CASE STUDY

A64 York

Bridge Deck Replacements

Project Particulars:

Project Value: £700k

Programme: 2000

Client: Highways Agency

Two post tensioned, single span concrete deck structures over the A64 dual carriageway were removed and replaced with new twin span steel composite decks, including new central columns. The existing abutments, bearing shelves and wing walls were modified to accept the new decks. A temporary footbridge was required for one of the bridges as this linked two sides of the Fulford Golf Course. The existing decks were transported to a temporary yard area, adjacent to the A64 in a field, for demolition. The new decks were constructed in the same yard, where they were jacked to their full height prior to transportation.

Our Involvement

Birse Civil Engineering were assisted by Birse Technical Services providing the temporary works design expertise at tender and contract stages. Duncan Noble led the engineering team on behalf of Birse Technical designing or approving all the temporary works on the project. Halcrow designed the permanent works for the project.

The temporary footbridge was provided by a sub-contractor who also erected and dismantled the bridge. This bridge required an Approval in Principle document which we contributed to in order to ensure that the design elements were properly described and approved by the Technical Approval Authority. We also inspected and logged geotechnical trial pits to enable the design of the reinforced concrete footbridge foundations and designed steel guy wires to attached adjacent bridge foundations and buried kentledge to resist lateral wind loading.

One of the critical items of the temporary works was the protection of a high pressure sewerage rising main (pressurized to 14 bar). This lay across the only access to the temporary yard area which had to be crossed by the SPPT’s carrying a 400T bridge deck! We had to prove to the water authority that the protection we designed would not load the pipe (900mm below ground level) at all. Our solution was a reinforced concrete slab cast on strip footings each side of the main. The slab was cast on “clayboard” void former under the slab which was subsequently wetted through holes in the slab to ensure a clear void between the slab and the main to allow any settlement under load without affecting the main.

The geotechnical aspects of the temporary works were critical to the overall success of the project as many of the temporary works schemes had a geotechnical basis. The large yard area required extensive treatment to enable the SPPT’s to operate. The high applied loads, stringent settlement limits and consequences of failure made the assessment a high risk issue for both cost and operational criteria. Duncan Noble assessed the geotechnical report and devised a suitable, cost effective scheme, then personally monitored the preparation of the sand bearing stratum and temporary stone mat, advising the site team on the suitability of clay pockets in the formation and soft areas. We commissioned and analysed plate bearing tests of the finished yard to ensure that the bearing assessments were valid and induced settlements would not be excessive.

We carried out an extensive route/configuration planning exercise for the SPPT’s to limit costly temporary highway widening, eventually settling on a double trailer unit under each deck and a diagonal configuration whist running on the highway. In addition to limiting the highway widening this also reduced costly crash barrier removal. As part of the assessment we carried out structural analysis to ensure the existing post tensioned decks, of poor quality, could withstand the long cantilevers beyond each temporary support position. We also designed temporary concrete walls to support the new decks during construction, after our cost exercise found that this solution was cheaper than using proprietary steel trestling. Our innovative falsework solution to the deck edges allowed the deck to settle under concrete loads (ie removing some pre-camber) whilst being supported by simple acaffolding from the ground.

The project was very successful, all possession work was completed as planned and the contract was delivered on time and to budget.