Woodsmith Mine

We were initially responsible for the first -120m of three foreshafts; excavating over 90,000m3 , pouring 45,000m3 of concrete and using 4,000t of reinforcement. Thorough research and planning allowed us to implement various techniques and procedures to achieving this as quickly, efficiently and safely as possible. Our various excavation techniques and equipment were modified and deployed to suit the specific requirements of the topography, timeline and conditions, such as the hydrogeologists’ specification that the earth was to remain dry to a depth of -120m. We created sophisticated 3D, 4D and VR simulations to examine a wide range of variables and hone the slipform technique. The technique not only saved time and costs but also increased worker safety by avoiding dangerous working conditions in any unfinished shaft.

The material transport system (MTS) shaft, which is designed to launch the tunnel boring machines (TBMs) that will drive one section of the 37km tunnel to transport the materials to the processing plant at Wilton, was excavated to -115m using a Herrenknecht vertical shaft sinking machine (VSM) and a

slipform secondary lining. This was the first use of this method and machinery in the UK and we broke the world record for the deepest shaft sunk using a VSM.

The slipform rig was set up near ground level, adding the top section of the shaft gradually over 12-hour shifts. The weight of the constructed section was hung by 9 sets of hydraulically controlled strands, with the rate of descent controlled until it reached -120m below ground. By using this system to continuously pour the concrete walls, simultaneously with the VSM’s excavation activity, we were able to create the MTS shaft more safely, rapidly and cost-effectively.

The service shaft was excavated to -120m and was sunk using a more traditional method with excavators, Brokks, concrete precast segments and underpinning. It was then lined from -120m to -48m using conventional sinking segmental lining with a slipform secondary lining. We created the first 52.7m – a depth equal to the height of a 12-storey building – in only 15 weeks. The production shaft was excavated to -120m, also using a traditional methodology. The inner shaft excavation was carried out remotely using a Brokk and a 5t excavator; the material was then loaded into a 4m3 skip which was removed to a muck bay at the surface using a 250t crane. At the -45m level, we installed 6 piles in both the production and the service shaft.

To de-risk the excavation programme for the construction of both the service and production shafts, we used telescopic excavators rather than the more customary crane and skips. We also used a 40m teledipper – the largest of its kind ever used in the UK to date – to ensure that our programme would not be negatively impacted by adverse weather. We also planned round the clock shifts and worked continuously over the Christmas and New Year periods to expedite delivery.