An innovative approach backed by extensive experience and expertise allowed Murray & Roberts Cementation to provide a cost-effective solution for the equipping of a backfill hole at Cullinan Diamond Mine.
"The scope of work was to equip a raisebored hole measuring 1.8 metres in diameter to a depth of 730 metres below surface. We completed the raiseboring in September last year - the sole purpose of the hole was to allow underground access for backfill aggregate," Neil Lane, engineering service executive, Murray & Roberts Cementation, explains.
"In itself, the request to raisebore a hole to accommodate backfill pipes was unusual, but it proved to be a far more economical solution than installations in traditional shafts," he says.
Murray & Roberts Cementation has a well established track record for raiseboring and completed the hole to its final depth safely and cost-effectively. Use of its RVDS technology ensured that the hole was drilled to a deviation of only 0.01% over its depth of 730 metres. It was considered essential that the hole be straight, as the backfill pipe column is essentially free hanging.
"Traditionally, we would require human access to equip a hole with backfill pipes. This necessitates the provision of some in-shaft infrastructure. Firstly, this is an expensive exercise and secondly, this shaft has a diameter of only 1.8 metres, which doesn’t give one much room to move," James Collins, engineering manager for Murray & Roberts Cementation, says.
In the traditional method, support structures are established at the bottom of the shaft, and the pipes are installed moving upwards.
The job entailed installation of two 150 NB schedule 160 high pressure backfill pipes in the hole.
"We came up with a new method of installation whereby the pipes were installed from the surface and lowered by means of a hoisting system," Collins says.
Pipes were supplied in 9.144 metre lengths with a screw-on configuration with a double diameter double start, which supplies increased torque ability on the thread. The torque allows sufficient pre-tension in the joint to carry the required loads. A total of 77 pipes per column were installed.
A number of innovative engineering methods were implemented to accomplish this job. Firstly, a purpose designed cranage system was engineered to accomplish the top down installation. The cranage was installed directly above the raisebore hole and collar support steel.
"We clamped the first pipe into position using a purpose designed hydraulic torque tool and continued the process until all 77 pipes had been installed for the first column. We then repeated the procedure for the second column. Our hydraulic torque tool ensured the effectiveness of each joint," Collins says.
Anchor steel will be installed at the shaft bottom to facilitate tensioning of the pipes between the bottom and the top support steel. "The backfill pipe columns are free hanging, and tensioning was essential to ensure that oscillation is dampened," Collins says.
The top support steel, which carries the greater load, comprised 1.1. metre deep plate girders with a rating of 900 kg/metre. "We used fairly substantial sections to facilitate the most appropriate support for the backfill pipe columns and to minimise deflection after we had tensioned the columns," Collins points out.
It was innovative thinking and experience that ensured an appropriate solution, particularly in terms of handling such large lengths and be able to rotate and lower the pipes simultaneously under controlled conditions. At the end of the screwing function, it was also important to be able to apply the correct torque to a 150 NB pipe.
These systems were developed at the Murray & Roberts Cementation’s Bentley Park workshops.
"Use of this method of equipping the raisebored hole with backfill pipe columns guaranteed that the job cost a fraction of the conventional method. We also achieved significant time savings and ensured safety," Lane comments.
"This has never been done before. After successfully completing the contract at Cullinan during the first quarter of 2006, we were awarded a contract at Black Mountain using the same technique," he says.
The installation crew at Cullinan comprised six people and the entire contract was completed without a single lost time injury.
"This is a good example of solving a complex problem with a simple yet foolproof solution," Lane concludes.