Chippenham Gateway Sewer (2022)

Gravity sewer pipe from Funke Gruppe - Courtesy of Envolve Infrastructure

Wessex Water were served under S98 of the Water Industry Act to provide an offsite foul water connection for St Modwen’s new Chippenham Gateway development. This commercial development consists of one million square feet of commercial development and potentially a 150-room hotel. An initial modelling appraisal identified insufficient capacity in the local public sewerage network to accept the additional flow and it was determined that connection would need to be made at Sutton Benger Water Recycling Centre (WRC) approximately 3.6km away.

Design evaluations

Whilst the high-level desktop design suggested a pumped solution would be suitable, subsequent topographic surveys indicated that a pressurised main could cause future operational issues due to an increased risk of air entrainment. A gravity solution would avoid these issues, however further site investigations would be required to value engineer the best route to minimise excavation depths within this undulating terrain.

Envolve worked with Wessex Water to undertake this further site investigation work, topographical surveys, and constructability reviews to develop the best route for a gravity solution, resulting in significant TOTEX savings.

Scope of works

Envolve Infrastructure was appointed by Wessex Water’s Engineering and Sustainable Delivery Team (ESD) to provide early contractor involvement (ECI) for the project. The successful delivery partnership collaborated on several value engineering and programme optimisation solutions in order to develop the project through optioneering, detailed design and construction. In March 2021 works commenced to deliver the scheme consisting of:

• 2.5km of 315mm OD Funke Gruppe plastic gravity sewer laid in field and approximately 1.1km in highway.
• 648m of 250mm OD Funke Gruppe plastic gravity sewer laid in fields, including twinning underneath Sutton Benger Brook.
• Construction of 31 (No.) 1200mm PCC manholes and 8 (No.) 1800mm PCC manholes from Marshalls CPM.

Easement for pipeline – Courtesy of Envolve Infrastructure

Delivery model

Envolve are an expertly experienced and industry respected civil engineering specialist organisation with a directly employed workforce consists of multidisciplinary highly skilled professionals who are proactive, experienced, and qualified in the delivery of safe, exceptional quality, value-for-money engineering solutions.

This direct delivery model not only ensured exceptional quality in the construction of the project but extended into first class stakeholder management and engagement with the local community and organisations. All factoring into the ‘One Team’ collaborative delivery strategy for the project.

Early contractor involvement

Through ECI collaboration, Envolve brought forward their specialist engineering knowledge and experience to guide the project team through the design phase to develop a more efficient gravity solution, following localised topography to improve constructability efficiency. The end solution utilised innovative construction techniques, materials, and efficient resource management to maximise engineering opportunities and cost savings for Wessex Water in tandem with achieving industry leading standards of safety, quality, time and cost, aligning the collaborative ethos of the long-term successful partnership between Wessex Water and Envolve.

Engineering challenges/risks

Envolve Infrastructure’s industry leading experience was fundamental in effectively managing challenges and risk throughout the life cycle of the construction project.

• Quality assurance: Envolve’s in house engineering solutions team demonstrated exemplary controls throughout the installation, integrating robust quality assurance processes and testing regime, ensuring challenging gradients and terrain did not affect the planned construction design.
• Terrain: The terrain continued to challenge the delivery team throughout construction. Innovative temporary works solutions were designed by an engineering team comprised of both in-house and external specialists and installed where necessary, supporting excavations in excess of 4m depth.

Sutton Benger Brook crossing – Courtesy of Envolve Infrastructure

• Geotechnical: Mechanical capability for assisting with excavation in rock was resourced from the Envolve fleet. 30T excavators and hydraulic breaker attachments along with specialist operators were required where geotechnical conditions demanded.
• Groundwater: In addition to rock, natural groundwater posed a significant risk on 1km of pipeline which was constructed in the River Avon floodplain. By pulling on the expertise of the collaborative partnership, Envolve worked closely with the Wessex Water environmental team to implement a successful pumping and discharge system that created a dry working area for the delivery team whilst preventing any adverse damage to the local natural environment.
• River crossing: The pipeline crossed Sutton Benger brook, a tributary of the River Avon. The pipeline was installed via a half and half crossing, diverting river flows via installation of a flume and cofferdam, dewatering the working area. Envolve worked closely with the Wessex Water environmental team to provide relevant advice and documentation to obtain the required Flood Risk Assessment Permits (FRAPs) from the Environment Agency. Stranded fish were removed with a hand-net and returned to safety downstream by the Wessex Water environmental team.
• Stakeholders: The proposed route had the potential to affect several highways and Public Right of Way (PROW) routes, landowners and local businesses. Envolve and Wessex Water, carried out early engagement with affected stakeholders including landowners, the parish council and local authority highways department, to understand local requirements and develop plans and procedures to minimise the impact to the community. This continued to be adapted throughout the project delivery, with site teams frequently communicating with affected landowners to minimise impact on farm operation and PROW closures as much as feasibly possible. Positive feedback from landowners from this effective communication from the site team was received by Wessex Water.

St Modwen’s Chippenham Gateway Sewer Supply chain – key participants

• Main contractor: Envolve Infrastructure
• Designer: Wessex Water
• Plastic pipe: Funke Gruppe
• ‘Perfect manholes’: Marshalls CPM
• Fencing & Landscaping: Huw Jones Fencing & Landscaping
• Resurfacing: Walton Civil Engineering & Surfacing Contractors
• Traffic management: Amberon Traffic Management
• Temporary works: MGF Ltd

(left) Gravity sewer pipe from Funke Gruppe and (right) Marshalls CPM ‘Perfect Manhole’ system – Courtesy of Envolve Infrastructure

Opportunities

During the ECI partnership, Envolve undertook a series of constructability workshops involving the entire ‘One Team’ approach to explore delivery opportunities and identify and manage the project risk characteristics. The innovative opportunities identified because of this effort resulted in potential savings in excess of £150k:

• Application of the ‘Perfect Manhole’ system from Marshalls CPM resulting in savings to time and materials and introduced off site construction.
• Reduced trench width with bespoke excavator buckets.
• Trenching machinery in the highway.
• Re-use and recycling of excavated materials utilising a U1 exemption.
• Reduction in pipeline cross sectional specification.

Environmental/carbon reduction

The importance of reducing the carbon and energy impact of services on the climate is an important and valuable objective for the strategic partnership between Wessex Water and Envolve. Civil engineering can also be obstructive to local environments; therefore, the delivery team formulated a strategy to ensure the project left a positive lasting legacy for the local environment and community:

• Inclusion of the Environment Agency into the stakeholder management strategy to plan works through classified Watercourses in line with most appropriate and complimentary methods and planned dates.
• Re-planting of hedgerows and trees with native species.
• Preservation and protection of local bat habitats and the newt habitats located in nearby water courses.
• Safeguarding of local otter families.
• Establishing tree protection zones and safety of trees subject to protection orders.
• Working with archaeological contractors for works subject to watching briefs and special construction methods.
• Re-use of excavated material on site, reducing both landfill and vehicle movements.
• Off-site manufacturing via procurement of the ‘Perfect Manhole’ system from Marshalls CPM.

Reinstatement work underway – Courtesy of Envolve Infrastructure

Social value

A ‘thank you’ present to the residents of Sutton Benger was gifted on behalf of the strategic partnership, by paying for and undertaking much-needed improvements to a community area in the Wiltshire village to mark the completion of the project. Identification of this community project followed conversations with Sutton Benger Parish Council over how best to give something back to the local community for their patience while the Chippenham Gateway scheme was completed. The revamped Neville Terrace ‘gossip area’ was presented to villagers with absolute delight and appreciation of its generosity and community spirit.

Summary

The construction phase was a success and completed on programme in October 2021. Despite many challenges along the way, the strength of the strategic partnership and application of the Envolve Infrastructure ‘One-Team’ approach demonstrates the power of collaboration between client, contractor, and the wider stakeholders. The success of the scheme is not only testament to the skill, dedication, and professionalism of the Envolve direct delivery team and Wessex Water but also demonstrates the benefit of ECI by creating strategic and effective relationships at conception of the project. The relationship between Envolve and Wessex Water is testament to the collaborative approach of the modern civil engineering industry and reinforces Envolve’s reputation for excellence in civil engineering and as the strategic partner of choice.