This thesis presents the results of research into the potential of underground mining methods applicable to inclined thick seams (thickness greater than 3.5m, and seam dip of 15 - 35 degree) in the Quangninh coalfield, Vietnam. The primary objectives of this research are to investigate the most suitable underground methods applicable to inclined thick seams in the Quangninh coalfield and to improve understanding of the operational and geotechnical issues associated with the application of chosen methods in thick seams with steeply dipping conditions. From a risk and operational assessment, the Longwall Top Coal Caving (LTCC) method is considered most appropriate for inclined thick seams under the current mining conditions in Vietnam. The LTCC method offers great potential for the efficient extraction of thick seams by caving coal from the upper section during the mining of the lower section. This significantly reduces the development cost per tonne. Compared to High reach Single Pass Longwalling, the LTCC method offers a low extraction height, resulting in smaller and less expensive equipment and better face conditions. Results from this study identified that for extracting an inclined thick seam, the face retreating along the strike has better operational advantages and better cavability than the face retreating updip or downdip of the seam. The operational issues of the LTCC method when extracting inclined seams are: the stability of the support, transport in the mine, and the difficulties in roof control at the transition between face ends and the gateroads. In terms of geotechnical issues, better cavability of the top coal is anticipated for flat coal seams compared to inclined seams. In addition, the chain pillar for flat coal seams is subjected to a higher vertical stress distribution than that of inclined ones. An improved cavability assessment method for the categorisation of the cavability of the top coal with four categories, ranging from 1 (excellent cavability) to 4 (very poor cavability), was suggested to assist the feasibility and design stages of the LTCC application. The cavability assessment method was conducted by numerical analysis combined with back analysis from the database obtained from past LTCC practices.

This thesis presents the results of research into the potential of underground mining methods applicable to inclined thick seams (thickness greater than 3.5m, and seam dip of 15 - 35 degree) in the Quangninh coalfield, Vietnam. The primary objectives of this research are to investigate the most suitable underground methods applicable to inclined thick seams in the Quangninh coalfield and to improve understanding of the operational and geotechnical issues associated with the application of chosen methods in thick seams with steeply dipping conditions. From a risk and operational assessment, the Longwall Top Coal Caving (LTCC) method is considered most appropriate for inclined thick seams under the current mining conditions in Vietnam. The LTCC method offers great potential for the efficient extraction of thick seams by caving coal from the upper section during the mining of the lower section. This significantly reduces the development cost per tonne. Compared to High reach Single Pass Longwalling, the LTCC method offers a low extraction height, resulting in smaller and less expensive equipment and better face conditions. Results from this study identified that for extracting an inclined thick seam, the face retreating along the strike has better operational advantages and better cavability than the face retreating updip or downdip of the seam. The operational issues of the LTCC method when extracting inclined seams are: the stability of the support, transport in the mine, and the difficulties in roof control at the transition between face ends and the gateroads. In terms of geotechnical issues, better cavability of the top coal is anticipated for flat coal seams compared to inclined seams. In addition, the chain pillar for flat coal seams is subjected to a higher vertical stress distribution than that of inclined ones. An improved cavability assessment method for the categorisation of the cavability of the top coal with four categories, ranging from 1 (excellent cavability) to 4 (very poor cavability), was suggested to assist the feasibility and design stages of the LTCC application. The cavability assessment method was conducted by numerical analysis combined with back analysis from the database obtained from past LTCC practices.

Longwall Top Coal Caving (LTCC) technology has great potential for more efficient mining of Australian thick coal seams. LTCC can potentially double (or greater) the longwall recoverable tonnes, per metre of gateroad development and improve the safety standards in longwall operation. Accurate cavability assessment of the coal seam is the key pre-requisite for successful application of LTCC method. Although some cavability criteria were developed in previous studies, their shortcomings limit their application. Apart from the lack of suitable cavability criterion, the caving principles and mechanisms in longwall operation (and LTCC) is still not well understood. The main objectives of this research work were: 1. to improve the general understanding about the caving mechanisms involved in LTCC operation, 2. to develop a new cavability assessment criterion, and 3. to characterise and classify the caving behaviour in selected Australian longwall operations. For this study a combination of analytical, observational and empirical engineering methods were used to reduce the shortcoming of individual design methods. For analytical study, the Distinct Element Method (DEM) was selected for computational analysis as the most suitable technique for this type of study. For observational methods, the size distribution of particles in goaf was used as a monitoring measure for caving performance evaluation and this method was performed in three Australian longwall operations (Ulan, West Wallsend and Broadmeadow). The Empirical assessment was performed by using the results of observational/computational analyses along with basic geological/geotechnical data from selected mine sites to back analyse and re-evaluate the results. The most significant outcomes of this study include: 1. a new cavability assessment system (Top Coal Cavability Rating, TCCR) that was developed based on computational analysis and by back analysis of past LTCC experience in China, 2. Improved understanding of caving mechanism, deformation mechanism and drawing mechanism of top coal in LTCC method, 3. advanced numerical models (UDEC, 3DEC and PFC models) that not only can be used for future research but can also be employed for LTCC design purposes, 4. characterisation and classification of caving behaviour in three Australian longwall operations. The results of this study can be effectively used by Australian coal industry in feasibility and design studies of LTCC operation. The new cavability criterion can be used to identify and quantify the major risks that may be involved in LTCC application.