Supply Chain - Wastewater Pumping Stations - Shafts
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(left) Pipework within the shaft and (right) precast concrete segmental shaft rings - Courtesy of envolve infrastructure[/caption]
ECI was integral to the final design solution from the outset, close collaboration with Rhondda Cynon Taf developed a more efficient and reduced cost engineering solution to meet the project objectives, providing a cost-effective and value driven solution. ECI began with a partnership with Hydrock developing the detailed design. Envolve’s ECI strategy was to further investigate and interrogate the outline design, whilst minimising risk and embodied carbon through streamlining constructability and increasing operational efficiency.
The design and delivery team undertook a review of the design philosophy in tandem with Rhondda Cynon Taf’s conceptual design. A proposal was drawn up, identifying the benefits of a deeper wet well shaft in lieu of a smaller shaft with accompanying storage tank. This approach mitigated the requirement for a reinforced concrete retaining wall adjacent to the existing highway.
Further investigation into the most efficient shaft construction identified the benefits of constructing a ‘wet’ cassion, in lieu of a conventional ‘dry’ construction. This construction method mitigated potential subsidence risk to 7 local residential properties and the main B4275 carriage way as there would no longer be a requirement to lower the existing high water table by our selected engineering solution.
In enlarging the proposed shaft, envolve was also able to house the proposed pumping equipment within the shaft itself, significantly reducing the overall footprint of the facility.
The final visual impact of the project was prioritised to provide adjacent residents with the least impactful and sustainable reinstatement on completion of the project. In lieu of a large tarmac area, a sustainable permeable hard standing was specified.
On site ECI works undertook ground investigation to determine the classification of waste material for excavation arisings including sampling and laboratory testing.
Envolve’s design solution successfully met Rhondda Cynon Taf’s requirement for a 250m3 storage tank with an outflow of 200 litres per second, achieved via a 6m diameter and 11m deep shaft with incorporated pumping station, all within the minimal footprint of the land available.
This significant design improvement ensured the final engineering solution worked within the site boundaries, also providing Rhondda Cynon Taf with an £800k cost saving for the overall project.
In summary, the outputs of our ECI expertise resulted in the following improvements to the final project design:
(left) Birdseye view of the wet caisson shaft under construction and (right) precast concrete segmental shaft rings at the base of the shaft - Courtesy of envolve infrastructure[/caption]
Birdseye view of the construction site - Courtesy of envolve infrastructure[/caption]
Completed shaft and cover slab - Courtesy of envolve infrastructure[/caption]
School visit to site from Glenboi Primary School and the footpaths and outdoor learning facilities - Courtesy of envolve infrastructure[/caption]
Following this, further support was provided to the school with the offer to complete their woodland walk and outdoor learning facility with accessible footpaths connecting the outdoor classrooms. A prestigious opening ceremony was hosted by the school for the grand opening with fantastic poems and songs performed by the pupils – there was even a special cake baked in appreciation of the relationship envolve developed with the school over the lifetime of the project. The woodland learning facility will hopefully provide years of outdoor learning opportunities for the school and its pupils.
Envolve operate a successful apprenticeship programme which sees a handful of placements throughout the business each year, these roles span from Commercial through to Engineering. This project presented a unique opportunity for an Engineering placement and a person from the programme was allocated to the project for its duration as part of their development plan.
(top left) Existing site prior to improvement works, (top right) site clearance, (bottom left) construction of temporary footpath and haul road to shaft and (bottom right) shaft cutting edge construction - Courtesy of Galliford Try[/caption]
(top left) Shaft collar constructed ready for sinking, (top right) shaft excavation with 27t excavator, (bottom left) first ring constructed ready for casting concrete jacking collar and (bottom right) shaft sinking in relation to public footpath - Courtesy of Galliford Try[/caption]
Ground conditions as identified in the borehole logs indicated a few meters of made ground and sub soil overlaying the Bude Metasandstone formation. The Bude formation had a reputation of being very hard and competent in places so Active Tunnelling Ltd opted to sink the shaft as a caisson with the option of underpinning if it could no longer be pushed down. Thankfully at this location the Bude formation did not prove too much for the excavators used to sink the shaft and it was successfully sunk as initially planned.
The arisings were removed from the shaft using an excavator mounted grab, before being loaded onto a tractor and trailer and tipped for drying further down in the field located adjacent to the pumping station.
One of the most challenging activities associated with the shaft was construction of a crane pad for the roof slab installation. A 180t mobile crane was needed to offload and install the various precast concrete panels which made up the shaft roof, however as the field was sloping steeply near the working area, a significant cutting around 3.5m deep had to be made in order to create an area level enough for the crane to set up on. Groundwater also became an issue at the cut edge into the hill and a drainage system had to be installed to divert the groundwater away from the crane pad to comply with the temporary works requirements on the design.
Directional drilling with slurry recycling operation - Courtesy of Galliford Try[/caption]
The river and A39 were not to only obstacles to overcome in this local area, there were sensitive fibre optic cables and also a medium pressure gas main. The chosen solution to overcome these obstacles was to directionally drill into the Bude Metasandstone and pass below everything at varying depths agreed with each stakeholder. Due to the location of the directional drill and also working on a flood plain next to a tourist ‘hotspot’, it was decided to minimise the construction site footprint west of the A39 as much as possible. To achieve this a slurry recycling plant was used to process drilling arisings with the processed arisings loaded straight into site dumpers and taken off to the tip.
The rising main itself was 180PE pipe which was butt fusion welded in advance of the drilling operation. The initial plan was to directionally drill the entire 486m of rising main however the Bude Metasandstone is a very strong rock and in places was less than 600mm below the field surface. The Gmac Utilities Ltd directional drilling team successfully drilled the road and river crossing at depth however the remaining section of rising main which was located at the interface between the sub-soil and the Metasandstone was almost impossible to control the drill line and level. It was therefore decided that traditional open cut excavation methods would be used to complete this section of the rising main.
(left) Shaft drop pipe and pump sump with baffle plate and (right) shaft fit-out - Courtesy of Galliford Try[/caption]
At pre-construction stage it initially made sense to locate the new pumping station on the western side of the existing building next to the existing wet well however this would have meant the felling of a very large and mature Ash tree (showing no signs of Ash die-back) and would have also resulted in around 2m of the new structure being visible above ground, due to its location on the downward side of the boat lift incline. Both the felling of a mature tree and the visual impact of a structure at this location was deem too significant to ignore and by moving the new pumping station location to the east of the existing building, constructability improve and therefore cost reduced.
An existing hedge bank and a few immature trees had to be felled in order to allow access to the site to undertake construction works. A replacement Cornish hedge bank was constructed to replace what was lost at the end of the project and replanted with native hedge and tree species with the aim of improving what was removed once the young plants had established.
Shaft crane pad construction - Courtesy of Galliford Try[/caption]
Through this close relationship, one of the landowners asked that a manhole cover be removed following abandonment of the soon to be redundant rising main and the site team suggested that they could fill in and re-profile a small depression left following the removal of an overhead power cable pylon. A few little gestures such as these went a long way with the landowner and were not underestimated by the site team.
Bulk excavation - Courtesy of BSG Civil Engineering Ltd[/caption]
Ards North WwTW - Inlet works overview - Courtesy of BSG Civil Engineering Ltd[/caption]
Inlet works side view - Courtesy of BSG Civil Engineering Ltd[/caption]
Screened raw sewage continues to the grit removal plant. The grit settles in a sump underneath the grit paddle. Any grit that settles in this sump is periodically aerated using the grit blower and then removed. From here the grit is transported out of the grit plant by the grit classifier and is deposited into the grit skip. From here it is removed off site for disposal.
After the wastewater has passed through the grit removal plant, flow continues to the FFT pumping station. Here influent is collected, and FFT of 48.2 l/s (summer) & 29.6 l/s (winter) is pumped to the PST flow splitting chamber.
(left) Ards North WwTW - Courtesy of NI Water, and (right) storm tanks tipping bucket installation - Courtesy of BSG Civil Engineering Ltd[/caption]
Aeration plant air manifold control valves and instrumentation - Courtesy of BSG Civil Engineering Ltd[/caption]
RAS/SAS pumping station - Courtesy of BSG Civil Engineering Ltd[/caption]
The thin sludge tank is equipped with an external mixer for the blending of the tank’s contents to feed the sludge thickening plant. A level ultrasonic with low and high-level floats are used in the control of the mixer.
Mixed sludge from the thin sludge tank is pumped to the sludge thickening plant via the two thickeners feed pumps. These pumps are progressive cavity pumps and are arranged in duty/standby configuration. The pumps are VSD controlled to allow the flow to the thickening plant to be controlled.
There are two sludge thickeners, with an individual throughput of 8.3 m3/hr @ 1.3% DS with a discharge of no more than 6%, based on a 5-hour day, 7-days per week operation at 100% of the maximum design load. The thickeners are arranged in a duty/standby configuration with two actuated isolation valves installed on the feed pipework allow this to be fully automated. If one thickener was to develop a fault, the standby thickener can resume operations without manual intervention.
Polyelectrolyte is made up on site to be added to the sludge fed to the sludge thickening plant. The addition of this chemical aids flocculation and makes thickening easier.
This plant is a liquid polymer set. The liquid poly is delivered within IBCs for the makeup process. The set mixes the poly from the IBC with water to a preset concentration rate and is then transferred to the poly storage tank. The duty poly dosing pump doses to the thickener’s sludge feed pipework from here.
Thickened sludge from the sludge thickening plant is transferred to the thick sludge tank. Here thickened sludge is stored for removal off site. A collection tanker removes the tank’s contents off site by coupling to the tank’s Bauer Coupler connector.
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PST No 1 & control building - Courtesy of BSG Civil Engineering Ltd[/caption]
The tank is equipped with a decant valve arrangement to facilitate settlement of the tank’s contents. The tank is also equipped with a level ultrasonic, low and high-level floats for level status and control of an externally fitted mixer for tank agitation.
The return liquors pumping station receives liquors from the following areas: inlet works drains, thin sludge tank decants and overflow, thick sludge tank decants and overflow, sludge thickening plant bund, FE and washwater drains and odour control plant drains. The station comprises of two fixed speed pumps, which are arranged in duty/standby arrangement. These return liquors to the activated PST splitting chamber.
Low level structures RC concrete installation - Courtesy of BSG Civil Engineering Ltd[/caption]
(left) Ballywhiskin TPS - Courtesy of NI Water, and (right) Carrowdore WwPS - Courtesy of BSG Civil Engineering Ltd[/caption]
(left) Sandycove WwPS and (right) Ballywalter Fowler Way TPS - Courtesy of NI Water[/caption]
NI Water, TetraTech, BSG Civil Engineering Ltd and Farrans along with local elected representatives to mark completion of the £18m Ards North Wastewater Improvement Scheme - Courtesy of NI Water[/caption]