Frome Valley

Location: Frome

Project Type: Relief Sewer

Client: Wessex Water

Project Duration: 52 weeks

Contract Value: £8m

The Frome Valley Relief Sewer scheme represents a £15million capital investment by Wessex Water to provide additional capacity for the continued development within the North Bristol area and will see flows diverted from South Gloucestershire to Avonmouth Treatment Works in Bristol with its improved treatment facilities.

he scheme consisted of approximately 5km of 1200 to 1800Ø pipe installed by open cut and tunnelling. The gravity pipeline connects Iron Acton (to the north of Frampton Cotterell), with Bradley Brook (on the north side of Bristol) and conveys flows of 400l/s from Hoovers Lane. Several major obstacles had to be overcome including crossings of the M4, 3No ‘main rivers’ including the river Frome, 4No HP gas mains including the 48” 80Bar Seabank main and a high-pressure ESSO Oil pipeline, alongside the usual services and major roads.

An £8m NEC3 Option A Design and Build contract was awarded to envolve in October 2016 with an onsite start date of March 2017, leaving only 6 months for the detailed design of the scheme. The original contract completion date of January 2019 was surpassed, the scheme was completed in July 2020 due to seasonal reinstatement work and delays with the Wessex Water inhouse M&E fit out and commissioning. The final account value was £8.5m, the award of compensation events was mainly associated with additional scope, requested by the Client.

In line with our collaborative approach, we engaged in Early Contractor Involvement with Wessex Water and all project stakeholders to conduct the following:

  • Development of access and egress routes
  • Consultation with landowner
  • Presentations at Council and Parish Council meetings
  • Submission of EA and Drainage Board applications for watercourse crossings
  • Application to Highways England for tunnelling under the M4 motorway
  • Agreement of mitigation works and post project improvements at the Local nature reserve
  • Installation of new water jumps and cross-country course features at an Equestrian Centre to minimize impact on the business
  • New pipe installation below the 80Bar 48” Seabank Main Gas crossing
  • Consultation with Fischer German relating to the Esso high pressure oil mains
  • Review and implementation of material re-use strategy, crushing excavated rock and certifying for re use as bedding material

During this ECI stage we also developed the delivery strategy.

Tunneled sections would be delivered by B&W Tunneling, a long-term strategic supply chain partner. Open cut sections were being delivered by our directly employed skilled workforce. Paramount to the success of the scheme was our ‘One Team’ approach between the stakeholders, this ensured absolute confidence from Wessex Water in our delivery strategy and structure.

Our Design Manager was responsible for the timely delivery of the design stage through regular design team reviews and ensured technical compliance with all Wessex Water design standards. Design was controlled using our design management plan. The plan controls the design services and sets out responsibilities, form of contracts, PI levels, document control and change management procedures.

The finalised design considered innovation, technology and construction efficiency and identified 1.6km of tunnelling, the remaining 3.4km was installed via open cut at depths of up to 7m in hard sandstone rock.

Three tunnel sections were necessary to eliminate impact of the project on landowners, environmental and 3rd parties, a primary driver in the route selection.

The first was a 410m 1800Ø section under the M4 motorway, the second a 150m 1500Ø section near housing and roads, and the final length totaling 1050m. Two intermediate shafts were required at change of directions at depths of 12m. The Open Cut section comprised 1200Ø and 1800Ø diameter pipework.

Works commenced on site ahead of schedule, we were appointed as Principal Contractor and Designer and Wessex Water retaining the role of Principal Designer. CDM15 regulations were discharged through the development of a detailed Construction Phase Plan and by collaborative working. The design was approved through a detailed design process review, regular meetings and a prestart conference were used to keep the wider team involved and informed. Stakeholders remained at the heart of the delivery, we managed this by conducting weekly progress meetings for the duration of the scheme covering design, construction progress, environmental and 3rd party interfaces.

Works were predominantly constructed offline on up to five work-fronts, three were open cut sections and two were tunneled progressing simultaneously.

Construction operations were managed by our own highly experienced directly employed site management team supported by our in-house SHEQ department.

The scheme comprised of three critical interfaces with Wessex Water live assets. In all cases we undertook ‘live’ connections to the existing network, minimizing any impact on the live network and the Wessex Water operational teams. Throughout the commissioning stage, we assisted the Wessex Water mechanical and electrical contractors, helping to ensure seamless interface between the specialisms.

The project was difficult and challenging but has ensured that housing development can continue in the North Bristol region with a robust strategic improvement to Wessex Water’s network. The scheme was delivered to outstanding quality and health and safety standards and is testimony to an aligned Project Delivery Team and our ‘One Team’ collaborative relationships.


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