er advance mining are low risk and consistently high output. However, there are factors, which limit the application of retreat mining. The most important of which is the development of high stress [24] levels due to the influence of nearby workings, which affect the stability [25] of development roadways in soft strata. The life of the coalface depends upon the life of the roadway gate support. Reinforcement [26] techniques are available to assist in stabilizing the mine roadways.A. A retreat system with a single entry: this system is similar to the advance system with one entry, except that the panel is fully developed before extraction starts. There is a problem of roadway maintenance near the gob.
This method has the advantages of economical use roadways and the efficient recovery of coal reserves. The mining direction is either down-dip [27] or along strike [28]. The disadvantages of the system are that the developed roadways in solid coal are liable to interaction from neighboring workings in the same seam: and the panel in extraction must be mined-out before the next one can start to avoid short circuiting ventilation.
B. Integrated advance and retreat system [29]: this system is used mostly in deeper and gaseous coalmines. Single entry is used resulting in limited development and easier face-end [30] operations. Alternate faces advance in opposite directions. This method, as in other single entry longwall mining methods, re-uses the roadway of the mined-out panel for extraction of the adjacent panel. In some countries, integrated single entry system has been used to control surface subsidence strains.
C. The retreat ��Z�� system [31]: this system has been used in coalmines to retreat the starting panel. One roadway is developed either the maingate [32] or tailgate [33] before the coalface commences production.
This system is applicable in weak friable strata because one roadway is formed behind the face-line and development work is reduced by 50%. The retreating maingate may use a bleeder roadway [34] where high methane emissions [35] in the face and gob are expected. The advancing main roadway may be used again for the next face. A disadvantage is that advancing of the roadway is a limiting factor in rapid face advance.
D. The retreat system with double entries: this system is the most commonly used longwall mining method in China where strata conditions are favorable. It is similar to the longwall advance system with double entries.
Ideally, work in the first retreat face should be finished before the headings for the next adjacent retreat face are driven to avoid roof control difficulties in the headings when they advance within 70m of the retreat face. This is not generally possible because of the need to have a replacement face ready to take over immediately each retreat face finishes.
Where double entry development headings [36] are driven by crosscuts, the outer headin
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