Mponeng Mine - shaft deepening using BASF Construction Chemicals SA admixtures and equipment
The challenge
At 3.461km, Mponeng is one of the deepest mines in the world. With temperatures reaching 60 degrees at the rockface, ventilation is critical to safety and sustained productivity. The Mponeng main shaft, which reaches to 85 level (some 2,300 metres below surface) was commissioned in 1986 and the sub-shaft in 1993. More recently, the shaft deepening project down to 123 level was undertaken to access the Ventersdorp Contact Reef (VCR) to extend the life of the mine by three years.
The project involves the deepening of the sub-shaft system and the development of access tunnels to the VCR horizon on 113, 116 and 120 levels (from 3,172 metres to 3,372 metres below surface). However, due to the inadequate air volume available to enable multi-blast development and a shortfall in the up-cast capacity, the deepening of the sub-ventilation shaft (SSV shaft) from 110 to 120 levels became necessary.
Explains Christo Delport, Project Engineer at Mponeng: “In the feasibility study we planned to use sub-shaft 1 (SS1#) as a down cast shaft and the SS2# as an up cast shaft. SS2# was, however, needed to transport men and material as the SS1# alone could not service the project and future mining from 113, 116 and 120 levels. Thus, multi-blasting could not be done, threatening to derail our development targets to reach the reef on certain dates.”
Booster fans were installed on 110 level with ventilation columns in the SS2# shaft in order to enable limited multi blast on 113,116 and 120 levels on the west side in the short term. However, these ventilation columns would need to be removed to facilitate the installation of 35 backfill ranges, resulting in a 163m3/s up-cast ventilation deficit. “A cost effective long term solution was needed that would maximise current resources and at the same time position the mine to exploit the reserves below its 120 level,” says Delport, “and a decision was taken to deepen the current 7,2m diameter SSV by 302m from 110 level down to 120 level. This had to be done without affecting the current multi-blast conditions and the production and various methodologies were considered.”
The solution
Mponeng management decided to make use of an Alimak Raise Climber to establish the centre hole and then to slipe and support the 7,2m diameter shaft from the top. The overall motivation for using Alimak technology was that it enabled the temporary support of the pilot hole, maintaining its integrity throughout the slipe and support phase.
A critical part of the project was strengthening and supporting the shaft walls. Time, cost and maintaining productivity were primary considerations. The admixtures and equipment presented to by BASF Construction Chemicals SA (previously Degussa) ensured success in difficult conditions.
Explains Robert Francke, client service and technical consultant at BASF Construction Chemicals SA: “A specially designed mix of wet concrete was pumped down a 65mm column from a platform to a maximum depth 160m for spraying on the shaft walls. The mix design consist of a reinforcing fibre, super plasticizer to enhance flowability and an accelerator (constituting 6% of the mix) to control and speed drying/setting times as needed. This enabled a team working on the Alimak, a lightweight hoist that could fit three people, to spray-crete an area of approximately 1-3 meters a day.
Unique enablers
“While the technique is not new, this is the first time that wet concrete has been used with a pump system to 160m,” says Francke. “The admixtures -- Glenium 27CH, Hyperplasticer; Delvocrete Stabiliser; Meyco SA 160, Alkali Free Accelerator; and Meyco Fib 540 HPP fibres -- were tested and found to ensure the proposed methodology worked.”
Delvocrete Stabiliser controls the dynamics of cement hydration by suspending the hydration process and re-activating it hours or even days later as needed without loss of quality of the hardened concrete. To reactivate concrete mixes stabilised with Delvocrete Stabiliser and to accelerate the hydration process, Meyco SA 160 is added at the spraying nozzle.
Glenium 27CH, a hyperplasticiser with a powerful dispensing effect is able to provide extended open times for wet mix sprayed concrete, especially for underground construction works where high early and final strengths are required. It is ideal for sprayed concrete linings, the extended open time to allowing for transportation and placing of the sprayed concrete even hours after batching. It also provides improved pumpability due to increased cohesion in the concrete mix, and reduces wear and tear on the pump. In addition, greater durability results from the improved mechanical properties in the concrete.
The Meyco FIB SP 530/540/550 and SP 640/650/665 fibres are user friendly and easy to dose into concrete mixes, reinforcing the concrete and providing toughness and ductility to the material.
Says Francke: “Previously, such a task would be done by blasting; drilling; lining the shaft with steel rings; pouring the concrete into the rings using a kibble, valve and hose system; and waiting approximately 8-24 hours for the cement to set and dry sufficiently before the next blasting session could take place. The logistics and length of time needed to complete the task in this manner made it unfeasible.
Outcomes
“BASF CC SA presented Mponeng with a proof of concept demonstration which resulted in acceptance, the purchase of the necessary equipment and admixtures. The task was completed in three phases of 100m each over a period of a year. The shaft was sealed as each 100m section was achieved, allowing ventilation to be pushed down to the new level.”
The project was completed in 12 months using BASF CC SA’s admixtures and equipment.
Says Delport: “Quality work was done in completion of this project. The methodology and materials used ensure the shaft is safe and will remain open for the life of the mine.”