Understanding all potential slope failure mechanisms is a pre-requisite for predicting the likelihood of batter movements during excavation in open cut mines. The tensile behavior of soils and rocks may be a significant contributor to a slope failure and must be known in order to quantify the risks of slope failure. The contribution can be particularly significant for Intermediate Geotechnical Materials (IGMs) that possess characteristics of both soils and rocks and where the failure mechanisms are complex due to the interplay between ductile and brittle behavior. Brown coal is such an intermediate Geotechnical Material. Recent batter movements in the brown coal mines in the Latrobe Valley, Australia have raised doubts about the current understanding of the mechanisms of slope failure in this material. Research is underway to re-evaluate all properties of the brown coal applicable to slope failure. This paper describes the investigation of brown coal tensile strength. There are alternative test methods available to determine the tensile behavior of materials, including direct tensile tests, beam bending tests and Brazilian compression tests. The applicability of each test method is material dependent, as such, it is necessary to confirm the validity of the methods for each material. Beam bending tests have achieved mixed results for both rocks and IGMs previously. Thus, the present work has explored only the use of Direct tensile and Brazilian test methods. Both methods were implemented using a modified direct shear apparatus and valid test procedures for both test methods were developed. Each test procedure has been verified by Finite Element Modelling (FEM) using ABAQUS 6.12.1 FEM code. The results from the laboratory test methods are in good agreement and show that brown coal is a predominantly brittle material with a peak tensile strength slightly greater than 100 kPa. The finite element analyses confirm that non-uniformity of the tensile stresses during sample loading tends to lead to the underestimation of tensile strength for both tests, but the Brazilian test has less bias for brown coal. It is observed that the rate of loading of low stiffness, low permeability, and saturated samples in the Brazilian test is an important test design parameter for the accurate determination of tensile strength of IGMs in the laboratory.
Brown coal deposits in the Latrobe Valley form part of the tertiary coal system of the Gippsland Basin, which is one of three major tertiary basins in Victoria, Australia. There are currently two operating brown coal mines in the Latrobe Valley (Yallourn and Loy Yang Mines) where coal is mined for power generation, with a third mine (Hazelwood) having recently ceased operations. An ongoing challenge in the mines is the management of geotechnical stability of the open pit batters. This includes the management of significant issues such as instability due to floor heave which is directly related to groundwater pressures of the underlying confined aquifers. The time dependent pressure distributions in the interseam layers are complex due to the complex heterogeneous stratigraphy of these layers. A model of the fine scale stratigraphy using Minescape has been developed to explore how pressure redistribution occurs and how the groundwater flow systems impact the interseam pore pressures due to pumping activity, leading to potential impacts on the mine batter movements. The objective of the preliminary groundwater modelling presented in this paper is to examine the hydraulic connectivity between the lower pumped aquifer layers and the upper sandy layers. The goal is to assess whether the connections are solely through vertical flows through the interbedded aquitard layers or whether there are lateral connections of the sandy layers that govern the vertical connections. A one-dimensional vertical flow model has been used for this purpose in conjunction with high quality groundwater head data from multiple depths in vertically sealed bores. The results suggest that the pressure redistributions vertically cannot be explained by vertical flows alone and that lateral exchange between layers is also occurring. This work will inform the next stage of modelling that will use the detailed stratigraphic modelling in three dimensions.