United efforts at Grane Valley reservoir26 May 2014
Recent safety recommendations involved addressing stability and overflow capacities at the Grane Valley reservoir system in Lancashire, England. Here Daniel Thomson from United Utilities gives more information about the construction of a wedge block spillway: the first of its size ever used at an impounding reservoir scheme in the UK.
The Grane Valley system in Haslingden, Lancashire in England is home to three impounding reservoirs (IRs) in a cascade system. The cascade comprises of Holden Wood IR, Ogden IR and Calf Hey IR.
Holden Wood IR lies at the bottom of the cascade, has a capacity of 367,000m3 at the top water level (TWL) and was constructed in 1841 by local mill owners. The reservoir has had several functions since it was built: originally supplying water to local mills, and then to the bleach works industry sited at the toe of the dam. It now supplies compensation water to the Ogden Brook to ensure water levels are suitable to sustain fish habitats.
The dam is retained by an earthfill embankment with a clay core, is 100m long and 18m high, with an upstream slope of 1:3 and a downstream slope of 1:2. The dam crest is 3.5m wide and carries an access road to a local property and farm land.
In 1900 Bury and District Joint Water Board decided to construct Ogden IR, which had been originally looked at in 1836 and again in 1872 to supply Manchester with water. The reservoir was commissioned for construction in 1902 for a tender price of £101,507 and was completed in March 1912.
Ogden IR has a capacity at TWL of 1,501,000m3 and is used to supply raw water to Haslingden Grane Water Treatment Works (WTW) located on the northern hillside downstream of the dam. It is a Pennine earthfill embankment 400m long and 15m high. With a crest width of 4.5m, it has a downstream slope at an incline of 1 in 2.5 with a berm half way down, and an upstream slope of 1 in 3. The reservoir has by-wash channels extending along both sides of the reservoir which discharge down cascades at each mitre.
Safety: designing a new scheme
The Grane Valley IR project was conceived in response to recommendations made under Section 10 of the Reservoirs Act 1975. Individual inspections were undertaken at Ogden and Holden Wood IRs by an independent inspecting engineer who needed to ensure that the dams were in good order to comply with the Act. Both of the reservoirs were provided with recommendations with regulatory dates to investigate the stability and overflow capacities.
As a result of flood studies and physical model testing, United Utilities (UU) commissioned remediation measures at both Ogden and Holden Wood IR, as they could not pass the design flood.
In Autumn 2009, UU Engineering in collaboration with MWH Engineering Consultants commenced the concept design of the scheme. By the summer of 2012, Eric Wright Civil Engineering (EWCE) was appointed as the main contractor and P&S Consulting Engineers were appointed as the detailed designers. UU Engineering and Construction departments played a substantial role in the review of the detailed design and ensuring the scheme was built to the specifications set out in the contract.
Work commenced on site in July 2012 and was completed in October 2013. All three reservoirs are back to TWL and Ogden IR is back to supplying Grane WTW at full capacity.
Teamwork between all parties enabled the project to be delivered 40 weeks ahead of its 104 week contract programme, 48 weeks before the regulatory deadline. With an overall delivery cost of £8.463M this project achieved completion within budget.
The works at Ogden IR included the construction of 21m wide, reinforced concrete (RC) diversion structure on the southern by-wash to divert flows from Calf Hey into Ogden IR under normal operation. This was under an advanced works contract which assisted the construction of the downstream main contract works by not flooding out the construction activities.
Detailed geological mapping of the local area also showed that the left and right abutments to the dam were an area of potential ancient post glacial landslips. It was considered that forming excavations to enable construction of deep overflow channels could re-activate these landslips. As a result the existing northern spillway was chosen for upgrading the overflow capacities. Due to the geometry of the existing masonry spillway and head over weir limitations, it was calculated that an auxiliary spillway was required.
Improvements to the existing spillway included construction of a 190m long, 4.5m wide new RC lining to increase its overflow capacity and resist the effects of masonry plucking of spillways. The lining increased the original wall heights. At the toe of the dam the existing tunnels were removed and replaced with a new 6m wide, 2.8m deep RC channel with an RC "bus shelter" (RC overhang) which prevents the supercritical flow rising up the wall.
The auxiliary overflow facilities involved the construction of a new 32m wide wedge block spillway. This included:
- A 14m long RC crest structure with access bridge and precast Ogee weir units.
- A 42m long wedge block spillway designed to CIRIA Guide 142 with 3m wide revetment block abutments at an incline of 1 in 3.
- A 20m RC stilling basin with baffle blocks and end sill.
- An overland flow channel protected with geotextile erosion matting.
In order to resist the loading effects from the spillway 52 individual, 10m long, 0.6m diameter contiguous flight auger (CFA) piles were keyed into the stiff glacial clays. The spillway consisting of over 1300 individual 1m x 1m wedge blocks was chosen for many of its design and construction advantages over a standard RC spillway on an earth embankment dam.
The benefits of this design included the ability to see any potential movements in the dam as the individual blocks move with the dam giving the ability to notice any sink holes forming or significant slippages.
Being a stepped structure, the main hydraulic benefit was that the energy would be dissipated on the sloped section of the spillway, which in turn reduced the depth and width of the stilling basin at the toe, lowering the risk associated with dam safety. From a construction perspective the benefits were that the blocks were easily lifted by a 13T excavator and were positioned by hand. The volume of concrete that was poured on a steep slope was also reduced and the formation was prepared without any problem due to it being granular.
The wedge block spillway was constructed from individual blocks, which were constructed as pre-cast elements with stringent quality checks to enable ease of installation on site. EWCE set up a quality system from mould construction to delivery on site that led to no wastage of unusable blocks.
New channel construction at Ogden IR
Other work which was carried out at Ogden IR included raising of the core. The existing core was raised in concrete in the 1990s and the scope of the new works included raising the core of the 400m long embankment in concrete.
Improvements were also made to the southern by-wash channel by constructing a 110m long, 4.5m wide RC lining prior to the tunnel entrance at the toe of the dam. This was to prevent the effects of masonry plucking and delamination of the masonry base. As part of these works there was also a 120m long, 4.5m wide RC channel constructed above the existing tunnel, which transports water from the southern by-wash from Calf Hey IR. This channel is 1.2m deep along its length until it passes through the end of the auxiliary spillway structure.
Due to potential settlement issues, the existing tunnel was filled with foam concrete along its entire length which was chosen due to its self-levelling properties and health and safety benefits during the construction phase. This new channel continues downstream where it widens to 8m and drops 2.5m into a deep structure with an energy dissipation wall and then connects into the northern spillway.
In order to construct the new RC channel at Ogden IR, substantial temporary works were required to route the 4.88m3/sec scour flow through to Holden Wood IR. This included a 50m long by 10m deep larson sheet piled wall to ensure that there was uninterrupted scour flows. The method of construction to pour the RC walls of the new RC channel utilised a 100T crawler crane that was sited on crane pads designed and constructed to BRE Report BR470, Working Platforms for Tracked Plant.
To construct the northern spillway lining at Ogden IR, a push pull system was utilised by EWCE to construct a 5m spillway lining section for both the western and eastern walls.
The work which was carried out also included the diversion of three pipes on site which were affected by the above construction activities. These pipes were a 600mm raw water pipe, a 450mm and 225mm potable water supply pipes.
Holden Wood IR
At Holden Wood a new 15m wide, RC overflow facility was constructed. This included:
- A 14m long RC crest structure with access bridge and precast Ogee weir units.
- A 49m long, 1.8m deep RC spillway.
- A 30m long, 4.5m deep RC stilling basin.
- A 25m long, 4.8m deep transition channel.
The crest structure was built through the existing RC clad wave wall which was cut using a diamond chainsaw with a large diameter saw to ensure that there was minimal restoration required between new and old structures. Approximately 120m3 of Japanese knotweed was also removed from the footprint of the new overflow and spillway facilities. This resulted in an uneven formation level underneath the new spillway and it was chosen to fill the void with foamed concrete to reduce the effects of differential settlement.
In order to meet UU's latest safety standards, a re-grade of the downstream slope from 1 in 2 to 1 in 3 with class 1A material was carried out in accordance with Specification for Highways Works (SPHW) classification. This work included the installation of a granular filter to combat the effects of internal erosion.
A granular filter was actually scheduled for installation in the future but due to re-grade and the filter requiring weight, it was constructed as part of this project. The filter was made up from two materials, a 300mm filter layer (6T) laid directly on the subsoil of the embankment and a 300mm drainage layer (6U) above it. Both had specific particle size distribution (PSD) curves for the geotechnical properties of the embankment. A drain was installed in the drainage layer to take away the filtered water to a suitable discharge point where it can be monitored by the headworks controller.
As part of the re-grade, a 40m long, 4m high RC retaining wall was required on the northern side of the spillway. This was to ensure that the slope slackening works did not impose on the entrance to the scour tunnel and meter house, which contains the controls for the existing and new scour operations.
The retaining wall was also essential to act as a barrier between the clean embankment material and the contaminated material to the east of the wall. As mentioned previously Holden Wood IR was home to a bleach works which had contaminated the ground with elevated levels of metals, while the demolition works contaminated the ground with chrysotile (white) asbestos. As part of the concept design the material excavated from the stilling basin and outlet channel contained the contamination and, to ensure environmental sustainablility, UU decided to be keep the material on-site.
To allow the safe re-use of the material, a Land Quality Risk Assessment and remediation method statement were prepared in accordance with appropriate guidance, and approvals from the local authority were gained. The remediation method involved 600mm clay capping over the entire 2500m2 area and a 10mm geo-synthetic clay liner with a permeability of 1x10-9m/sec along the northern wall of the stilling basin and outlet channel.
The scour facilities were also improved by removing the two single 300mm diameter pipes in the tunnel and replacing them with a 600mm diameter scour pipe. This improved the scour capacity from 490l/sec to 590l/sec at TWL, which was not a huge improvement but was the maximum which could be achieved with the existing constraints of the small diameter open faced rock tunnel.
At Holden Wood the main material used was reinforced concrete and was placed using a 90T crawler crane. The crane ran along a working platform adjacent to the excavation for the spillway. Due to the tight working area constraints, EWCE employed a king post wall system. It was a cost effective and fast installation involving augering a hole with a CFA piling rig which was then filled with concrete. An H pile was then installed to enable mounting of concrete/steel panels between the webs of the piles to create a robust embedded retaining wall, providing the support for the open excavation whilst providing resistance to the working platform.
Due to the number of design and construction constraints which had to be considered, this project was one of the most complicated IR construction jobs in UU's history. With excellent teamwork from UU Engineering, UU Construction and ECWE, the project was completed on time, within budget, with a good health and safety etiquette and to a high quality.
Water management was essential to the success of the project, as it had to be safely routed through all three reservoirs. This resulted in weekly meetings with EWCE, Headworks and UU Construction to ensure construction activities were programmed on a weekly basis. This was one of the biggest successes as the project was constructed without any flooding incidents.
Daniel Thomson, Civil Engineer and Construction Manager, United Utilities.