Supply Chain - Sewerage - Contractors
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Case Studies
- Sherston S101a - First Time Sewerage Scheme (2023)
- Norton, Presteigne & Weobley WwTWs (2023)
- Marhamchurch SPS (2023)
- Galmpton Catchment (2024)
- Marchmont Road Flood Alleviation Scheme (2024)
- Arklow Long Sea Outfall (2023)
- Chilton Foliat STW Flow Compliance (2023)
- Stratton SPS to Helebridge STW Rising Main (2023)
- Stroud Sewerage Strategy Project: Part 2 (2023)
- WINEP Durham Transfers (2024)
- St Augustines Road Flood Alleviation (2023)
- Ravenhill Avenue, Belfast, Flood Alleviation Project (2024)
- Ards North Long Sea Outfall (2023)
- Exmouth Offshore Project (2024)
(left) Schematic showing the route of the new gravity sewer and new rising main and location of the new SPS and (right), illustration from the Wessex Water leaflet to property owners showing responsibility split - Courtesy of Wessex Water[/caption]
Residents in Sherston applied for an FTS scheme due to this pollution, which led to an assessment by Wessex Water which concluded that a public FTS was viable and could be progressed to design and construction. Wessex Water carried this out using their in-house capabilities at a cost of £2m. Although only a few properties were causing environmental and public health issues, Wessex Water included as many properties as possible within the FTS scheme, taking into account the viability of doing so. As a result, 24 properties (estimated population equivalent of 60) currently on septic tanks will be able to connect to the new public sewerage system by July 2023. Wessex Water’s FTS policy is to construct the new public sewerage system up to the inside the property boundary, where it terminates within a demarcation chamber. This chamber denotes the boundary between the public and private drainage system.
Penstock ready to be installed over the outgoing pipe (on the left) within a manhole chamber upstream of the wet well - Courtesy of Wessex Water[/caption]
Several iterations of the design were undertaken to confirm the best route of the pumped rising main, location of the proposed new SPS and access road, as outlined below:
Connection to the existing sewerage network in Easton Town (B4040) - Option discounted
This solution provided the shortest route for the proposed new rising main and access track to service the new SPS. However, the rising main route would need to pass through a narrow driveway between existing houses.
Due to the depth of the excavation, an open cut construction method was discounted as it would be within the zone of influence of the building foundations and consequently had the risk of undermining and causing structural damage to these buildings. Trial holes indicated shallow rock which would have further exacerbated structural damage due to vibration when breaking out the rock.
Trenchless techniques were considered, however due to the rock, only micro-tunnelling would be suitable, and would require a reception pit within Easton Town road, which would cause significant disruption to the community. Micro tunnelling would have also resulted in the scheme becoming financially unviable.
Connection to the existing sewerage network in Sandpits Lane (access via Sandpits Lane) - Option discounted
This solution provided the next shortest route for the access track to service the new SPS, and although the rising main route was extended, it allowed for an additional gravity sewer to be laid within the same trench to connect four additional properties at minimal cost, which increased the cost viability of the scheme.
Prior to submitting a planning application for this option, Wessex Water consulted with the community. Significant public objections were raised due to the visual impact of the proposed new access track and SPS, particularly from those who were already on mains drainage and would not benefit from the FTS scheme. In addition, land purchase negotiations for the access track could not be resolved.
Connection to the existing sewerage network in Sandpits Lane (access via Tetbury Road) – Selected Option
The sewerage network for this option was similar to Option 2, however the access track route and SPS location changed to take into account the points raised from those objecting on the grounds of visual impact and land purchase issues.
This option was confirmed to be both technically and financially viable and no public objections were raised, which was valuable in securing planning permissions.
(left) Excavation of shallow rock for rising main and (right) internal view of the wet well before sump benching - Courtesy of Wessex Water[/caption]
(left) Shuttering around the wet well in preparation for pouring the final layer of concrete surround and installing dowels into the valve chamber and (right) construction of wet well - Courtesy of Wessex Water[/caption]
All pipework within the valve chamber was constructed in ductile iron DN80 and the rising main was constructed in MDPE pipe, SDR17, OD 110. The gravity pipework was constructed in PVCu Funke Gruppe pipe (100 mm and 150 mm internal diameter), which was selected to provide due to its light weight and construction durability compared to conventional vitrified clay pipework.
Penstocks were required so that incoming sewerage flows can be controlled, to allow isolation of the wet well for cleaning and maintenance. The penstock was designed to be located within the wet well, however concerns were raised by the site team that the proposed opening for the penstock spindle would be difficult to cast in the correct location during the off-site fabrication of the cover slab. To resolve this, the cover slab design was amended to remove the penstock spindle opening, and it was moved to an upstream manhole, where the penstock could be positioned within the clear opening of the manhole cover.
An overpumping connection allows for the temporary pumps to be installed and connected to the rising main via a Bauer coupling on the pipework from within the valve chamber. The valve chamber cover was designed to allow for the safe operation of the Bauer coupling by having a split walk-on grid over the coupling.
This allowed the operator to place the foot on one of the closed flaps, whilst the other flap is lifted, thereby allowing them to position themselves directly over the Bauer coupling. This avoids the operator having to lean over an opening or handling the tankering hose with one hand.
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Valve chamber cover, with split walk-on grid over the Bauer connection - Courtesy of Wessex Water[/caption]
(left) Q-bic system construction and (right) concrete slabs being cast over the crates to form the turning area - Courtesy of Wessex Water[/caption]
Public drop-in session and (inset) 3D model of the new sewage pumping station - Courtesy of Wessex Water[/caption]
All temporarily affected habitats were seeded with a species-rich grassland and wildflower mix, managed to avoid cutting during the late spring and summer to benefit invertebrates and other wildlife. Off-site biodiversity enhancement measures were also undertaken in Sherston Primary School, including planting of additional trees, wildflower meadow, and providing bird nesting pockets to students.
Close interaction with residents is a key feature of FTS schemes due to the work within private gardens. All residents were contacted prior to construction via face-to-face meetings and community drop-in session to give them the opportunity to ask any queries direct to the Wessex Water Project Team. A 3D model of the proposed sewage pumping station was created and used during the drop-in session to provide more clarity for the public on the proposed solution.
Norton SPS progressive cavity pumps - Courtesy of Morgan Sindall Infrastructure[/caption]
Norton SPS: MCC kiosk and Haigh Engineering inlet screen - Courtesy of Morgan Sindall Infrastructure[/caption]
Spider plough operation to construct the Norton WwTW pump away rising main - Courtesy of Morgan Sindall Infrastructure[/caption]
Presteigne WwTW: Ferric point of application (PoA) unit - Courtesy of Morgan Sindall Infrastructure[/caption]
Alkalinity dosing was required as a result of the new ferric dosing. The identified solution for this was a small bag dosing plant with on-site mixing to create a liquid dose. A new point of application facility located on the side of the existing PST which discharges to the biological filter. It was determined that this point had enough hydraulic mixing for an effective alkalinity dose.
The new works is achieving its ammonia consent with the current loading, but it had been identified that ammonia compliance could be a risk during AMP9. Nitrifying sand filters have been proposed as a technology to treat the ammoniacal nitrogen as a tertiary treatment solution. This has an added benefit of tertiary solids removal to give confidence in the total phosphorus consent. However, given the excellent performance of the works against consent and nitrifying capacity of the existing biofilters, the nitrifying sand filters will be phased into the works when required.
Works included a new sampling arrangement with sampling kiosk, complete with an upstream diversion and reception chamber for when the nitrifying sand filter is constructed.
Presteigne WwTW: Ferric dosing system and safety shower - Courtesy of Morgan Sindall Infrastructure[/caption]
Weobley WwTW: (left) Huber Technology inlet screen and (right) flow to full treatment pump station - Courtesy of Morgan Sindall Infrastructure[/caption]
This provided significant cost and embodied carbon savings compared to a completely new screening installation. Downstream of the screen, an inlet flume was provided for flow measurement and control of the downstream lift pumping station. The flume was constructed off-site, allowing for greater dimensional accuracy than an on-site constructed flume, as well as reduced downtime for the site.
A new lift pumping station for all incoming flows for treatment was required to provide a separate lift to the balancing tanks then the unscreened storm flows. The existing inlet pumping station was re-purposed as the storm pumping station, meaning the more efficient pumping station was operating for more of the time compared to the older storm pumping station, which only operated intermittently - hence saving on energy usage and operational carbon.
An existing multi-use returns pumping station was used to pump humus returns and storm returns to the head of the works. This asset consisted of outdated and poorly operating pumps and valve work. A solution was developed to replace mechanical equipment within the existing dry-well, saving the need for a new complete pumping station.
Weobley WwTW: Ferric point of application units - Courtesy of Morgan Sindall Infrastructure[/caption]
(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.
Aerial shot showing Galmpton STW on the right and rising main scar through the fields on the left - Courtesy of Galliford Try[/caption]
Galmpton stormwater storage tank following the installation of the flushing bell and air valve release pipework - Courtesy of[/caption]
Storm tank roof installation main beam lift - Courtesy of Galliford Try[/caption]
Galmpton storm tank with the precast concrete roof installed - Courtesy of Galliford Try[/caption]
Google Maps image showing location of the site[/caption]
Storm tank under construction - Courtesy of Paul Milligan & CWA[/caption]
Marchmont FAS project site - Courtesy of Paul Milligan & CWA[/caption]
Storm tank roof installation - Courtesy of Paul Milligan & Caledonia Water Alliance[/caption]
Customer information event - Courtesy of Scottish Water[/caption]
Outfall pipe arrival from Norway by sea to the dock in Arklow for preparation and assembly - Courtesy of Van Oord Ireland[/caption]
The proposed outfall pipeline was required to discharge the treated effluent from the wastewater treatment plant to receiving waters. Both marine and river outfall options were initially considered for the proposed development.
However, hydrodynamic and water quality modelling demonstrated that the wastewater discharge standards required for a river outfall would be much more onerous than that of a marine outfall. For that reason, the river outfall was not considered further.
Outfall pipe floated along the quay wall in Arklow prior to preparation and assembly works - Courtesy of Pipelife Norge AS[/caption]
The marine outfall was also designed to be stable in the ‘worst case’ design loads for current and wave loading conditions provided. It was therefore necessary to weight the pipeline with concrete collars and subsequently concrete kennels after installation (float and flood method), to provide a specific gravity of not less than 1.35 that could be achieved without any reliance on backfill material.
This installation weight was provided with 1015kg (290kg/m) (in air) fixed weight concrete collars at 3.5m centres and to satisfy the permanent on bottom stability, 2292kg (654.86kg/m) (in air) concrete kennels were provided at 3.5m centres in between the fixed weight concrete collars. The collars and kennels were provided by Tracey Concrete.
Welding of risers for diffuser section of outfall pipe - Courtesy of Van Oord Ireland[/caption]
Excavation of trench at the onshore connection point using the backhoe dredger ‘Razende Bol’ - Courtesy of Van Oord Ireland[/caption]
(left) Outfall pipe during sinking operation and (right) outfall pipe aligned prior to sinking operation - Courtesy of Van Oord Ireland[/caption]
Outfall pipe concrete collars being fitted in water with special lifting equipment - Courtesy of Van Oord Ireland[/caption]
Once the outfall pipeline had been sunk to the sea bottom and pressure tested, concrete kennels, each weighing 1 tonne were placed in between the concrete collars at 3.5m centre to centre spacings to provide additional permanent weighting. Additionally, six diffusers were installed from the backhoe dredger platform by divers. Each diffuser comprised of a flanged PE pipe, fittings and a riser with a four-port crosshead, each with a flange allowing for Tideflex S35 valves to be fitted directly. Included as part of the diffuser installation, was a removable access flange at the top of the riser to enable access inside the riser.
After completing the diffuser and kennel installation, the pipe was backfilled over its full length to provide 1.5m cover. Rock armour protection was further provided extending approximately 7m around the diffuser section from its centre to protect the area around the diffuser from scour. Also provided was a steel protection frame for the diffuser, fitted with sacrificial anodes to allow a 50yr design life for protection from dropped objects, anchors, trawlers and fishing nets. Finally, on the offshore end a marker buoy and on the onshore end a Saint Andrews Cross were installed to mark the pipeline location.
Due to the turbulent weather at the site location, including hold points in the working method, so that each installation step could be paused at planned intervals, allowed Van Oord to work around the weather. Additionally, working with local contractors who helped to adjust and adapt work methods and techniques within the limited amount of space, allowed the team to prepare and install the outfall system using specialised equipment and marine vessels, with minimal disruption to the environment and public. In particular, a bespoke rigging system was designed and fabricated using a local yard to assist with the efficient installation of the collars whilst working from the water.
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Outfall pipe during sinking operation - Courtesy of Van Oord Ireland[/caption]
Aerial view of setup - Courtesy of A. Thomas Plant Hire Ltd (ATP)[/caption]
Assessment of the existing pump arrangement at Mill House SPS confirmed that they would be suitable to transfer flows along the new rising main pipe alignment which had a reduced head to that of the previous arrangement. However, the existing pumps were determined to be nearing the end of their asset life and their replacement was also included.
The gravity sewer section was designed to maintain self-cleansing for the expected pass forward flow of up 8 l/s with the fall gradient varying between 1 in 90 and 1 in 60.
(top left) winch/anchor unit, (bottom left) bedding conveyor and (right) test pit on installed pipe - Courtesy of MMB[/caption]
The pipe plough - Courtesy of MMB[/caption]
(left) Pipe plough in progress and (right) reducer installed - Courtesy of A. Thomas Plant Hire Ltd (ATP)[/caption]
Stratton SPS temporary works installation - Courtesy of Galliford Try[/caption]
Washout chamber construction - Courtesy of Galliford Try[/caption]
Directional drilling remote field set up - Courtesy of Galliford Try[/caption]
Directional drilling helped minimise impact on farmland - Courtesy of Galliford Try[/caption]
Along the rising main route there were sensitive fibre optic cables to cross and additional control measures were required to safely locate and expose these through hand excavation with a full watching brief put in place when drilling the new main past these services.
The A39 which is a major road in this area, also had to be crossed. The local council were not keen for the rising main to be drilled below this section of the road as it was constructed on an embankment, there was however an agricultural culvert nearby. Through conversations with the local council structures department, it was established that the concrete running surface in the culvert was not the structural floor but was actually built on top of a thin layer of Type 1 stone. The acceptable solution to both the council and land owner, who used the culvert, was to cut a trench through this concrete running surface down the middle of the culvert, dig out the stone and place the pipe in trench and backfill with structural concrete. This provided just enough protection to the new main and allow the existing culvert clearance to be maintained.
Directional drilling recycling unit - Courtesy of Galliford Try[/caption]
Directional drilling below sensitive gallop track - Courtesy of Galliford Try[/caption]
As previously mentioned, one of the landowners had a gallop track which the new rising main had to pass under. Part of the farmer’s business was specialising in race horse breading and training, and the gallop track was a highly engineered training surface where race horses could reach speeds of around 30 mph. The landowner was concerned that by open cutting the track, groundwater paths could change and result in a boggy area developing and this could cause significant injury to a jockey or race horse. It was agreed that directionally drilling below the track was an acceptable solution and whilst working in this area, additional controls were put in place as the gallop track remained operational through the construction work. On a few occasions, the site team got to hear and feel what it was like when race horses were running at top speed just a few meters away from where they were working, something which none of them will forget.
(left) The launch of TBM Marvellous Mining Molly and (right) shaft with pipe jacks installed about to be benched - Courtesy of Galliford Try[/caption]
Due to varying local topography and numerous obstacles along the route the team opted for a combination of open cut and pipe jack tunnelling techniques for the trunk sewer - details below.
TBM Marvellous Mining Molly in position to start mining - Courtesy of Galliford Try[/caption]
(left) Installation of prefabricated chamber within live sewer line and (right) inside the prefabricated chamber - Courtesy of Galliford Try[/caption]
Construction of bypass channel and (inset) installation of the temporay PortaDam from OnSite Central Ltd - Courtesy of Galliford Try[/caption]
(left) Excavation of shaft and (right) preparations for open cut rising main installation - Courtesy of Galliford Try[/caption]
Piling operations by Francis & Lewis International Ltd, for the construction of the pipe bridge substructure - Courtesy of Esh-Stantec[/caption]
Active Tunnelling constructing the new wet well shaft at Pity Me STW
Construction of new valve chamber and associated pipework at Pity Me STW - Courtesy of Esh-Stantec[/caption]
Aerial view of working area at Brasside STW - Courtesy of Esh-Stantec[/caption]
Construction of pipe bridge structure by Francis & Lewis International Ltd - Courtesy of Esh-Stantec[/caption]
(left) The preferred option and (right) flooding on St Augustines Road - Courtesy of Galliford Try[/caption]
Outfall under construction - Courtesy of Galliford Try[/caption]
Poor ground and underfoot conditions adjacent to the River Rother proved too risky and problematic for traditional mobile crane lifts, so all lifts of the precast sections were to be undertaken with a tracked excavator. This further constrained the design and restricted the maximum mass of each section to 13 tonnes. Consequently, in its final iteration the precast headwall was designed in two sections and was combined with an in situ toe and infill slab/wall which completed the structural design.
(left) Installation of drive pit on St Augustines Road, (middle) guided auger bore rig, and (right) the transition between 600mm and culverts - Courtesy of Galliford Try[/caption]
Settlement calculations were undertaken using the “Guide to best practice for the installation of pipe jacks and microtunnels technique” and a predicted settlement of <3mm was deemed to be acceptable. The pre-condition surveys highlighted only minor cosmetic issues and were used as reference to the state of repair of the properties prior to construction work commencing.
The majority of the civil design was undertaken in house. This element included a design coordination role combining and processing the information received from the consultants and stakeholders. An example of the numerous stakeholders which were consulted is that of the Coal Authority. Initial searches indicated the presence of historical coal workings. More detail was sought and records indicated the presence of an adit within 20m of the proposed pipeline. A coal permit was sought from the Coal Authority and the design progressed whilst this was processed.
Service detection - Courtesy of Galliford Try[/caption]
Cofferdam and silt curtain - Courtesy of Galliford Try[/caption]
Cofferdam
A cofferdam was required to retain the River Rother and this was designed and supplied by National Trench Safety UK and installed by the primary subcontractor Arthur Civil Engineering. The cofferdam was unique in design in that it utilised Larsson piles in a trapezium shape (plan) to minimise over-dig yet still accommodate the headwall and first culvert.
The cofferdam was installed by Ivor King Piling using a 35t excavator with a Movax side grip pile driver attachment. The cofferdam was effective but 4” end suction pumps supplied by Selwood Ltd were still required to maintain the cofferdam and keep water levels low.
Headwall installation
Once the cofferdam was watertight, the headwall toe was cast in situ, after which the precast headwall arrived in two sections as planned. It was lifted into position by Arthur Civil Engineering using a 50t tracked excavator. An administrative issue led to the need to negotiate a 2-week FRAP extension with the EA and the headwall was completed and the cofferdam removed by 14 October in accordance with the conditions of the revised FRAP.
Pipeline works
Construction works on the remainder of the project continued in March 2022. A temporary one-way system was implemented to provide a safe working area to install the remainder of the pipeline with open cut works commencing on 14 March. The open cut works progressed steadily with over 100 services safely crossed. The later spring and summer months provided excellent conditions for online sewer relay with drought-like conditions, culminating in hosepipe bans for the majority of the UK.
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Open cut pipelaying - Courtesy of Galliford Try[/caption]
Prior to the tunnelling works commencing, concern was raised over the proximity of the tunnel as it crossed under a 225mm foul water sewer in Derby Road. With a clearance of just 300mm, the existing foul sewer was inspected by CPS Drainage Solutions using CCTV and a precautionary 5m cured-in-place patch liner was installed to strengthen the sewer prior to the guided auger bore operation. No-dig works commenced in early June and progressed well without incident and were completed towards the end of the summer. No settlement was detected during the installation and no visual deformations were reported from the nearby structures.
Open cut works continued late into the summer with flooding benefits delivered by 5 October. Significant rainfall events have since occurred and the feedback from residents previously affected by the flooding has been positive.
900mm Ridgistorm pipe from Polypipe Civils ready for installation - Courtesy of Galliford Try[/caption]
Laying storm sewer on Ravenhill Road - Courtesy of GEDA Construction[/caption]
Manhole construction – Courtesy of NI Water[/caption]
Reference design cross section sketch - Courtesy of AECOM[/caption]
Revised design cross section sketch - Courtesy of AECOM[/caption]
Storm separation at Millar Street - Courtesy of GEDA Construction[/caption]
(left) Preparing for reinstatement at Bakers Court and (right) Permanent reinstatement on Ravenhill Avenue - Courtesy of GEDA Construction[/caption]
Tow and sink operation - Courtesy of NI Water[/caption]
Aligning pipeline over pre-dredged trench - Courtesy of NI Water[/caption]
A range of pipeline diameters and materials were considered to achieve optimum velocities, a balance between pipe diameter and TWL in the proposed header tank. PE pipes were selected for the outfall with varying SR ratings used to offer material cost savings using only higher rated pipes for the marine sections to take account of installation (towing and sinking) stresses on the system.
Specialist hydraulic modelling was then completed by Hydraulic Analysis Limited (HAL) to provide confidence in the performance of the system against all combinations of design flows and tidal levels. The analysis confirmed the pipe diameter selection, siting of air and scour valves and provided specific specifications for pipeline gradients, bends and air valves installation to ensure stable free surface flow conditions were achieved within the mains gravity driven sections.
Barge mobilisation from Bangor Marina - Courtesy of Farrans[/caption]
The final solution was therefore to discharge treated effluent directly from the sample chamber to a gravity outfall which acts in part as a pressurised system. This amendment to the design approach was verified by HAL and removal of the pumping station and header tank was agreed by all prior to finalising the construction details including:
The outfall was installed off the coast at Ballyferris adjacent to a caravan park - Courtesy of NI Water[/caption]
(left) Onshore pipeline assembly - Courtesy of Farrans, and (right) installation of concrete kennels - Courtesy of NI Water[/caption]
Re-excavating the proven trench - Courtesy of Farrans[/caption]
At high tide, the pipe was floated, towed and positioned above the pre-dredged trench. Connection was made to the pre-installed land section of the outfall and the pipe was filled with water allowing a gradual sinking and pipe placement along the trench. Backfilling operations commenced once the pipe was surveyed and verified.
Professionally qualified divers installed the prefabricated diffuser head. Live underwater video surveys were conducted to monitor the installation of all equipment.
The execution of these activities adhered to stringent health and safety protocols, strict environmental considerations, and project timelines, ensuring the successful completion of the project on time and within budget.
NI Water and Farrans held a joint safety initiative for children in caravan parks - Courtesy of NI Water[/caption]
(left) Pipe string along coastal path and (right) Pushing pipe lower car park and compound - Courtesy of South West Water[/caption]
(top left) 500t drill rig compound and entry point and (bottom) pipe pushing operation at sunrise - Courtesy of South West Water[/caption]
500 T drilling rig - Courtesy of Galliford Try[/caption]
(left) Drill pipe and pilot drill bit and (right) outfall pipe, roller and guide - Courtesy of Galliford Try[/caption]
900mm diameter pipe welder - Courtesy of Galliford Try[/caption]