Supply Chain - Reservoirs & Storage - Services

The existing single cell, below ground 4.5 ML clear water storage tank dates from the 1930s. It is, therefore, an ageing asset whose deterioration has recently led to some failures in water quality and general lack of resilience in the system. These drivers led to a decision by Scottish Water to commission a £6.5m replacement. Forecast modest rises in local population numbers was a secondary consideration.

The principal challenges were:

The contract specified the replacement of the existing almost 90 year-old tank with a 3 ML, twin-celled treated water storage tank to be constructed on the same site, contiguous with the structure of the old tank.

The dimensions of the new asset are 29m x 21.5m x 7m to be covered by a reinforced concrete roof with waterproof membrane.

All aspects of the design of the new tank were undertaken by ESD JV’s design and engineering department. Work started on-site in July 2022. In total, approximately 850m3 of concrete was poured to construct the new structure.

In order to protect the viability of the existing tank, and as a consequence of above average rainfall in the winters and springs of 2022, 2023 and 2024, a complex bulk excavation operation was undertaken which included the extensive use of soil anchors to stabilise the ground.

This solution included some in-depth collaboration from the site team and specialist sub-contractor to design a solution to ensure no current live network pipes (which were only 6m from the top of the batter of the excavation) were damaged during the installation of the anchors. In order to achieve this, the site team undertook a survey of the existing network to establish the precise coordinates of each pipeline.

This allowed for the design to be altered to a smaller length of anchors with a revised slope angle ensuring no pipes would be damaged as part of the TW solution. All excavated material (spoil) was used as backfill for the new asset and also for the existing asset which is scheduled to be de-commissioned and removed with minimal excess being removed from site. In total, circa 10,500 tonnes of material were excavated to construct the new asset.

A hydraulic assessment to decide the level of the tank resulted in the need to bury it underground. Given the damp ground conditions on-site, a water impermeable concrete was used to construct the base and walls. This maximises the lifespan of the asset and represents the best carbon solution.

The heavy clay soil resulted in the rapid build-up of water within the excavated area leading to significant drainage issues. This resulted in some pre-excavation cut off drainage ditches being constructed at the high level of the excavation along with gravel filled silt ditches and fencing installed around the site to mitigate the impact of any run-off. Due to the inclement weather encountered throughout the winter of 2022, this led to some initial delays to the programme.

However, any delays were mitigated by innovative solutions to other tasks. The team at Kerse are proud of their ‘championing’ of the use of an electric pedestrian tower crane. Operated at ground level, this led directly to accelerating the construction programme and a significant cost reduction. The selection of the pedestrian crane, which was initially proposed to be an operated crane via operator at high level, brought significant health and safety benefits by removing the need for working at height.

Also, the site team installed cameras at critical inspection points on the crane which meant all daily inspections could be carried out at ground level by the operator, again eliminating the requirement to work at height. All statutory inspections were undertaken by a third party under a safe system of work which was included as part of the sub-contracted scope.

The additional benefit from selecting this method came from a reduction in carbon output with the initial temporary works reinforced concrete pads (which were due to be in excess of 95m3) and the selection of the pedestrian ground operated crane reducing this concrete requirement to only 8m3 to fulfil the temporary works requirement.

A rechargeable electric mobile elevated working platform (MEWP) was employed, providing further benefits. The use of the pedestrian crane and MEWP significantly reduced the carbon footprint on the project in comparison to using alternative options such as mobile cranes and access scaffold which are commonly used on this type of project. The use of the tower crane to facilitate lifting operations throughout the project, along with the MEWP for high level access, proved to be a time saving, cost effective and carbon friendly solution to our buildability requirements on site.

Another innovation was the temporary works wall truss system employed.

Reinforcement for the walls of the new tank was prefabricated on-site. In itself, this produced significant cost, time and safety benefits. This solution reduced the requirement for access scaffolding to be erected, this being substituted by the MEWP system. In addition, personnel did not have to construct the reinforcement whilst working at height or manually handle the reinforcement and place it into position. Instead, this was achieved via pre-fabrication at low level, subsequently allowing full panel reinforcements to be lifted and installed into position providing further safety, time and cost benefits to the project. Another reason for employing this system was that no additional temporary support system was required to support the reinforcement.

Throughout the project, there was regular liaison between ESD JV and Scottish Water with weekly and monthly collaboration meetings held leading to a close and productive working relationship on the project mitigating potential delays. All issues and challenges have been met in a collaborative environment.

At Kerse, Scottish Water have introduced a new telecoms telemetry system using 4G connectivity monitoring all instrumentation, including water level sensors, alarms and floats, which replaces the old PSTN line.

This option was the result of a business approach change from Scottish Water to provide either VDSL (fixed line) or LTE (4G mobile) for IP enabled communications. In order to prevent the time and cost involved in installing a VDSL line, a decision was made following a successful survey to utilise a new 4G solution for the project’s telemetry communications.

Water into supply was achieved on schedule in May 2024. This was followed by reliability testing and sampling by Scottish Water before the facility was declared fit for operation which was achieved on 30 May 2024, with the old asset now drained and ready for de-commissioning.

Currently, a new site access road is being completed and various measures of landscaping, including grass seeding, are underway. A new perimeter fence will also be installed as per SW security requirements.

Towards the end of the project, ESD JV was asked by Scottish Water to undertake the decommissioning and demolition of the now-replaced clear water tank. This was not part of the original scope of the contract. These works will be completed in the summer of 2024.

The twin cell reservoir is 100m by 75m x 9m tall with a storage capacity of 43 ML. The construction of the reservoir is semi-precast, utilising precast wall panels with an in situ concrete base, in situ infills between the walls, precast columns and roof planks and an in situ screed placed over the top of them. The site is comprised of a sloping grassland field with a fall of 1 in 10 from north to south.

The new service reservoir and pipework was subject to planning and full statutory Environmental Impact Assessment. Environmental assessment and planning submission was undertaken by others during the definition design stage. Environmental constraints and any required mitigation were passed to MMB for consenting and implementation. This included the discharge of numerous planning conditions from local Councils, applications for: protected species licences (including great crested newt); prior consent under Section 61 of the Control of Pollution Act, and Scheduled Monument Consent from Historic England for tunnelling work under the Bowes Railway.

Landscaping upon completion of works will implement a non-statutory biodiversity net gain on the reservoir site through planting other neutral grassland using a locally sourced seed mix on the reservoir banks and surrounding land, the reinstatement/gapping up and planting of new species rich hedgerow, and the planting of new native trees within the site. A retention basin will be constructed within the site boundary to manage runoff.

Once appointed, MMB began to consult with the pre-construction designers at WSP and the client to further develop and detail design the structure and pipe network. MMB worked with FLI Precast at an early stage of the development to engage with them to design a semi-precast solution for the main structure, utilising lessons learned, and knowledge gained by both organisations from constructing multiple service reservoirs in this manner.

From the beginning of their appointment, MMB has engaged with all stakeholders to develop solutions through collaborative working and to ensure that water quality standards are met throughout the construction process. MMB has particularly engaged with WSP and NWG, who have had an on-site presence throughout the works and have been able to identify opportunities within the design for cost and carbon effective design amendments, with shorter approval windows due to the collaborative nature of the design.

One opportunity highlighted by the design team was the proposal to remove the reservoir underdrainage from the scope. By studying available guidance, in particular the UK Water Industry Research Limited, Treated Water Storage Assets: Good Practice for Operation and Management, Version 2 report.

Underdrainage is more prevalent on structures that are not considered monolithic. Due to the construction sequencing and the type of jointing techniques considered, with no movement joints along with the required fall and depth of the drainage system due to project constraints, reservoir fall direction, the lack of presence of groundwater and subsequent uplift concerns, the underdrainage system would not have been beneficial to the structure. This was presented at an early stage to the Qualified Civil Engineer appointed to oversee the structural design and was deemed surplus to requirement and omitted.

The use of BIM360 as a shared working environment allowed the client, their consultants, MMB and their subcontractors to collaboratively work on drawings, showing real time changes to the design in 3D and allowing quicker design discussions, with the aim to shorten the design program and keep the site program on target. This worked particularly well when MMB re-designed the valve house layout and sizing using virtual reality.

It was noted on receipt of the outline design that the valve house was not sufficiently designed for operation and maintenance of the assets housed within it. Therefore, MMB had to increase the size of the building itself, change pipework arrangements to ensure adequate access to assets. To make this process as efficient as possible the 3D model was used in a VR headset to aid meetings and acceptances with the client. This was better than using 2D drawings as the team could all see in real space what the proposals were, and pick up any issues to be addressed.

Due to constraints regarding planning consents, works on the project began in earnest in May 2023, whilst the detail design of the reservoir was still ongoing, leading to a truly ‘design and build’ approach on the scheme. This approach sharpened attention on the project’s critical path and detail design works proceeded with construction programme in mind.

Significant design works were conducted in relation to the geotechnical operation, which required approximately 55,000m3 of sandstone and superficial materials to be excavated. The excavated materials will be reprocessed at a local quarry to become the embankment backfills, reducing waste.

One of the initial challenges with the construction of the scheme was the variance in geotechnical properties of the founding material. Covering approximately 80% of the reservoir footprint is an area of sandstone, and the remaining 20% an area of superficial materials, comprising weathered sandstone and granular glacial till to the south side of the reservoir.

Due to this variance, there was a significant risk of differential settlement of the base, which was highlighted during the definition phase. Initial settlement calculations led to large differential settlements which would have created intolerable stresses within the structure, so a ground improvement solution was sought. The solution adopted was to use a large temporary berm of material, comprising of the excavated material for the reservoir, to surcharge the softer material to the south. This berm of material (measuring approximately 8m tall, generating greater bearing pressures than the final structure would when filled to maximum capacity) was constructed, along with monitoring points using settlement gauges within this material, to drive out the anticipated settlement.

The monitoring results were analysed showing that the settlement was achieved quickly, with a significant tail off from any additional settlement recorded. The trendline of the settlement indicated granular properties over more cohesive, which confirmed that rebound of the material was unlikely. The berm was removed and then an assessment of rebound of the material was made. Initial programmed time for this berm was significantly longer than eventually needed and due to the close monitoring of this settlement it resulted in some programmed time savings to the works.

The use of the precast walls reduced the scale of wall reinforcement fixing reducing both a risk of the reinforcement cages moving during the temporary construction phases and the time duration of works at height for setting the reinforcement and pouring operations, making construction safer. The reduction in concrete pours reduced the number of deliveries to site and the number of concreting operatives at any one point.

Off-site production of the walls aided in finish control of the concrete and allowed for higher accuracy of the placement of pipe wall couplers and setting in submarine door frames with less likelihood of cracking around these openings.

SMR Projects Ltd supplied a Xypex additive for the concrete mix, which reacts with water and forms crystalline structures closing small voids/cracks that may occur from various thermal effects. Controlled permeability formwork was used to densify the surface of the concrete by limiting evaporation of water from the concrete surface. This increases resistance to soft water attack which is prevalent in this area of England. An impermeable sheet was also used over the infill following striking of the formwork for the curing process to prevent the concrete from drying out too quickly, again to reduce early cracking of the concrete surface creating a higher quality product.

From an operational perspective, using submarine doors to access the reservoir cells through the valve house allowed MMB to design out a requirement for access directly through the reservoir roof, thus reducing the number of openings in the roof helping to prevent contamination ingress to the reservoir via additional roof hatches and reducing fall from height risks associated with such access.

A further saving was made during design by ensuring that the valve house was capable of housing the electrical and signal board components needed for monitoring of flows and operation of valves. This removed the need for an external electrical control building that was considered in the planning application.

The Springwell Project, located near a new housing estate close to the village, has led the team to focus on early engagement with local stakeholders to minimise disruption, including road closures, bus and footpath diversions, construction traffic, and the impact on local businesses, as well as noise and dust concerns. To ensure the community feels informed and involved, MMB have participated in meetings and workshops with residents to provide project updates, get local input and answer questions.

The approach to community engagement has also included regular drop-in sessions, quarterly information events, newsletters, and site tours to keep residents up to date throughout the project. These initiatives have helped build a strong, positive relationship with the community, reflecting MMB’s commitment to minimising disruption and improving the local environment.

There has been significant emphasis on supporting education, particularly in STEM. By partnering with Newcastle College, we’ve organised site visits and provided two students with work experience, while local primary schools have been engaged through activities such as site safety aiming to spark interest in engineering and construction. Beyond education, MMB have collaborated with local organisations and the project’s subcontractors have also contributed to the community by installing a reading area at Springwell Village Primary School, demonstrating how MMB have harnessed resources and partnerships to deliver meaningful improvements.

The reservoir is part of a broader package of works to improve the resilience of Northumbrian Water’s clean water network and the reservoir aspect of the scheme is approximately 70% complete at the time of writing (August 2024).

Through MMB’s commitment to protecting the environment and social engagement the project has limited disruption where possible and aimed to build a stronger connection with the local community.

Direct engagement with the client during the enabling works and beyond has expedited the programme of works, allowing longer lead items to be procured earlier and aid in achieving this challenging programme.

The use of a large berm of excavated material during the enabling works for the scheme has induced settlement within the ground prior to the structure installation, thus reducing stresses on the structure and allowing a more efficient design to be made.

Engaging with the precast concrete panel designer at early stages of the construction programme allowed construction efficiencies to be achieved with respect to structural arrangements and thickness, reducing embedded carbon for the structure. Through the team’s collective experiences of building modern service reservoirs, MMB have endeavoured to create a structure which should be less prone to leakage. Further works and testing are ongoing to realise this so far successful project.