SHAFTS, CAVERNS - BTS HARDING PRIZE COMPETITION | TECHNICAL
Movement of the rig Left:
Photo showing (circled red) the area of the rig that was pulling inwards Middle:
Ovalising of the rig was measured
Right (top, bottom): Revised support solution to help counter ovalising
The most productive shift achieved 13 wagons, nearly 100m3, due to reliable concrete supply and a simpler reinforcement arrangement. Steel fixers installed around six tonnes of reinforcement per shift, contributing to a total of 350 tonnes of conventional reinforcement fixed during the full lining. Carpenters worked from the top deck to install cast-in items and cleaned the shutter with scrapers, a critical activity to maintain a high-quality concrete finish. Any hardened concrete left on the shutter risked scouring through fresh concrete, resulting in avoidable repairs. During the initial jacking of the rig, once the shutter
had been filled by 800mm, premature movement up the shaft caused a minor spillage as the concrete at the base had not yet achieved sufficient early strength, Figure 5. This was attributed to slower curing than indicated by site trials. I monitored for leaks during the initial jacking at pit bottom and instructed the operator to delay jacking until curing was complete. On the subsequent attempt, the concrete had set adequately, and the slipform progressed without incident. Concrete ordering and coordination were managed
through a WhatsApp group, providing visibility to surface teams, shaft operatives, and the following shift. This ensured all information was in one location, including slump test results, which allowed adjustments to be made to achieve the desired consistency. While on the rig, I pre-warned operatives about load consistency and advised them of wagon arrival times, supported by Tarmac’s GPS tracker. This system proved valuable during delays, enabling the slipform supervisor to adjust jacking speed accordingly. Depending on supply frequency and site progress, the section manager controlled the addition of retarders to improve finish quality and reduce the need for repairs. Concrete testers measured every wagon upon arrival.
Slump values were targeted to within the range of 140mm to 230mm, and strike cubes were taken every 50m3. If a load failed the slump test, the procedure was to allow it to stand for 30 minutes before retesting.
Loads failing a second test were rejected. This rigorous quality control ensured that all concrete poured exceeded the specified cube strength of 50 MPa. When higher slump concrete loads arrived on site,
the material required longer to achieve initial set. As a result, the rate of rise had to be carefully managed to prevent bulging at the base of the shutter. Conversely, concrete that was too dry produced a poorer surface finish. Achieving the correct balance between consistency and rate of rise was therefore critical. For the fibre reinforced mix CM4a, the typical rise rate was maintained at approximately 200mm/hr, which provided sufficient clearance at the top of the shutter while allowing adequate curing time. The slipform supervisor controlled this by jacking the rig in 50mm increments every 15 minutes, with the resulting layers illustrated in
April 2026 | 23
– Left:
Close-up of the bracket to support the climbing tubes
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