Project Sheet: A5 Berwyn Street Culvert, Llangollen
- Martin Wright Associates (for Denbighshire County Council)
- Dawnus Construction Ltd
- Production of Preliminary Sources Study, Geotechnical Design report and Geotechnical Feedback Report in accordance with HD22/08,
- Supervision of site investigation
- Development of the construction method and temporary works with the designer and the Contractor.
Description of Project
Following flooding during heavy rains in 2000, it was proposed to construct a flood alleviation scheme in Llangollen.
The scheme consisted of an intake structure built to the south of properties known as Craig view and Gaerwen. This was then connected with an in-situ culvert system along Hall road to the A5 junction. Beneath the A5 a 1500mm wide and 1000mm deep rectangular, pre-cast concrete box culvert was installed, which connected to a headwall. This then connected to an open channel leading to the River Dee.
In 2009 GroundSolve Ltd. were appointed to assist Martin Wright Associates with the production of the HD22 documentation for the section of culvert which passed beneath the A5 Berwyn Street.
At the location of the culvert crossing there were numerous services in the carriageway, which ran perpendicular to the line of the culvert. An excavation around 3.0m deep and 3.0m wide was required for the construction of the culvert. Hence the options available for both the installation of the temporary works and the construction of the culvert were restricted.
By working closely with the designer and the Contractor, GroundSolve Ltd developed a safe method of construction for both the temporary and the permanent works.
GroundSolve Ltd also assisted the Contractor with the design of the temporary works.
Project Constraints & Solutions
There were a number of constraints which had to be considered when developing the construction method for the project. These included:
- Maintaining traffic flow on the A5 trunk road at all times
- Numerous live services in the eastbound carriageway, which had to be protected and remain live throughout the works
- Installing temporary works beneath the services
- Significant groundwater flows in a northerly direction towards the River Dee
- Excavation of up to 1.7m of bedrock at the southern end of the culvert
All of these issues needed to be given careful consideration when designing the scheme.
In order to maintain traffic flows on the A5, a lightly reinforced concrete beam was constructed perpendicular to the line of the culvert. A traffic barrier was drilled into the wall. This provided support to the running lane of traffic, hence allowing the excavation for the culvert to extend right up to the traffic barrier.
In order to install the culvert section beneath the services, the material was excavated by hand using a heading type approach. Each time the heading was advanced by 300mm trench sheets along with walers and props were installed to provide side support to the excavation, hence providing a safe working area. When this work was complete the box culvert section was jacked into place beneath the services.
The groundwater was controlled by pumping from the excavation into sedimentation tanks. Flocculating agents were used to increase the rate of sedimentation. Once sedimentation had occurred, the water was discharged into the open channel downstream of the culvert, where it was passed through straw bales, and finally discharged into the River Dee. The Environment Agency visited site and were satisfied with the procedure being undertaken.
The works were completed successfully and on time in order to allow the temporary traffic light control to be lifted before the embargo commenced.