Two Rocks Pump Station Trench Sheet & Bracing Solution
As part of the new Two Rocks Metropolitan Wastewater Works, our customer needed to construct a 7.0m deep Pump Station in unstable sand with a low water table presenting itself within a 2m thick floating limestone rock layer. There was no room to bench at the required shallow angle, so an alternative shoring solution was required. The excavation size needed to be 7m x 7m on plan to cater for the 5m square pump station.
The customer wanted to self-perform and were on a tight budget to complete the works and had ruled out traditional shoring boxes due to the depth and weight of system, and a slide rail system couldn’t take the loading at the clear opening required.
To provide a self-performing solution, Mabey’s Engineering team suggested that the customer use a sheet and frame solution. This required the sheet length being kept down so that a manageable 6m long sheet could be used with a traditional EMV and a reasonable sized excavator.
The solution was to dewater by spears to 1.0m below formation level so that a sheet toe-in water cut off was not necessary and the top of the excavation was locally reduced by 1.5m so that the earth retained height was 5.5m. The retaining wall would then be made up of 6m long M12 trench sheets and four Multibrace support beams.
Engineering & Project
Mabey’s Engineering Manager provided site installation support with the RCR crew for 5 days. From hooking up the EMV to building up and putting in the sheet and frame system. This support was pivotal to the successful outcome for the customer to ensure that the job ran smoothly.
The floating rock layer added another layer of complexity to getting the sheets down, but this was easily managed on site by using a drive cap on top of the sheets and a dig and push sequence was adopted as required.
The shoring was installed on time and within budget and was a very successful outcome considering this was the first time the contractor had undertaken this specific type of work.