Doug Stead

Numerical analysis of initiation and progressive failure in natural rock slopes—the 1991 Randa rockslide

The 1991 Randa rockslide in the Swiss Alps involved several complex mechanisms relating to geological, mechanical and hydrological processes for which no clear trigger can be asserted. This paper investigates the concept of progressive failure and the numerical modelling of rock mass strength degradation in natural rock slopes using the Randa rockslide as a working example. Results from continuum (i.e. finite-element) modelling are presented to illustrate a hypothesis, suggesting that initiation of a progressive rock mass degradation process, ultimately leading to failure, began following deglaciation of the valley below. Discontinuum (distinct-element) modelling is then applied to investigate the underlying mechanisms contributing to the episodic nature of the rockslide. Finally, the use of a hybrid method that combines both continuum and discontinuum techniques to model fracture propagation are discussed in the context of modelling progressive slide surface development linking initiation and degradation to eventual catastrophic failure.

Numerical Simulation of Squeezing Failure in a Coal Mine Roadway due to Mining-Induced Stresses

Squeezing failure is a common failure mechanism experienced in underground coal mine roadways due mainly to mining-induced stresses, which are much higher than the strength of rock mass surrounding an entry. In this study, numerical simulation was carried out to investigate the mechanisms of roadway squeezing using a novel UDEC Trigon approach. A numerical roadway model was created based on a case study at the Zhangcun coal mine in China. Coal extraction using the longwall mining method was simulated in the model with calculation of the mining-induced stresses. The process of roadway squeezing under severe mining-induced stresses was realistically captured in the model. Deformation phenomena observed in field, including roof sag, wall convexity and failed rock bolts are realistically produced in the UDEC Trigon model.

Applications of Finite/Discrete Element Modeling to Rock Engineering Problems

AbstractIn this paper, the authors review recent applications of an integrated numerical modeling approach based on the analysis of the mechanical behavior of discrete systems. The numerical analysis includes both a more realistic representation of fracture networks and the simulation of rock mass behavior as a combination of failure through intact rock and displacement/rotation along predefined discontinuities. Selected examples are presented with respect to a variety of engineering problems, including shear testing, failure of hard-rock pillars, slope stability, and block/panel cave mining. The results clearly illustrate the importance of including natural jointing to better capture rock mass behavior in response to loading and unloading. Particular emphasis is given to modeling cave development and surface subsidence, and the proposed numerical method is shown to capture fully the complex rock mass response to caving associated with multi lift extraction. Whereas the use of relatively complex numerical...

A case study integrating remote sensing and distinct element analysis to quarry slope stability assessment in the Monte Altissimo area, Italy

Abstract Over last decade geomatic techniques have been increasingly used for the geometrical characterization of rock slopes. Terrestrial laser scanning and digital terrestrial photogrammetry in particular are now frequently used in the characterization of joint surfaces and slope geometry. Although the use of these techniques for the structural characterization of slopes is widely documented, limited research has been undertaken to improve our understanding of the importance of the derived data quality in the construction of slope geometry imported into 3D numerical models. One of the most common problems encountered in the use of these techniques, especially in case of slopes with complex geometry, is the presence of occlusions. In this context, the aims of this paper are to describe how the integrated use of terrestrial laser scanning, digital terrestrial photogrammetry and topographic surveys can mitigate the influence of occlusions and how the slope geometry gained from these surveys can be important in slope stability analyses. For this purpose a case study in the Monte Altissimo area (Apuan Alps, Italy) will be presented. Several geomatic techniques were used for studying a slope overhanging the Granolesa quarry. Special emphasis will be given to the importance of using Total Station and Differential GPS surveys as tools for data fusion. Moreover, in order to validate this procedure, the accuracy and precision of the output were determined through comparison of 3D models derived from laser scanning and digital terrestrial photogrammetry. Furthermore, two different analyses with the three-dimensional distinct element code, 3DEC, were carried out in order to highlight the advantages and limitations of using data obtained from terrestrial remote sensing techniques as opposed to models based on topographic maps.

Slope tectonics: a short introduction

Geomorphology,structuralgeologyandengineeringgeology allow description of the main character-istics of a slope in distinct ways that can be com-bined to provide a complementary view of theoperative slope processes. The subjects presentedin this Special Publication include: slope mor-phology and evolution; mechanical behaviour ofthe material; modes of failure and collapse; influ-ence of lithology and structural features; and therole played by controlling factors. This Slope Tec-tonics volume comprises a series of very differentcontributions that attempt to underline a multidisci-plinary approach thatshould form the framework ofslope instability studies.Slope Tectonics is adopted in this volume tomeandeformationthatisinducedorfullycontrolledbytheslopemorphologyandthatgeneratesfeaturesthatcanbecomparedtotectonicfeatures.Thestressfield in a slope is the result of gravity, topographyand the geological setting created by an ensembleof geodynamic processes. Active tectonics (alsocalled neotectonics) generates a stress field thatcancontrolslopeprocesses;astrongfeedbackexist-ingbetween geological history, tectonics, lithology,geomorphological evolution and topography.As a consequence, a list of factors and theirrela-tive influence can be presented.(1) Fabric induced by a local stress field within aslope:† discontinuities and local faults with cata-clastic bands of variable thickness;† folds (Fig. 1), associated predominantlywith brittle structures;† complex failure paths (stepped or multi-surface);† localfailures:rockbridgefailuresorexten-sional failures (graben-like or pseudo-graben-like);† subsidence due to weak or soluble mate-rials causing complex sliding–topplingphenomena.(2) Reactivation of pre-existing faults, disconti-nuities, joints, foliations or rock anisotropies:† surfaces characterized by residual or lowerthan peak strength;† formation of composite failure surfaces.(3) Regional tectonic movements inducing newslope morphologies:† uplift;† major fault movement;† pull-apart zones:† folding.The boundary between classical tectonics andslope deformations, especially at a large scale, hasalways been indistinct as emphasized by Antoine(1988) (Schultz-Ela 2001). In his paper Antoinediscussed mechanisms like ‘diverticulation’ def-ined by Lugeon (1943) and Badoux (1963), wherepart of Pre-Alpine nappes were reversed in geo-metry by huge landslides inverting the stratigraphy(Antoine 1988). A remarkable geometrical analogyexists between basin extensional tectonics (Wer-nicke 1981) and certain landslide spreading inclays (Voight 1973; Varnes 1978; Hutchinson1988); this is despite the fact that Wernicke’shypothesis demonstrated that Basin and Rangeregions were not produced by huge landslides, butby low-angle faulting induced by geodynamic pro-cesses. Regional extension has produced changesin topography and, as a consequence, significantgravity-induced deformations.Therefore, we suggest that the term ‘slope tec-tonics’ is justified and must be recognized as animportant component in slope deformation. Slopeinstability implies movements driven by gravitythat can produce irreversible deformations. In thepast, slopes were viewed as privileged erosionzones (De la Noe & De Margerie 1888; Strahler1950), whereas few erosion processes were attribu-ted to landslides (Young 1972). Since the work ofSimonett (1967) and Hovius et al. (1997), the link

Evaluating Kinematic Controls on Planar Translational Slope Failure Mechanisms Using Three-Dimensional Distinct Element Modelling

This paper investigates the importance of kinematic release mechanisms on planar translational slope failure using three-dimensional distinct element codes. The importance of the dip and dip direction of the rear, basal and lateral release surfaces and their influence on failure mechanism, dilation, and the development of step-path failures is illustrated. The three-dimensional block shape and volume of the unstable rock masses simulated with the different discontinuity set geometries are characterized. Two assumed three-dimensional slope models are investigated in order to assess the importance of varying kinematic confinement/release mechanisms. These two assumed boundary conditions are shown to be critical in the development of asymmetrical rock mass deformation patterns. Scale effects due to the block size and discontinuity persistence are shown to control the calculated displacement and failure mechanisms. The numerical modelling results are also demonstrated to be sensitive to the assumed normal and shear stiffness of the discontinuities. The influence of the factors investigated on the failure of a single rock block versus a rock mass are compared and discussed.

Back Analysis of the 2014 San Leo Landslide Using Combined Terrestrial Laser Scanning and 3D Distinct Element Modelling

Landslides of the lateral spreading type, involving brittle geological units overlying ductile terrains, are a common occurrence in the sandstone and limestone plateaux of the northern Apennines of Italy. The edges of these plateaux are often the location of rapid landslide phenomena, such as rock slides, rock falls and topples. In this paper, we present a back analysis of a recent landslide (February 2014), involving the north-eastern sector of the San Leo rock slab (northern Apennines, Emilia-Romagna Region) which is a representative example of this type of phenomena. The aquifer hosted in the fractured slab, due to its relatively higher secondary permeability in comparison to the lower clayey units leads to the development of perennial and ephemeral springs at the contact between the two units. The related piping erosion phenomena, together with slope processes in the clay-shales have led to the progressive undermining of the slab, eventually predisposing large-scale landslides. Stability analyses were conducted coupling terrestrial laser scanning (TLS) and distinct element methods (DEMs). TLS point clouds were analysed to determine the pre- and post-failure geometry, the extension of the detachment area and the joint network characteristics. The block dimensions in the landslide deposit were mapped and used to infer the spacing of the discontinuities for insertion into the numerical model. Three-dimensional distinct element simulations were conducted, with and without undermining of the rock slab. The analyses allowed an assessment of the role of the undermining, together with the presence of an almost vertical joint set, striking sub-parallel to the cliff orientation, on the development of the slope instability processes. Based on the TLS and on the numerical simulation results, an interpretation of the landslide mechanism is proposed.

Discrete Fracture Network approach to characterise rock mass fragmentation and implications for geomechanical upscaling

Abstract Natural fragmentation is a function of the fracture length and connectivity of naturally occurring rock discontinuities. This study reviews the use of a Discrete Fracture Network (DFN) method as an effective tool to assist with fragmentation assessment, primarily by providing a better description of the natural fragmentation distribution. This approach has at its core the development of a full-scale DFN model description of fracture orientation, size and intensity built up from all available geotechnical data. The model fully accounts for a spatially variable description of the fracture intensity distribution. The results suggest that DFN models could effectively be used to define equivalent rock mass parameters to improve the predictability achieved by current geomechanical simulations and empirical rock mass classification schemes. As shown in this study, a mine-scale DFN model could be converted to equivalent directional rock mass properties using a rapid analytical approach, allowing the creation of a rock mass model that incorporates the influence of a local variable structure with continuous spatial variability. When coupled with more detailed numerical synthetic rock mass simulations for calibration and validation, a balanced and representative approach could be established that puts more equal emphasis on data collection, local- and large-scale characterisation, conceptualisation and geomechanical simulation.

A Combined Remote Sensing–Numerical Modelling Approach to the Stability Analysis of Delabole Slate Quarry, Cornwall, UK

Rock slope geometry and discontinuity properties are among the most important factors in realistic rock slope analysis yet they are often oversimplified in numerical simulations. This is primarily due to the difficulties in obtaining accurate structural and geometrical data as well as the stochastic representation of discontinuities. Recent improvements in both digital data acquisition and incorporation of discrete fracture network data into numerical modelling software have provided better tools to capture rock mass characteristics, slope geometries and digital terrain models allowing more effective modelling of rock slopes. Advantages of using improved data acquisition technology include safer and faster data collection, greater areal coverage, and accurate data geo-referencing far exceed limitations due to orientation bias and occlusion. A key benefit of a detailed point cloud dataset is the ability to measure and evaluate discontinuity characteristics such as orientation, spacing/intensity and persistence. This data can be used to develop a discrete fracture network which can be imported into the numerical simulations to study the influence of the stochastic nature of the discontinuities on the failure mechanism. We demonstrate the application of digital terrestrial photogrammetry in discontinuity characterization and distinct element simulations within a slate quarry. An accurately geo-referenced photogrammetry model is used to derive the slope geometry and to characterize geological structures. We first show how a discontinuity dataset, obtained from a photogrammetry model can be used to characterize discontinuities and to develop discrete fracture networks. A deterministic three-dimensional distinct element model is then used to investigate the effect of some key input parameters (friction angle, spacing and persistence) on the stability of the quarry slope model. Finally, adopting a stochastic approach, discrete fracture networks are used as input for 3D distinct element simulations to better understand the stochastic nature of the geological structure and its effect on the quarry slope failure mechanism. The numerical modelling results highlight the influence of discontinuity characteristics and kinematics on the slope failure mechanism and the variability in the size and shape of the failed blocks.

Assessment of rock slope stability using remote sensing technique in the Gold Coast area, Australia

Rock falls and landslides along major roads cause significant damage to infrastructure in the Gold Coast area, Australia. Current methods of hazard assessment, which mostly include field mapping and data collection for site characterization, are inherently labor intensive and subject to bias due to safety issues and time constrains. However, many of these problems have recently been addressed through the development and deployment of digital imaging technology based on photogrammetry. This method involves the use of high-resolution digital stereo-photographs, from which a three-dimensional image of the slope can be constructed. Such images can highlight the surface texture of slopes and identify potentially unstable zones, thus providing engineers with valuable information regarding the slope design. Photogrammetry is still a relatively new remote sensing technique in Australia and has mostly been used in the mining industry. Little has been done to study the feasibility of its application in civil engineering to solve geotechnical problems related to the stability of natural slopes and road cuts. This paper presents the results of a pilot study aimed at assessing the stability of rock slopes in the Gold Coast area. Field surveys including photogrammetry were performed to study the geological settings of the sites, and characterize the slopes topography and type of discontinuities. Based on the 3-D models, the potentially unstable zones were identified, and slope stability analysis of those areas was performed. The obtained results indicated that photogrammetry can be a helpful tool in assessing geohazard related to slope stability problems.