Bypass linked to regeneration in Littlehampton and Bognor Regis.
Our impact
225m
Length of viaduct over floodplain
121
Number of concrete beams to construct the viaduct
650t
Weight of crane to construct viaduct
A key element of the scheme was construction of a 225m-long viaduct over a flood plain area known as Black Ditch.
Building across a floodplain
Working on behalf of West Sussex County Council, Jackson was Principal Contractor on this project, responsible for the design and build of the northern section of the A284 Lyminster Bypass, to alleviate traffic issues through Lyminster village.
The £30m bypass is 1.1km in length, consisting of a 7.3m-wide single carriageway road with a shared 3m footway/cycleway facility.
A key element of the scheme was construction of a 225m-long viaduct over a flood plain area known as Black Ditch.
The scheme was constructed off-line from the existing road network to minimise disruption to road users during the works.
An array of 96 foundation piles had to be drilled through soft ground into the chalk base, each 750mm diameter and up to 27m depth.
A total of 121 concrete beams were pre-cast off-site before they were lifted into position using a 650t mobile crane.
Viaduct construction
Construction of the viaduct required extensive temporary works across the floodplain to provide a competent working platform.
The substructure of the viaduct consisted of 12 reinforced concrete abutments and intermediate piers. These structures were supported on 96 bored piles up to 27m long.
A total of 121 concrete beams were pre-cast off-site before being delivered and lifted into position using a 650t mobile crane. The viaduct consisted of 11 sections, with each section comprising 9 intermediate beams and 2 heavier edge beams, weighing up to 55 tonnes.
The bridge deck was cast in a specific sequence to suit the design parameters and avoid excessive temporary strain.
Ground settlement
The flat, coastal plain that extends south of the South Downs contains a mixture of soil types over a chalk base. The land is low lying and prone to flooding.
To accelerate ground settlement, wick drains were installed and temporary surcharge of chalk placed to the north end of the site.
Piezometer monitoring equipment and settlement plates were installed to measure settlement to determine when targets were achieved and the surcharge could be removed.
Soil mixing was also used to improve ground conditions in several areas of the site, which minimised the need for waste removal from site and reduced import requirements.
To accelerate ground settlement, a temporary surcharge of chalk was placed to the north end of the site.
