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Parallel piles save Bristol quay Old quay walls in the middle of a Bristol redevelopment site had to be made secure, but any disturbance might have caused catastrophic failure.

TEMPLE QUAY is a non-tidal harbour in the middle of a major redevelopment in central Bristol, adjoining but elevated above the well-known Floating Harbour.

Of greatly varying age, its walls were in poor condition and probably close to failure, but precise construction details were unknown.

These were the problems facing client English Partnerships and engineer WS Atkins (South West) when remedial works were required to support redevelopment of the adjacent city centre site.

In partnership with principal contractor Kier Construction, foundations specialist Stent Foundations explored the full range of foundation/retaining wall options - bored piles, driven precast and cast insitu piling, diaphragm walling, as well as steel combi walls incorporating raking steel tubular piles, and hybrid schemes.

All these solutions hit two fundamental difficulties:

the ground was soft alluvium sitting directly on hard (Redcliffe) sandstone. Soft bore techniques needed to be combined with rock penetration.

any disturbance of the existing wall, whose precise position was unknown, might cause cata- strophic failure. Any solution must avoid excavating through the wall.

Stent designers analysed the quay wall using the FLAC computer program to assess the situation. They then developed a cost saving alternative design based on a double-line sheet pile retaining wall system. The complete alternative structure was analysed in construction and in service to ensure suitability.

Costed at a final value of pounds800,000, this alternative was approved for construction by the client and principal contractor.

The scheme involved vibrating and driving two parallel lines of LX25 steel sheet piles along the line of the existing wall. In part of the works, the new retaining walls were both located in the harbour, in other locations they ran either side, straddling the existing gravity wall. Following install- ation, the two walls were tied together at high level, prior to excavating the material between thm and backfilling with an underwater concrete.

A reinforced concrete retaining wall, incorporating a harbour-side walkway, was then con- structed on top of the sheet piles, and faced with a brick cladding.

In all, 7,000m2 of permanent LX25 sheet piles in lengths averaging 13m were installed by Stent using two pile vibrators, and driven home using 1t hydraulic drop hammers. The piles were largely installed over water, up to 8m from the edge of the quay.

Stent replaced a total of 200m of Temple Quay wall and strengthened a further 100m over a 17 week period beginning in March. Scheduled for 21 weeks, successful completion of the programme four weeks early was attributed to innovative design and close co-operation between the engineer, principal contractor and foundation specialist.

The Floating Harbour floats to see another day, and another millennium.

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