Digging for victory

Getting ready to welcome our tunnel boring machine in Paull

When the tunnel drive under the Humber came to an end in 2019, our Tunnel Boring Machine (TBM) made its way to a specially made shaft in Paull, on the North Bank of the Humber. The shaft is a 13.5 m deep and 15 m wide sophisticated hole in the ground, and when it was completed a critical milestone of the project was achieved.

The story of this shaft started in the summer of 2018 when the its walls were pre-installed into the ground. This was done by a machine known as a piling rig, which installed 86 secant piles. The rig used a drilling bit known as a corkscrew to excavate the ground, making room for a new pile to be poured.? The piles are 20 m deep, which is deeper than the shaft. This depth difference is to anchor the piles into the ground and prevent the walls of the shaft collapsing once complete.

A month after the piles were poured the top metre of the shaft was excavated and all the piles were tied together with a newly poured concrete ring, or capping beam. The ring acts like a rubber band around a bunch of match sticks, stopping the piles from failing in.

One further step was taken before excavation of the shaft began. The UK is blessed with its water resources, however this water, more specifically groundwater, brings challenges. In the case of the shaft, if the groundwater was not controlled it would flood and collapse the its walls. To prevent this from happening a specially engineered dewatering system was put into place. Only when there was certainty that the system worked did excavation of the shaft finally commence.

The digging of the shaft was carried out by an excavating machine, while the soil was carried out of the shaft by a 150 tonne crawler crane. Working together, the two machines navigated through clay, ancient river bed, sand, glacial till and peat, with skill, speed and great care. Once the desired depth was reached, the excavator was lifted out of the shaft and concrete was poured in to form the base slab.

Excavation of the shaft

This base slab was key to the completion of the shaft. Without it the water and earth from the surrounding ground would rush into the shaft once the dewatering system is shutdown. The slab must be heavy enough to prevent the water pressure from pushing the shaft up, but light enough not to sink the shaft. The best way to explain it is putting a cork in a water vat, if the cork is too heavy it will sink, if it’s too light is will fall on its side, but if it’s perfectly weighted the top of the cork will float just on top of the water.

Operatives are busy at work tying together the steel bars to create the reinforcement for the base slab

Finally, to prepare the shaft for the arrival of the Tunnel boring machine, a specially designed concrete eye was built. The eye was where the TBM entered the shaft and it was designed to safely guide the machine in and stop the earth from outside the shaft from entering it. At the base of the slab a cradle was poured to be the TBM’s final resting place before it was dismantled and sent back to Germany.