CROSSRAIL EASTERN RUNNING TUNNELS

In March 2015 PJ Davidson was engaged on the Crossrail C305 project to slipform the track sub-bed and walkways of the Eastern Running Tunnels under London. The challenges of a total of 72km of slipforming concrete in over 12km of twin-bored tunnel were tackled head on by the team in an innovative way, drawing on their existing tunnel experience.

Prior to starting the project, PJ Davidson undertook a number of trials above ground, to identify the most suitable way to optimise the operation below ground. This involved different concrete mixes, alternative ways to ensure the concrete adhered to the tunnel segments, as well as plant configuration options to reduce health and safety risks to an absolute minimum.

Using two of our own Mobile Concrete Batching Plants to service the materials production requirements, one was set up at Limmo Peninsula and the other at Stepney Green. Our self-owned concrete plants gave us the capacity to produce over 1,400m³ of concrete in a 24-hour shift on this project. The plants were within the project working areas and ensured we could avoid potential delays associated with concrete supplies from external ready-mix plants.

We developed an innovative approach to delivering the concrete to the work face. The concrete was batched adjacent to the access shaft in a towed mixer barrel. The barrel was then lowered into the shaft by the gantry crane and landed on a bespoke cradle. Here it was able to be inspected, and the consistency altered to suit the slipforming requirements, before being discharged into one of our fleet of tunnel vehicles to be transported to the paving gangs.

Through our previous experience of concrete construction within tunnels, we had developed plant solutions to concrete supply and placement at the rate required by the paving team. The narrow concrete delivery wagons in our fleet were designed to be able to pass each other within the tunnel bore at designated raised passing points (California Bridges). Their unique side-cab design with lockable steering axles offered a safe way for the concrete to be delivered from the batcher to the work face and back again. The logistics of the concrete supply process were managed from start to finish by P J Davidson.

With the concrete delivered to the paver, the specially adapted paving machine was able to pave the material to the required specification, guided by the Leica 3-D stringless guidance system, to control line and level.

Due to the automation, the knowledge of slipforming concrete and the innovative approach to the project, PJ Davidson was able to complete the works with no reportable accidents or incidents, at a rate that exceeded the preceding works, ensuring that the tunnel was handed back for follow on activities, without delay. The rate of progress achieved by a small team ensured that production was achieved with an efficient size of gang which remained consistent throughout the works, with minimal staff turnover, despite such an intense working environment.

The quality afforded by this process ensured that the work was handed back defect free. In addition, since there was no need for temporary materials, this was a more sustainable solution than alternative construction methods employed elsewhere within the project.