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Compact rig called in for a tight spot

Geotechnical contractor May Gurney used one of its most compact piling rigs to cope with extremely restricted access on a recent slope stability contract in Sheffield.

The firm, working for SOL Civil Engineering and Sheffield City Council, installed piles as part of the permanent solution to a natural embankment slippage along a stretch of the A6102 Middlewood Road where it runs alongside and 12m above the River Don.

Temporary repairs had already been made after a major slip undermined the footway on the road.

A structural steel walkway supported on piles 3.6m apart was installed, with gabion baskets placed up to 3m apart next to the slip to improve slope stability. These proved a significant obstruction to subsequent boring activities.

Because the road is the only major link between Sheffield city centre and its northern suburbs and has important services running along it, the council wanted to keep traffic flow as normal as possible during the work.

The city's in-house engineering consultant, Sheffield Design & Property, developed a bored pile retaining wall solution to solve the long term stability problem, while still maintaining traffic and services during construction.

May Gurney used a Funda Mait 24t piling rig to install 51, 450mm diameter and 12.4m long piles through layers of sandstone and mudstone. These were reinforced with a heavy duty steel H-section to 'stitch' the unstable zone to the stable zone beneath. About four were installed each day.

Piling was restricted to one half of the carriageway and an area of slippage, to minimise disruption.

The steel walkway was used to support the front end of the piling rig, after being load tested to confirm safe working. Working restrictions were made worse by overhead high voltage cables just 17m above the road, which meant the steel beams had to be very carefully installed, even with the power turned off.

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