Discrete (spaced) pile rows are an established method of improving slope stability, or 'dowelling' an existing slip. The piles predominantly provide horizontal restraint to the potentially unstable mass of the slope. The method becomes more cost effective as the pile spacing increases, but there is also increasing risk that the soil will 'flow' through the gap between piles, rather than arching across it. Two- and three-dimensional numerical analyses of a generic slope with piles at various locations are undertaken. A simple model of the stabilising force required for a given increase in factor of safety of the slope is then combined with a model of limiting pile row interaction to allow direct estimation of the effect of piles at a particular spacing ratio in improving the factor of safety.

This paper presents results from a 180 degrees(cf. 360 degrees) axisymmetric model, which allows viewing of soil movement as a circular penetrometer advances into the granular soil. The model is tested in a geotechnical centrifuge, with digital photographic techniques used to track soil movement. Selected results are presented with initial discussion.

Lateral pile-soil interaction arising from relative pile-soil movement can be modeled using a horizontal 2-d 'plan' section of the pile translating through the soil (or vice-versa). The ultimate lateral pressure on such a pile is well understood for an 'undrained' Tresca soil. However, behavior is not so firmly established for a purely frictional ('cohesionless') soil strength, or in any 'drained' situation. A series of FLAC analyses has been undertaken to determine if and how this 2-d situation can be modeled. The results demonstrate the effect of restraint on the section in the 'out of plane' direction, ultimately leading to the use of a 3-d analysis. The effect of pile spacing along a row (normal to the pile-soil movement), and in particular the phenomenon of 'arching' between adjacent piles, are also considered.

For the optimum design of a rock crushing plant the breakage characteristics of the rock under the loading conditions imposed by the crusher are required. Often the determined static mechanical values fail to characterise the energy required and degree of fragmentation of the material within the crusher. This paper describes a laboratory and numerical investigation into the affect of strain rate on the energy requirements and breakage characteristics of three different sedimentary rocks. For each rock type high speed uniaxial compression tests were carried out at six different strain rates over the range 2.57 × 10 –5 strain/s to 0.02 strain/s. Changes in the stress-strain behaviour were discovered indicating the importance of strain rate on breakage characteristics. For each of the rock types the stress/strain curves showed an increase in ductility as the strain rate increased and it was calculated that the energy used in bringing the specimen to fracture increased by up to 50%. The increase in energy was also accompanied by an increase in the degree of fragmentation achieved. The numerical modelling of the high speed compression test was undertaken for the sandstone using the discrete element method code PFC 3D produced by Itasca. The numerical modelling reproduced the increase in energy requirements for fracture and an increase in the degree of fragmentation. The modelling also predicted that this trend would continue at strain rates of 1 strain/s that would be anticipated in rock crushers.

Laboratory drop weight tests rig and numerical modelling have been used to investigate the impact energy effect on the degree of dynamic fragmentation of cylindrical rock samples. The drop weight tests indicated that the degree of dynamic fragmentation of the Daley Dale sandstone formed a non-linear relation with impact energy. In the numerical modelling of the drop weight test, the shear localisation developed in the Daley Dale sandstone was assumed to represent discrete fracture planes. Shear bands, outputs of the image analysis of the numerical model, were used to separate solid material from shattered or broken material, allowing the FLAC output from the dynamic modelling to be processed into black and white fragmented images. This was undertaken to determine particle size distributions. Corresponding grading curves from the numerical modelling were then validated against the laboratory derived curves indicating the potential of numerical modelling to simulate the process of rock fragmentation.

Many studies have been performed to predict the strength and deformation moduli for both rock masses and discontinuities. However, in most empirical equations in the literature that have been proposed for the estimation of the strengths and deformation moduli, the rock mass is assumed isotropic. Although it is known that the strength and deformation of a stratified rock mass varies depending on the loading direction in relation to the orientation of the lamination planes, there exists at present no commonly accepted method for characterising this anisotropy. This paper outlines the development of a rock mass rating system for coal measure rock masses that can be used to empirically predict the engineering properties of stratified rock masses. The output from the classification system is two numerical ratings representing the different engineering properties of the strata in directions parallel and perpendicular to stratification.A detailed description on the methodology used to develop the classification system has been given and the derivation and rating of the various rock mass parameters is outlined. The methodology of deriving the in situ stratified rock mass input parameters and geomechanical computational analysis is illustrated for the case of two rock bolted roadways within a representative deep UK coal mine. The model predictions were validated against the actual in situ monitoring data of displacements within the immediate roof of the roadway.

Extensive numerically based modelling has been conducted to simulate surface and sub-surfàce subsidence due to longwall mining in UK Coal Measure rocks. The results have been validated, against the Subsidence Engineer's Handbook (SEH) surface subsidence prediction method. A Rock Mass Classification Rating (RMR) has been used to derive pre and post failure input parameters for both strength and stiffness properties. RMR has been found to be the most acceptable approach through a thorough review of methodologies and approaches to the determination of numerical modelling input parameters. Longwall panels of 200m width and 2m extraction thickness at various depths have been simulated using Fast Lagrangian Analysis of Continua FLAC (Version 3 3), A strain softening constitutive model with user defined pre failure stifmess and strength parameters in the input data file, and post failure values for stifmess have been incorporated in a separate function that is activated by plasticity during the computer rua The final version of the model has been validated through a combination of the pattern of stress redistributed around longwall panels and the displacement distribution at the surface. Sequences evaluating the variation of subsidence with extraction thickness and panel width have been run for typical but idealised UK longwall excavations at 400m depth. An interesting relationship between depth and RMR has been explored and a modification to Serafim and Pereira's expression for in-situ rock mass Young's Modulus derivation has been suggested. Based on the experience in validating the model for UK Coal Measures it is proposed to adapt the model as a tool for surface subsidence prediction in various international coalfields having significantly different rock mass and subsidence characteristics.

For this paper multistage triaxial compression tests were conducted under various saturation conditions aimed at quantifying the relationships between the water content and the mechanical properties of fractured siltstone. Samples were examined with reference to the evolvement of the failure surface with the changing water content. It was observed in the experiments that the peak shear strength quickly reduced to the corresponding residual value under the combined effects of normal stress relief and the degradation of the shearing surface. Reasonable explanations are given through use of the modified Patton's model. Correlations between the degree of saturation Sw and the shearing surface parameters, i.e. the residual friction angle and the dilation angle have been studied and a theoretical model is developed. The modified strength envelopes predicted by the model are validated by the testing data, showing a good agreement with the testing results.

Water is one of the many factors influencing the strength of geomaterials. In this paper, multistage triaxial compressions were performed to investigate the weakening effect of water on intact and fractured sandstone specimens. Laboratory testing results are presented and discussed with particular reference to the change of the mechanical properties of the rock. A trend of strength reduction for both intact and fractured rocks can be generalized from the testing results. It is also observed that the fractured rock is more reactive to the water even though no apparent soluble gouge is sandwiched in the related rock fractures. Additionally, theoretical formulations are derived to characterize the strength reduction on the basis of the understanding of the physical process. Properties of a single rock joint are correlated with the tangential plastic energy of the shearing plane with the non-associated plastic flow rule during shearing. The degree of saturation S w is introduced into Patton’s model as an additional parameter to predict the shear strength evolvement with the degree of water saturation.

Natural rock's geomechanical properties are inherently variable so geotechnical engineers are unavoidably faced with decision making under a degree of uncertainty. However, a traditional single property value numerical modelling design does not address this uncertainty other than by applying safety factors to the answer or using worst assumptions for parameters. With complex problems, these approaches are often impractical. To address the uncertainty involved in a roadway design, the authors have combined the popular Monte Carlo uncertainty analysis technique with a powerful non-linear numerical modelling method by means of a controlling Visual Basic program. The numerical model used is a 20 148 element representation of a stress-loaded, bolt-supported, coal mine roadway in a state of plastic yield. The combination of the time-consuming looping of the Monte Carlo technique with the already time-intensive non-linear numerical model is challenging and raises a number of issues the authors explore to assist others with this type of analysis. With a practical run-time limit of a month on a fast personal computer, the Monte Carlo analysis approach needs to be carefully controlled as less than 1000 Monte Carlo parameter combinations are available for the analysis. It should be noted that both geotechnical and geometrical parameters are varied in the analysis. The input parameters whose uncertainty has a significant impact on the numerical model output are first screened out by a one-at-a-time analysis and are then treated as random in a follow-up analysis. All the remaining inputs are assigned nominal mean values. The application of this hybrid technique to an underground coal mine roadway design is introduced in this paper. The form and distribution of the analysis results is shown to provide useful uncertainty information about the potential roof deformation of the roadway with respect to input parameters' uncertainty. (C) 2005 Institute of Materials, Minerals and Mining and Australasian Institute of Mining and Metallurgy. Published by Maney on behalf of the Institutes.

An historical review of the geotechnical behaviour of the Northwich Rock Salt is presented as a forerunner to a numerical modelling analysis of the current stability of the Winsford salt mine, Cheshire. Extensive laboratory and in situ tests have been historically undertaken by the mine to characterise the strength and stiffness behaviour of the rock salt. Recent proposals to store waste within the Bostock No. 5 panel of the mine have lead to increasing concerns as to the current stability of the workings, as well as to the long term stability of the mine. This present study uses the wealth of geotechnical data to assess the current mine stability using numerical modelling techniques and validates the results against in situ roof to floor convergence data. The results indicate that the mine structures are stable. Convergence simulation using the numerical model compare favourably with the in situ monitoring data allowing greater con.dence to be placed in future predictions.

Laboratory drop weight tests and numerical modelling have been used to study the effect of impact energy on the degree of fragmentation of cylindrical granite specimens. The drop weight tests indicated that the degree of fragmentation of the granite formed a non-linear relation with impact energy. In the modelling of the drop weight test, the shear localisation developed in the granite was assumed to represents discrete fracture planes. Image analysis of the model outputs was undertaken to determine particle size distributions. Corresponding grading curves from the numerical modelling were then validated against the laboratory derived curves indicating the potential of numerical modelling to simulate the fragmentation process of brittle rocks.

In order to provide more reasonable support design for shallow tunnels in inclined stratum, it is necessary to investigate the collapse of shallow tunnels in inclined stratum. A new collapse mechanism of shallow rectangular and circular tunnels in inclined rock stratum is constructed, which includes four curves. On the basis of the upper bound theorem of limit analysis and the nonlinear Hoek-Brown yield criterion, obtaining the precise expressions of four curves by means of the variational principle, and deriving the equations of the range and total gravity of the potential collapsing block of rectangular and circular tunnels can be solved respectively. The results are consistent with those published and numerical simulation, which confirms the rationality of this research. The parametric sensitivity analysis of the collapse of rectangular and circular tunnels is carried out, which shows that rock mechanics parameters and geometric parameters have a significant impact on the range and total gravity of the potential collapsing block is obvious. Especially, the dip angle of rock stratum plays a key role in the symmetry of tunnel collapse and the progressive collapsing block is unsymmetrical unless the rock strata are horizontal.

The influence of electric field strength on the microwave treatment of ore is elucidated. The ore consisted of a microwave-absorbing mineral in a low-absorbing matrix, and the influence of electric field strength was assessed by numerical simulation. Simulations were undertaken using finite difference modelling techniques for a theoretical 15×30 mm sample of calcite host rock containing 10 vol.%, 1-mm2 particles of pyrite. The simulations modelled the microwave heating, thermal conduction, expansion, thermally induced fracturing and strain softening and, finally, uniaxial compressive strength to predict the effect of microwave heating on the strength of the ore material. Standard correlations were then used to develop specific comminution energy verses t10 relationships for the treated and nontreated samples. It is shown that microwave power density is vital to the fracturing of the rock, and it is suggested that by utilising high power densities, the microwave fracturing of rock to reduce grinding energy requirements may be economically viable.

This paper presents results from, and theory to support the anisotropic testing of a series of laminated Coal Measure siltstones. It was found that laminations within the siltstone units created an anisotropic fabric with varying strength and stiffness properties in relation to the orientation of the planes of lamination. New results are presented illustrating changes in triaxial, unconfined compressive, elastic modulus and tensile strength for ten orientations of the lamination plane. A number of anisotropic failure criteria are reviewed and then compared to the test results allowing the selection of the optimum method of characterising the siltstone strength anisotropy. The influence of lamination plane orientation on the tensile strength and Elastic Modulus was also assessed and analysed in relation to theoretical models.

Numerical models have been developed for an UK based colliery to study the strata behaviour around a longwall face. The modelling indicated that the failure of the roof strata would become more extensive and periodic as the face advances, with vertical shear mechanisms being the dominant form, The failure zones would propagate through the overlying the Sandstones, and well into the strata above, allowing water ingress into the workings. A good correlation has been observed between the numerical results and microseismic monitoring data.

This paper describes work undertaken within the Geotechnics Research Group at the University of Nottingham on the development of a methodology for simulating the compaction of a collapsed goaf with its associated deformations, stress redistribution and shearing along bedding interfaces and the matrix of the rock strata surrounding the longwall panel. The application of the modeling methodology to a longwall panel in a UK coal mine is described where a series of models representing distances behind the active coal face have been constructed to simulate the movement of the coal face and progressive goaf compaction. The outputs from this modeling, relating to shear plane development and principal stress magnitudes around the panel, were subsequently used as an aid in the prediction of methane flows into the active workings. The modeling was undertaken using the continuum code FLAC Ver. 3.3. To simulate the anisotropic stress-strain behavior of the strata FLAC's ubiquitous joint model was utilized with a fish function providing post failure strength reduction of the ubiquitous joints and rock matrix, typical of the highly strained and failed strata around longwall panels. To simulate the progressive compaction of the goaf and lowering of the roof beds behind the coal face, models were generated representing various stages of convergence of the extracted thickness. Goaf convergence was correlated to distance behind the face line by the modeling of the sequential extraction of the coal face, movement of the powered supports and progressive goaf compaction behind the face line. An important aspect in the predictive modeling methodology was the identification of the mechanical properties of the major lithological horizons within the immediate and proximate roof and floor strata. Cores of the immediate strata adjacent to the panel and surface to seam boreholes were used to provide information in relation to the major lithological horizons. To predict the in-situ strata properties a dedicated rock mass classification system developed specifically for characterizing coal measure strata has been utilized. This allows a strata quality rating to be determined for each horizon. The rating was then used within established empirical relationships to determine the in-situ strength and stiffness properties of the rock matrix and bedding.

Extensive in-situ and laboratory-based experimental work formed the basis for an assessment of the stability of an abandoned panel of Winsford salt mine, Cheshire, England, conducted by numerical modelling with the finite-difference code FLAC. The final model is composed of a Burger rheological component to represent the time-dependent deformation of the salt sequences and a Mohr-Coulomb constitutive model to simulate the behaviour of the non-creeping marl strata. The results of the creep model are compared with actual convergence monitoring data for validation purposes. The results indicate that the section of the mine is stable and confirm the presence of an inherently stable core in each of the internal pillars within the panel.

Research has been carried out to develop a practical computer modelling approach integrating geotechnical and computational fluid dynamics (CFD) techniques to maximize the potential of the existing firedamp drainage technology, through the determination of likely fracture patterns and methane flow directions. The approach allows the simulation of the dynamic nature of stress/methane flow fields around a modem longwall face and the evaluation of the effectiveness of a drainage system at both operational and designs stage.

The use of yield in supports to control the final loading that develops upon a support system has been one of the most important deformation control techniques used by tunnelling engineers, both historically and currently. Successful use of this approach requires a thorough understanding of the process of rock–support interaction as it is an approach that can fail dramatically if incorrectly applied. There is a fine line between the yield support technique improving the conditions, and the approach resulting in the development of a large area of failed rock, which could ultimately be detrimental. The relationship between the support action and the rock has historically been studied using analytical approaches with the application of significant simplifying assumptions.