Loading characteristics of mechanical rib bolts determined through testing and numerical modeling

Underground coal mines use mechanical bolts in addition to other types of bolts to control the rib deformation and to stabilize the yielded coal ribs.Limited research has been conducted to understand the performance of the mechanical bolts in coal ribs.Researchers from the National Institute for Occupational Safety and Health(NIOSH)conducted this work to understand the loading characteristics of mechanical bolts(stiffness and capacity)installed in coal ribs at five underground coal mines.Standard pull-out tests were performed in this study to define the loading characteristics of mechanical rib bolts.Different installation torques were applied to the tested bolts based on the strength of the coal seam.A typical tri-linear load-deformation response for mechanical bolts was obtained from these tests.It was found that the anchorage capacity depended mainly on the coal strength.Guidelines for modeling mechanical bolts have been developed using the tri-linear load-deformation response.The outcome of this research provides essential data for rib support design.

A review of methods,applications and limitations for incorporating fluid flow in the discrete element method

The past decade has witnessed the substantial growth in research interests and progress on the subject of coupled hydro-mechanical processes in rocks and soils,driven mainly by the surge of research in unconventional hydrocarbon reservoirs and associated hazards.Many coupling techniques have been developed to include the effects of fluid flow in the discrete element method(DEM),and the techniques have been applied to a variety of geomechanical problems.Although these coupling methods have been successfully applied in various engineering fields,no single fluid/DEM coupling method is universal due to the complexity of engineering problems and the limitations of the numerical methods.For researchers and engineers,the key to solve a specific problem is to select the most appropriate fluid/DEM coupling method among these modeling technologies.The purpose of this paper is to give a comprehensive review of fluid flow/DEM coupling methods and relevant research.Given their importance,the availability or unavailability of best practice guidelines is outlined.The theoretical background and current status of DEM are introduced first,and the principles,applications,and advantages and disadvantages of different fluid flow/DEM coupling methods are discussed.Finally,a summary with speculation on future development trends is given.

Slope stability assessment of an open pit using lattice-spring-based synthetic rock mass (LS-SRM) modeling approach

Discontinuity waviness is one of the most important properties that influence shear strength of jointed rock masses,and it should be incorporated into numerical models for slope stability assessment.However,in most existing numerical modeling tools,discontinuities are often simplified into planar surfaces.Discrete fracture network modeling tools such as MoFrac allow the simulation of non-planar discontinuities which can be incorporated into lattice-spring-based geomechanical software such as Slope Model for slope stability assessment.In this study,the slope failure of the south wall at Cadia Hill open pit mine is simulated using the lattice-spring-based synthetic rock mass(LS-SRM)modeling approach.First,the slope model is calibrated using field displacement monitoring data,and then the influence of different discontinuity configurations on the stability of the slope is investigated.The modeling results show that the slope with non-planar discontinuities is comparatively more stable than the ones with planar discontinuities.In addition,the slope becomes increasingly unstable with the increases of discontinuity intensity and size.At greater pit depth with higher in situ stress,both the slope models with planar and non-planar discontinuities experience localized failures due to very high stress concentrations,and the slope model with planar discontinuities is more deformable and less stable than that with non-planar discontinuities.

Comparison of lattice and pseudo 3D numerical simulation of tip screen out operation

Hydraulic fracturing(HF)is a commonly used technique to stimulate low permeability formations such as shale plays and tight formations.However,this method of well stimulation has also been used in high permeable unconsolidated sandstone formations to bypass near-wellbore formation damage and prevent sand production at some distance apart from the wellbore wall.The treatment is called frac-pack completion,where a short length but wide width fracture is formed by injecting aggressive concentrations of proppant into the fracture plane.This operation is known as tip screen-out(TSO).Detailed design of fluid and proppant,including an optimal pump schedule,is required to achieve satisfactory TSO.In this study,we first assess the lattice-based numerical method's capabilities for simulating hydraulic fracturing propagation in elastoplastic formation.The results will be compared with the same case simulation results using a pseudo 3D(P3D)model and analytical model.Second,we explore the Nolte(1986)design for frac-pack and TSO treatment using lattice-based software and the P3D model.The results showed that both models could simulate the hydraulic fracturing propagation in soft formation and TSO operation,while some differences were observed in generated geometry,the tip screenout time and net pressure profiles.The results are presented.It was noted that fracture propagation regime(viscosity/toughness),nonlocality and nonlinearity had an influence on the different geometries.The advantages of each model will be discussed.

Heat advection and forced convection in a lattice code–Implementation and geothermal applications

A three-dimensional thermo-hydro-mechanical numerical model has recently been enhanced with thermal capabilities to study the response of geothermal reservoirs to stimulation and production.In this paper,we present an effort to consider three relevant thermal mechanisms in an existing lattice code initially designed for hydraulic fracturing:a)thermal advection in the fluid;b)heat transfer by forced convection from the rock to the fluid;and c)accurate thermal conduction in the rock matrix considering the thermal boundary layer effect.A numerical implementation of the new coupled advection-forced convection logic as well as the coupling with the existing conduction logic in the commercial code XSite is summarized.The numerical solution is compared to analytical solutions for simple simulation cases.The new simulation capability is applied in a large-scale geothermal example to illustrate its performance.