Supply Chain - Stormwater Storage - Retention Ponds

In recent years, the village of Boherbue in north-west Cork has undergone considerable expansion, resulting in increased demand for housing. This growth has presented challenges, particularly for the existing wastewater treatment facility, which has struggled to cope with the surge in population. Additionally, the plant has fallen short of the Environmental Protection Agency’s treatment standards, underscoring the pressing need for an upgrade. Of particular concern is the discharge of treated wastewater into the River Brogeen, a protected conservation area that serves as the habitat for the freshwater pearl mussel. In collaboration with Uisce Éireann (Irish Water) and Cork County Council, a comprehensive project has been undertaken to modernise and enhance the capacity of the treatment plant. This initiative not only addresses the immediate needs of Boherbue’s growing community but also ensures compliance with environmental regulations. Early Contractor Involvement

The project, delivered under the innovative Early Contractor Involvement (ECI) framework, aimed to address the overloaded and outdated infrastructure while ensuring compliance with regulatory standards and meeting the evolving needs of the community. The innovation in this project lies in its multifaceted approach to efficiently treat wastewater, reduce carbon emissions, and protect natural ecosystems.

Leveraging the ECI framework allowed for the integration of innovative solutions such as solar energy, natural sludge drying reed beds, and the preservation of existing constructed wetlands for stormwater overflow. This innovative approach not only sets a new standard for sustainable wastewater treatment but also ensures long-term environmental benefits for Boherbue and its natural surroundings.

[caption id="attachment_16527" align="alignnone" width="1200"] Google Earth image of Boherbue[/caption]

The scope of works for the upgraded facility included:

A new inlet works. A stormwater storage tank. Biological treatment processes. Tertiary solids removal cloth filtration system. Sludge drying reed beds.

A description of these key features follows.

New inlet works

An inlet works with fine screening to 6mm in 2D and a manual bypass screen to 19mm with a stone trap upstream of the screens were installed. The screens were designed to cater for the Phase 2 Formula A flows of 23.2 l/s screens are to be designed to cope with a minimum of 150% of the design capacity for works under 2,500 PE, i.e. a design capacity of 35 l/s based on the Design Formula A. The hydraulic design of the inlet works was also based on the fine screen(s) being blinded by a minimum factor of 70%. The screen selected by the ECI team was the Definitive Ecology SF/T3 in-tank screw screen, with SDACA (specially designed anti-clog auger) system to ensure compliance with the screening handling requirements.

[caption id="attachment_16526" align="alignnone" width="1200"] Inlet works preliminary treatment - Courtesy of Glanua[/caption] Stormwater tank

Flows exceeding the plant’s peak design flow, overflow by gravity (weir discharging via overflow screen) to the stormwater holding tank, which has been designed to provide a retention time of two hours for the difference between the Design Formula A and FFT flows plus 40 l/PE for SWO compliance as only 1:7.1 dilutions will be available for the WwTP effluent in the Brogeen River (based on DWF and 95th percentile river flow).

The stormwater holding tank is provided with a Eliquo Hydrok CWF flushing bell in order to automatically clean the tank once the storm flows have abated and the tank has been emptied; which is an innovative approach to this, requiring no energy input.

The CWF Storm Flush system is a highly effective non-powered cleaning system for all stormwater retention tanks, utilising the sewage stormwater for the flushing process thus requiring no additional fresh water. Storm return pumps located in a sump within the storm tank, which will reduce retention of larger volumes of waste and reduce the risk of odour.

Biological treatment processes

The secondary treatment stage for Boherbue includes the following:

Two anaerobic selector tanks with three separate selector cell volumes. Two 624m3 aeration tanks with 109m3 of anoxic volume each for denitrification. Settlement provided by two 5m diameter/3.5m side wall depth secondary settlement tanks. [caption id="attachment_16524" align="alignnone" width="1200"] Secondary aeration treatment - Courtesy of Glanua[/caption]

Each aeration tank is 15.6m length x 5m width x 4m working depth. Aeration is provided to the tanks using a fine bubble diffused aeration system designed to achieve an overall peak standard oxygen transfer rate of 66.15 kgO2/h (i.e. 33kg O2/h per cell).

The aeration system comprised three screw blowers from Kaeser Kompressoren operating on a duty/duty/standby basis, with automatic changeover of the standby blower on a time-basis; ensuring equal use of the equipment. Air from the blowers is forwarded to each aeration cell by an air header, which feeds individual branches into the aerobic cells. The diffusers are disc membrane units from Xylem Water Solutions.

Tertiary solids removal systems

Following settlement, secondary treated effluent is passed onto a tertiary solids removal treatment stage for final effluent polishing prior to discharge to the Brogeen River. The TSR process was required in order to ensure compliance with the WwDL ELVs, and in particular with the phosphorus ELV of 0.25mg/l, through coagulation/flocculation and filtration treatment.

[caption id="attachment_16528" align="alignnone" width="1200"] Cloth filter tertiary treatment - Courtesy of Glanua[/caption]

During the design stage and evaluation of the ECI team, the supplier Eliquo Hydrok was found to have the most reliable, energy and cost-efficient equipment that includes both flocculation and filtering stage. The package unit includes a flocculation stage comprised of one stream with two chambers that split into the cloth filters.

Sludge drying reed beds

The implementation of sludge drying reed beds (SDRB), which significantly increases sludge concentration to up to 40% dry solids (DS) after 10 years of operation is a key innovation. This revolutionary approach reduces sludge export volumes by more than tenfold compared to conventional methods, thereby minimising transportation and dewatering costs while substantially reducing the carbon footprint of the Boherbue WwTP. SDRB sludge can also be directly land spread without the need for additional chemical conditioning, further enhancing its environmental benefits.

There are numerous benefits in the inclusion of the sludge drying reed beds vs traditional sludge management practices. These include but are not limited to the following:

Reduction of carbon footprint of the plant by approximately 45 T over a 10-year period. Carbon capture via natural plant growth. Up to 25% organic matter removal due to mineralisation. Dewatering process completed with zero energy consumption. At full design loads, the SDRBs will remove over 49 (No.) 20,000 litre tankers from the road per annum. [caption id="attachment_16533" align="alignnone" width="1200"] Sludge drying reed beds to process wastewater sludge naturally - Courtesy of Glanua[/caption]

The construction of the sludge drying reed beds (SDRBs) commenced in early February 2023, incorporating earth embankments and precast concrete wall sections for formation. Each cell was meticulously lined to ensure water-tightness, with feed and drainage pipework installed for optimal functionality. Reeds were planted in late April to establish and grow during the summer months, aided by the importation of sludge to promote growth. This comprehensive approach ensures the effectiveness and sustainability of the SDRBs in treating wastewater and reducing the carbon footprint of the Boherbue WwTP.

Solar technology

The use of solar technology underscores the project’s commitment to renewable energy sources, contributing to further reductions in carbon emissions. By leveraging these innovative technologies and practices, the initiative sets a new standard for sustainable wastewater treatment, offering a replicable model for other treatment plants to follow. This holistic approach not only improves operational efficiency but also promotes environmental stewardship, paving the way for a cleaner and more sustainable future in the wastewater sector.

50Kw solar panels from The Low Carbon Energy Company Ltd have been installed which will produce 42,500Kwh of electricity per year. This alone will introduce a carbon saving of 13.7T/CO2 per year which shows significant savings versus a traditionally built wastewater treatment plant of this kind.

[caption id="attachment_16532" align="alignnone" width="1200"] Solar panels for clean, renewable energy - Courtesy of Glanua[/caption] Reuse of constructed wetlands

The integration of the existing constructed wetlands into the wastewater treatment facility which was core to the design development brought a multitude of benefits, ranging from low operational costs, support for diverse ecosystems as well as sustainable water management. The wetlands improve water quality of streams and rivers in surrounding area creating conditions for aquatic life to thrive, and also create a scenic area in the village, promoting biodiversity and providing a habitat for small animals.

Boherbue WwTP: Supply chain - key participants Precast concrete: Glanua Group Designers: Jennings O’Donovan & Partners Designers: Construct Engineering Precast concrete: Shay Murtagh Precast Precast concrete: Carlow Tanks Precast concrete: FLI Precast Solutions Precast concrete: Tracey Concrete PLCs & automation: UMAC Systems Inlet screens: Definitive Ecology Tertiary solids removal cloth filters: Eliquo Hydrok Ltd Aeration blowers: Kaeser Kompressoren Aeration diffusers: Xylem Water Solutions Solar panels: The Low Carbon Energy Company Ltd CWF Storm Flush system: Eliquo Hydrok Ltd Steel modular buildings: Shanette [caption id="attachment_16530" align="alignnone" width="1200"] (left) Precast concrete aeration tanks reduced construction time and safety risks and (right) view of final clarifiers and cloth filters - Courtesy of Glanua[/caption] Innovative construction methods

Throughout the entire design process, Glanua Group and Uisce Éireann’s PMO team evaluated several project aspects such as energy efficiency (BEP), TOTEX costs, and environmental impact.

Energy efficiency of all motors and equipment (BEP). Process and equipment selected on TOTEX costs (including CAPEX, OPEX and capital replacement at end of life). The impact to the local environment was carefully assessed for all design options considered during the planning phase. Modular and off-site construction

Modular/off-site construction was used to reduce the carbon footprint and on-site safety risks, with precast panels and BCAR-compliant steel modular buildings used to promote efficient delivery and enhance safety.

Precast panels were instrumental in constructing the main aeration tank, reducing construction time and mitigating safety risks compared to traditional in situ methods. Precast wall sections were used to separate the sludge drying reed beds sections to reduce the footprint size and again to reduce construction time and safety risks. Throughout the project, precast process tanks were used extensively contributing to streamlined construction processes and improved safety measures.

A BCAR-compliant steel modular building from Shanette was used for the control room, laboratory, and caretaker’s office facilities. This has a significantly lower carbon footprint than that of a conventional block-built structure and in turn reduced health and safety risks significantly during construction due to fabrication being completed before assembly on site.

[caption id="attachment_16529" align="alignnone" width="1200"] Off-site fabrication of steel building, fully assembled as works have been completed (April 2024) - Courtesy of Glanua[/caption] Sustainability

The construction of the Boherbue Wastewater Treatment Plant is an example of Uisce Éireann and Glanua Group’s commitment to sustainability across various dimensions, including environmental, social, and economic considerations.

The inclusion of sludge drying reed beds demonstrates dedication to environmental sustainability. These beds offer an innovative solution for sludge management, reducing the environmental impact of waste disposal while also providing opportunities for ecological conservation.

By using nature-based solutions to treat and manage sludge, the carbon footprint associated with traditional disposal methods is minimised; thereby mitigating environmental harm. The SDRBs also significantly reduces operational costs, providing a long-term cost-effective solution.

The integration of solar technology underscores the commitment to renewable energy and carbon reduction. By harnessing solar power to supplement the energy needs of the wastewater treatment plant, reliance on fossil fuels is reduced, contributing to the transition towards a more sustainable energy future. This not only reduces operational costs but also mitigates greenhouse gas emissions, thereby supporting climate action efforts and environmental preservation.

Socially, the decision to reuse the existing wetlands demonstrates dedication to maintaining and sustaining biodiversity in the Boherbue community. By incorporating natural elements into the design, not only is disruption to local ecosystems minimised, but also to the overall health and vitality of the local environment.

Darran O’Leary, Programme Manager at Uisce Éireann said:

“We are delighted to have delivered this important project for the community of Boherbue. The modernisation and improvement of the wastewater infrastructure will accommodate further growth in the area and ensure that cleaner and safer treated wastewater is being discharged into the Brogeen River.

“It also adds biodiversity value to the area and remains sensitive to the surrounding landscape. This, coupled with the use of solar energy, demonstrates our commitment to putting sustainability at the heart of everything we do”.

Boherbue WwTP upgrade works is a flagship project for sustainable and carbon saving treatment processes and will serve as a reference project for future similar projects within the industry.

By demonstrating the feasibility and benefits of integrating renewable energy, advanced treatment technologies, and ecological conservation practices, this project sets a precedent for industry-wide innovation and best practices.

It complies with stringent Environmental Protection Agency (EPA) discharge consent standards, while minimising impact on the existing landscape, demonstrates a commitment to environmental sustainability, operational efficiency, and community well-being, making it a catalyst for positive change within the wastewater treatment sector and beyond.

The topography in and around the city of Hull makes the city extremely susceptible to flooding. Derringham is a residential suburb to the west of the city, which was heavily impacted by the 2007 floods. Significant flood alleviation works are therefore required in the area to manage stormwater to prevent an event on this scale from having such an impact when it inevitably occurs again in the future. The Derringham Aqua Greens Project has been delivered by the Living With Water Partnership, Stantec and Mott MacDonald Bentley (MMB) to provide flood alleviation in the heavily residential area. The Living With Water Partnership

The Living With Water (LWW) Partnership was formed following the 2007 floods when 16,000 properties in Hull and East Riding were flooded. It became clear that no single agency accepted any responsibility outside of their own term of reference, but that each had a part to play in maintaining the waterways in the city of Hull which contributed to the floods. The partnership brings together Yorkshire Water, Hull City Council, East Riding of Yorkshire Council, Environment Agency and University of Hull to enable them to collaboratively deliver flood alleviation schemes in the city. The first of its kind in the UK, the partnership aims to reduce flood risk in high-risk areas, while combining blue-green principles to deliver sustainable, low carbon solutions, encouraging knowledge share between all parties, building community resilience, improving local areas by providing biodiversity net gain.

[caption id="attachment_21743" align="alignnone" width="1200"] Location plan of the aqua green sites within the Derringham area - Courtesy of Mott MacDonald Bentley[/caption] The partnership has proved paramount throughout the design and construction phases as collaboration between all parties has been needed throughout. This included having the expertise of Yorkshire Water on the technical elements of drainage design, discussions with Hull City Council on how highways will be reinstated, proposed planting arrangements and gaining access to council owned green spaces and collaboration with the University of Hull to incorporate the requirements for testing & monitoring equipment within the design from the start. Project scope

The principles of the project are to allow exceedance flows from heavy rainfall, for which there is no capacity in the sewer network or nearby watercourses, to flow above ground via existing road accesses between properties, into detention basins retrofitted into existing green spaces in the area. The stormwater can be retained away from properties and returned to the sewer network once there is available capacity and the network is no longer surcharged.

The road access, or ‘10fts’, between properties are lowered to create a channel for stormwater to flow from the highway into the Aqua Green. A headwall and return sewer are installed from the low point of the Aqua Green, connecting into the existing sewer network in the main highway. Flows leave the Aqua Green under gravity, meaning that no mechanical or electrical elements are required for the operation of the solution. The lowering of the 10fts meant that property protection measures were required to the properties adjacent to the 10ft to prevent the solution resulting in them to be at greater risk of flooding. Existing boundary fences were replaced with new fence panels installed on concrete gravel boards to provide a waterproof barrier between the 10ft and the property.

Across the three Aqua Greens, over 1800m3 of storage has been proved. With a traditional grey infrastructure project, providing the same storage would have had a much higher embedded carbon measure, could not have been constructed in the small areas that were available and would have required some form of M&E installation with associated ongoing maintenance and running costs.

[caption id="attachment_21751" align="alignnone" width="1200"] Property protection on properties adjacent to the 10fts and (inset) position of manholes within narrow 10ft - Courtesy of Mott MacDonald Bentley[/caption] Derringham Aqua Greens: Supply chain - key participants Client: Yorkshire Water Principal designer & contractor: Mott Macdonald Bentley Outline design: Stantec UK Academic partner: University of Hull Local Authority: Hull City Council GCL liner & underdrainage: SIG Construction Accessories Headwall: Marshalls Water Management Hinged pole for testing & monitoring: Fabrikat (Nottingham) Pipework: John Davidson (Pipes) Ltd Flap valve: Affco Flow Control (UK) Ltd Surfacing: Calder CAD Ltd Landscaping: Craven Forestry Limited Aqua Greens design

The concept design was carried out by Stantec, who produced a full hydraulic model of the whole area to determine where the highest flood levels are seen, to identify the green spaces that would provide the most benefit if they were developed. A total of 11 green spaces in the area were investigated, with cost benefit analysis carried out to establish which of those spaces would provide the most benefit for the community. Three sites were selected to be developed in the first Work Package (WP1) with a further three sites to be developed in Work Package 2 (WP2).

[caption id="attachment_21746" align="alignnone" width="1200"] Installation of the liner and underdrainage layer - Courtesy of Mott MacDonald Bentley[/caption] One of the main aims of the detailed design phase was to produce standard details that could be implemented at all Aqua Green sites, to streamline both the design and construction phases. As the return sewers and associated chambers were to be laid in the 10ft between two properties, a standard 1200mm precast concrete manhole could not be installed as there was not space to do so. A 1200mm x 750mm standard product was therefore developed and agreed with Yorkshire Water as it was a much safer and simpler install.

The overall project programme meant that long term groundwater monitoring could not be carried out prior to the design process for WP1. For design calculations, groundwater therefore had to be assumed to be at ground level, which resulted in flotation of the liner and as a result, an additional drainage later needed to be installed below the liner to prevent flotation.

During the construction of WP1, it was observed that the Aqua Greens were retaining water before the liner was installed and that the as clay of very good quality. For WP2, permeability testing and longer-term groundwater monitoring were able to be carried out. This determined that the natural clays were of higher impermeability than the geosynthetic liner and that the groundwater was not as high as was believed during the design phase for WP1. It was therefore discussed and agreed with all relevant parties that the additional geosynthetic liner did not need to be included on site. This saved a total of two weeks on the programme for WP2, approximately 60 tonnes of carbon, material costs of approximately £60,000 and removed the risk of delays associated with the liner install in wet weather.

Construction for WP1 was from October 2023 to December 2024, with works still progressing on the WP2 sites. Sole access to the green spaces via 10ft wide access tracks introduced some difficulties to the construction. Only small plant could be used and the 5m wide liner had to be cut to 2.5m for delivery to the greens. Limited space on site also meant that excavated materials could not be stored at the greens and had to be stockpiled at the site compound, requiring a significant number of dumper loads of materials to be moved on the local road network. The nature of the project and works commencing in winter meant the Aqua Greens frequently flooded during the construction, with water regularly needing to be pumped to the drainage network before further progress could be made.

[caption id="attachment_21741" align="alignnone" width="1200"] Retention of water in Aqua Greens seen during construction prior to the installation of the liner - Courtesy of Mott MacDonald Bentley[/caption] Community engagement

Due to the location of the Aqua Greens in such close proximity to residential properties, significant community engagement was needed throughout the project, during both design and construction phases, to ensure that the community were kept up to date with what the project would deliver, how the construction works would impact them and ultimately how the SUDs solution would operate and be maintained into the future. Over 30 engagement sessions have been held with the community to date, from the start of the design development to the end of the construction phase. This has been coupled with regular letters and updates on the LWW website, providing the residents with an opportunity to raise any concerns about the works as well as to help to convey the importance of flood alleviation in the area.

To aid conversations with the local community about the Aqua Green developments, computerised visualisations were produced which showed how the Aqua Greens would look upon completion. While the existing greenspaces were mainly neutral grassland before works commenced, the solution was to plant wildflower seeds in the greenspaces upon completion, to increase biodiversity of the area. To provide habitat for wildlife, wildflower areas are maintained less frequently than grassland spaces and this therefore needs to be portrayed to the residents.

[caption id="attachment_21753" align="alignnone" width="1200"] (left) Visualisation image used to aid communication with the public and (right) installation of community noticeboard on site - Courtesy of Mott MacDonald Bentley[/caption]

The visualisations also helped the MMB team to discuss the proposals with the residents to see if there was anything additional, they would like to see of change as part of the design. Some residents said that they particularly liked the shrubs at the back of one of the green spaces as they provided them some security for their garden, so these were retained as part of the design. Others suggested that the proposed parking areas would not be used and that they did not want to encourage parking on the green spaces, so these were reduced in width to just passing places on the narrow roads.

Continued engagement with the community has been needed throughout the construction phase to answer any queries that the residents had about the works and rectify any problems they had been experiencing. Upon completion of the Aqua Greens, permanent notice boards have been installed at each of the sites to inform the residents of the benefits of the Aqua Green.

Continued testing & monitoring

The University of Hull have installed testing and monitoring equipment at all three of the Aqua Green sites in WP1 to understand how these Aqua Greens perform. The testing and monitoring equipment included soil moisture probes located above and below the liner; conductivity, temperature, and depth (CTD) probes in the return sewer and a weather station at a local school. The aim is that this data will be analysed and used to improve the design to enhance these assets for future installations. The weather station data is also accessible by the local school so that the data can be used for educating the students about the weather and flood risk of their local area and how we can safely live with water.

The intention is that further testing and monitoring equipment will be included on the WP2 sites so that a comparison can be made between the lined and unlined solutions to see if one of them is more effective at providing the required storage or improving the quality of the water that enters the sewer network.

[caption id="attachment_21745" align="alignnone" width="1200"] Installation of community noticeboard on site – Courtesy of Mott MacDonald Bentley[/caption] Summary

Since the installation of the first three Aqua Greens was complete, the benefits provided by the Aqua Greens has been visibly clear during periods of heavy rainfall, with quantitative data expected from the testing and monitoring installations within the next few months. These large-scale SuDS basins have provided flood protection to hundreds of properties in an area which otherwise would be at very high risk. Providing the protection via blue-green methods rather than traditional grey infrastructure has resulted in a solution with a much lower carbon impact, while also providing the areas with an improved biodiversity, making it a much more sustainable solution. The blueprint of this project, along with that of the Living With Water partnership, can be applied to other areas across the UK where properties are at high risk to deliver more types of these projects in the future.

Lower Woodburn Wastewater Pumping Station (WwPS) serves the Greenisland Drainage Catchment and is located close to the town of Carrickfergus on the foreshore of Belfast Lough. The storm outfall from the WwPS flows directly into the lough. Although recognised that discharges have minimal impact on the Belfast Lough Shellfish Water Protected Area (SWPA), there is localised impact and a need to protect and preserve amenity on the accessible shoreline. Upgrading of the WwPS was needed to comply with the Environmental Regulator’s (NIEA) consent requirements. Living with Water Programme

As part of Belfast’s strategic ‘Living with Water Programme’ (LwWP), NI Water has invested £1.8m to upgrade Lower Woodburn WwPS by increasing the volume of storm emergency overflow storage and installing a static screen. The solution also facilitates proposed housing (315 units) and hospitality/recreational developments that discharge directly to the WwPS and delivers storm separation of 17.8 hectares of impermeable ground. The project delivers against the LwWP goals:

PROTECT against Flooding. ENHANCE the Environment. GROW the Economy. [caption id="attachment_12584" align="alignnone" width="1200"] Lower Woodburn WwPS site pre-construction - Courtesy of DAWSON-WAM Ltd[/caption] Project delivery

Delivery partners for this project were appointed through existing NI Water frameworks. Tetra Tech was appointed to provide professional services, (including project management, cost management and site supervision) and to act as NEC Project Manager. The Murphy/ DAWSON-WAM Ltd JV (MDW) was appointed as main contractor.

Building on earlier work done to investigate catchment-wide solutions for Carrickfergus as part of the LwWP, MDW was engaged to develop the design and build solution under an Early Contractor Involvement (ECI) NEC contract. The design was reviewed and approved in collaboration with NI Water, and budgetary approval was granted for construction works under an NEC-ECC Main Option A contract in summer 2023.

Delivery was a collaborative effort between client, consultant and contractor teams to improve water quality and environmental conditions in the receiving Belfast Lough. The works comprised addition of surface water storage in a new concrete cofferdam constructed alongside, and connected to, the existing pumping station wet well. The constrained nature of the site lead to the need for access to neighboring lands from which to operate construction plant and machinery.

The scope was further developed during construction to improve the overall solution performance by installing a non-return valve on the storage overflow to enhance separation between storage and the marine environment and to improve the design life of the pre-existing pumping station.

Change was addressed using the provisions of the NEC contract and was managed within the overall budget for construction works. The new storage tank was integrated from an ICA perspective by linking to NI Water’s telemetry system for effective monitoring of storage levels and overflow incidents.

Throughout construction, regular site inspections were undertaken by Tetra Tech’s Site Supervisor and Health & Safety Manager to affect joint inspections with NI Water’s and MDW’s H&S Managers. This made sure of a combined approach to H&S management; issues were addressed face to face in real time and resulted in an excellent H&S performance throughout, with no accidents or incidents.

[caption id="attachment_12583" align="alignnone" width="1200"] 3D models of new storm storage tank - Courtesy of RPS[/caption] Early contractor involvement (ECI)

The ECI process proved its worth as extensive design and build challenges were overcome and a buildable solution was developed that satisfied the construction brief and costs criteria. This led to Murphy/DAWSON-WAM Ltd being appointed as principal contractor for the design and build of Lower Woodburn WwPS upgrade in August 2023. Contract works involved the design, supply, installation, testing and commissioning of an additional 317m3 of additional gravity storm storage on the existing site.

Certain elements of the project proved extremely challenging, none less than the design and construction of the additional storage tank on an existing site footprint that was extremely compact. Construction was further restricted as access to the site was from the busy A2 road, (the main road corridor from Belfast to Carrickfergus), immediately to the east of the site was a live petrol station with underground fuel tanks, on the west was an existing domestic occupied property and the access to the property and to the local strand, straddled the site on the east and south side. This was further complicated by the fact the existing pumping station had to be kept operational during the entire construction process.

The extensive ECI (which included site investigations, topographical surveys, network modeling, liaison with key stakeholders and utility providers, design development and costs approval), was completed post contract award. MDW appointed RPS as their civil design partner and through collaboration with NI Water and Tetra Tech, overcame the buildability challenges of this compact site.

The ground investigation report undertaken on the footprint of the only available space on the site identified glacial till overlying bedrock which varied from 6 to 8m below ground level. Early tank storage design calculations indicated that a tank depth of approximately 6.1m deep below ground level would maximise the footprint available within NI Water’s site, and enabled a gravity return into the existing pumping station.

[caption id="attachment_12588" align="alignnone" width="1200"] (left) CFA cased auger secant piling cofferdam installation and (right) excavation of storm tank exposing secant piles - Courtesy of DAWSON-WAM Ltd[/caption] Temporary works

Several temporary works solutions were considered to facilitate the tank construction including the use of a steel sheet pile cofferdam. This solution was omitted for several reasons but predominately because of the vibration risk to the surrounding properties and being unable to achieve sufficient embedment into the mudstone bedrock.

For the solution, DAWSON-WAM Ltd developed a secant piled cofferdam design which deployed a hard/hard continuous flight auger with reinforcement cages in every male pile (second pile). Utilising a double auger drive motor, it was capable of coring into the mudstone while maintaining pile verticality.

Working with piling design partner, Cassidy Geotechnical Ltd, a 450mm diameter secant piled cofferdam with 9m long piles was developed as the most viable solution. This was further valued engineered by incorporating the piles as part of the permanent works and introducing a single face, in situ, internal reinforced concrete lining to form the internal wall of the tank. Through design analysis, it was identified in the temporary condition the secant piled cofferdam would require propping during excavation, but this was omitted by collaboration between the piling designer and contractor by introducing a reinforced capping beam that was poured prior to excavation which reduced the deflection enough during excavation to eliminate the requirement of the temporary prop. The capping beam would also provide support the permanent in-site reinforced roof structure.

Pre-construction structural surveys were undertaken on all the surrounding properties and vibration monitors were installed at two locations adjacent to the site during piling operations. The secant method proved a success as the piles were installed to the designed length without triggering any vibration limits and when excavated provided a watertight cofferdam.

[caption id="attachment_12586" align="alignnone" width="1200"] Aerial view of site during storm tank reinforced concrete base slab construction - Courtesy of DAWSON-WAM Ltd[/caption] Lower Woodburn WwPS: Supply chain - key participants Main contractor: Murphy/DAWSON-WAM Ltd JV Project management & professional services: Tetra Tech Civils design: RPS Secant piling design: Cassidy Geotechnical Ltd Civils contractor: DAWSON-WAM Ltd MEICA: J Murphy & Sons Shuttering contractor: Form Builders Mixer: Xylem Water Solutions Static screen: Eliquo Hydrok Ltd Washwater booster pumpset: Dutypoint Ltd Combisafe fall protection: Honeywell Construction Due to the storage requirements of the tank, and given the small site footprint, there was not adequate space on the site to undertake the construction works safely. Therefore, a detailed construction phasing plan was developed by the contractor at the ECI stage which incorporated keeping the existing pumping station in service during construction. This plan identified that by temporarily incorporating the existing access lane (leading to the beach and neighbouring property) as part of the site, construction could be undertaken. An agreement was reached with the owners of the house - who had just recently purchased the property – to delay their relocation to the dwelling until works had been completed, and the council agreed to temporarily close access to the strand to aid construction. [caption id="attachment_12587" align="alignnone" width="1200"] Single sided reinforced concrete wall pours against secant piles to form internal tank walls - Courtesy of DAWSON-WAM Ltd[/caption]

Consultations were carried out through NI Water’s internal communications team and members of the project management team, and included public drop-in meetings and approximately 100 letter drops.

Due to site constraints, the establishment of an off-site construction compound also facilitated just in time delivery of the construction materials to the site, which maximized the small amount of on-site space and reduced site congestion.

MEICA installation included a single Xylem Water Solutions mixer to prevent silt build up in the tank, an Eliquo Hydrok 6mm screen was fitted on the existing 315mm emergency overflow pipe from the pumping station and a new 6 bar washwater booster pumpset was commissioned on site for maintenance of the screen.

Site finishes included a new perimeter fence, EMS area and a wildflower mix was used to finish final surfaces to enhance biodiversity.

Works programme

Construction works commenced on site in August 2023 and were completed on programme and within budget by late April 2024.

The collaborative approach of the project team effectively delivered a solution to improve the water quality of Belfast Lough, supporting NI Water’s agenda to Protect, Enhance and Grow through the delivery of the Living with Water Programme.

The project required the contractor to excavate within 25m of Beaumaris Castle – a UNESCO World Heritage site and one of the best examples of 13th and 14th century architecture in Europe attracting over 100,000 visitors last year. Faced with the challenge of installing 600m of 1.5m diameter polypipe in 6m sections between existing manholes, John Kelly Construction spent time researching the products available on the market, before finding that MGF’s High Clearance Trench Boxes were ideal for the job. [caption id="" align="alignnone" width="1200"] Courtesy of MGF Ltd[/caption] The 7m x 2.9m wide High Clearance Trench Boxes coupled with Top Boxes saved time and money to the contractor when compared to sheets and frames. The strength of the box allows suitability and capability of going to 5m depth and having high clearance struts on both ends of the box allows access at both sides – ideal for installing larger diameter pipes. [caption id="" align="alignnone" width="1200"] Courtesy of MGF Ltd[/caption] Paul Cowen, Technical Sales for MGF commented:

“Beaumaris’ most recent floods in 2017 was the catalyst that something needed to be done regarding the surface water drainage system. MGF became involved to supply the excavation support equipment to alleviate the threat of flooding to protect the natural beauty and tourism trade in the area”.

MGF’s stock logistics team confirmed availability of the kit at our Dartford depot and coordinated transfer to the Astley depot before delivery to site with three 40ft articulated vehicles and two 15t wagons to ensure the order was met. [caption id="" align="alignnone" width="1200"] Courtesy of MGF Ltd[/caption] John Kelly, Managing Director of John Kelly Construction, stated:

"I was very impressed with MGF as a company as they were very quick to respond to our queries, provided designs and all equipment ordered arrived on site with no delays."

Sector: Water & Utilities Client: Isle of Anglesey County Council Contractor: John Kelly Construction MGF depot: North West - Astley Products hired: High clearance trench box, top boxes & titan manhole box   For more information: MGF Ltd | +44 (0)1942 402700 | www.mgf.co.uk