Stoford Properties Limited
The scheme required BWB to complete a detailed model of the existing Barbourne Brook corridor to inform a scheme for both managing flood risk within the site and to reduce the existing flood risk downstream of the development. We also undertook to design a flexible site wide infrastructure network of highways, drainage, and earthworks to ensure maximum flexibility to accommodate changing market conditions and demand.
The proposed works comprised the installation of services, by trenchless technology methods, beneath the M5 motorway near Junction 6. This enabled the proposed Worcester Technology Park, located on the eastern side of the M5 to be a fully serviced site.
The Geotechnical Classification, as defined in BS EN 1997:1(2004) and the Highways Agency, HD22/08 Managing Geotechnical Risk was designated as Category 2 for the site, as the development and construction comprised conventional geotechnical structures with no exceptional risks. McPhillips were the Main Contractor for the duration of the project.
The process BWB followed to successfully complete the project was in line with Design Manual for Roads and Bridges (DMRB), HD22/08 Managing Geotechnical Risk. BWB undertook the following tasks as part of the geotechnical certification process:
Statement of Intent (SoI);
Preliminary Sources Study Report (PSSR);
Ground Investigation Report (GIR);
Geotechnical Design Report (GDR); and
Geotechnical Feedback Report (GFR).
BWB designed works to enable two 7m by 3.5m launch pits and two 4m by 3.5m reception pits excavated to depths of approximately 4m below original ground level. The launch and reception pits were located to the east and west of the motorway respectively at a distance approximately 15m away from the motorway edge.
Two 1200mm internal diameter and 1430mm external diameter pipes were installed in a 1450mm diameter bore created via the pipejacking technique. The linear distance covered by jacking pipes is of the order of 80m.
For monitoring purposes, a number of survey points were marked by the Main Contractor (McPhillips) between the edges of the launch and reception pits and the toe of the motorway embankment. Allowable settlements in relation to the works was agreed between BWB and necessary project stakeholders.
The major challenge for this scheme was to avoid excesses settlement of the live carriageway of the M5 motorway; a worst case scenario would be based on the full collapse of the overlying material and as such could equate to potential collapse of the existing carriageway level by up to 1.45m (based on maximum diameter of pipe jack equipment). However, utilising the pipe jack solution, the tunnel is created behind the digger shield as the excavation is taking place within the shield. This method results in a flexible, structural, watertight tunnel as material is excavated, which therefore generally reduces the amount of overbreak that can occur from other techniques. Additionally, as the pipe jack is advanced the final pipes are placed as the excavation occurs and therefore it is considered to provide better ground support as there is no downtime between completing the excavation works and the installation of the pipes. This equally reduces any potential issues of heave at the base of the excavation.
For monitoring purposes, a number of survey points were marked by the Contractor between the edges of the launch and reception pits and the toe of the motorway embankment. Monitoring was continued during works as well as for a duration of 10 weeks following completion in order to capture any post completion settlement. Based on monitoring data no excessive settlement was recorded.
BWB managed the geotechnical certification process effectively and ensured timely project completion. Efficient design and supervision of the works including adoption of trenchless technology proved to be both cost and time savings for the Client. It was ensured that there were not disruptions of the live infrastructure of the M5 motorway.
Following this project the HE recommended BWB to service providers on other schemes.
Principal Geotechnical Engineer
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