The BLS were originally pioneered in the RSA at Middlebult Colliery by Voest-Alpine commencing in 1984 and later trialled in several Australian collieries commencing in 1987. Their subsequent use in pillar extraction panels has become widespread with some 60 units in operation by 1991.
A BLS units is illustrated to the right. They are mobile electro-hydraulic self contained chock units. When set to the roof they act as a hydraulic four leg chock but as the base is fitted with caterpillar tracks, when lowered, the units are able to readily tram forward to a new position. Each unit has its own electrically powered hydraulic power pack to provide for all hydraulic functions. Each unit is supplied with electric power through a trailing cable and the operations are controlled by radio remote control so that the operators are able to lower, tram and reset the units from a safe location removed from the goaf edge.
The BLS units perform exactly the same function as the breaker props but do so with the following advantages.
They are able to offer greater positive setting loads and roof support on the goaf edge than can be achieved normally with reasonable quantities of timber props.
They create a cost saving in timber usage to defray their capital cost.
As a consequence of reduced timber usage they can contribute to reduced accidents associated with the transport, handling and use of timber.
They may be able to be reset to the new breaker line position as extraction takes place faster than timber breaker lines can be set.
This increases potential continuous miner cutting time and productivity.
Through remote control they remove the active goaf edge area personnel who would be required otherwise to set the breaker-line props thus further improving safety.
They may also have application in some partial
extraction methods to allow wider headings to be worked
on the retreat by providing temporary roof support
across the wide headings or “rooms”.