GHERG has undertaken a wide range of projects since its inception in 2010, some of which are described below.
Batter Stability Project
On 14 November 2007, the northeast batter of the Yallourn East Field brown coal mine failed. The failure extended laterally for 500 m and involved about six million cubic metres of an 80 m high open-cut slope.
An inquiry was set out to establish the facts, circumstances and causes surrounding the failure. The inquiry concluded that the batter failure occurred by block sliding along an interseam clay under the coal, and was directly influenced by existing joints and the Latrobe River. A Technical Review Board, formed in 2009 in response to the failure, specified that geotechnical studies needed to be undertaken to better understand batter stability and re-establish a robust technical platform to manage geotechnical risks of open-pit brown coal mines in the Latrobe Valley.
The Batter Stability Project (BSP) grew from this initial motivation, which was later ratified by the recommendations arising from the Hazelwood Mine Fire Inquiry.
Seven research sub-projects have been motivated by this project, as follows:
- Laboratory testing of brown coal and interseam materials, Mr Mojtaba Karami
- Geotechnical modelling of the NE batter failure, Mr Mohsen Ghadrdan
- Hydrogeological modelling of the NE batter failure, Mrs Tahereh Shaghaghi
- Stochastic modelling of soil and parameter heterogeneity, Mr Ashley Dyson
- In situ pressuremeter testing of the brown coal and interseam layers, Dr Zhang Tang
- Aquifer depressurisation modelling, Mr Sid Rastogi
- Performance of horizontal bore drains at Yallourn, Ms Christiana Perdigao.
Tensile strength of organic soft rock
The tensile strength of the Victorian brown coal is around 100 kPa. It influences the distribution of stresses in the coal during stress unloading associated with pit excavation. Understanding the magnitude and variability of coal tensile strength provided information on the strength of the coal and its potential for tensile cracking. A range of laboratory tests were completed using direct tensile testing on bone-shaped coal samples, and Brazilian testing of cylindrical samples. To investigate the effect of stress concentration, a brittle crack model was also calibrated in ABAQUS/Explicit.
Large deformation ring shear testing
A new ring shear test apparatus was constructed to investigate the residual shear strength of coal at large shear displacements (larger than 1 m). Shear tests of a broad range of the interseam and coal samples have been conducted to examine the residual shear stress versus normal effective stress envelope over the full range of field conditions. Multi-reversal direct shear tests were also conducted and compared with the results from the new ring shear tests.
Residual shear strength of cohesive soils
This research work developed an effective method of measuring the residual shear strength of an interseam clay from Loy Yang mine, using the direct shear apparatus. Two modifications were introduced to the DS apparatus to prevent rotation of the loading platen, rotation of the upper shear box half and specimen loss. Test results and finite element method modelling show that the modifications minimise or remove these common sources of error. The results require the friction to be corrected. Residual friction angles obtained from the modified apparatus (ranging from 14.6° to 16.6°) were comparable to the results from the ring shear apparatus (12.1° and 14.0) but slightly higher, as expected.
Effect of lignite content on the properties of interseam clays
As the transition between the interseam clay and lignite seams is not always distinct, but rather gradual, the effect of organic content (lignite content) on the geotechnical properties of a fine grained cohesive soil (interseam clay) was investigated using a multi-reversal direct shear apparatus. The study showed that an increase in organic content of overconsolidated, remoulded specimens results in a decrease in plasticity index but an increase in residual friction angle, liquid limit and plastic limit. The shear deformation required to reach the residual shear strength was shown to decrease.
Coal permeability at low confining stress
Previous permeability tests on brown coal samples were carried out under high confining stresses and under high pressure gradients. While these conditions are applicable to the coal at large depths, they are less applicable to the coal near the batters and in areas with significant depressurisation. A laboratory-based investigation was undertaken to measure the saturated hydraulic conductivity of the coal under low confining stress conditions. The study also provided useful insights into the solids compressibility and gas presence at low fluid pressures.
Lugeon (Packer) tests on Victorian brown coal
The hydraulic conductivity of the Victorian brown coal was measured in situ from a series of Lugeon packer tests conducted at a range of depths of a continuous coal seam located at the Yallourn open cut mine in Victoria, Australia. The in situ test results were interpreted using both analytical methods and numerical simulation. They were also compared with laboratory test results. Field testing resulted in permeabilities several orders of magnitude higher than laboratory testing, suggesting the existence of macro fractures on a scale greater much than that observed in the lab.
In situ pressuremeter testing of brown coal open-pit mine
A series of in situ pressuremeter tests were completed at Yallourn mine to interpret in situ stresses conditions and obtain deformation and strength parameters for the overburden, coal and interseam materials. Technical services and equipment were provided by CMR Technical Services based in Perth, Australia. The pressuremeter device used in the field is a custom-made cylindrical probe with a nominal diameter of 71-mm, designed by CMR for pre-bored holes. Soil samples collected at the test intervals were tested in the lab to provide support for the interpretation of these results. Pressuremeter testing in the coal and interseam materials provided evidence of variation of stress with depth and between the two sediments.
Automated mini-pressuremeter probe for unconfined tensile testing
Unconfined expansion tests (UET) were conducted to measure the tensile strength and shear modulus of the Victorian brown coal. The test uses an automated mini-pressuremeter probe and was designed to mimic in situ pressuremeter testing. The new test reduces the potential for bias in the test results arising from predefinition of crack orientation. The tensile strengths obtained from UETs were consistent with previous strengths determined from direct tensile and Brazilian tests. Numerical modelling using ABAQUS was undertaken to further investigate the lab tests. The analyses provided confidence in the validity of the laboratory test results.
Brown coal composites as alternative landfill liner materials
Lignite composites have been investigated as an alternative material for use in landfill liners, in place of clays, which are scarce and expensive in Australia, particularly for the mines in the Latrobe Valley. The composites consisted of brown coal, and either polyacrylamide (PAM) or sodium bentonite, as additive. The additives were distilled with water and sodium chloride solution (50 g/l) to investigate the effects of pore fluid chemistry. This study focused on the determination of liquid limit, swell index and hydraulic conductivity of the coal composites as a measure for landfill liner use. It was found that the liquid limit, swell index and hydraulic conductivity do not deteriorate significantly in the presence of highly concentrated electrolyte solution (50 g/l NaCl). Such distinct properties will be beneficial over currently used landfill liner materials.
Impact of ambient oxidation on brown coal
This study adopted a molecular dynamics simulation technique to investigate the characteristics of chemically bound water in brown coal and the effects of ambient oxidation and temperature at varying total system water contents. Elemental analysis indicated that the oxidised system was equivalent to about 103-day aerial oxidation of the coal at 35°C. Shear viscosity and diffusion of water in brown coal were investigated and linked to chemically bound water contents. The study indicated that the characteristics of the chemically bound water are influenced by ambient oxidation, temperature and total system water contents.
Deformation of unbound granular materials under repeated load
Conventional pavement design methodologies used in Australia assume a linear elastic model or sub-layering rules for the unbound granular materials (UGMs) and fine-grained soils (subgrade layer). They disregard the nonlinear stress-dependency of the resilient modulus under repeated transient traffic loads. Nonlinear analyses result in better prediction of pavement response compared to linear analyses. This study characterised the resilient and permanent deformation behaviours of four road UGMs sourced from quarries in Victoria, Australia, using triaxial testing under repeated load.
Cutter soil mixing technique for stabilisation of Coode Island Silt
This research investigated the effectiveness of cutter soil mixing (CSM) using lime (or lime/cement) in the stabilisation of Coode Island Silt (CIS). It studied improvements on the strength, stiffness, permeability and durability of treated soil. Advanced numerical techniques were also used to gain further insight on the CSM process. One of the most challenging issues with CSM applications is to the amount of injected slurry. Most mixing practices resort to lab testing for arriving at design parameters. However, it is impractical to capture the real mixing pattern in the lab as CSM cutter wheels are different from the typical lab mixers. It is believed that an advanced numerical method can more accurately simulate realistic mixing and disaggregation of soil particles.
Aquifer depressurisation and groundwater recharge
This project extended previous modelling of groundwater depletion in the Latrobe Valley to improve understanding of the relationship between aquifer depressurisation and groundwater recharge. The project was split into four components. The first concerned the modelling of the hydrology of the upland catchments at the valley sides to provide evidence for the range of potential deep recharge arising in these areas. The second concerned the field investigation of stream bed recharge in the region using thermal signature methods. The third addressed the unsaturated zone characteristics of the shallow soils to examine the potential for direct recharge to the aquifer through the land surface. Finally, hydrological modelling was undertaken with SHETRAN to identify the interactions between groundwater depressurisation and recharge under a range of climate conditions.