Jennmar Australia now offers the Big John Mine Prop for use as standing support. The Big John can be configured to be a stiff or yielding system depending on mine requirements.
The Big John is currently available in 70 and 90 tonne capacity. If a yielding system is desired, the prop can be configured to yield from 0 to 35 tonnes. Total deflection during the yield process can also be set to mine requirements.
The concept is simple in design, with an inner pipe sliding inside an outer pipe creating a telescoping effect that allows for extension of the prop from a collapsed position used for transport into the mine. The Big John prop is adjustable from 300 to 600 mm.
Each prop is fitted with an end plate, designed for optimum mine performance, to distribute the load to the mine roof and floor. The top and bottom plate can be custom designed for mine requirements.
The detached swivel top and bottom plate design allows for adjustment to uneven floor and roof conditions including heave. The prop can be extended to the roof height with the aid of a setting tool.
Controlled yielding is provided by a split piece of pipe that is clamped to the smaller diameter section of the prop. The nuts on the machined clamp section are torqued to provide a clamping force.
It is the clamping force and clamp configuration that provides the controlled yielding of the prop as the split section of pipe slides under frictional contact along the smaller bottom prop section.
Two tests were conducted on 2.1 m JM Big John Yield Props. In
the first test the steel end plates of the prop itself established
the roof and floor contact. In the second test, three 125 x 150 x 750 mm oak crib timbers were used to establish roof and floor contact.
A comparison between the two tests is shown on the left. It is seen from this figure that the oak timbers had little effect on the performance of the prop. Both props began to yield at about 27 tonnes with a gradual increase until the outer pipe contacted the floor plate at about 400 mm of convergence.
The buckling load on the prop with timbers was slightly less (66 tonnes) than the prop with only the steel endplates (72 tonnes). The oak wood was deformed only slightly from the prop buckling load, resulting in only mild distortion of the steel endplates and a fairly uniform loading of the prop.