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END OF THE PIER SHOW

PILING & DEEP FOUNDATIONS

Missing foundation records meant building a replacement pier from scratch on a Yorkshire motorway bridge reinstatement.

Lack of pile records for a sixlane motorway bridge on the main route south from Leeds meant repair work had to start from scratch.

The bridge, at junction 6 of the the M621, crosses a short stretch of urban dual carriageway. It had suffered severe salt attack to its abutments and central support pier.

Foundation contractor Systems Geotechnique has been working on the repairs in partnership with framework contractor Birse CL and Carillion WSP, which acts on behalf of the Highways Agency in Yorkshire.

As always with main highway reinstatement projects, many pressures apply. These include maintaining traffic flow, ensuring safety for road users and repair team workers and completing the project within the shortest possible time.

While the abutments could be repaired by hydro jetting, concrete crack injection and sprayed concrete, it was decided to take down and replace the support pier.

The relatively weak fill, gravel, clay and mudstone ground conditions were not ideal for the high load requirements of the pier, so the design team decided to use 600mm diameter piles socketed into the mudstone.

Two independent test piles based on the initial design of a 4.5m socket were installed into the underlying mudstone, to provide a maximum working load of 2000kN at about 9m depth. Although the piles achieved the desired 2.5 factor of safety, they demonstrated higher settlements than predicted as a result of the weaker than expected mudstone.

As a result, the piles needed to be deeper. However, at one end of the site this greater depth took the toe of the piles to the top of a previously grouted coal seam which meant using longer piles - 14.5m deep - in this area.

Work was carried out largely beneath a bridge span, so headroom was often limited to 5.5m. This could have made tripod piling necessary, but Systems Geotechnique decided this would be too slow and that Klemm KR709 restricted access piling rigs operating in CFA mode were a better answer. This was despite the fact that the 14.5m increased depth requirement of piles was at the very limit of the piling rigs' capacity.

Two KR709 rigs were used on site, together with a drum mixer and concrete pump. A telescopic forklift installed the 8m cages supplied to site in two sections with threaded bar connectors.

This plant, together with small excavators and dumpers to remove piling spoil, was located between two open, functioning road carriageways with temporary fencing barriers to maintain safety.

Even though the site threw up unforeseen obstructions, the subcontractor says careful planning ensured it could install the piles around the bridge without delay.

In restricted access situations such as this, where the ability to add or move piles is severely limited, the test installation proved its worth as it allowed production piles to be redesigned to accommodate a revised understanding of the ground conditions.

The design stage of the project took from February until August last year, with budgeting and approvals continuing until October.

The following month saw piling work begin with the two test piles and the first of the 36 production piles going in during December. Rig crews completed the last of these at the start of this year.

The value of the piling project to Systems Geotechnique was £110,000. Its sister business, Structural Repair chnique, carried out the abutment concrete repairs under a £12,000 contract.

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