Petts Hill railway bridge carries the Chiltern Line over the road on the boundary between Harrow and Ealing, immediately south of the junction of the A312 (Northolt Road) and A4090 (Alexandra Avenue). It was a ‘bottleneck’ for all traffic and had been identified as a source of delay to buses because of the restricted width of the carriageway under the bridge.
Birse Rail was appointed to replace the existing structure in order to accommodate a four lane carriageway with footways and cycleways on both sides.
Works included the installation of four 3.6m overall diameter caisson-constructed reinforced concrete piles. The piles, which were approximately 22m deep, carried the transverse precast reinforced concrete crosshead beams and provided direct support to the bridge superstructure. The bridge abutment walls and wing walls were formed by contiguous bored piles which provided earth retention and resistance to train surcharge loading.
Demolition of the existing structure and installation of the new bridge deck took place during a 60 hour possession over Christmas 2008.
Innovation
The original design for the works to Petts Hill railway bridge was based on using four large diameter bored piles installed during possession to support the new bridge deck. The use of piling rigs to install the piles would necessitate possession access requiring additional cost and programme time to complete.
Birse Rail’s alternative solution involved replacing the existing bored pile solution with four 3.6m diameter Caisson piles which allowed the works to be undertaken in normal working hours.
The piles were constructed using low headroom equipment and were positioned such that a fenced 'green zone' could be established adjacent to the running lines.
The benefit of this approach was a significant reduction in the number of possessions required to construct the caissons. Furthermore, during the tender process Birse Rail was able to develop the alternative solution to AIP status and offer this to the client. This approach led to a reduced design programme for Birse Rail’s alternative solution.
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