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Pontoon excavator - Courtesy of Mott MacDonald Bentley[/caption]
The design of the upstream leg of the siphon included a 70m length of 2000mm Ø flanged steel pipework leading from the inlet to the crest, with excavation of the reservoir bed to install at the designed depth necessary to attain the required submergence depth of the inlet end of the siphon. Excavated material could then be utilised as backfill and provide sufficient resistance to thrust forces about bends near the crest. Prior to passing through the crest, the siphon tapers from 2000mm Ø to 1600mm Ø.
Offloading pipes - Courtesy of Mott MacDonald Bentley[/caption]
Cofferdam installation around the concrete core - Courtesy of Mott MacDonald Bentley[/caption]
Working closely with Groundforce, MMB assessed different phases of the work and how the cofferdam could be optimised to suit construction requirements. The existing concrete core was utilised in the design to resist frame loadings throughout, alongside whilst the bottom frame was designed to be removed following the casting of the slab to facilitate further construction of the siphon.
Installation of the large diameter siphon pipes - Courtesy of Mott MacDonald Bentley[/caption]
Installation of the large diameter siphon pipes - Courtesy of Mott MacDonald Bentley[/caption]
Screenshot of Synchro software showing workings around the crest - Courtesy of Mott MacDonald Bentley[/caption]
Concrete core: (top left) before and (bottom left) after wire cutting with 82t of concrete removed, and (right) Reinstated concrete core following installation of 1600mm pipe prior to connection with the upstream leg - Courtesy of Mott MacDonald Bentley[/caption]
Field where the lagoon will be sited - Courtesy of Mott MacDonald Bentley[/caption]
Early construction of the lagoon - Courtesy of Mott MacDonald Bentley[/caption]
Lagoon liner installation - Courtesy of Mott MacDonald Bentley[/caption]
Security gate installed to allow access to lagoon area - Courtesy of MMB[/caption]
Pipework and valve arrangement within the valve chamber – Courtesy of MMB[/caption]
The tunnel cross-section is a 2.4m diameter horseshoe shape. Access to the upstream end of the dry side of the tunnel is necessary to operate, inspect and maintain the new pipework and valve arrangement. The pipework arrangement and diameter needed to consider the ability to remove sections of pipework if required at a later date.
A long list of options were developed, however the chosen solution was to replace the twin 18-inch pipework with a single larger diameter pipe, offset to one side of the tunnel. This option maximised the outflow from the reservoir and maximised space for access, inspection and maintenance. To merge the two 18-inch pipe sections from the tunnel plug, an asymmetric manifold was designed to combine the flow, with guard and duty valves upstream of this. Enhanced releases are provided by two flow control valves installed offline to the new drawdown pipework, that could be remotely operated using a telemetry system located in a new control kiosk.
Other options considered during the optioneering included:
(left) Gate valve transportation and (right) bulkhead valves with concrete encapsulation - Courtesy of MMB[/caption]
Following the feasibility stage, the pipework system was further optimised to improve hydraulic efficiency whilst maintaining velocities to a suitable level. The manifold was optimised to achieve balanced flows between both legs, to prevent significantly higher velocities within a single leg. The results of the optimisations enabled the diameter of the larger pipework to be reduced from the initial proposal at the feasibility stage, leading to a reduction in material costs and embodied carbon by approximate 10%.
The diameter of the larger pipework was 800mm, and the flow rate measured in the commissioning phase was approximately 5m3/s.
Siphon and over pumping intakes - Courtesy of MMB[/caption]
Valve chamber and tailbay - Courtesy of MMB[/caption]
The historic valve house was demolished and replaced with a new buried valve chamber, and the tailbay was relined with a new reinforced concrete structure. This was due to the visual condition of the historic structures, to create a larger area for the new valve and access arrangements, and to facilitate the installation of the new pipework and valves.
The first task within the tunnel was to remove the three existing pipes. During the feasibility surveys it was found that the existing pipework and fittings contained asbestos, MMB commissioned a specialist asbestos contractor to remove the pipework adopting a pipe wrapping containment and use of a wire saw cutting technique. Once the pipework was removed all existing concrete pipe supports were also removed and the existing tunnel invert made good. Once all of the existing pipework had been removed up to the existing guard valves (at the tunnel bulk head), works to remove the existing duty valves and replace with new continued under client Gold Command control due to the single isolation nature of the works.
The works then progressed onto installing the new twin 450mm to 800mm diameter carbon steel pipework in tandem with installing a new level concrete tunnel floor. Due to the limited space within the tunnel and lack of lifting facility MMB used a small pick and carry crane which was able to transport the new pipework into the tunnel for installation. The tunnel was 180m in length with two 45° bends, this presented some construction challenges due to the limited flexibility in the flanged system.
The new valve house arrangement was then installed and incorporated the compensation, enhanced releases, and scour valves complete with extension spindle and pedestal arrangements for operation via electrical actuation on the new raised flooring arrangement.
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(left) Scour pipework arrangement at the bulkhead and (right) scour pipework & thrust block - Courtesy of MMB[/caption]
(left) Condition of the spillway blocks prior to the works and (right) new concrete slab with stainless steel protection to the joint - Courtesy of Esh-Stantec[/caption]
Tunstall Reservoir dam - Courtesy of Esh-Stantec[/caption]
Tunstall Reservoir spillway - Courtesy of Esh-Stantec[/caption]
(left) View in the unlined section of the tunnel (notice the beams across the soffit) and (right) view of pipe inlet and partial headwall (notice standing water on the invert) - Courtesy of Esh-Stantec and CC Informatics[/caption]
Various options were considered during the Concept stage. The option of not filling the tunnel was disregarded as it would not eliminate the potential risk to the integrity of the dam embankment and spillway channel. Filling the entire extent of the tunnel was considered, however it was not preferred due to the significant constraints associated with it. These constraints include the difficult access and requirement to work in a confined space, potential land ownership issues, the presence of a SSSI immediately to the east of the abutment, and the increasing depth to the tunnel as it continues beyond the spillway. For all of these reasons, it was agreed with that filling the extent of tunnel beneath the dam and spillway would be sufficient as that would eliminate the risk to these structures.
(left) View of the geopolymer in the tunnel, and (right) view of the pea gravel stop end - Courtesy of Esh-Stantec and CC Informatics[/caption]
Overview of the siphon diversion route in preparation for the works - Courtesy of Esh-Stantec[/caption]
Summary chart from the reservoir drawdown hydraulic analysis - Courtesy of MMB[/caption]
Pipes and thrust restraint
Hoops stresses within the DN700 mild steel siphon pipes were checked and a DWI Regulation 31 coating specified. The stainless steel pipe supports were analysed for bending and shear and concrete thrust blocks were designed for the system test pressures. The pipe wall thickness was reduced from 10mm to 8mm as an efficiency. The pipework arrangement drawing was uploaded into the federated model in BIM360 to check for clashes and when installed on site was found to fit perfectly.
Ground investigation and geotechnical analysis
Following the original construction of West Hallington Reservoir (between 1884 and 1890) the Resident Engineer Charles Henzell wrote a report to the Institution of Civil Engineers describing the works and detailing a number of challenges which included:
(left) Cable percussive borehole rig on the crest of the embankment and (right) scaffold decked area required for rotary bore works on the slope - Courtesy of MMB[/caption]
A desk study and intrusive in situ ground investigation (cable percussive and rotary bored boreholes, standard penetration tests (SPTs), piezo installation, lab testing) were carried out to classify and determine the thickness of the superficial and solid geology along with groundwater levels, identify any contaminated or aggressive ground and determine the engineering properties of the native soils. Slope stability and ground bearing pressure checks were carried out to ensure large plant would not compromise the stability of the embankments.
The clay core and embankment material of the Pennine-type dam was found to be very silty and exhibit poor engineering qualities and so was disposed of, and a source of good quality puddle clay and Series 600-2C material was found locally at a brick factory in Birtley, Gateshead. This was installed in compliance with the test and inspection plan (T&IP) to a good standard.
Thankfully the three springs in the west embankment mentioned in the Henzell report were not encountered.
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Extreme weather conditions during the construction stages: (left) winter 2022 and (right) flooding from Storm Babet (October 2023) - Courtesy of MMB[/caption]
Temporary works
A borehole at the toe of the embankment on the dry side indicated the presence of water-bearing sands and gravels and so interlocking sheet piles were specified that punched through the sands and gravels into the underlying clays and so cut off groundwater flows into the excavation.
A Portadam supplied and installed by OnSite Central was employed to provide a dry area to construct the intake bay. The Portadam was erected in the wet by operatives wearing buoyancy suits, and then pumped out and an excellent seal was achieved. The Portadam also allowed plant access around the pipework whilst the stone pitching was reinstated.
Flood risk assessment
A flood risk assessment (FRA) was carried out to support the planning permission and, as due diligence, to ensure MMB would not flood a property by accident. The assessment looked at the extent of flooding during operation of the siphon along with fluvial, surface water, groundwater and finally reservoir flood risk due to an uncontrolled release (which of course the siphon is designed to help prevent). The FRA concluded that:
A screen dump from the SYNCHRO 4D simulation - Courtesy of MMB[/caption]
The simulation highlighted a number of pinch points when materials must be delivered or when water levels must be lowered or raised and helped drive efficiencies. SYNCHRO 4D was also useful for explaining the construction process to the client, to site operatives and to visitors.
Solar power
The mechanical equipment associated with the siphon is housed within a high security kiosk. Due to the remote site location, the cost of a permanent power supply for the kiosk building services was prohibitive. A solar powered solution has been installed utilising roof mounted PV cells, DC/AC converter and battery storage.
This sustainable solution provides sufficient energy for lighting and heating within the kiosk with negligible running costs.
Solar panel display - Courtesy of MMB[/caption]
Siphon intake bay constructed within a Portadam barrier supplied and installed by OnSite Central - Courtesy of MMB[/caption]
Stone pitching
Reinstatement of the original stone pitching (to resist wave action) took seven weeks longer than planned; mainly down to availability of stone masons. Productivity increased as the works progressed and the three man gang went from replacing 3 to 5m2 per day. The stone pitching was reinstated to a good standard, but it was felt that the detailing around one of the pipe supports would be built differently next time.
The 2,500m perimeter of the reservoir had been faced with stone pitching down to original ground level; installation of which in the 1880s without an excavator and selector grab must have been a job and a half!
Ecology
Prior to the construction works, NWG employed OS Ecology to undertake trapping for white clawed crayfish, a native species that is in decline due to the introduced North American signal crayfish. No white clawed crayfish were found within West Hallington Reservoir but three were trapped within the adjacent East Hallington Reservoir. A pied wagtail nest was spotted by eagle-eyed site workers next to the engine of an excavator. The excavator was withdrawn from service until the eggs had hatched and the fledglings left the nest.
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Reinstatement of the stone pitching - Courtesy of MMB[/caption]
DN700 siphon pipework during installation - Courtesy of MMB[/caption]
The original spillway channel - Courtesy of MMB[/caption]
Completed masonry cladding on spillway, and (inset) wall raising showing new and existing masonry - Courtesy of MMB[/caption]
Upstream leg of siphon - Courtesy of MMB[/caption]
Due to the levels of communication between Stantec and MMB, this additional chamber was designed out and the chamber was moved into the stilling basin, prior to it being backfilled to contain the spillway flows. This reduced additional excavation works and aided in the construction programme, promoting some carbon efficiencies. Good communication between both parties ensured that both projects were capable of being delivered within programme and reduced abortive works.
Fibre reinforced concrete tumble bay slab - Courtesy of MMB[/caption]
The site team and stone masons chosen for the scheme comprised of the majority of those who had worked on the previously delivered Swinsty Spillway structure and had an in-depth working knowledge of constructing these structures. Their knowledge and experience greatly aided with the construction of the scheme and achieving the challenging programme.
The siphon works required a significant draw down of the reservoir level to be able to work on the inlet structure safely. As this was quite a long duration for the works, it would impact the programme significantly and equally impact the spillway structure. It was decided to work from both ends of the pipe where practicable and to use a make-up piece at a bend on the downstream side of the embankment. This would aid in any minor tolerance issues in the pipework and ensure that the pipe closed correctly.
To aid the construction of the pipework, the ICON system was used for setting out. This involved directly using the 3D modelling from the design of the pipe into a handheld tablet device and enabled accurate setting out, without the need to update drawings or continually check that the drawings were the most relevant. This also aided in delivering the as built positions of the pipework for the updates to the completion drawings.
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3D model showing the interaction between the spillway and siphon schemes - Courtesy of MMB[/caption]