inclined and creep-type open-pit mine slopes with weak interlayers.

The stability of open pit rock slopes is commonly assessed using deterministic methods. A single value for the Factor of Safety of the slope is calculated, and is assumed to represent the overall stability of the slope. A limitation of the deterministic approach is that it does not account for the natural variability of the input parameters or the uncertainty caused by sampling. The uncertainty and variability of input parameters such as frictional strength, cohesive strength, and discontinuity orientations are often accounted for by selecting values that incorporate conservatism based on limited laboratory testing or other data collection methods. The confidence in the Factor of Safety calculated using deterministic analyses remains a matter of engineering and experiential judgment. Reliability based methods, now adopted by many practitioners, overcome some of the limitations of deterministic methods by incorporating the natural variability of key input parameters into the calculation methods. If the natural variability is included, then a range of Factors of Safety can be calculated for the various combinations of the input variables. This range represents the probability density function of the continuous distribution of Factors of Safety, from minimum to maximum values. The cumulative distribution of values can then be used to quantitatively describe the likelihood of achieving a certain Factor of Safety. Understanding the distribution of Factors of Safety, and the reliability of the results, provides a method to make reliability based decisions. This paper presents a feasibility level study undertaken for Diavik Diamond Mines Inc. using point estimation methods in combination with limit equilibrium methods to evaluate the Factor of Safety of a proposed open pit slope. Point estimation involves the use of sample statistics to estimate population parameters. Confidence intervals are constructed that include the value of an unknown variable—in this case Factor of Safety—with high probability. The confidence intervals are developed based on the method of moments from probability theory, which is used to describe random variables of the sample population using the expected value of the random variable and the square of the expected value. A Monte Carlo random sampling method is used to develop a simulated population to approximate a normal distribution, which in this case represents the probability of slope performance defined in terms of the Factor of Safety. This paper describes the process followed to develop the probability density and cumulative distribution functions for the Factor of Safety. The statistical distribution of Factor of Safety is presented and is used to evaluate the pit slope stability in terms of probability of failure and acceptable risk tolerance.

The rock mass consists of heterogeneous and anisotropic materials, with smaller and larger blocks of rock, and the presence of structural discontinuities is a significant concern. Characterising the rock mass is crucial for the success of engineering excavations in such areas. A detailed study of joints, their orientation, and discontinuities in the exposed rock mass is crucial as they greatly influence stability and fragmentation. Existing rock mass classifications like Rock Mass Rating (RMR) and Q-classification require various geological parameters and physico-mechanical properties of the rock. However, determining these parameters conventionally can be time-consuming, requiring careful on-site measurements. The number of opencast coal mines is increasing compared to underground mines due to shorter gestation periods, higher productivity, and quicker returns. However, opencast mining raises environmental concerns such as air and water pollution, solid waste management, land degradation, and socio-economic issues. Additionally, many opencast coal mines, regardless of size, are reaching greater depths, making analysing bench slopes and ultimate pit slope design crucial. Slope failure in these mines leads to production loss, additional costs for recovery and handling of failed material, pit dewatering, sometimes mine abandonment or premature closure, and loss of life. A study was conducted at Prakash Khani Opencast Mine – IV, Manuguru area, M/s The Singereni Company Collieries Limited (SCCL), to investigate the influence of structural discontinuities on slope stability. SIROVISION software was used to assess rock mass characterisation, while PLAXIS-2D software was employed to analyse the influence of structural discontinuities on slope stability. A comparative conclusion was drawn based on the results obtained from SIROVISION and PLAXIS-2D analyses. The study revealed that the RMR of the mine ranged from very poor to fair due to numerous discontinuities. It was also found that discontinuities in the slope decrease the Factor of Safety (FOS), indicating an impact on slope stability.

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.