Following extensive development work, Murray & Roberts Cementation’s team of engineers has unveiled its innovative pre-sink gantry. This 21st century technology was successfully implemented at the Venetia Project.

This unique shaft sinking methodology is engineered to deliver optimal safe working conditions and comprises a single rail mounted gantry which combines the stage and kibble hoists as well as the blast barricade. The stage is suspended from the gantry on steel wire ropes attached to two 8t stage winders on purpose-built platforms to the sides of the main girders.

Significantly contributing to enhanced safety and productivity, man and material loading is handled on one side of the gantry with waste rock being dumped from the other side. “This is achieved with the gantry traversing between these two points,” says Allan Widlake, business development director at Murray & Roberts Cementation.

Main hoist

The main hoist of the gantry, used for kibble hoisting and slinging, was custom engineered to allow a pre-sink of up to 80 metres below the collar elevation. On the Venetia Project an actual depth of 60 metres below collar elevation was sunk.

The hoist is able to raise and lower a kibble with a 10t payload at a conveyance speed of 0.5m/s. The gantry system incorporates an automatic tipping frame. The kibble is slewed into its docking position where it is automatically positioned and hooked onto the frame. By lowering the hoist, the kibble’s payload is discharged into a truck waiting below.

Allan says that this system significantly reduces risk thereby enhancing safe working while reducing tipping cycle times resulting in increased productivity.

The height of the gantry structure is matched to the height of the stage which allows the stage to clear the collar when raised to its upper limit. Once the stage has been raised in this upper position, the long travel wheel drive motors are energised to move the gantry, complete with suspended stage, away from the shaft. The blast barricade is then drawn over the excavation which effectively prevents fly rock from leaving the shaft barrel during blasting.

First and second shaft positions

After blasting and clearing the shaft of the blast fumes by means of forced ventilation, the gantry rolls back to its position over the shaft, and the fully-equipped stage is automatically aligned and positioned using a fully-integrated PLC, and then lowered back into the shaft to the required depth.

An innovation which further facilitates productivity applies to projects where more than one shaft needs to be sunk. The pre-sink gantry offers the ability to pull itself along the rails between the first and second shaft positions.

“Being able to rapidly move from the one shaft to the second during the pre-sink phase offers major time saving advantages and further reduces risk,” Allan adds. Traditionally, set up for a pre-sink can take between one to three months but with this innovative technology it’s now possible to do so in as little as two to three days.

Pneumatic air leg

Further innovations on the pre-sink stage include integral mechanised drilling systems. This consists of six vertical drill rigs supported on swivel arms suspended under the stage. Each operator guides the drill and manoeuvres it to match the pattern of holes required for the blast. An inline pneumatic air leg on the rock drill extends to create the necessary thrust between the stage and the floor and for drill retraction after drilling is completed.

“This technology has reduced the physical effort involved in the drilling operation and most importantly there are no longer any manual drill operators in the shaft bottom.”

Shuttering system

The shaft lining process has also been simplified. A proprietary shuttering system is now suspended from the sinking stage. The shutter depth is 6 metres and after each 6 metre excavation the shutter is positioned 12 to 18 metres above the shaft bottom. This means that the shutter can be left in position during drilling and blasting operations.

“Importantly, the pre-sink gantry was not engineered for a single project and is able to handle shaft pre-sinks from 6 metre diameters up to a large 10 metre diameter shaft,” Allan concludes.

Murray & Roberts Cementation

Allan Widlake, Tel: (011) 201-5000