Bre-Anne Sainsbury

Investigation of caving induced subsidence at the abandoned Grace Mine

Abstract The Grace Mine, located in southeastern Pennsylvania, USA, was owned and operated by Bethlehem Steel Corporation during 1951–1977. During this time, iron ore was extracted using the underground panel caving mining method that has resulted in significant surface subsidence. Upon recovery of the water table after mining, a lake has formed over much of the subsided area. Before redevelopment of the abandoned mine site for residential and light industrial usage, an investigation of the subsidence zone of influence, and the potential for further subsidence has been undertaken.

Three-dimensional analysis of complex anisotropic slope instability at MMG’s Century Mine

Anisotropic and foliated rock masses present particular difficulties in the assessment of pit slope stability. Although many attempts have been made to describe the strength of rock masses that exhibit a preferred orientation of weakness, no general methodology has emerged throughout the literature to simulate anisotropic behaviour in a three-dimensional numerical model of pit slope stability. In order to simulate the effect of the anisotropic shale rock mass on pit slope stability at MMG Limited’s (MMG) Century Mine, the Ubiquitous Joint Rock Mass (UJRM) method has been applied. This paper outlines the methodology that was used to assist mine personnel in the management of complex anisotropic slope instability that jeopardised the recovery of 1.8 million tonnes of zinc ore.

Practical use of the ubiquitous-joint constitutive model for the simulation of anisotropic rock masses

Anisotropic rock masses, the behavior of which is dominated by closely spaced planes of weakness, present particular difficulties in rock engineering analyses. The orientation of discontinuities relative to an excavation face has a significant influence on the behavioral response. At the present time, discontinuum modeling techniques provide the most rigorous analyses of the deformation and failure processes of anisotropic rock masses. However, due to their computational efficiency continuum analyses are routinely used to represent laminated materials through the implementation of a Ubiquitous-Joint model. The problem with Ubiquitous-Joint models is that they do not consider the effects of joint spacing, length and stiffness. As such, without an understanding of the limitations of the modeling approach and detailed calibration of the material response, simulation results can be misleading. This paper provides a framework to select and validate ubiquitous-joint constitutive properties.

Consideration of the Volumetric Changes that Accompany Rock Mass Failure

List of symbols d Dynamic rock mass density (kg/m3) s Initial rock mass in situ density (kg/m3) Porosity eqiv Equivalent porosity max Maximum porosity achieved by a rock mass. Default 0.4. vsi Volumetric strain increment vsi(max) Maximum volumetric strain achievable by a rock mass Ψ Dilation angle (degrees) dyn Dynamic dilation angle (degrees) φ Friction angle (degrees) ci Intact Unconfined Compressive Strength (MPa) 3 Minor principal stress magnitude (MPa) Edyn Dynamic deformation modulus (GPa) Ein situ Initial peak in situ rock mass deformation modulus (MPa)

A Sub-Level Caving Algorithm for Large-Scale, Small-Strain, Numerical Simulations

In block and panel caving, mobilization of the ore is achieved without drilling and blasting. The disintegration is brought about by natural processes that include the in situ fracturing of the rock mass, stress redistribution, the limited strength of the rock mass, and gravitational forces. Sub-level caving (SLC) requires the transformation of in situ ore into a mobile state by conventional drilling and blasting. This may be a result of a high rock mass strength or strategy to reduce dilution. The SLC method is thought to have evolved as an upscaling technique to the top slicing mining method (Peele 1918). Block caving, in turn, was the logical scale-up from sub-level caving. In the first application of sub-level caving, the ore was not drilled and blasted completely between two sub-levels, but only parts were broken by induced caving; hence the name sub-level caving (Janelid 1972). At current day SLC operations, the ore mass between the sub-levels is blasted. As a result of this, the primary concern with SLC mining methods is not the strength of the orebody itself but the competency of the hangingwall material (for subsidence and dilution predictions) and prediction of fragmentation and gravity flow of the blasted ore material through the SLC rings. Existing caving algorithms described by Sainsbury et al. (2010) have been developed based on a block and panel caving scenarios. To simulate sub-level caving within a largescale, small-strain numerical models, modifications to the block/panel caving algorithm are required. They are detailed herein and generally relate to: