Groundforce Shorco supplied a range of specialised equipment to support a complex excavation during work at the Ynys Y Fro reservoir near Newport, Gwent.
Contractor Mott Macdonald Bentley (MMB), a member of Welsh Water’s Capital Delivery Alliance constructed a new spillway at the reservoir.
The project involved a large, irregularly shaped excavation which is open at either end and required lateral support throughout its length.
Having worked with Groundforce previously, MMB contacted the company with a request for a design solution. Groundforce were supplied with the Autocad drawing showing the section for the new spillway installation and were asked to give 1.0m clearance along each side.
Due to the awkward shape of the excavation it was decided to split this into bays: the tumble bay, clay core and upper radius.
The largest section was the tumble bay, which measures 18m x 14m x 4.46m deep; the clay core comprises two bays measuring 7m x 4m x 3.3m deep; and the upper radius is 15m long x 9m x 4m deep.
Irregularity wasn’t the only challenge for the designers, however. Hard ground meant that in several places it was impossible to achieve a toe-in for the Larssen L603 steel sheet piles, supplied by Piletec which would line the excavation.
The solution was to change the frame layout and alter the lateral restraints
The structure proposed by Groundforce comprised a combination of hydraulic Mega Brace waling beams, cross struts, knee braces and sheet piles.
The tumble bay was supported with a sheet-piled cofferdam comprising 7m-long L603 sheet piles braced at two levels. A single-frame solution was used to brace the 8m-long sheet piles in the clay core, while in the upper radius the excavation was lined with GFi trench sheets and supported with two levels of bracing.
The original concept proposed the use of knee braces with lateral restraints for the largest section [the tumble bay] in order to limit the number of struts needed
However, due to the hard nature of the ground, they were unable to achieve the required toe-in for this solution to work. This was solved by adding a knee brace in the corner where the sheets had been driven to full depth and straightened out the remaining struts
Due to the sealed reservoir and the potential for cracking if water were to seep in, MMB were unable to install slit trenches. Instead the Groundforce engineers suggested that it would be best to dig and drive sheets for some areas rather than pre-drive/slit trench in areas that had not achieved the correct toe-in. In some areas, the sheet piles could not even reach formation level due to the hard ground conditions.
While technical challenges of this sort are not unusual in the civil engineering industry, the project team at Ynys Y Fro had to also work around the unprecedented disruption caused by the Covid-19 lockdown.
Due to the Covid-19 Lockdown only one physical site meeting took place at the back end of February. This made it even more difficult to discuss the relevant changes that were required to enable the MMB site team to install the new reservoir spillway with the maximum amount of working room.
All of the design discussion meetings were held via online video conferencing platforms as all of this was taking place in April and May at the height of the lockdown.
Despite these difficulties, Groundforce finalised the temporary works design in time for the equipment to be delivered to site in mid-May and the innstallation went according to plan.