Introduction
Piletec supplied their ICE8SG Side-Grip vibrating hammer to main contractor JN Bentley who needed to extract sheet piling from a site with some very challenging ground conditions.
Installing steel sheet piles can be difficult, particularly in stony ground when some extra effort is often required to drive them to full depth.
But in forcing their way through difficult ground, the piles can themselves suffer damage and if the damage results in significant deformation, the task of removing them can be just as challenging.
The Project
This was the situation facing JN Bentley while carrying out work in February at Gouthwaite Reservoir on behalf of Yorkshire Water.
Bentley’s task was to install siphon pipes and their associated valve equipment to allow Yorkshire Water to lower the levels in the reservoir in times of high rainfall.
The siphon installation was instructed under the ‘The Reservoirs Act 1975’. This act places a duty on the Environment Agency in England and Wales and LAs in Scotland to maintain a register of large raised reservoirs which can contain more than 25,000m3 of water above the natural level of any part of the land adjoining the reservoir.
Challenge
JN Bentley employed a specialist contractor to install the sheet piled cofferdams for the project. “The ground was generally good, but we knew there were some issues and there was some difficulty driving some piles the last metre or so,” says site manager Peter Middlebrooke.
A total of 78 sheet piles were installed to depths up to 9.5m using a Movax side-grip vibrating hammer. But when it came to extracting them, 15 of the piles refused to budge.
“This is no reflection on the contractor – I watched them try to extract the piles and they really did try – but their standard practice is to stop if the sheets haven’t moved more than about 100mm in a certain amount of time,” explains Mr Middlebrooke.
When sheet piles cannot be extracted using normal methods, alternatives (such as excavation or simply cutting the piles off below ground level) are usually employed.
However in this case, leaving part of the piles in the ground would have created other problems so JN Bentley chose to persist and try to extract the sheet piles itself.
Solution
Last year JN Bentley had invited Piletec, a division Groundforce, to one of its monthly site managers’ meetings where teams exchange knowledge and share best practice. At the meeting, Piletec demonstrated its ICE8SG Side-Grip vibrating hammer made by the Dieseko Group in the Netherlands.
Recalling this demonstration, Mr Middlebrooke contacted Piletec to enquire about hiring the ICE8SG.
“This sort of equipment is very specialised and is usually used only by specialist contractors. It’s unusual to be able to hire it,” says Mr Middlebrooke. “But we saw this as an opportunity to give our machine operator some experience and training with this equipment and, as time was on our side, we chose to go ahead and experiment,” he adds.
After consulting the hire company that had supplied the 35-tonne Caterpillar excavator being used on the job, Mr Middlebrooke satisfied himself that the ICE8SG side-grip hammer was compatible and called Piletec in.
“Piletec were really good. They sent an engineer out to provide training and technical back-up and they remained on-site throughout the first day,” he says.
The first pile proved very resistant but JN Bentley eventually brought it to the surface. As predicted, it was badly curled-up at the bottom, having impacted hard material. “The second one was easier, then the third was difficult. They were obviously binding to each other,” says Mr Middlebrooke. However, after a week, all the piles had been successfully extracted.
“It wasn’t easy and it took some time, but the good thing is that we now know how to do this and how to operate this very specialised piece of equipment,” says Mr Middlebrooke.
The ICE8SG vibratory hammer grips the pile from the side, meaning that sheet piles can be used that are much longer than the reach of the excavator boom on which it is mounted. Its unique linear side-clamping system ensures a highly efficient clamping load transfer.
The hammer is a recent addition to the Piletec hire fleet, says the company’s Northern area manager Jade Walker: “We allow the customer to use their own machine and hire the ICE8SG unit only.
“In this case, JN Bentley wanted to demonstrate a successful extraction of the sheets and do this in-house, by using their own operators.
“This was also a perfect opportunity to upskill their own workforce,” she adds.