Evaluation of the application of the Longwall Top Coal Caving (LTCC)
Method in Australia

 

B K Hebblewhite & Y J Cai

 

UNSW Mining Research Centre

School of Mining Engineering

The University of New South Wales,

SYDNEY, AUSTRALIA

 

ABSTRACT

 

The very significant production improvements achieved in the Chinese coal mining industry over the last decade, as a result of development and application of the LTCC method, has prompted Australian mines to examine the method and its potential, for Australian application.  UNSW, together with the various research collaborators listed above, has assisted in identifying potential sites where LTCC might be applicable in Australian mines.

 

The major thrusts of the current UNSW research are in three areas:

-        continual updating of a thick seam minable reserves database, relative to potential mining methods

-        investigation of the geomechanical factors that affect the safe and efficient performance of LTCC, relative to Australian mining conditions

-        optimization of the LTCC face configuration, performance and management with respect to caving and coal handling capacities.

 

This paper presents the key issues and latest findings from each of these three areas of research.  Under the geomechanical research, factors such as coal strength and rock mass characteristics are considered with respect to caving potential and face support performance.  The caving and coal clearance simulation/optimization research includes evaluation of a range of front and rear conveyor capacities relative to different caving strategies, as well as alternate panel conveying options.

 

INTRODUCTION

There has been considerable interest in underground thick seam mining methods in Australia for many decades.  As outlined later in this paper, Australia has significant reserves of thick coal seams that require the application of alternative mining methods – beyond the conventional bord and pillar or standard longwall systems.  The incentive for identifying or developing new methods for underground thick seam mining is primarily optimising resource recovery.  However, the Australian coal industry is an export-dominated industry where high productivity, sustainable financial viability and the highest safety standards are paramount.  As such, the Australian requirement is for appropriate methods which meet the Australian safety and productivity/financial performance criteria, or preferably improve on them, at the same time as achieving improved resource recovery.

As a point of definition, the term thick seam has been applied to any minable seam thickness greater than the reach of existing development and longwall systems.  In the 1980s and 1990s, this was interpreted as 4.0m.  However, with higher reach continuous miners ands longwall systems, an arbitrary figure of  4.5m has been adopted for all recent studies.

Earlier studies by UNSW and others (Hebblewhite, 1999 & Hebblewhite et al., 2002) reviewed the Australian opportunities and available methods and technologies for underground thick seam methods.  Arising from this review, four generic methods were identified as having thick seam potential.  These were:

·        extended height single pass longwall (SPL)

·        multi-slice longwall (MSL)

·        hydraulic mining (HM)

·        caving longwall systems (CL), including longwall top coal caving (LTCC).

The option of extending the height of a conventional single pass longwall was considered to have limited possibilities.  It was apparent that technology was already gradually increasing both shearer and support heights from 4m to 4.5m and now up to 5m and above (Hamilton, 1999).  However, limitations such as equipment size, weight and stability, plus face conditions were considered to limit the application of this method to no more than 6m height, for many years to come.

Multi-slice longwall was also reviewed in detail, with consideration of experiences from both Europe and China with this method.  The potential to apply modern paste fill technologies for septum formation and stabilisation was investigated (Palarski, 1999, Bassier & Mez, 1999).  However, through a risk assessment process, some of the other issues such as mining under goaf areas (water and gas hazards), and general stability concerns, ruled this method out as a viable option for Australia at the present time, quite apart from the very limited gains in productivity anticipated.

Hydraulic mining, as practised in New Zealand and elsewhere previously, was investigated.  It was found to be a method with a significant potential in a limited range of suitable mining conditions.  It offered significant financial benefits, but limited large scale production potential.  It was therefore considered to be suitable as a “niche application” method, but not a universally applicable option.

This then left the range of longwall caving options.  Evaluation of different European and early Chinese experiences with the original “soutirage” mining concepts and equipment showed promise, but performances were below the level required to be viable in Australia, not to mention concerns over issues such as dust and spontaneous combustion.

THE LTCC METHOD

However, during the first of the ACARP projects, the UNSW/CMTE team, plus others at CSIRO became aware of the significant developments and impressive performance improvements being achieved in China with the development and application of the LTCC method (Xu, 1999).  The method is essentially an extension of the original soutirage concept, but with significant equipment and face operational changes related to the use of the second rear AFC behind the face for handling the caved coal (see Figures 1, 2 and 3).

In terms of equipment innovation, the more recent Chinese developments have relocated the top coal draw points to the rear of the longwall supports, rather than bringing coal through the roof canopy of the shield onto a conveyor within the shield structure.  These previous methods were quite clumsy and mechanically complicated, quite apart from the excessive dust-make within the face area and the ‘cluttering up’ of the already limited space within a line of shield supports.  The Chinese equipment has a pivoting supplementary goaf or tail canopy behind the support.  Beneath this is a retractable second AFC.  With the rear AFC extended and the rear canopy lowered/retracted, caved top coal can be loaded onto the rear AFC, whilst production continues conventionally in front of the supports.  In the retracted rear AFC position with the rear canopy raised, the supports and face operation can function conventionally.

The Chinese industry had reported averages of 15,000 to 20,000 tpd from an LTCC face; up to 75% recovery of 8m+ thick seams using a 3m operating height longwall; and +5 MTPA face production.  There are now well over 70 LTCC faces in China.  A new semi-automated 300m long LTCC face was installed at the Xinglongzhuang Colliery of the Yankuang Group, in Shandong Province, in August, 2001, with production capacities of at least 7MTPA.

The major perceived benefits of the LTCC method for Australia include:

·        Operating Cost Reductions: The LTCC method enables potentially double (or greater) the longwall recoverable tonnes, per metre of gateroad development, thereby reducing the development cost/tonne significantly, and reducing the potential for development rate shortfalls leading to longwall production disruption.

·        Resource Recovery and Mine Financial performance: The LTCC method offers a viable means of extracting up to 75% to 80% of seams in the 5m – 9m thickness range.  Single pass longwall is considered to be limited to an upper height of 6m, and is currently only operating at or below 5m.

 

Figure 1.  Conceptual Model of LTCC System (after Xu, 1999)

 

 

 

 

 

 

 

 

 

 

 

 


Figure 2.               Typical LTCC Face Support (note articulated rear canopy)

 

 

 

 

 

 

 

 

 

 

 


Figure 3.               View along rear conveyor

 

·        Mine Safety: 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.

A joint ACARP research project between UNSW and CSIRO was undertaken to further investigate the LTCC method for Australian application, and was reported in 2003 (Kelly et al., 2003).  In parallel with the ACARP study, UNSW and CSIRO jointly developed a relationship with the Yankuang Group in China, one of the leading operators of the LTCC method.  Various study visits by UNSW, CSIRO, CMTE and industry representatives from Australia visited China to inspect the LTCC operations of Yankuang and other companies over the past five years.  All groups have returned with very favourable impressions and views about prospects for the method in Australia. 

 

AUSTRALIAN THICK SEAM RESERVES

 

Table 1 summarises the extent of thick seam reserves in Australia – both in terms of measured reserves  and measured plus indicated (Hebblewhite et al., 2002).  These figures confirm that there is at least 6.4 billion tones of minable underground thick seam reserves.  Some significant features of these minable reserves are:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Table 1. Australian thick seam reserves (after Hebblewhite et al., 2002)

 

The above parameters confirm that the majority of thick seam reserves are in the 6m – 9m range, beyond the scope of Single Pass Longwall, but eminently suitable for the LTCC method.  Furthermore, the seam dips and depths are also within the range of current LTCC technology.

 

GEOTECHNICAL ISSUES FOR LTCC

 

The success of an LTCC operation – from all three perspectives of safety, resource recovery and productivity – depends to a large extent on having an appropriate geotechnical environment, and then successful geotechnical management within that environment.  This has been recognized by Chinese operators who have developed a number of reliable empirical classification and design schemes.

 

The geotechnical factors considered to be of most importance for safe and effective implementation of LTCC in Australia are considered to be the following:

 

 

 

Coal seam cavability

The consistent cavability of the top coal in an LTCC operation is crucial to its success, particularly with respect to adequate resource recovery.  If the coal caves, but in too large a pieces it can cause blockages and handling problems both feeding onto, and traveling along the rear conveyor.  Of even greater problem is if the coal hangs up, even only for a short time, such that it caves but beyond the reach of the rear AFC.  On the other hand, if the coal is too weak and friable, there is the potential for the coal roof to commence breaking up too far in advance of the rear support canopy, leading to potential face and roof problems.  The main geotechnical components affecting coal cavability are uniaxial compressive strength (UCS); cleat, bedding and other discontinuities; and vertical stress on the coal.

 

 

Figure 4.               Australian thick coal seam strength distribution (after Kelly et al., 2003)

 

In the case of UCS, Chinese experience is understood to be that a range of 15 MPa to 25 MPa is well suited to good caving conditions.  Above 30 MPa, caving can become problematic, subject to the other parameters of discontinuities and stress.  Figure 4 presents a chart showing the distribution of coal strength, in UCS terms, for thick coal seams considered suitable for LTCC methods on all other grounds.  This indicates that at least 29% of seams fit into the middle category (15 – 25 MPa), with only 14% greater than 25 MPa.  Some potential sites within this strong coal category (with UCS values in the range 30 – 40 MPa) are currently subject to further laboratory and in situ investigation, in order to assess the other factors and to what extent they might compensate for the higher strength range.  The 57% of coal seams in the sub-15MPa range (predominantly in Queensland) will undoubtedly cave well, but will require more detailed, site-specific studies in relation to immediate roof integrity above and in front of the supports.  This may also require adjustment of the initial coal cutting height to secure a stable immediate roof.

 

The question of depth is also an important issue for Australia.  As the figures in Table 1 indicated, 51% of the measured reserves are below 150m in depth.  The effect of this is that the amount of vertical stress due to overburden cover, acting on the top coal may be insufficient to fracture the coal sufficiently, particularly if it coincides with a stronger than average coal seam.  Once again, it is the combination of all three sets of parameters (and possibly others) which is likely to determine the final cavability assessment.  This whole question of cavability is the subject of ongoing research in Australia, both from the point of view of coal cavability classification systems, and also in terms of appropriate stress analysis modeling techniques.

 

Massive roof strata units

The issue to be considered here is the one which a number of Australian mines experience, in terms of periodic weighting, or delayed, cyclical caving of massive roof horizons.  The effect of these can be no more than nuisance value, right through to damage to face support systems and face/roof instability ahead of the supports.

 

It is the opinion of a number of geotechnical specialists who have visited Chinese LTCC faces, that the “typical” Chinese roof geology (above the coal seam) is more benign than Australia, with respect to these massive units, i.e. the Chinese stone roof is typically weaker and softer and more amenable to caving.  The effect of such differences is still the subject of investigation in the form of parametric numerical stress modeling.  It is speculated that the massive units may produce both benefits and problems on an LTCC face.  Benefits from the perspective of additional loading on the top coal above the supports as the massive units cantilever back into the goaf; and problems if the delayed caving also inhibits the coal from caving immediately.

 

High horizontal stress

On this issue also, there remains a need for further investigation.  The available data on pre-mining stress magnitudes and directions in Chinese mines has not, as yet, allowed quantitative comparisons.  However, visual evidence from underground inspections would indicate that the horizontal stress regime is less hostile in Chinese mines than many Australian mines, where ratios of between 2:1 and 3:1 are not uncommon (horizontal to vertical stress).

 

The consequences of such stress fields will obviously depend on many factors including face orientation, discontinuities, massive units etc. The concern is that high stresses could inhibit caving by locking the top coal together until after the face and rear AFC has passed.  Again, this is an area where further work is required.

 

 

FACE AND OPERATIONAL ISSUES

 

There are a number of operational issues that have already been alluded to above, such as face orientation, selection of mining horizon, etc. In addition to these, the operational areas considered important for successful Australian LTCC implementation relate primarily to the gate end area (face end support, equipment configurations and coal clearance); face ventilation (gas/dust management); caving management (support operation and dilution control); cutting sequences; and overall coal clearance systems (AFC capacities and compatibility with cutting and caving sequences, BSL, panel belts and outbye coal clearance systems).  There is ongoing research being conducted into the overall cutting, caving and clearance options in order to gain maximum productivity from the LTCC system.  One interesting option under consideration is the use of two panel belts – one in each of the maingate and tailgate – to separate the coal flow from the two AFCs.  This has obvious benefits and applications, at least in non-gassy mines, and particularly where the coal from the lower horizon may be of a different quality to that within the top caved coal horizon.

 

CONCLUSIONS

 

In summary, Australia certainly has extensive underground thick seam reserves that are well suited to the use of the LTCC mining method, particularly in the seam thickness range between 6m and 9m.  It is also clear that there is considerable experience to be gained from the impressive Chinese developments with this method and the results that have been obtained to date.

 

In terms of implementation within Australia, there appear to be no insurmountable impediments to the introduction of the method, although there are a number of operational, geotechnical and equipment issues that do require further investigation and development, as well as some site specific design issues.

 

ACKNOWLEDGEMENTS

 

The authors wish to acknowledge the financial support of various funding bodies for the work conducted to date (including ACARP, UNSW, ARC and Yankuang Group), as well as the co-operation and valuable input from their collaborative research partners (CMTE and CSIRO) and various industry representatives in both Australia and China.

 

 

 

 

 

 

 

 

 

 

REFERENCES

 

Hebblewhite B,   Technology and feasibility of potential underground thick seam mining methods.

Simonis A &        School of Mining Engineering, UNSW/CMTE ACARP Project C8009 Final Report Cai Y (2002)   UMRC 2/02.  ISBN  0 7334 1945 3.            

 

Kelly M,               Application of longwall top coal caving to Australian operations.  CSIRO

Wright B, Cai Y,                Exploration and Mining/School of Mining Engineering, UNSW ACARP Project

Hebblewhite B,   C11040 Final Report 1137F.

Onder U & Xu B

(2003)  

 

Hamilton N          Single pass thick seam longwall experience at West Wallsend            Colliery.  2nd Intl

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Hebblewhite B    Overview of Australian thick seam mining prospects. 2nd Intl Underground Coal (1999)                              Conf., UNSW, Sydney, Australia, 15-18 June, 1999, pp29-36. ISBN 1 876315 17 2.

                                                                                                        

 

Palarski J (1999)               Multi-slice longwalling with backfill.  2nd Intl Underground Coal Conf., UNSW,                               Sydney, Australia, 15-18 June, 1999, pp37-46. ISBN            1 876315 17 2.

 

Bassier R &         Application of paste fill in active longwalls and for stowage. 2nd Intl Underground

Mez W.                Coal Conf., UNSW, Sydney, Australia, 15-18 June, 1999, pp47-54.  

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Xu B (2001)        The longwall top coal caving method for maximizing recovery at Dongtan Mine.                                       3rd Intl Underground Coal Conf., UNSW, Sydney, Australia, 12 - 15 June, 2001,                        unpag. ISBN 0 7334 1812 0.