Lateral thinking solves vertical problem

31 Mar

Supporting the sides of a 7m-deep square shaft is usually a fairly straightforward affair for Groundforce Shorco. First you install interlocking steel sheet piles around the perimeter, then you brace the top with a waling beam before commencing excavation.

But what happens if you can’t use sheet piles?

This was the challenge facing Groundforce Shorco and its client, specialist contractor Barhale, on a sewage pumping station project for Stafforshire County Council in Wolverhampton. 

Barhale (working for main contractor Amey) had already installed a 5m-diameter wet-well using the underpinning method – excavating the ground incrementally and jacking down a series of precast concrete rings to line and support the excavation.

The next job was to install a 1.8m-diameter inlet chamber adjacent to the wet well and break through so that services could be installed.

This meant that operatives and their equipment would have to enter the excavation in order to carry out the work. To allow them to do this safely, the excavation would need temporary support for its entire depth.

Exceptionally hard, rocky ground made the use of steel sheet piles impossible. But although hard, the ground was not necessarily self-supporting, and therefore some form of support was required to stabilise the sides of the excavation.

“Normally we’d vibrate Larssen piles into the ground and brace them with a hydraulic frame,” explains Groundforce Shorco’s technical sales representative Will Wain. “But we couldn’t do that here. So instead we proposed using our large manhole boxes to provide support,” he says.

The problem here was that while Groundforce Shorco’s largest manhole box was quite capable of supporting a 5m x 5m excavation, it is designed for a maximum depth of 5.5m and this excavation extended to just over 7m.

The solution was to create a ‘dig within a dig’ by installing a 5m x 5m manhole box, comprising a 2.5m deep base and 1.5m deep top, and then, inside that, installing a 4m x 4m box, also comprising a 2.5m deep base and 1.5m top, to take the excavation to its final depth.

The excavation itself was dug by a 50-tonne excavator using a hydraulic breaker and a standard bucket to scoop the rocky spoil out of the hole.

“It’s solid sandstone, and very hard,” says Luke Wilden, Barhale’s sub-agent on the project. “Whereas the 5m-diameter wet well excavation was underpinned, the inlet chamber is only 1.8m and there wasn’t enough space in there to use underpinning.”

Since the shaft was dug in solid sandstone the risk of the ground collapsing was minimal, says William Wain. “But we couldn’t count on the ground supporting itself and there was always a risk of loose rock falling from the side. The manhole box was therefore the ideal solution”.

The Groundforce Shorco manhole boxes used here are among the largest available and few other equipment hirers can supply units of this size. “I don’t know how they’d have done this any other way,” says William. But although the equipment was adopted at his suggestion, he refuses to take personal credit:

“The engineer would have come up with this idea eventually,” he says. “Given the lack of options, we were sent down that route.”

Once the connections linking the inlet shaft to the wet-well were completed, standard concrete manhole rings were installed, the boxes removed and the excavation back-filled around the new 1.8m-diameter concrete shaft.