Footwall Slope Research Articles

Disturbed zone calculation and stability evaluation method of footwall slope with slip-shear failure under excavation

Disturbed zone calculation and stability evaluation method of footwall slope with slip-shear failure under excavation

IW Sentinel-1 satellite scenes for the investigation of mine slope stability: experiences from the Riacho dos Machados gold mine (Brazil)

In the last decade, the Persistent Scatterer Interferometry – PSI have been largely employed to predict instabilities and failure in open pit mines. The PSI is a powerful technique, which combines radar satellite data in order to detect and monitor tiny surface displacements over vast areas. In the last years, the Sentinel-1 radar mission have produced images of the globe acquired with different spatial and temporal resolutions that are now freely available. In recent years, the footwall slopes of the Riacho dos Machados Gold Mine – MRDM (Minas Gerais state, southeastern Brazil) have recorded large planar failures controlled by foliation planes. Therefore, the focus of this paper is to evaluate a stack of 39 Interferometric Wide Sentinel-1 scenes, spanning from January 2018 to April 2019, acquired in descending orbit geometry, for the detection and monitoring of surface displacements in the MRDM. The results have shown that descending IW Sentinel-1 scenes can be used to provide a broad picture of the Line-Of-Sight - LOS deformation phenomena. In order to monitor the evolution of the deformation phenomena induced by mining activities, LOS deformation maps with millimeter accuracy could be only delivered at least each 12 days.

Physical model test study on shear strength characteristics of slope sliding surface in Nanfen open-pit mine

A physical model for the footwall slope of Nanfen open-pit mine, China was established using a self-developed deep geological engineering disaster model test system. A thermosensitive similar material, paraffin, was selected to simulate a weak structural plane in the slope to reproduce the landslide process. From an experimental perspective, the variation trend of shear strength parameters of weak structural plane and the mechanical support characteristics of NPR (negative Poisson’s ratio) anchor cable under the condition of a large landslide deformation and failure were examined. The results of this model test showed that slope failure has four distinct stages: (1) soil compaction stage, (2) crack generation stage, (3) crack propagation stage, and (4) sliding plane transfixion stage. According to the test results, the rock mechanics parameters of weak surface in the footwall slope of Nanfen open-pit mine were calculated. The cohesion is approximately 1.35 × 105 Pa, and the internal friction angle is approximately 6.33°. During slope failure, the NPR anchor cable experiences a large deformation but no damage occurs, indicating that the NPR anchor cable can be continuously monitored and reinforced during the deformation and failure of landslide. The stress characteristics of NPR anchor cables during the test are consistent with the monitoring results of Newtonian force at the landslide site, proving that NPR anchor cables are effective and reasonable in landslide monitoring and early warning.

Modeling the Shape and Evolution of Normal‐Fault Facets

Facets formed along the footwalls of active normal‐fault blocks display a variety of longitudinal profile forms, with variations in gradient, shape, degree of soil cover, and presence or absence of a slope break at the fault trace. We show that a two‐dimensional, process‐oriented cellular automaton model of facet profile evolution can account for the observed morphologic diversity. The model uses two dimensionless parameters to represent fault slip, progressive rock weathering, and downslope colluvial‐soil transport driven by gravity and stochastic disturbance events. The parameters represent rock weathering and soil disturbance rates, respectively, scaled by fault slip rate; both can be derived from field‐estimated rate coefficients. In the model's transport‐limited regime, slope gradient depends on the ratio of disturbance to slip rate, with a maximum that represents the angle of repose for colluvium. In this regime, facet evolution is consistent with nonlinear diffusion models of soil‐mantled hillslope evolution. Under the weathering‐limited regime, bedrock becomes partly exposed but microtopography helps trap some colluvium even when facet gradient exceeds the threshold angle. Whereas the model predicts a continuous gradient from footwall to colluvial wedge under transport‐limited behavior, fully weathering‐limited facets tend to develop a slope break between footwall and basal colluvium as a result of reduced transport efficiency on the rocky footwall slope. To the extent that the model provides a reasonable analogy for natural facets, its behavior suggests that facet profile morphology can provide useful constraints on relative potential rates of rock weathering, soil disturbance, and fault slip.

Limit-Equilibrium Analysis of Stability of Footwall Slope with Respect to Biplanar Failure

AbstractBiplanar failure is the most common instability type of footwall slopes, with a steeply dipping joint (typically the bedding plane) and a gently dipping joint (i.e., the daylighting toe joi...

Numerical and theoretical study of bi-planar failure in footwall slopes

Bi-planar failure is the commonest type of failure in steep and high footwall slopes. This paper presents an investigation of the bi-planar failure mechanism and a stability analysis model for footwall slopes based on the study of eleven diabase mines in Hunyuan County, Shanxi Province, China. First, a detailed investigation of the site and tests were carried out to obtain a geological model and calculation parameters for the slope. Then, the failure process in footwall slopes was investigated by a trigon approach performed using the Universal Distinct Element Code (UDEC) software package. A mechanism was then obtained for bi-planar compression–shear failure by combining the results obtained for the stress, displacement, and crack development. Subsequently, based on the limit equilibrium and frictional plasticity theories, a new columnar mechanical model was proposed for bi-planar footwall slope failure. The results demonstrate that the UDEC trigon approach was well suited to simulating bi-planar failure in steep and high footwall slopes. Employing this approach, a sliding failure surface that initiates, develops, and propagates in the discontinuity and intact rock could be correctly captured and the evolution of the damage in the toe of the slope could be quantitatively investigated. The results also indicate that shear failure first occurs at persistent discontinuities and then compression–shear failure occurs in the intact rocks. In addition, the failure surface of the rock emanates from the toe of the slope, is oriented upwards at an angle to the persistent discontinuity thus forming the through sliding surface (this angle can be determined using frictional plasticity theory). The critical heights and safety factors of footwall slopes could be readily obtained by employing this new columnar mechanical model giving results that are consistent with the results of numerical simulations. Furthermore, the results of a parameter-sensitivity analysis show that the dip angle of the toe breakout surface increases as the slope angle increases. However, the angle between the toe breakout and internal shear surfaces remains unchanged as the slope angle increases. Moreover, the critical height of the footwall slope decreases linearly as the slope angle increases. The method proposed provides theoretical guidance for the stability analysis of steep and high footwall slopes. The results produced should also help engineers to gain a better understanding of the mechanisms of bi-planar failure in such rock slopes.

Rock structure characterization of a magnetite gneiss with foliation-parallel discontinuities for footwall slope design

The Warramboo magnetite iron ore deposit in South Australia is predominantly hosted by gneiss. Design of the footwall slope requires an assessment of the role of foliation-parallel discontinuities as planes of weakness parallel to the slope. A review of published studies of joint spacing and joint persistence in gneiss shows spacing and persistence of foliation-parallel discontinuities relative to non-foliation-parallel discontinuities varies greatly. Foliation-parallel discontinuities form a spectrum between simply being one joint set in a blocky rock mass through to being a significant anisotropic weakness. Based on detailed analysis of the characteristics and frequency of foliation in drill core in the Warramboo magnetite deposit, it is inferred, based on analysis of spacing, that foliation-parallel discontinuities are no more frequent or persistent than other joints. Therefore the rock mass is blocky rather than anisotropic and footwall slopes steeper than the foliation dip are viable.

Geological structural controls on stability of footwall slopes: an example from the Bowen Basin, Queensland

Abstract Surface mining of coal can involve extensive footwall slopes parallel to shallow to moderately dipping coal measures strata. Footwall failure mechanisms typically invoke bedding-parallel defects but also require either the existence of flatter structures, which cross-cut bedding, or require break-out through the rock mass to allow failure surfaces to emerge. Permian-aged Baralaba Coal Measures of the Bowen Basin, Queensland, are prospective for coal with extraction by open-pit methods. The Baralaba Coal Measures contains multiple seams within an interburden sequence comprising sandstone, siltstone, mudstone and carbonaceous variations. The coal measures sequence has been deformed into a complex pattern of NW-striking folds which has resulted in bedding dip ranging from 15° to 60°. Bedding has been classified as shallow (10–30°) to moderate (30–60°). Geotechnical investigations conducted to support coal extraction up to depths of 200 m suggest that structural controls strongly influence footwall slope design. For the purpose of footwall slope design, a distinction can be made between deposit areas of relatively simple structure (uniformly dipping bedding on fold limbs) and structurally complex areas (where layer-parallel shortening close to fold hinges has resulted in a system of low-angle thrusts and asymmetrical minor folds).

Characterization of bi-planar and ploughing failure mechanisms in footwall slopes using numerical modelling

Footwall slopes refer to unbenched rock slopes in which the slope face is parallel to a set of persistent discontinuities (e.g. bedding planes, foliation, faults). These are commonly encountered in weak, thinly bedded, orthogonally jointed, sedimentary rock sequences. Common failure mechanisms include bi-planar failures where shallow dipping crosscutting structures daylight near the slope toe, enabling sliding to occur along steep dipping bedding planes. In the absence of crosscutting structures, failure occurs through deformation and rock mass yielding involving the formation of inter block shear and toe breakout surfaces. Because of the complexity of the toe breakout mechanism, evaluation methods are not well understood. An improved understanding of the failure mechanism, the role of adverse discontinuities, and characterization of the discontinuity, intact rock and rock mass strength properties are key for a successful footwall stability analysis. This paper investigates the development of the inter block shear and toe breakout surfaces with three approaches: i) continuum-based frictional plasticity theory; ii) discontinuum-based distinct-element modelling with Voronoi tessellation using the commercial software UDEC; and iii) hybrid continuum/discontinuum finite-/discrete-element brittle fracture modelling using the commercial software ELFEN. Numerical simulations using ELFEN and UDEC demonstrated a good agreement with frictional plasticity theory. Ploughing failure of footwall slopes is also evaluated, specifically the influence of cross-cutting discontinuity dip angle relative to the slope face. The effects of different geometrical parameters (e.g., slope angle and depth/height ratio) on bi-planar and ploughing failure are assessed using a sensitivity analysis approach. A “Damage Intensity” parameter is introduced and used to quantify damage in the numerical simulations using ELFEN.

Underground stope optimization with network flow method

A new algorithm to optimize stope design for the sublevel stoping mining method is described. The model is based on a cylindrical coordinate defined around the initial vertical raise. Geotechnical constraints on hanging wall and footwall slopes are translated as precedence relations between blocks in the cylindrical coordinate system. Two control parameters with clear engineering meaning are defined to further constrain the solution: (a) the maximum distance of a block from the raise and (b) the horizontal width required to bring the farthest block to the raise. The graph obtained is completed by the addition of a source and a sink node allowing to transform the optimization program to a problem of maximum flow over the graph. The (conditional) optimal stope corresponding to the current raise location and height is obtained. The best location and height for the raise are determined by global optimization. The performance of the algorithm is evaluated with three simple synthetic deposits and one real deposit. Comparison is made with the floating stope technique. The results show that the algorithm effectively meets the geotechnical constraints and control parameters, and produce realistic optimal stope for engineering use.

Footwall slope stability analysis with the numerical manifold method

This paper extends the numerical manifold method (NMM) for footwall slope stability analysis. A fracturing algorithm based on the Mohr–Coulomb criterion with a tensile cutoff is implemented into the NMM code. The developed program is first calibrated by simulating four typical crack problems. It is then applied to analyze the potential footwall slope instability. Parametric studies with respect to dip of a bedding plane, orientation of a predominant joint set, the bedding profile, etc., are carried out. Numerical results indicate that the developed program can simulate the opening and sliding along pre-existing discontinuities, fracturing through intact rock, as well as kinematics of the failed slope, and can also reproduce the major failure mechanisms observed in footwall slope collapses. The NMM is promising for such problems and deserves to be further developed to be practically used in natural/excavated rock slope stability analysis and open pit slope design.

Field-scale assessment of effective dilation angle and peak shear displacement for a footwall slab failure surface

When designing high steep footwall slopes, an accurate assessment of the dilational shear strength component is necessary. Bedding surface profiles taken from a detailed digital terrain model of a slip surface exposed by a large slab-type footwall failure were used to characterize the geometric roughness and dilation angles in the sliding direction. The dilation angles predicted from best-fit dilatancy curves are sensitive to the base distance or shear displacement. The dilation angle for a given shear displacement (or base distance) is not sensitive to the profile length if it exceeds the typical in situ block size. For the case history, a 2D finite element analysis was performed to back-calculate the effective dilation angle assuming no cohesion was present and the basic friction angle was 25°. This analysis suggested that a dilation angle of 12° was required to create a condition of incipient slab failure. Using dilatancy curve equations, it is possible to convert dilation angles back-calculated from finite element stability analyses into shear displacements required to mobilize the peak strength. The shear displacement required to mobilize a dilation angle of 12° is estimated to be 0.02–0.15 m for the slab failure that was studied. Analysis of profile geometries to estimate the shear displacement needed to reach peak shear strength will improve the interpretation of slope displacement monitoring data.

Geomorphic significance of postglacial bedrock scarps on normal-fault footwalls

The existence of well-preserved Holocene bedrock fault scarps along active normal faults in the Mediterranean region and elsewhere suggests a dramatic reduction in rates of rock weathering and erosion that correlates with the transition from glacial to interglacial climate. We test and quantify this interpretation using a case study in the Italian Central Apennines. Holocene rates are derived from measurements of weathering-pit depth along the Magnola scarp, where previous cosmogenic Cl-36 analyses constrain exposure history. To estimate the average hillslope erosion rate over similar to 10(5) years, we introduce a simple geometric model of normal-fault footwall slope evolution. The model predicts that the gradient of a weathering-limited footwall hillslope is set by fault dip angle and the ratio of slip rate to erosion rate; if either slip or erosion rate is known, the other can be derived. Applying this model to the Magnola fault yields an estimated average weathering rate on the order of 0.2-0.4 mm/yr, more than 10 times higher than either the Holocene scarp weathering rate or modern regional limestone weathering rates. A numerical model of footwall growth and erosion, in which erosion rate tracks the oxygen-isotope curve, reproduces the main features of hillslope and scarp morphology and suggests that the hillslope erosion rate has varied by about a factor of 30 over the past one to two glacial cycles. We conclude that preservation of carbonate fault scarps reflects strong climatic control on rock breakdown by frost cracking.

Comparison of limit-equilibrium, numerical and physical models of wall slope stability

We review commonly observed failure mechanisms associated with footwall slopes, and examine the basic concepts underlying the limit equilibrium approach assessing footwall slope stability. The failure mechanisms described are those, where failure follows pre-existing joints or occurs in intact rock, namely, fully and partially joint-controlled bilinear and ploughing slab failures. Numerical models implemented using UDEC illustrate the code’s potential for analysing these failure mechanisms. Physical models representing the studied instability mechanisms were studied in a tilt-test set-up in the laboratory, with the empirical results comparing fairly well with the theoretical approaches.

Geotechnical study for optimizing the slope design of a deep open-pit mine, India

The study was undertaken to determine the optimal overall slope angle of a lead–zinc mine, which could extend down to a depth of 170 m. Detailed geotechnical investigations were conducted, including geotechnical mapping of existing benches to establish the geometrical and mechanical properties of the discontinuities, the drilling of eight boreholes and the establishment of the physico-mechanical properties of the intact rock. Based on these data, limit equilibrium and numerical simulation techniques were applied to assess the stability of the slopes and determine an optimal slope angle. It was concluded that the overall foot-wall and hanging-wall slopes should be 42 and 48° respectively.

Cretaceous (post-Valanginian) sedimentation and rift events in NE Greenland (71–77°N)

A tectono-sedimentary model is proposed for the Cretaceous (post-Valanginian) succession of NE Greenland which provides insights into the evolution of adjacent margins. Subsequent to a major reorganization of fault blocks during rifting in the Volgian–Valanginian a seafloor topography controlled by block faulting was created and was maintained by faulting until the end of rifting in the Middle Albian. This topography was infilled after the Middle Albian and a shelf break margin was created and maintained until at least the Campanian. After this time there is no onshore sedimentary record in NE Greenland to constrain interpretations. Post-Valanginian sedimentation was dominated by the accumulation of fine sediments. However, significant thicknesses of coarse clastics are recorded at several stratigraphic levels in East Greenland. The primary controls on sandstone occurrence are steps in the basin bounding fault. The greatest accumulation of sandstones is associated with the largest of these steps. Sandstone supply to basinal areas was controlled by periods of lowstand. These have been identified in the Barremian, Late Aptian and Coniacian. Sandstone deposition occurred in fiuvio–deltaic, tidally influenced shallow marine and deep marine environments. The degradation of sandy footwall slopes did not lead to the formation of sandy sequences in the adjacent hanging wall successions.

Developments in the analysis of footwall slopes in surface coal mining

Surface mining of coal, particularly in areas of mountainous topography, often involves the formation of extensive footwall slopes parallel to the strata dip. Due to structural deformation steep dips on the limbs of folds may be encountered in association with thrust faults, jointing and residual shear strength conditions. Such an environment necessitates a rigorous assessment of footwall stability in order to ensure safe and economic exploitation of the coal. This paper provides a detailed review of the factors influencing footwall slope instability in surface coal mining and the major instability mechanisms which have been recognized within the published literature. The analysis of footwalls in the design stage and the back analysis of footwall slope failures has in general been undertaken using predominantly two-dimensional limit equilibrium techniques often incorporating a simplistic elastic column analysis. The application of numerical modelling techniques to surface coal mine footwalls has received little attention. In this paper the authors attempt to illustrate the potential for investigating footwall failure mechanisms and stability using principally the distinct element method.

Rock anchor support systems at Smoky River Coal Limited, Grande cache, Alberta: R. F. Dawson, A. S. Bagnall & K. Barron, CIM Bulletin, 88(992), 1995, pp 60–65

At Smoky River Coal Limited (SRCL), economic occurrences of structurally thickened coal deposits form attractive surface mining targets. These steeply dipping, structurally complex deposits require that special attention be given to pit wall design and operational practices. Where anchor support is feasible, there is potential for reducing footwall waste requirements and/or increasing coal recovery. SRCL has used rock anchors to support footwall slopes since 1985. This paper outlines design considerations and performance of rock anchor support systems at SRCL. Three case histories are included that illustrate the design process for supporting steeply dipping footwall slopes using rock anchors.

Optimization of slope angle of a lead-zinc mine

The present study represents an attempt to optimize the overall slope angle of a lead-zinc mine in Rajasthan state, which could extend to a depth of 170 m during the first phase of mining. Detailed geotechnical investigations were conducted. These included geotechnical mapping, determination of the physio-mechanical properties of intact rock, determination of the rock mass rating and estimation of rock mass properties. Information was collected from mapping of benches and borehole logs. Based on these data, limit equilibrium and numerical simulation techniques were applied in order to assess the stability of the slopes and determine an optimum slope angle. From these investigations it has been inferred that the overall footwall and hangingwall slopes should be 42° and 48° respectively.

Reinforcement and control of footwall slopes in an open pit coal mine (In French) : Soukatchoff, V; Hantz, D; Mathieu, E; Paquette, Y Proc 7th ISRM International Congress on Rock Mechanics, Aachen, 16–20 September 1991V2, P965–969. Publ Rotterdam: A A Balkema, 1991

Reinforcement and control of footwall slopes in an open pit coal mine (In French) : Soukatchoff, V; Hantz, D; Mathieu, E; Paquette, Y Proc 7th ISRM International Congress on Rock Mechanics, Aachen, 16–20 September 1991V2, P965–969. Publ Rotterdam: A A Balkema, 1991