Chester Northgate Tunnelling – Chester City Centre
Client: Vinci PLC / Cheshire West & Chester City Council
Project Description/Main Operations:
V J Donegan & Co Ltd were approached to work alongside Vinci PLC and Cheshire West & Chester City Council to provide a constructable solution for a new surface water outfall, from the Northgate Development Phase 1 in the heart of Chester City Centre to the River Dee. Given the historic importance of Chester, the project and route presented many challenges that required specific solutions.
Having been engaged by Vinci pre-contract award, V J Donegan & Co Ltd provided upfront consultancy in the early design stages, which included advice to their designers of preferred shaft locations, drive lengths, undertaking trial holes and assisting ground investigations and undertaking S185 diversion works before contract award.
To complete the excavation works, V J Donegan & Co Ltd worked closely with Cheshire West & Chester City Councils archaeological team who provided watching briefs when excavating in the made ground.
The complex route of the tunnel required daily settlement and 3D monitoring of the existing highways, monuments, adjacent buildings, existing car park entrance underpass and enhanced monitoring on the 11th century medieval city walls which required secretary of state approval for tunnelling under.
To enable the traffic to flow through the city centre during the construction phase, a city-wide clockwise diversion was implemented to minimise disruption to the residents of Chester, city centre workforce and visitors.
V J Donegan’s solution included:
- Construction of 4nr drive shafts of 5.50m & 6.00m diameters to depths varying from 5.0m to 12.0m in glacial till and sand stone.
- Construction of 3nr reception shafts 5.00m diameter to depths varying from 8.0m to 12.0m in glacial till and sand stone.
- Construct 4nr 1200mm dia. tunnels totalling 590mtrs via pipejacking, using a Herrenknecht AVN1200TC MTBM.
- Construct 2nr 1000mm dia. tunnels totalling 225mtrs via pipejacking, using a Herrenknecht AVN1000XC MTBM.
- Covert all drive and reception shafts in to access manholes to Welsh Water specification and install pre-cast concrete cover slabs.
- Construct 120m of 675mm gravity sewer via open cut trench technique and install 2nr manholes to Welsh Water Specification.
The project was successfully delivered over a 10-month period to programme.