To date, research has not focused on the behavior of concrete block stoppings subjected to excessive vertical loading due to roof to floor convergence. For this reason, the failure mechanism of stoppings under vertical loading has not been fully understood. Numerical models were used in combination with physical testing to study the failure mechanisms of concrete block stoppings. Initially, the behavior of a single standard CMU block was observed and simulated using FLAC. Full-scale stoppings were then tested in the Mine Roof Simulator and modeled using UDEC. Through a combination of physical testing and numerical modeling a failure mechanism for concrete block stoppings was established. This failure mechanism consists of development of stress concentrations where a height difference as small as 1/32â exists between adjacent blocks. These stress concentrations lead to tensile cracking and, ultimately, premature failure of the wall. / Master of Science

Foundation performance is controlled significantly by the stress-strain behavior of the underlying soils. For geomaterials, the small-strain shear modulus Gmax is a fundamental stiffness applicable to both monotonic static and dynamic loading conditions, as well to both drained and undrained loading. Yet, Gmax is too stiff for direct use in computing foundation displacements. The main objectives of this research are to: (1) explore the scaled parallelism between the stress-strain-strength behavior of the single soil element response and the load-displacement-capacity of a shallow foundation system supported on soil; (2) develop a methodology for evaluating the performance of vertically-loaded footings using a rational framework based on the small-strain modulus Gmax, large-strain strength ( and #964;max or su) and strain at failure ( and #947;f); and (3) calibrate the proposed method using a foundation database of full-scale load tests under both undrained and drained conditions. In geotechnical practice, foundation bearing capacity is handled as a limit plasticity calculation, while footing displacements are evaluated separately via elastic continuum solutions. Herein, a hybrid approach is derived that combines these two facets into a closed-form analytical solution for vertical load-deflection-capacity based on numerical studies. Here, a non-linear elastic-plastic soil model was developed to simulate the stress-strain-strength curves for simple shearing mode (LOGNEP) for each soil element. The model was encoded into a subroutine within the finite difference program FLAC. A large mesh was used to generate load-displacement curves under circular and strip footings for undrained and drained loading conditions. With proper normalization, parametric foundation response curves were generated for a variety of initial stiffnesses, shear strengths, and degrees of non-linearity in the soil stress-strain-strength response. Soil stress-strain non-linearity is described by a logarithmic function (Puzrin and Burland, 1996, 1998) that utilizes a normalized strain xL that relates strain at failure and #947;f, shear strength ( and #964;max or su), and small-strain stiffness Gmax, all having physical meaning. A closed-form algorithm is proposed for generating non-linear load-displacement curves for footings and mats within an equivalent elastic framework. The proposed method was calibrated using a database of well-documented footing load tests where soil input parameters were available from laboratory and/or in-situ field test results.

Stability of underground excavations is a prerequisite for the proper functioning of all other systems in a mining environment. From a safety point of view, the lives of people working underground rely on how well the support systems installed underground are performing. The ground control engineer cannot design an effective support system unless the area of the rock mass around the opening, prone to failure, is well identified in advance, even before the excavation of the tunnel. Under high stress conditions, usually experienced at deep mining levels, stress-induced rock failure is the most common type of instability around the underground openings. This thesis focuses firstly on the use of the finite difference numerical tool FLAC to simulate brittle rock failure under static in-situ stresses. Brittle failure of the rock mass around underground openings is a particular type of stress-induced failure, which can result in notch-shaped breakouts around the boundary of the tunnel. Generation of these breakout zones is a discontinuum process and approximating this process using FLAC, which is a continuum tool, requires careful consideration of the stress conditions and the stress related behavior of rock material. Based on plasticity theory, this thesis makes an effort to estimate the breakout formation using an elastic – brittle - plastic material model. Due to seismic challenges that deep mining operations are currently experiencing, rockbursting is a major hazard to the stability of underground structures. Therefore in this research, brittle failure of rock in the vicinity of the underground excavations is approximated also under dynamic loading conditions. The numerically modeled results of two different material models iv are compared with each other along with a previously developed empirical graph. This assessment, when further validated by field observations, may provide a different perspective for underground support design under burst-prone conditions.

Dans le cadre d’un programme de recherche mené par le GISOS (Groupement d’Intérêt Scientifique de Recherche sur l’Impact et la Sécurité des Ouvrages Souterrains), le site pilote de Cerville-Buissoncourt (Lorraine, France) a fait l’objet d’une importante instrumentation géophysique et géotechnique pour assurer la surveillance d’une cavité saline à 200 m de profondeur, depuis son état stationnaire jusqu’à l’effondrement des terrains du recouvrement. Les objectifs principaux de cette thèse consistaient à : 1) valider la technique de surveillance basée sur l’écoute microsismique dans un contexte salin, et 2) modéliser numériquement le comportement mécanique complexe du recouvrement, particulièrement l’initiation des microfissures et leur propagation. L’analyse de la sismicité induite enregistrée a permis de caractériser l’état initial de la cavité en terme d’activité microsismique. Deux types d’événements ont été identifiés : les événements isolés correspondant aux ruptures localisées, et les événements en rafale, d’une dizaine de secondes de durée. D’après les résultats de localisation d’hypocentres, la totalité de la sismicité enregistrée est générée au niveau de la cavité dans le gisement de sel, ou bien dans les faciès marneux qui composent le toit immédiat de la cavité actuelle. Les déclenchements en rafale seraient liés à des phénomènes de délitement puis de décrochement de blocs de marne, suivis des chutes de blocs dans la cavité remplie de saumure. Le travail de modélisation numérique a été focalisé sur la possibilité de rendre compte de l’endommagement dans les couches fragiles du recouvrement. Nous avons mis en oeuvre un modèle géomécanique hybride à l’échelle du site pilote qui intègre les différentes formations géologiques présentes dans le recouvrement, ainsi que l’initiation, la propagation et la coalescence des microfissures dans le banc raide, à l’aide des logiciels FLAC et PFC2D. La calibration du modèle discret PFC2D pour reproduire le comportement en traction du banc raide a été vérifiée numériquement à l’échelle du site pilote. Cette vérification a été basée sur la comparaison, en termes de la réponse élastique et d’apparition des ruptures dans le banc raide, entre l’approche hybride FLACPFC 2D et la modélisation purement continue avec FLAC. Le modèle hybride ainsi défini pourra être utilisé dans le cadre d’une retro-analyse une fois que les mesures in-situ, notamment les enregistrements microsismiques et les données de déformation, seront disponibles à Cerville-Buissoncourt / Within the framework of a research program carried out by the GISOS (Scientific Grouping of Research Interest on the Impact and Safety of Underground Works), the pilot site of Cerville-Buissoncourt (Lorraine, France) was the subject of a large geophysical and geotechnical instrumentation to ensure the monitoring of a salt cavity at 200 m depth, from its stationary state to the final overburden collapse. The main objectives of this work consisted on : 1) the validation of the microseismic monitoring technique in a salt mine environment, and 2) the numerical modelling of the mechanical behavior of the overburden, particularly the initiation and the propagation of microcracks. The analysis of the recorded induced seismicity allowed to characterize the initial state of the cavity in terms of microseismic activity. Two types of events were identified : isolated events corresponding to localized ruptures, and swarms of events, of tens of seconds of duration. According to hypocenter location results, the totality of the recorded seismicity is generated either in the cavity surroundings within the salt layer, or in the marly facies of the current cavity roof. Swarms would be related to delamination of clayley marls in the immediate roof, followed by rock debris falling in the brine filled cavity. The numerical modelling was focused on the possibility of accounting for the damage in the fragile layers of the overburden. We implemented a hybrid geomechanical model of the pilot site which integrates the various geological formations present in the overburden, as well as the initiation, the propagation and the coalescence of microcracks in the stiff layer, using FLAC and PFC2D softwares. The calibration of the discrete PFC2D model to reproduce the tensile behaviour of the stiff layer was numerically checked on the site scale. The validation was based on the comparison, in terms of the elastic response and the damage onset in the stiff layer, between the hybrid approach FLAC-PFC2D and the purely continuous modelling using FLAC. The hybrid model thus defined would be used for back-analysis studies once in-situ measurements, in particular microseismic recordings and deformation data, will be available at Cerville-Buissoncourt

The company Rio Deserto is currently mining the famous Barro Branco coal seam in the state of Santa Catarina located in the south of Brazil. One of their coal mines, #101, is experiencing problems related to the pillars in one panel. The coal seam is slightly inclined and several pillars have developed damages on the down-slope side with focus in the top corner. Damage inspections revealed a thin clay layer located between the coal pillar and the overlying siltstone. The clay layer is believed to affect the pillar strength and possibly be a source for the observed damages. Aim of this report has been to evaluate different theories behind the damages, focusing on the clay interface using numerical modelling with FLAC. Using convergence data, a calibration of the model is initially done before evaluating the combination of different interface and coal strength while observing the pillar. In addition is an evaluation of influence from structures such as cleats/joints. The results clearly show that with a small shear displacement (1-4 mm) the pillar damages are almost symmetrical on the up-slope and down-slope side of the pillar. Structures can influence and contribute to non-symmetrical pillar damages although not perfectly matching the field observations. Furthermore, the results show that a forced shear movement (8-25 mm) best reproduce the observed damages. A shear movement along the interface is therefore believed to be source mechanism behind the pillar damages. The forced shearing can potentially be explained by factors not considered in the model such as horizontal stresses, disturbances by mining and presence of water within the clay.

Tese de mestrado integrado. Engenharia Civil. Estruturas. Faculdade de Engenharia. Universidade do Porto. 2010

Current design guidelines for reinforced soil walls and slopes recommend the use of granular soils such as gravels and sands as select fills. Cost savings could potentially be realized by using on-site clays and silts. Some clays are swelling and silts are frost susceptible. When considering the use of swelling clays and frost susceptible silts as fills, environmental loading due to swelling-shrinkage and freeze-thaw effects from environmental changes could become a design issue. To examine the hypothesis that consideration of environmental loading during design will produce improvements in the performance of geosynthetic-reinforced soil structures that use clays or silts as fill materials, experimental and numerical studies were undertaken. Geosynthetic-reinforced clay specimens were subjected to wetting and drying in a model test apparatus developed and commissioned for this study. In separate test set-up, reinforced silt specimens were subjected to freezing and thawing. Tests on unreinforced specimens were also performed in otherwise identical conditions for comparison purposes. Movements of the specimens, soil strains, reinforcement strains, soil suctions and soil temperatures were monitored during the application of environmental loading in addition to mechanical loading from external stresses. The results of the laboratory model tests showed that reinforcements reduced horizontal displacements of the clay specimens during wetting and drying. The same is true for the case of silt during freezing and thawing. The environmental loading induced strains, and therefore stresses in the reinforcements. The measured geogrid strain during the wetting-drying of reinforced clay specimen was up to 0.75%. Similarly, the measured geogrid strain in the reinforced silt specimen during freezing-thawing cycles was up to 0.57%. The strains were greater than the strains generated by mechanical loading for the range of applied stresses used in this study. Numerical models were developed to simulate wetting only induced swelling of reinforced clays and freezing only induced expansion of reinforced silts specimens. They were used to simulate the results of laboratory model tests. The performance of geosynthetic-reinforced soil slopes with swelling clay fills and frost susceptible silt fills was evaluated. Parametric studies were performed to determine important parameters affecting the performance of reinforced clay and silt slopes.

Current design guidelines for reinforced soil walls and slopes recommend the use of granular soils such as gravels and sands as select fills. Cost savings could potentially be realized by using on-site clays and silts. Some clays are swelling and silts are frost susceptible. When considering the use of swelling clays and frost susceptible silts as fills, environmental loading due to swelling-shrinkage and freeze-thaw effects from environmental changes could become a design issue. To examine the hypothesis that consideration of environmental loading during design will produce improvements in the performance of geosynthetic-reinforced soil structures that use clays or silts as fill materials, experimental and numerical studies were undertaken. Geosynthetic-reinforced clay specimens were subjected to wetting and drying in a model test apparatus developed and commissioned for this study. In separate test set-up, reinforced silt specimens were subjected to freezing and thawing. Tests on unreinforced specimens were also performed in otherwise identical conditions for comparison purposes. Movements of the specimens, soil strains, reinforcement strains, soil suctions and soil temperatures were monitored during the application of environmental loading in addition to mechanical loading from external stresses. The results of the laboratory model tests showed that reinforcements reduced horizontal displacements of the clay specimens during wetting and drying. The same is true for the case of silt during freezing and thawing. The environmental loading induced strains, and therefore stresses in the reinforcements. The measured geogrid strain during the wetting-drying of reinforced clay specimen was up to 0.75%. Similarly, the measured geogrid strain in the reinforced silt specimen during freezing-thawing cycles was up to 0.57%. The strains were greater than the strains generated by mechanical loading for the range of applied stresses used in this study. Numerical models were developed to simulate wetting only induced swelling of reinforced clays and freezing only induced expansion of reinforced silts specimens. They were used to simulate the results of laboratory model tests. The performance of geosynthetic-reinforced soil slopes with swelling clay fills and frost susceptible silt fills was evaluated. Parametric studies were performed to determine important parameters affecting the performance of reinforced clay and silt slopes.

abstract: The objective of the research is to develop guidelines for identifying when settlement or seismic loading presents a threat to the integrity of geosynthetic elements for both side slope and cover systems in landfills, and advance further investigation for parameters which influence the strains in the barrier systems. A numerical model of landfill with different side slope inclinations are developed by the two-dimensional explicit finite difference program FLAC 7.0, beam elements with a hyperbolic stress-strain relationship, zero moment of inertia, and interface elements on both sides were used to model the geosynthetic barrier systems. The resulting numerical model demonstrates the load-displacement behavior of geosynthetic interfaces, including whole liner systems and dynamic shear response. It is also through the different results in strains from the influences of slope angle and interface friction of geosynthetic liners to develop implications for engineering practice and recommendations for static and seismic design of waste containment systems. / Dissertation/Thesis / M.S. Civil Engineering 2013

Since its development in the 1920s, bolting has become the most dominant support method in underground construction. However, because of the geological environment, the design process for roof bolt systems is an art rather than a science. To quantify the selection of bolting systems a MSBT (minimum solid beam thickness) approach was developed. The ultimate goal of this bolt design paradigm was achieved by optimizing bolt length, bolt density, and bolt pretension during installation. The impact of the number of strata layers within bolting range and pretension applied to bolts upon the stability of an opening was investigated using FLAC model. Four statistical models for predicting optimum bolt supports using a minimum solid beam thickness were established, and based on these results, a design criterion was proposed. To meet support needs in various geological and geotechnical settings, a variety of bolt types have been developed. The installation of such bolt-based support systems is often complex and specialized, and thus imposes a challenge for engineers to identify the specific cause and to take appropriate remedial measures once problems arise. To solve these problems, a knowledge-based expert system (KBES) has been developed. The knowledge base includes the data accumulated from years of laboratory and field investigations conducted by the Mine Safety and Health Administration of the US Department of Labor. A user-friendly Windows-based program was implemented using KAPPA environment. After identifying the problem, the KBES searches its knowledge base and reasons out the most likely, secondary, and other potential causes, then provides solutions according to users' input. The results of this research are validated and demonstrated using case studies. / Ph. D.