Groundforce Shorco and sister division Piletec have come together to provide a turnkey ground support package on a mains water diversion project in the UK Midlands. 

The project involved rerouting the strategic water main under a new railway line, currently under construction, at Lavender Hall Farm at Berkswell, near Coventry.

Contractor J Murphy & Sons is carrying out the complex operation for customer Severn Trent, which has involved eight different designs that overlap to allow the installation of the pipework and UTX (under track crossing) chamber.

Installing 110m of 1,200mm concrete sleeves

Murphy is installing 110m of 1,200mm concrete sleeves in a 6m-wide, 6m-deep open trench under a railway line to allow for future maintenance and removal without causing disruption to the railway line operations.

“The new pipelines run parallel to the existing water main about 30m to the south. But they need to be deeper to allow the railway to be built over the top,” said Murphy’s project manager Sam Woodyatt.

The length of UTX concrete sleeve being installed under the railway is 110m. The length of each water main diversion is 220m. Support for the 6m-deep trench is provided by Groundforce Shorco’s rolling strut trench boxes extended to their full width of approximately 6.3m.

Deployed in ‘leap-frog’ fashion

The trench boxes avoid the need to instal steel sheet piles along the full length of the excavation. Instead, Murphy is using 11 rolling strut boxes which are deployed progressively, in ‘leap-frog’ fashion.

As work progresses along the route of the pipeline, Murphy backfills the trench as it goes, removing each trench box in turn from the rear of the excavation and reinstalling it at the forward end.

Groundforce Shorco trench boxes are a popular choice for pipeline installation in relatively shallow trenches due to the speed and simplicity of installation. Rolling strut boxes differ from conventional fixed-strut boxes in that the rolling struts enable both plates to move independently of each other during installation and removal.

This has the benefit that during the installation process, parallel plate movement vastly reduces the potential for horizontal soil movements compared to more conventional box strutting systems where the installation process involves the shifting or tilting of the box plates, causing the width of the trench to vary which in turn allows the soil to move.

Construction of a 12m-deep in situ concrete chamber

While excavation of the trench continues, Murphy is using Groundforce Shorco’s MP150 hydraulic struts to support the UTX chamber excavation. This excavation will facilitate construction of a 12m-deep in situ concrete chamber which will house valves and washout equipment essential to allow Severn Trent Water to maintain the water main.

Measuring 15.2m x 13m x 9.5m deep, this excavation is lined with 10m-long Larssen sheet piles and supported by Mega Brace hydraulic waling beams and four MP150 modular hydraulic props. The props are installed as knee-braces, spanning the corners of the excavation and braced against each other in the middle of the waling beams.

This excavation comprises two rectangular bays adjacent to each other and braced at the interface with Mega Brace units.

The main challenge was the poor ground conditions, with landfill over bedrock and the uncertain location of underground services, including the existing water main. There are also overhead power cables and a large embankment that cannot be disturbed due to the risk of undermining structures built on top of it.

Therefore, instead of pre-driving sheets to full depth, Groundforce Shorco proposed using a 'dig-and-drive' method whereby sheets are driven a short distance into the ground which is then excavated in stages to allow the identification of buried obstacles before the sheet piles encounter them.

In each excavation, large services must be avoided, and so Murphy has installed Groundforce Shorco’s End Closure Panels above the services to prevent any soil entering these locations.

Murphy also employed Piletec’s new side-grip excavator-mounted vibrating piling hammer to install the sheet piles, with Piletec providing on-site training for the excavator operators in how to use the new device.

The great advantage of this new hammer is that it enables the use of sheet piles that are longer than the reach of the excavator. The hammer’s linear side-clamping system also results in a more efficient clamping load transfer.

With the UTX chamber now fully supported and progress on the trench excavation progressing quickly, Murphy will soon be ready to excavate the second UTX chamber on the opposite side of the proposed railway line where Groundforce Shorco also proposes to provide structural support.

Another large excavation

The new pipelines will then be reconnected to the existing water main on the western side of the railway inside another large excavation, known as the UPT chamber.

“The water makes a 90° turn here where it meets the existing pipeline and we have to instal what’s known as a thrust-block. This will resist the force imposed by the volume of water turning that corner otherwise the pipeline will fail,” said Sam Woodyatt.

Groundforce Shorco was involved at an early stage in the design process, allowing them to fully assist with solving the challenges of the project.

“Groundforce Shorco were very helpful. They turned designs around very quickly and I was in direct contact with Zoe [Holmes, design engineer] at all times,” said Woodyatt.

“They were instrumental in helping us to minimise the amount of material excavated. We are on a landfill site and all arisings must be disposed of in a licensed tip, which is very expensive and less environmentally friendly.”