Somerset: Durleigh Flood Defence

Durleigh Water Treatment Works, located adjacent to Durleigh reservoir on the outskirts of Bridgwater, Somerset, is owned & operated by Wessex Water. The predominant requirement for DAWSON-WAM during construction was to provide fluvial flood defence to the Works and surrounding properties which was delivered on time/budget.  

DAWSON-WAM was contracted by Wessex Water to design & construct 810lm of secant piled walls adjacent to the substantial watercourse of Durleigh Brook. The flood walls were capable of retaining an internal excavation within the Works of 5.5m to allow construction of underground storage tanks, pump dry well, low lift sump and interstage pumping building. DAWSON-WAM mitigated operational risk, utilised Interface Protocols requiring Wessex Water’s ongoing permissions, consents and approvals.

DAWSON-WAM installed 630mm diameter Cased CFA piles to a depth of 11.5m through firm mudstone, forming a robust hard/hard secant wall which, coupled with the construction of a capping beam and installation of temporary propping, supports the excavation and installation of internal structures.

Our high frequency resonance free piling rigs/plant with effective silencers in tandem with Acousti-bloc noise barrier maximised the acoustic shadow mitigating all potential noise impact. Noise/vibration monitoring ensured regulations met and the residents/community were never impacted.

Existing boundary walls and fences were demolished to provide access for installation of secant piles. Flood wall works conducted under Flood Risk Activity Permit(FRAP) approved by EA.  DAWSON-WAM compiled FRAP demonstrating the avoidance and mitigation measures against increasing the flood level, polluting the watercourse and damaging surrounding flora/fauna. Sorbent booms placed in watercourse, ensured any potential spillages from piling plant were contained and allowed for its recovery and removal.

Following installation of the secant piled flood wall, DAWSON-WAM carried out native riverside tree planting to rehabilitate and enhance the fish habitat within the river. Landscaping and construction of new pathways also completed.

Controlled pumping ensured groundwater, collected by 890m of back drainage, required EA Approval and Discharge Consent/Permit. Silt traps/filtered groundwater, permit and monitoring by EA. Existing drainage ditch relocated along site boundary and flood gate controlling flows from the reservoir into Durleigh Brook replaced.

In order to protect employees and reduce HAVS activities during the construction process, Wessex Water proposed the innovative use of Ancon Shear Connectors within the steel reinforcement of the secant pile flood walls to eliminate the requirement for post-installation dowelling of the secant piles. Typically used within traditional concrete wall/slab construction, Ancon Shear Connectors transfer shear loading and enable movement across construction joints.

Working with their supply chain, DAWSON-WAM proposed construction details to ensure sufficient shear capacity within the connection and developed installation methods to enable the shear connectors to be carefully installed to the correct position and orientation.

Exemplary Customer Care delivered by: early and sustained consultations with local community, businesses, landowners and elected representatives via information events/evenings; project letter drops and council briefings. DAWSON-WAM Site Agent’s personal contact with impacted third parties, progress notice board and informed site visits for invited community members augmented Stakeholder consultations.