The solution was the Rolling Strut Double Slide Rail system from shoring specialist Groundforce Shorco.
Trant’s task is to excavate the 5 m wide, 5.5 m deep, trench and install a 3 m diameter concrete-steel-concrete sandwich pipeline, welding and grouting the 4 m long sections together as work progresses. The pipe will supply cooling water from the sea to a new combined-cycle gas fired power station currently under construction by main contractor Siemens for power company Marchwood Power.
“The traditional method of supporting a trench of this size would be to drive steel sheet piles and brace them with hydraulic struts” explains Trant’s contracts director, Simon Trant. This would have involved piling and shoring a short length of trench, excavating between the piles in order to install the pipe, then backfilling, removing the piles and repeating the process for the next run of pipe.
“That option would have been very slow and very expensive” says Mr Trant.
Groundforce Shorco, which has a history of involvement with Trants, was asked to price the shoring package for the subcontract.
“I looked at the design and thought ‘this would be a good application for the Slide Rail system” says Groundforce technical sales executive Keith Brown. “It’s not a common piece of equipment – in fact this was the first time I’d supplied it – but I could see immediately that this was the ideal solution” says Mr Brown.
Despite its exotic title, the Rolling Strut Double Slide Rail system is a simple and robust piece of equipment purpose designed to support large excavations in poor soils. “It is a modular system not dissimilar to a run of trenchboxes” explains Groundforce technical sales executive Keith Brown.
The system comprises four vertical heavy-duty steel rails, driven into the ground at each corner of the system, with steel side panels which slide down between the rails to support the sides of the excavation. Lateral bracing is provided by heavy steel struts which are also slid into place within the rails and which – crucially – can slide up and down the full depth of the system. This allows the struts to be ‘rolled’ up the excavation to allow the installation of the large pipe sections.
Being modular, the system is easy to assemble and install. It is also quick to strike and remove from the finished excavation. Each module, or ‘bay’ measures 4 m in length but can extend to as much as 12 m in width. At Southampton, Trant is using eight bays, thus allowing pipe runs of up to 30 m to be installed at a time.
According to Simon Trant, the Groundforce Shorco system has been a revelation. “Apart from the traditional sheet-piled option, this was the only system available that could do the job” he says. “It’s very quick and easy to use and extremely robust. It’s built to withstand rough treatment ” he adds.
In fact, Trant’s excavator is used not only to excavate the trench but also to lift and install the system. “If they’d used sheet piles and hydraulic struts, they’d have needed a piling rig to install it and a crane to lift it” observes Mr Brown.
Although the hire cost of this system is significantly higher than that of the piled system, the speed of installation and reduced manpower required will deliver major savings on this project, says Mr Brown. “Trant did their own calculations and worked out that this was the cheapest option” he says.
Work on the pipeline began in July and has progressed extremely well; “We’ll be finished by Christmas”, says Mr Trant. So far the company has installed 58 pipe sections (totalling 232 m) with no interruptions or snags. “It’s exceptionally efficient” he says.
For further information please contact Lynn Campbell, TLC pr. Tel 0151 227 4957 email lynn@tlcpr.co.uk
