Project Sheet: Shoreham Bridge
- Cass Hayward LLP Consulting Engineers
- Geoffrey Osborne Ltd
- Supervision of a site investigation
- Ground investigation
- Schedule laboratory testing
- Produce an interpretive geotechnical report
- Preliminary Pile Design
- Check contractors final pile design
Description of Project
West Sussex County Council appointed a contractor to replace the existing footbridge over the River Adur which is about 90 years old. The bridge links Shoreham beach to Shoreham by Sea town centre.
The proposed footbridge was in an environmentally sensitive area so a detailed ground investigation was necessary before construction could begin. GroundSolve Ltd organised a site investigation which involved the excavation of exploratory holes, boreholes and window samples which were sent to the laboratory to be analysed in order to advise the contractors about the suitability of the pier foundations.
Boreholes were undertaken on land on either side of the site and also on a jack up rig in the estuary.
The bathymetry of the river was profiled using a Multi-Beam Echo Sounder, and Sub Bottom Profiling was used to record site conditions.
The replacement bridge was designed by Cass Hayward to be a combined pedestrian and cycle bridge, constructed to the east of the original footbridge. It was also designed to be a swing bridge which balances on two central spans and is supported from a reinforced concrete pier. The swing bridge provides a 15.24m navigation channel with unlimited headroom when in the open position.
The project involved designing the new bridge, its realignment, the replacement of the abutments and improvements of the flood defences along the southern side of the river.
The flood defence work was being carried out by the Environment Agency and resulted in the construction of a 4.83m tall defence wall which provides 1 in a 50-year level of protection.
The finished bridge has a sheet pile causeway on the northern abutment extending 13.65m into the estuary. In total, seven piers were constructed, the fourth on being the central support for the swing bridge. The piers were built on piled foundations, designed by GroundSolve Ltd.
To the west of the old footbridge is a SSSI. There are mudflats and salt marshes which needed to be considered because numerous protected species were identified in the estuary such as lizards, protected fish and wading birds.
Another issue in need of consideration was the presence of a highly permeable chalk aquifer beneath the site.
Ground conditions comprised made ground and river gravels overlying weathered chalk bedrock. Following the interpretive geotechnical report, a preliminary pile design was undertaken for costing. GroundSolve were then responsible for checking the contractors final pile design.