Advancing and Retreating Longwalls

Advancing Longwalls

In this method, the face start point is close to the main headings, usually leaving a barrier pillar to protect them. Once the face equipment is installed, extraction commences working away from the main headings towards the block limit. Obviously the main and tailgates do not exist prior to the start of extraction and have to be formed at each end of the face as mining progresses. The gate roads are effectively in the goaf and a false rib has to be installed on one side, usually by constructing a small pillar, sometimes using stone cut from the roof in thin seams or using some type of cementitious material brought into the mine. Such gate roads tend to require a very heavy support system (yielding steel arches have often been used).

Principle of advancing longwall

Advancing longwalls were once common in Europe in relatively thin seams where packs were constructed using stone, which had to be cut in order to produce sufficient height for the gate roads, and sometimes using coal fines which were not very marketable at one time.
Usually a pillar of coal referred to as a "chain pillar" would be left between adjacent longwall blocks, wide enough to remain intact when carrying the load between two goaves and protect the gate road. Occasionally two longwalls would be operated simultaneously, one each side of a shared maingate (in this case referred to as a "mother gate").

Retreating Longwalls

In this method, the gate roads are first driven from the main headings to the block limits and then connected with a roadway to install the face equipment. The gate roads may be connected to another set of roadways at that point for ventilation/gas control purposes. Once the face equipment is installed, production commences with the face retreating from the limit back towards the main headings, usually to finish at a position so that a barrier pillar is left to protect the latter headings.

Principle of retreating longwall

Because the gate roads are long, it is normally necessary to drive at least two (sometimes more) at each side of the block. That set of roadways which will be used for access onto the longwall and for coal clearance off the longwall are typically called the "maingate roadways" whilst the other roadway or set of roadways is typically referred to as the "tailgate roadways". The latter are used for primary access on occasions, but this is not generally the case.
As the face retreats, the roadways forming the face ends are destroyed and become part of the goaf. The other roadways will remain open if adequately supported and it is common practice for one of the remaining roads at the maingate end (usually only one in any case) to become the tailgate of the next block. The gate road first working pillars then become the chain pillars between the blocks.

Comparison of Advancing and Retreating Longwalls

The advantages of retreating longwalls compared to advancing are:

• Gate road formation is remote from face operations (less congestion at face ends, less supplies into longwall face area, face not held up waiting for gate road preparation or vice versa, no problems of dust production from gate road workings affecting longwall personnel).
  
• No gate or roadway side packs required, so less supplies overall
  
• Longwall block is surrounded by roadways before the longwall starts so knowledge of strata conditions is much better
  
• Gas drainage of adjacent blocks can be carried out starting during development; with longwall advancing the drilling can only be done behind the face after longwall extraction, allowing less drainage time before the next block commences production
  
• With retreat longwall mining, additional gateroads or bleeder roadways behind the goaf area can be developed for ventilation by the development unit if required. Such additional roadways are much more difficult to mine with an advancing longwall
  
• There are more options for ventilation/gas control using additional roadways at the limit of the block
  
• Advancing longwall gateroads typically require extensive maintenance to maintain the roadway cross-section (roof and floor brushing) during the life of the longwall block, whereas retreat longwall gateroads are allowed to collapse behind the retreating face
  

The only real advantages of advancing longwalls are:

• Production can begin earlier as the mine does not have to wait for the gate roads to be developed before longwall production can commence (provided development rates are adequate this should only apply for the first longwall in a mine)
  
• It provides an opportunity for disposal of stone which has to be excavated into gate side packs (this benefit is probably more than offset by the costs involved in pack construction)
  

As far as the writer is aware, only one advancing longwall has been worked in Australia, a hand worked face at Stockton Borehole Colliery operated from the late 1890's to the mid 1950's.