Developing a complete geotechnical model for brown coal slope stability analysis
Brown coal slope stability is a concern for the large brown coal mines of the Latrobe Valley. The coal is a very light intermediate geotechnical material that presents brittle as well as ductile modes of deformation and failure. Long timescale, large deformations arise as a result of stress relief and dewatering, with the consequence that vertical joints can open and permit water inflows, which lead to significant hydraulically-driven movements.
The permissible extent of deformation before a slope becomes unstable and fails is not well understood and the relative importance of different processes leading to failure remains largely undefined. Internal and external infrastructure in close proximity to the current mine boundaries can be at risk from both chronic and catastrophic slope failure.
Previously collected geotechnical data for the coal do not explain all the observed processes or the potential risks. Given this background, a broad suite of laboratory experiments, field measurements and numerical modellings are under way to build new geotechnical knowledge of the materials in the batters of the coal mines and to apply this to predict slope failure risks.
- Tensile strength testing
- Coal creep testing
- Large deformation ring shear testing
- Investigating coal permeability at low confining stress
- Application of pressuremeter testing in open-pit mining
- Analysis of time-dependent geotechnical factors affecting brown coal mine batter stability
- Ash disposal into mines
- Artificial soils development and testing for mine rehabilitation
- Characterisation of the deformation behaviour of unbound granular materials using advanced repeated load triaxial testing
- Evaluation of cutter soil mixing technique for stabilisation of Coode Island Silt
Research team: Dr Ali Tolooiyan
The tensile strength of brown coal is significant and influences the distribution of stresses in the brown coal during stress unloading associated with pit excavation. Understanding the magnitude and variability of coal tensile strength will provide information on the strength of the coal during deformation and the potential for tensile cracking. A range of tests are under way using direct tensile testing on bone-shaped coal samples as well as Brazilian testing of cylindrical samples. Additional testing is investigating the role of pre-existing cracks in precipitating tensile failure.
Research team: Dr Edward Kim
GHERG's initial creep tests demonstrated that brown coal moved extensively under relatively small environmental changes and that there was a clear requirement to understand the environmental factors affecting the coal behaviour. The environmental effects on the coal are being studied using a combination of computer simulations and controlled laboratory testing reproducing the field conditions. It is clear from the results to date that the magnitude of the responses may be influential in contributing to the slope movements observed. Volume changes derived from environmental changes and associated chemical processes are significant. Thermal contraction is also observed as the coal cools down. Future tests will determine the stress responses of confined coal to chemical processes to provide information needed for stability analysis.
Research team: Prof Rae Mackay
A new ring shear test apparatus based on an old design by Bishop (1971) has been constructed to investigate the residual shear strength of coal at large shear displacements (>1m). The apparatus is designed to accommodate the compressive loads predicted at depth in the coal adjacent to the current pits, where depths of excavation are up to 200m. Shear tests of a broad range of the interseam and coal samples are under way with the equipment to examine the residual shear stress versus normal effective stress envelope over the full range of conditions identified in the field. Comparative testing with Multi reversal direct shear testing is also ongoing to explore the consistency of the large deformation shear results with those obtained over much shorter shear path lengths.
Investigating coal permeability at low confining stress
Research team: Prof Rae Mackay and Dr Jianfeng Xue, Mr Liu Kan (Tongji University, China)
The permeability of intact coal is very low. However, observations of coal permeability have been made under high confining stresses and under high pressure gradients. While these conditions are applicable to coal in the deeper profiles adjacent to the mine, they are less applicable near the mine batters and in areas where significant depressurisation of the coal has taken place. To improve our understanding under these conditions a laboratory based investigation of the saturated hydraulic conductivity is being undertaken. Owing to the physical conditions under which the experiments are being carried out, the study is also providing useful insights into the solids compressibility and gas presence at low fluid pressures. This research overlaps with the consolidation studies on brown coal.
Application of pressuremeter testing in open-pit mining
Research team: Dr Ali Tolooiyan, Dr Jianfeng Xue and Mr Zhan Tang
Increasing use of stress-strain analyses and availability of Finite Element Analysis (FEM) techniques in geotechnical and mining engineering has led to better appreciation of the importance of ground in situ stresses. Open-pit design practise assumes that the effect of in situ stress is an important issue when the stresses induced in pit wall slopes are large enough to approach and exceed the material strength. This issue can be even more serious for brown coal open-pits when mass blocks with several joints and light weight are involved. Even if the horizontal in situ stress in Latrobe Valley brown coal mines has not been considered to be large enough to cause coal mass damage and produce an enlarged zone of weakened material, this stress is surely one of the main causes of long-term batter deformation, hence it is needed to be measured and understood accurately.
Pressuremeter is a geotechnical in situ measuring equipment constructed to measure the ground in situ horizontal stress. The probe of the equipment is inserted or pushed into the borehole and is fixed at test depth. Pressurised Liquid or gas is applied into the probe and inflate the flexible membrane which applies the input pressure to the walls of the borehole and causes borehole cylindrical expansion. Volume, deformation and pressure data are recorded and used for calculation of some valuable geotechnical parameters such as horizontal in situ stress, stiffness, shear strength, friction angle and cohesion.
In 2013, GHERG started to develop few novel FEM techniques for post processing of pressuremeter test results in brown coal and inter-seam clay. Also, a laboratory scale pressuremeter chamber which will furnish GHERG scientists to run the laboratory scale pressuremeter test has been designed and being constructed in the GHERG geomechanic laboratory. As a part of this project, GHERG is currently planning to perform pressuremeter test in Latrobe Valley brown coal mines.
Analysis of time-dependent geotechnical factors affecting brown coal mine batter stability
Research Team: Prof Rae Mackay, Dr Ali Tolooiyan and Ms Catherine Couling
In 2007, a portion of the nearly complete north-eastern batter of the Yallourn brown coal mine collapsed. The focus of this current research is the case study of a selection of time-dependent geotechnical factors believed to have influenced the long-term stability of the brown coal batters. This research will investigate the relative significance of some of the causes contributing to the batter failure, particularly the long-term geomechanical behaviour of materials, influence of geologic structures and groundwater conditions. Both two- and three-dimensional finite element modelling, combined with existing data recorded at the time of failure and new data collected from geomechanical testing of materials, are being used in this investigation.
Research Team: Dr Barbara Panther, Prof Rae Mackay and Mr Michael Taylor
A large volume of ash is produced by coal burning.
A new approach to ash disposal is proposed that has the potential to remove the long-term environmental liability of disposal into caissons in the overburden dumps adjacent to the mine. The ash provides valuable fill for the open-cut mine for final rehabilitation. The approach is presumed to be to pump the ash from the current ash detention ponds directly onto the mine floor, drain excess moisture and then cover with overburden materials. The ash would be deposited in layers and the water flows at the base of the mine would be managed during the rehabilitation period to allow chemical interaction between the ash and the other mine wastes. The objective is to stabilise the materials so that the major contaminants are permanently locked up and/or neutralised. The ash layers will provide the major pathway for horizontal flows to ground control wells that will facilitate water management within the ash sequence.
The study is designed to look at the environmental impacts of ash disposal over the full life cycle of the mine from preliminary emplacement to final rehabilitation of the mine void.
Research Team: Prof Rae Mackay, Dr Barbara Panther, Dr Mohan Yellishetty (Monash University), Mr Arun Vappaladadiyam.
The overburden (OB) and interseam waste materials that are disposed in or adjacent to a brown coal open cut will be the main constituents of any cover developed during intermediate and final rehabilitation of the mining area.
Sections of the overburden profile contain pyrites that are potentially acid generating when exposed to the atmosphere. Acid control is important to reduce metal leaching from the overburden and to restrict acidic waters entering the surface water system. Also these overburden materials are largely devoid of nutrients and regeneration of a vegetative cover to these materials is found to be difficult, thus leading to a poorly protected surface that is erodible, potentially acid generating and structurally weak. Fire hazard is high under such circumstances, rehabilitation strategies require a chemically safe, structurally stable surface that can minimise the risk of brown coal exposure at the surface and facilitate rapid infiltration of surface run off into coal joints, which may reduce the ground stability.
Field trials at Loy Yang Mine, Victoria will be undertaken to test the performance of synthetic soils built from overburden waste, power station fly ash and composted sewage sludge. Performance will be assessed in terms of chemical stability, leachate quality, erosion rate, plant growth, and moisture retention. New rehabilitation soils are essential to meet the large predicted shortfalls at open pit coal mines. Past work has investigated single additives to develop soils, but with only partial success. This work investigates combinations of additives to improve soil development and aid mine rehabilitation for future uses. The research will provide detailed guidance on synthetic soil development for long-term mine rehabilitation.
Characterisation of the deformation behaviour of unbound granular materials using advanced repeated load triaxial testing
Research Team: Dr Ali Tolooiyan & Professor Rae Mackay and Ms Negin Zhalehjoo
Industrial Partner: Australian Road Research Board (ARRB Group)
Currently, pavement design process in Australia assumes a linear elastic model or sub-layering rules for the unbound granular materials (base/sub-base layers) and fine-grained soils (subgrade layer). This method disregards the nonlinear stress dependency of the materials resilient modulus under repeated transient traffic loads. Nonlinear analysis tends to predict more accurate prediction of the pavement responses, such as stresses, strains, and displacements in the critical components compared to the linear analysis. The objective of this research is to replace the existing linear elastic model with a nonlinear stress-dependent resilient modulus model for the pavement materials to predict the realistic performance of Australian roads pavement and minimise the risk of excessive permanent deformation.
Evaluation of cutter soil mixing technique for stabilisation of Coode Island Silt
Research Team: Dr Ali Tolooiyan & Professor Rae Mackay and Mr Kaveh Ranjbar Pouya
Industrial Partners: BAUER GmbH., Germany & Wagstaff Piling Pty Ltd., Australia
The main part of the current research is to investigate the effectiveness of CSM using lime (or lime/cement) in the stabilisation of Coode Island Silt (CIS) by studying the improvement of strength, stiffness, permeability, and durability of treated soil. Also, this study aims to develop and apply advanced numerical techniques to evaluate and simulate the CSM process. One of the most challenging issues for CSM applications is to optimise the amount of injected slurry. Conventionally, most mixing practices resort to laboratory testing. However, it seems impossible 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.
- Coal consolidation behaviour
- Advanced techniques in measuring residual shear strength parameters of cohesive soils
- Investigation of groundwater/surface water interactions
Research team: Dr Jianfeng Xue, Prof Rae Mackay and Mrs Fatemeh Moein
Through a program of short and long time consolidation testing, integrated with permeability testing, new results that characterise the nature of the consolidation and fluid flow responses of the brown coal and interseam materials to dewatering are being produced and the implications of the results for long term subsidence and rebound are being developed. A new mathematical model of coal consolidation is being developed that includes descriptions of the impact of trapped gas and coal compressibility to explain their contributions to the large scale subsidence and dewatering rates note in the vicinity of the mines.
Research team: Dr Ali Tolooiyan and Mr Luke Tatnell
The objective of this research was to find an effective method of measuring the residual shear strength of fine grained cohesive soils (specifically interseam clay from Loy Yang mine) using the direct shear apparatus. Due to multiple sources of error, which arise during large deformation testing of fine grained soils in the standard direct shear apparatus, results obtained from this test method are generally of low quality. Two simple modifications are introduced 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 sources of error. When the tests are conducted with the modifications in place, the residual shear strength results obtained are generally improved. The test results require a simple correction for the friction which occurs between the two shear box halves. Residual friction angles obtained from the modified apparatus (ranging from 14.6° to 16.6°) are compared to results from the ring shear apparatus (12.1° and 14.0) and are, as expected, slightly higher.
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) is analysed. An increase in organic content results in an increase in residual friction angle, liquid limit and plastic limit, but a decrease in plasticity index. It also decreases the shear deformation required to reach the residual shear strength, in overconsolidated, remoulded specimens.
Research team: Prof Rae Mackay and Mr Chow Hirunteeyakul
The present project is designed to extend previous modelling of groundwater depletion in the Latrobe Valley by improving understanding of the relationships between aquifer depressurisation and groundwater recharge.
The project is split into four components. The first concerns 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 concerns the field investigation of stream bed recharge in the region using thermal signature methods. The third is addressing 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 is being undertaken with the physically-based modelling system, SHETRAN, to identify the interactions, if any, between groundwater depressurisation and recharge under a range of climate conditions.