INFRASET PERFECTS BRATTICE WALL BEAM MANUFACTURING:

The advantage of the brattice wall concept is the cost savings that accrue from sinking one instead of two shafts on the same mine. The wall divides an oval-shaped and slightly larger than normal shaft into two sections, the larger of which is used for moving men and materials into and out of the mine, while the other is used to expel hot air from underground.

Four brattice walls have been built to date using Infraset’s precast concrete beams, two by Grinaker-LTA Underground Mining Contracting at the Zandfontein and Frischgewaagd mines in the Secunda area, and two by Shaft Sinkers, also in the Secunda region.
 
Manufactured at Infraset’s Brakpan plant, the beams contained reinforcing cages of high tensile steel weighing from 270kg to 350kg each. Once cast, the beams were subjected to 12 hours of steaming, a process which speeds curing to a point where 90% of the final 45MPa rating is achieved within seven days of casting.

“We cut our teeth on the Zandfontein project which was completed two years ago,” said Giovanni Vasta, sales and marketing manager, Infraset Infrastructural Products. “Due to the considerable air pressure variance between the two sections of the main shaft, accuracy in beam casting was crucial to keeping both sections of the shaft air-tight and discrete entities,” said Vasta.

CMA director, John Cairns, says using precast beams as opposed to building the brattice wall in-situ offers several advantages , the major one being the speed of placement of the prefabricated units, as opposed to slip-forming or in-situ casting of the dividing wall.

Both Zandfontein and Frischgewaagd were completed within the required, fairly tight, schedules.

One hundred and ten beams measuring 9.2m x 1.2m x 0.25m thick were lowered into Zandfontein’s 132m deep shaft at an angle. This was achieved with the aid of M24 sockets which were secured to the reinforcing cages before casting. In the Frischgewaagd project, which was completed in July this year, 174 beams were used to build a brattice wall 208m high. 

Once in their horizontal resting position, the beams were held secure and grouted into two vertical supporting chases which ran from top to bottom on both sides of the shaft. And, to avoid excessive weight being applied to the lower beams every fifth beam was used as a bearer beam. These rested on special supports cast into both chases at six meter intervals. This process necessitated a high degree accuracy, as an additional 5mm in beam height can cause “creep” of half meter over a total height of 150m.

On the three subsequent projects the beams were cast at a slightly lower height of 1.195m because the Zandfontein project demonstrated that it is much easier to compensate for under rather than over-sizing. This was achieved by inserting a 5mm plate in the moulds.

Vasta said it would have been prohibitively expensive to cast beams of these dimensions with a much higher degree of accuracy, and normal casting is affected by continually varying ambient temperatures.

Picture shows the Zandfontien coal mine where 110 brattice wall beams measuring 9.2m x 1.2m x 0.25m and weighing seven tons, were lowered into Zandfontein’s 132m deep shaft.