Project Sheet: Loughor Viaduct
- Cass Hayward LLP Consulting Engineers
- Desk Study report
- Site Investigation
- Geotechnical Interpretive report for permeant and temporary works
Description of Project
The Loughor viaduct crosses the Loughor estuary between Carmarthenshire and West Glamorgan.
Network Rail have been upgrading the South Wales mainline and proposed the replacement of the Loughor viaduct in South Wales. Cass Hayward were appointed to design the new viaduct, and GroundSolve Ltd were appointed to undertake the desk study and site investigation for the proposed development.
The Grade II listed viaduct was built in 1852 by the South Wales Railway Company and consisted of 17 timber pile-driven piers (40’ spans) supporting wrought iron decking. However, the piles were poorly maintained and over time additional piles had to be inserted for extra support resulting in a conglomeration of different kinds of timber all of which were at different stages of deterioration.
Sections of the viaduct were supporting higher stress levels than the safety limit, causing two of the piles to lean slightly. It was suggested that the inclining of the piles was as a result of movement in the river bed.
The embankments at both ends of the viaduct were supported by masonry walls. The eastern abutment wall was showing large cracks and there was evidence of ground instability. Temporary boulders were placed in front of the abutment, however, material continued to erode from the embankment.
The site investigation involved shell and auger boreholes extended by rotary coring to prove the underlying bedrock and the potential for any shallow abandoned mineworking’s beneath the structure. Trial pits were also undertaken on land to investigate the existing masonry abutments. The main section of the works was undertaken from a jack up rig in the estuary.
The works were supervised on a full-time basis by GroundSolve Ltd.
GroundSolve Ltd came up with the following three geotechnical options:
- Demolition of the existing viaduct and replacing it with a new bridge
- Building the new bridge on line around the existing viaduct
- Building the new bridge around the existing viaduct.
The chosen method was to build the replacement bridge adjacent to the existing viaduct by installing the foundations of the new structure either side of the old bridge allowing the viaduct to remain operational. The new bridge was then jacked into position.
Due to the high foundation loads of the piles, the loads needed to be transferred to the underlying glacial deposits and Coal Measures bedrock. However, as glacial deposits weren’t present across the whole length of the viaduct, it was decided that all of the piles should be constructed with rock sockets which would be installed into the Coal Measures bedrock.