Supply Chain - Project Planning & Surveying - Archaeology

The water network in the western end of the Thames Valley Supply Area is dominated by local borehole water sources. Over time, population growth coupled with general environmental drivers to reduce reliance on groundwater sources in sensitive chalk stream areas, Thames Water has sought to improve supply resilience and deployable output by linking local sources via trunk mains. This allows movement of water from one area to another as well as to reliably supply the intermediary supply zones. Typically, water from Oxford that is produced at Farmoor Water Treatment Works is transferred via a series of trunk mains to maintain resilient customer supplies to approximately 90,000 people in Swindon and the surrounding supply areas.

Background

The existing Faringdon Water Booster Station (WBS) to Blunsdon Reservoir transmission system comprises a 700mm diameter asbestos cement pipe and a 900mm diameter glass reinforced plastic (GRP) pipe built in 1987 and 2001 respectively. This transmission system is approximately 15.8km long and has various interconnections between the pipes to provide resilience in service. The Faringdon WBS to Blunsdon Reservoir 900mm GRP main represents one leg of this network. Due to the high frequency of failures, which had a significant impact on Thames Water’s ability to transfer treated water into the Swindon area, work began in AMP6 to replace the highest risk section in the Highworth area and 2.8km was completed, including a crossing of the River Cole. Following the successful replacement in AMP6, and an increased deterioration of the existing 900mm GRP main, the remaining balance of scope (13.7km) was approved for delivery in AMP7. To mitigate the frequency of bursts and the impact of the deteriorating asset on customer supplies, temporary emergency abstraction licences at Baunton WTW and Latton WTW were secured with the Environment Agency to allow the existing GRP main to be operated at a lower duty.

Project summary

The principal drivers for the project are:

• To reduce the supply interruption risk to customers in Swindon and the Cotswold area. In a reasonable worst-case scenario, without the current mitigations it could equate to over 14 million minutes.
• To minimise the risk of customer complaints associated with low pressure and/or no supply. In a reasonable worst-case scenario, without the current mitigations this could equate to over 90,000 properties.
• To eliminate reliance on groundwater sources under emergency licences to cover the loss of flow during a burst.
• To reduce reliance on groundwater sources that draw from chalk streams and the River Kennet during high demand and drought conditions.
• To increase trunk main capacity and transfer rates to accommodate increasing flows for future population growth (principally in the immediate Swindon area).
• To eliminate the frequent and expensive repair costs of the existing GRP main sections.
• To restore Thames Water’s trunk main network resilience and bulk transfer capability in this area, which is fundamentally impaired when the Faringdon to Blunsdon main is operating at a reduced duty rating.

[caption id="" align="alignnone" width="1200"] Google Maps image showing pipeline route[/caption]

Planning and design

Pipeline route: Initial design work was completed during AMP6 by Stantec UK as part of the Phase 1 delivery. The proposed route closely follows the existing 700mm asbestos cement and 900mm GRP mains, and there are several advantages to following this existing route:

• This represents the most direct route between Faringdon WBS and Blunsdon Reservoir thus minimising the overall length of pipework required.
• It is mainly through farmland and minimises the need to work in residential areas.
• It minimises impact on landowners.
• Ease of access for Thames Water through use of existing way leaves
• Existing information from previous investigations can be utilised.

The AMP7 project was awarded to Kier Infrastructure and Overseas Ltd when detailed design of the pipeline route was reviewed and refined, taking account of ecological constraints, requirements from landowners and planned developments along the route. Archaeological mitigation: A programme of archaeological mitigation was undertaken by Wessex Archaeology following consultation with the relevant county archaeological advisors. A mitigation strategy comprised excavation, recording and the strip, map, and sampling. The archaeological excavations formed part of a staged approach consisting of an archaeological desk-based assessment, geophysical surveys, and archaeological evaluations. Through collaboration with the approving authorities, Thames Water adopted targeted route changes to avoid known areas of high archaeological interest. Generally, a small number of finds were found with possibly the most significant being a crouched burial in the Blunsdon section of the pipeline. [caption id="" align="alignnone" width="1200"] Archaeological investigations and (right) crouched burial - Courtesy of Kier Infrastructure and Overseas Ltd[/caption] Pipe material: As part of the design deliverables a technical note was prepared comparing ductile iron, carbon steel and HDPE as the three commercially feasible materials for this diameter range. The recommendation was to adopt HDPE PE100 as the preferred pipe material, using 1200mm OD SDR17 from Aliaxis UK. Pipework used in cross connections, existing utility crossovers, shafts and other special cases was designed using ductile iron supplied by Saint Gobain PAM UK, with mechanically restrained VE joints as required. Furthermore, for the tunnel sections, steel pipework with the E-Joint system was utilised supplied by Noksel España. To ensure asset design life, steel pipework was maximised for the critical tunnel sections with pipe wall thickness specified at 10mm, which was greater than Thames Water’s asset standard of 6mm. Based on the pH levels of the water the pipe would be conveying, a far superior asset design life of 100+ years could be theoretically achieved. Environmental impact assessment (EIA): During the initial stages of the project, several desktop and detailed environment, archaeological and third-party assessments and reports were compiled. This highlighted areas where further surveys and investigations were later completed ahead of an Environmental Impact Assessment Screening Opinion being submitted to the relevant councils for review. A full EIA was requested but this was, however, successfully challenged with the Secretary of State who did not consider the environmental impact would be of a magnitude necessary to suggest a scheme of this scale would result in significant environmental effects, which would exceptionally necessitate an environmental statement. This successful challenge still obligated Thames Water to work closely with the local approving authorities to deliver a comprehensive programme of environment and archaeological mitigation works for the route. [caption id="" align="alignnone" width="1200"] Pipes laid along route - Courtesy of Kier Infrastructure and Overseas Ltd[/caption]

Faringdon to Blunsdon Trunk Main Phase 2: Supply chain - key participants

• Project delivery: Kier Infrastructure and Overseas Ltd
• Designers & specialist consultants: WSP
• Sahara survey: Water Research Centre Ltd
• Archaeological investigations: Wessex Archaeology
• Civil Engineering Contractor: CPC Civils
• Civil Engineering Contractor: OCC Services Ltd
• Trenchless crossings: HB Tunnelling Limited
• Shoring/temporary works: GroundShore Ltd
• Tunnel boring machine: Herrenknecht
• Precast jacking pipes: FP McCann
• HDPE PE100 pipe: Aliaxis UK
• Ductile iron pipes: Saint Gobain PAM UK
• Steel pipework: Noksel España
• Tunnel roller spacers: FT Pipeline Systems
• Pipe fittings, bends, tees & collars: George Green (Keighley) Ltd
• Pipe fittings: Wolseley
• Pipe fittings: Electrosteel Castings (UK) Ltd
• Pipe fittings: PTS (SA) Limited
• Valves: AVK UK Ltd
• Welding: Fabricon Ltd
• Welding: PTS Group
• Butt fusion welding machine: McElroy Manufacturing
• Independent weld inspector: Technical Innovation Servis Ltd
• Temporary fencing & gates: RF Gardiners Ltd

Construction

The project was classified as a ‘high consequence project’ requiring an enhanced level of quality assurance. To achieve this, several measures were implemented. As part of this strategy, the delivery contractor Kier Infrastructure and Overseas Ltd was briefed to deliver the project in phases to tactically reduce Thames Water’s operational risk until the project was fully commissioned. The project brief also included decommissioning of the existing fragile 900mm GRP which will be safely decommissioned following commissioning of the new DN1200 pipeline. The contractor will make allowances to ensure that the existing 900mm GRP main can be slip-lined in the future for additional resilience. [caption id="" align="alignnone" width="1200"] (left) Drive shaft, (middle) tunnel pipe insertion and (right) the completed tunnel - Courtesy of Kier Infrastructure and Overseas Ltd[/caption] Tunnelling: Open cut techniques were the predominant pipe installation methodologies and were undertaken by CPC Civils and OCC Services Ltd. However, several trenchless crossings were required for the major roads and rivers crossed by the pipeline route. The relatively straight forward digger shield tunnel boring machine was the predominant method used for trenchless crossings, including Bydemill Brook and the A361. The crossing of the B4019 proved more challenging however, when hard rock was discovered. This was because of a last-minute change in the location of the micro-tunnel shaft to avoid a new attenuation pond being built by a developer in the immediate area. Following a review of the new geotechnical investigation completed, it was agreed that the previous digger shield tunnel boring machine was not suitable to overcome the hard rock. Instead, the more robust variable density tunnel boring machine was employed with a specialist drill head brought in from Herrenknecht in Germany to cope with the density of the rock. Although the deployment of this complex machine took four weeks to set-up, the 180m long, 1.5m diameter tunnel was constructed in no time at all and was successfully completed in just over eight weeks. The trenchless crossings were undertaken by HB Tunnelling Limited. Pipe welding: An automatic butt fusion welding machine was utilised on the project to enable the joints to be made in what was an industry first. The machine from McElroy Manufacturing was shipped from the United States for the project, and enabled butt fusion joints to made up to 1200mm in diameter; a first of its kind in Europe. The automatic nature of the machine enabled far higher quality welds to be made and on a consistent basis as the human element of checking temperatures, raising of the welding plates etc. was eliminated. [caption id="" align="alignnone" width="1200"] Pipe welding operation and (inset) McElroy Manufacturing welding machine - Courtesy of Kier Infrastructure and Overseas Ltd[/caption] Welding work was undertaken by Fabricon Ltd and PTS Group with Technical Innovation Servis Ltd employed as an independent weld inspector to provide an additional level of assurance on the quality of the butt fusion joints made on the 1200mm OD PE100 SDR17 pipework.

Commissioning

Due to the large volume of water involved in commissioning each section of the new pipeline and significant flow rates required to achieve water quality standards, flushing lagoons were constructed to allow the safe and controlled disposal of the flushing water.

Working in the community/customer communications

Following disruption to landowners over many years due to frequent bursts on the existing GRP pipe, all contractors have been fully focused on maintaining and improving relationships with all landowners. At the time of writing (May 2023), Kier Infrastructure and Overseas Ltd and CPC Civils have worked very successfully with all landowners along the new pipeline route developing great working relationships. This has been demonstrated on a few occasions particular during the funeral of one landowner who CPC Civils had developed a great relationship with, when works were stopped so all workers could pay their respects. [caption id="" align="alignnone" width="1200"] (left) Funeral cortege - Courtesy of CPC Civils, and (right) Salvation Army Christmas present appeal 2022 - Courtesy of Kier Infrastructure and Overseas Ltd[/caption] Kier have also worked well with local councils donating materials for community initiatives, with CPC Civils also donating two portable defibrillators to support local campaigns, as well as supporting a Salvation Army present appeal on the run into Christmas 2022 to support families who would otherwise be unable to afford gifts for their children. The project has also given the opportunity to work with the royal agricultural university and bring students to site to understand the ecological and environmental impact and considerations on a project of this scale, as well as visiting their workshops to build owl boxes to be installed along the route of the pipeline.

Summary

Design of the £45m Faringdon to Blunsdon Pipeline Project commenced in November 2020 followed by a construction start in May 2021. [caption id="" align="alignnone" width="1200"] Flushing lagoons - Courtesy of Kier Infrastructure and Overseas Ltd[/caption] Commissioning and beneficial use of the pipeline is expected in November 2023 with all site works being completed by August 2024. This will complete replacement of the 15.8km transmission system providing resilience to supplies to help meet future supply and demand requirements.

Essex and Suffolk Water, part of Northumbrian Water Ltd, supplies drinking water to 794,000 properties. Maintaining a supply to those customers requires the distribution of up to 670 Ml/d of high-quality drinking water. In Essex, the two primary treatment works are Hanningfield WTW, a physio-chemical works near Chelmsford and Layer WTW, a biological works, near Colchester. These are supplied by raw water stored in two large reservoirs at Abberton and Hanningfield. In 2016, and more recently in 2022 low levels of rainfall and hot weather compromised the raw water quality in Abberton Reservoir which led to significant challenges to the biological process at Layer WTW. This meant unprecedented demand was placed upon the chemical works at Hanningfield WTW, which in turn drew down the water level in Hanningfield Reservoir to levels not seen since the summer of 1996.

The challenge

Whilst this low water level at Hanningfield Reservoir was observed, the level in Abberton was healthy due to the process constraints of the biological works at Layer. To mitigate the scenario of having two reservoirs, one relatively full, and another extremely low, occurring in the future, Northumbrian Water applied for enhancement funding in AMP7 to construct a 20km pipeline between Layer WTW and Langford WTW, also a physio-chemical works, where water from Abberton could be reliably treated. This in turn would allow water from the River Chelmer and River Blackwater that would normally be treated at Langford, to augment the water level in Hanningfield Reservoir by utilising existing transfer pumping facilities between Langford and Hanningfield Reservoir. Providing this connectivity, albeit by substitution, will allow Northumbrian Water to balance the water levels in both Abberton Reservoir and Hanningfield Reservoir, and mitigate a potential scenario where no water is available to supply Hanningfield WTW.

The project

In 2019 Essex & Suffolk Water teams began to plan a pipeline route between Layer WTW and Langford WTW for a pipe that could support flow rates up to 50 Ml/d. This figure was determined as the optimum flow rate to allow balancing of the reservoirs to take place and ensure security of supply when faced with long term drought conditions. The pipeline will take advantage of the capacity and capability of the existing pumping station at Abberton which can deliver up to 255 Ml/d to Layer WTW. The route would begin with a strategic connection into the existing raw water mains that deliver water from Abberton Reservoir into Layer WTW. From this point a route was sought that would allow water to run under gravity conditions to Langford WTW where it would be discharged into two existing bankside storage reservoirs each with a capacity of 35 Ml. The pipeline from Layer to Langford would operate as a gravity main, with the upstream head being set by the pressure in the rising main at the take-off point. With approximately 5 bar of available driving head, the flow out of the pipeline will be controlled by a downstream flow control valve. The pipe sizes have been selected to ensure a positive pressure is available throughout the pipeline and to provide sufficient differential pressure at the outlet at maximum flow.

Hydraulic profile

The hydraulic profile shown below (pipeline pressure is shown in red) extends from Abberton Pumping Station to the outlet at Langford WTW. Chainages are measured back from the Langford outlet (at chainage 0m) and the start of the new pipeline construction is at approximately chainage 20,000m. Note the falling available head due to friction losses along the pipeline and its change in gradient at approximately chainage 12,000m. This change in gradient is due to a change in pipe size which is the result of the pipeline optimisation. [caption id="" align="alignnone" width="1200"] Hydraulic profile - Courtesy of Stantec UK[/caption] The direct connection from the pumped rising main to Layer introduced a number of hydraulic complexities. A key issue was the control of transient pressures, particularly associated with a sudden loss of output at Abberton Pumping Station. Existing surge vessels provide robust surge control, however, the addition of the 20km extension to Langford would require careful consideration. The time to close the outlet valve at Langford, particularly following a sudden loss of pumping at Abberton, would need to be sufficiently long to avoid exacerbating transient pressures while a closing time too long would risk depressurising the pipeline and the risk of drawing air into the pipeline. The occurrence of transient air pockets at high points could create further transient pressures if encountered by the initial pressure waves. While these challenges could be largely avoided by separating both pipelines entirely with a break tank, the tank would need to be sufficiently large and elevated to provide the same driving head. The design team are, instead, progressing an optimised solution to avoid these additional capital works through thorough detailed understanding of the problem, careful selection of the control valves and the implementation of a control system that remains simple yet capable of maintaining pressures within acceptable limits.

Ground investigations

To support the design of the pipeline Essex & Suffolk Water has now completed ground investigation work along the route to determine ground and groundwater conditions and help inform excavated material management decisions. The ground investigation work comprised:

• 63 dynamic sampler boreholes up to 6m in depth.
• 11 cable percussion boreholes up to 10m in depth.
• 27 soil resistivity tests.
• Post-installation groundwater monitoring at approximately 12 locations.

This work was completed by Essex & Suffolk Water’s framework partner Allied Exploration & Geotechnics (AEG) in October 2022 based on a specification written by Stantec UK. This ground investigation work will help Stantec to determine the appropriate pipeline material. [caption id="" align="alignnone" width="1200"] (left) Ground investigation work and (right) pipe installation - Courtesy of Essex & Suffolk Water[/caption]

Pipe material selection

The use of steel, ductile iron (DI) and polyethylene (PE) were all considered. Due to aggressive ground conditions, appropriate corrosion protection would be needed for ferrous materials. PE would remove the corrosion risk but the large diameters needed and the associated pipe thickness would require careful control of welding during construction. At the time of writing (June 2023), final selection is ongoing but, following initial assessment of cost, constructability and maintenance issues, either an all DI pipeline or a mix of DI and PE are considered most favourable.

Project planning

The project is currently moving through the detailed design phase and one activity that has been conducted concurrently is the liaison with the council districts that the pipeline crosses, both Colchester City Council and Maldon District Council. In the experience of Essex & Suffolk Water, large diameter cross country pipelines have typically required Environmental Impact Assessments to support the planning process. For this project, an alternative approach was taken where the team designed the pipeline and its construction methods, including associated compound areas, to have the least possible impact from a number of perspectives including ecology, visual impact and traffic and transport in advance of submitting our EIA Screening Request. This design, including examples of how measures were introduced to provide a less impactful route than one that may have been presented prior to completing these mitigations, helped support the EIA Screening request. In response to that request both Councils viewed the screening request favourably and allowed the team to complete the work using permitted development rights recognising the efforts of Essex & Suffolk Water to plan and design the least impactful route possible.

Archaeology

Essex & Suffolk Water now plans to complete the archaeological work associated with the project over the summer period of 2023. Throughout the early phase of the project Essex & Suffolk Water has worked closely with AECOM to complete a desk based assessment and then develop a Written Scheme of Inspection. This has now been approved by the council archaeologist and comprises 552 trenches measuring 2m x 30m across a 40m corridor along the 20km pipeline route. This work will be completed over a 16 week period, 8 weeks for the field work and a further 8 weeks for recording and report preparation. Due to the archaeological history in the area it is expected that some archaeological artefacts of interest will be uncovered. This may in turn necessitate further mitigation trenching along the route.

Layer to Langford Pipeline - Supply chain (as of June 2023)

• Principal designer: Stantec UK
• Ground investigation: Allied Exploration & Geotechnics
• Land owner engagement: Savills
• Technical input: Saint Gobain PAM UK
• Archaeology: AECOM
• Pipe installation: Northumbrian Water
• Pipe supplier: Electrosteel Castings (UK) Ltd

Schedule

The installation of the majority of the 20km pipeline will begin in February 2024 in order to complete the work in March 2025, allowing Essex & Suffolk Water’s in house team to complete the installation in one pipelaying season. Work has already commenced on a number of minor road crossings to coincide with the work of other developers in the area and minimise impact on our customers. Work is also scheduled to commence on the directional drill crossing that will be required beneath two main rivers as part of the project, the River Blackwater and the Langford Cut in September 2023.