Published in September 2005 Australian Longwall Magazine
Longwall Top Coal Caving (LTCC) has been developed and operated in China for the past 20 years, and with over 100 faces in operation producing over 200 million tonnes of coal it is a mature technology in Chinese conditions.
In Australia there is a large potential for an underground thick seam mining method such as LTCC to be introduced as currently there are measured underground thick seam (greater than 4.5m) resources of 6.4 billion tonnes and an indicated reserve of 17.5Bt situated in both New South Wales (25%) and Queensland (75%). The Australian Coal Association Research Program (ACARP) has recognised this potential and has funded studies, one completed in 2003, into the feasibility of introducing LTCC into Australia.
According to the 2003 ACARP report, the major perceived benefits of the LTCC method for Australia were LTCC enables potentially double the longwall recoverable tonnes per metre of gateroad development, thereby reducing the development cost per tonne significantly. LTCC offers a viable means of extracting up to 75-80% of seams in the 5–12m thickness range. Plus, lower face heights (relative to high reach single pass longwall) result in improved face control, smaller and less expensive equipment and improved spontaneous combustion control in thick seams, through removal of the majority of top coal from the goaf.
LTCC employs both coal-cutting of the lower portion of the coal seam accompanied by caving and reclamation of the “top” coal. Coal is first cut from the longwall face using a conventional shearer and AFC arrangement working under hydraulic face supports that incorporate a rear coal conveyor and cantilever/flipper arrangement.
Face-cutting heights are generally in the range of 2.8-3m. As the support is advanced forward after the shear, the rear conveyor remains in place in preparation for the caving sequence.
The caving sequence allows the broken coal at the rear of the supports to flow from the goaf onto the rear conveyor. Once an area has been caved, the rear cantilever is extended back out into the goaf stopping any further influx of goaf material.
The main challenges in introducing LTCC into the Australian mining industry fall into three categories: geological/geotechnical (caving assessment, coal recovery, subsidence); mine environment (gas, dust, ventilation and spontaneous combustion); and equipment design and mining systems (equipment design requirements, automation and communications).
Geological/Geotechnical
Scientific studies and detailed investigation is required to determine a particular sites’ potential for LTCC. The theory of Top Coal Caving must be understood and applied to Australian conditions in order to assess a particular site’s potential.
Top coal fracturing: The process of fracturing and crack evolution in the top coal is critical to the success of LTCC. The process is dependent on abutment stress and coal mass strength, according to Jin Zhongming in his paper Longwall Top Coal Caving Theory. The fracturing process begins ahead of the LTCC face when the coal seam is acted on by abutment stress. Secondly, the top coal undergoes horizontal dilation as it is acted upon by vertical stress when little or no horizontal confinement is present (over the top of the supports) before final caving.
Estimation through modelling of the degree of fracturing occurring during the different stages of this cycle is at the core of predicting LTCC production. Figure 1 is a simplified illustration of the stress regime surrounding an LTCC face and its effects on the top coal.
Caving assessment: Chinese research institutes have developed numerous methods for assessing the cavability characteristics of their mines based on empirical methods, laboratory testing and experience. The methods for calculating cavability rely primarily on coal strength, stress (vertical), top coal thickness, interburden/stone band thickness and degree of fracturing.
Once the caving conditions have been assessed, the appropriate mining equipment is selected and caving technique applied. The caving assessment or “index” relates directly to an assessment of the possible percentage of seam recovery and a generic assessment of the mining conditions.
Australian mining conditions, however, vary markedly from the Chinese coalfields. Chinese LTCC mines are deeper at around 600m depth with moderately hard coals, whereas Australian mines are relative shallow at 250-350m in depth and with relatively softer coals. The ratio of vertical to horizontal stress also differs, with Australian mines typically experiencing higher horizontal stress conditions
Parameters such as the “degree of fracturing” are difficult to observe, measure or calculate, and for Australian mines this information is not currently available.
Tools are being developed in the current ACARP project to enable a comparison of Australian mining parameters against current Chinese LTCC measurement indices to obtain an indication as to whether LTCC is applicable to prospective Australian sites. Using this approach a relationship between known Australian parameters and the Chinese rating system or index in this case can be developed.
Numerical Modelling approaches: Modelling of the caving process is crucial to (a) understanding the caving behaviour that can be expected under Australian conditions so as to be able to develop our own specific classification system, and (b) to model the performance of the mining equipment (in particular the face supports) in these conditions. Only by being able to model these two criteria can a realistic prediction of the LTCC system be developed.
Modelling of LTCC is complicated further by the fact the strata must be modelled and assessed through the various abutment stress stages that result in increasing fracturing of the top coal as shown in Figure 1. This process necessitates the use of different modelling approaches to represent the various top coal behaviours.
CSIRO utilises COSFLOW (an in-house package) to model the LTCC scenario as a continuum. The results from this modelling are then fed into a discrete element model (PFC2.0) to observe and analyse the effect of the amount of fractures or “broken bonds” within the top coal due to abutment stress on caving performance. Figure 2 shows a PFC2.0 discrete element model with top coal being fractured and caving at the rear of a LTCC support.
Mining environment
With the introduction of LTCC, changes in...click here to read on.
