Introduction
Installing a sheet-piled cofferdam was traditionally a specialist operation and many piling subcontractors still offer this as a specialist service.
However, modern piling technology has made the process of installing steel sheet-piles a task well within the capabilities of any competent excavator operator, resulting in an increasing number of contractors now choosing a ‘self-delivery’ option: hiring-in specialised piling equipment and using their own on-site plant to carry out the operation themselves.
Project
Design and build company Mott MacDonald Bentley (MMB) – which brings together expertise from Mott MacDonald and JN Bentley - is currently working on a £16.4M project to upgrade the Mossley wastewater treatment works near Ashton-under-Lyne in Greater Manchester for client United Utilities.
The scheme forms part of a wider £230m programme by United Utilities to improve water quality in the North-West region’s rivers and reduce sewage spills by more than 10 million tonnes annually.
As part of the Mossley WTW upgrade, MMB is installing a new combined sewer overflow (CSO) with integral detention tank on the site. This is designed to retain surcharges and prevent pollution of the River Tame that flows past the Mossley WTS.
Challenge
CSOs are usually quite small precast concrete structures delivered to site in kit form. But Mossley WTW’s new CSO detention tank is a large structure and will be built using in-situ concrete. The excavation required to accommodate this structure is 27m long, 10m wide and 6m deep.
“The ground conditions here are quite challenging,” explains MMB site agent Andy McBride. “It includes made ground with existing buried structures over hard mudstone”.
The task was to install a sheet-piled cofferdam more than 7m deep and support the sides using a proprietary shoring system to allow the 700 cubic metres of material to be excavated to formation level.
While a specialist piling subcontractor would certainly be able to carry out the piling work, MMB chose to do this itself.
“Because we knew about the ground conditions, we decided it would be better for us to do the piling as well as the excavating,” says McBride. “If we’d used a piling specialist and they had encountered refusal due to the buried obstructions, we’d have had to come in and clear the obstruction for them. It made more sense for us to do it all ourselves.”
Solution
MMB has worked numerous times in the past with Piletec’s sister company Groundforce Shorco and here again, they were selected to provide the structural support for the excavation.
“Groundforce Shorco did the design for the sheet piling and the frames and through them we contacted Piletec,” says McBride. “Because of the poor ground conditions, we decided to pre-auger the pile-line with a Piletec auger to loosen the soil prior to installing the sheet piles,” he adds.
To install the GU13 Larssen sheet piles, MMB hired an ICE8SG side-grip hammer from Piletec.
“We were installing 7m, 7.5m and 8m-long piles. Handling sheet piles of that length is not easy using a conventional hammer unless you have a very long boom. But the side-grip hammer allows you to grip the pile anywhere along its length and pitch it into position. Then you can start driving the sheet, again holding it at any point along its length,” explains McBride.
“Piletec came to site and installed the hammer to the boom of our 36-tonne Komatsu excavator. They then spent two days providing on site familiarisation for our operators. We’re keen to up-skill our operatives and this was a useful learning-curve. They remained in contact with us throughout and were happy to field any technical enquiries and return to site if we needed any assistance.”
MMB hired the ICE8SG hammer for a total of three weeks. Then, with the sheet-piled cofferdam in place, started excavating and installing the support frame from Groundforce.
“We are now in the process of casting the in-situ concrete. The tank should be finished by the end of November. We will then back-fill and hire in the side-grip hammer again to remove the sheet piles,” concludes McBride.