Sewer Rehabilitation

Project Driver:

The host pipe was subject to severe H2S attack (a loss of approximately 30mm of wall thickness) and on the point of collapse.

Solution:

Carrying out a detailed survey in advance of the works enabled all liners to be designed, ordered and delivered at the same time thus reducing individual deliveries. The surveys identified the high levels of corrosion within the pipe and thus areas whereby the diameter was 780mm not the original 750mm, this then triggered the need for an innovative fold down liner. Starting at the furthest upstream end the programme allowed for one length after another to form a near continuous length. 

As a result, an innovative fold down structural UV liner was manufactured to accommodate the variance in the standard diameter ensuring a close fit to the host pipe. The rehabilitation took place in 11 sections ranging from 15 to 120mtrs in length with a liner thickness of 7.4mm. 

Delivered as principal contractor, the scope also included design, significant over pumping capable of maintaining flows of 500ltrsp/s, civils enablement, traffic management and key stakeholder engagement.

Delivery:

McAllister’s Project Manager owned the project from inception and managed it through to delivery giving the client comfort that we understood the project and could deliver what was promised with no internal hand-offs. Working during the COVID-19 pandemic with restrictions on movement is a situation that no organisation wants however, after careful planning and by following government guidelines allowed work to continue. The health and wellbeing of all colleagues and general public was paramount, COVID-19 risk assessments took place daily with a clear reporting processes should someone be concerned.

Working collaboratively with the client through early engagement allowed us to not only confirm the scope by carrying out detailed surveys but plan for all elements of the work including traffic and stakeholder management for increased efficiency by being able to plan works effectively and mitigate risks associated with this.

Successful flow management was critical, McAllister worked with the client pump supplier to design a system capable of not only managing storm condition flows but also to maintain access to the beach for local residents. Maintaining access to the beech was deemed to be valuable to both visitors and residents and as such gained approval from customers early on. The overpumped section included for: overpumping flows for the entire length, 4no 12” pumps, all steel flanged pipework to prevent potential spillages, primary and stand-by pumps running on variable speed drives to match the incoming flow, powered by primary and stand-by generators and more importantly telemetry and remote monitoring to allow for full control.

Traffic management was installed as a rolling solution whereby the controls moved with the works and was not static at each end, this allowed the restricted area to be reduced resulting in minimised impact for local traffic.

Complexities

As with all projects, stakeholder engagement is vital to its success, this includes not only Local Council, Highways etc but more importantly the local customers. Due to its sensitive location (environment and residential) extra attention was given to early engagement with the appointed residents association, the benefit being a single point of contact for multi occupancy dwellings whom is obliged to raise concerns/issues from all residents. The proactive nature of the planning allowed for information to be shared early and in returned we received customer feedback/concerns which contributed towards mitigating potential customer issues. As a result of the engagement, concerns were raised as to the footpath closure and the approved alternative pedestrian route over an existing grassed footpath. The issues related to providing additional considerations to the elderly demographic and to ensure that an even and stable footway was provided. In response to the concern we installed even and stable temporary pedestrian matting along the length of the works. The demographic also had also had an impact on not being able to suspend bus stops during this install which had a direct impact in planning the works all overcome due to valuable stakeholder engagement.      

Working in a coastal resort comes with it challenges, not only are we working in an environmentally sensitive location along the seafront but during late summer/early autumn weather can bring a rapid unexpected change in conditions. Although we carry out an assessment of the long and short-term weather forecasts during planning stage nothing can guarantee the actual conditions of the day. Early October 2020 bought some difficult conditions to the site with high winds and heavy rain. The rain itself wasn’t an issue for flow management as we had planned for storm conditions when designing flow management solutions however, the wind creates a different issue. With hundreds of meters of fencing and barriers on site running parallel with the seafront the safe maintenance of the barrier systems was of upmost importance to protect our site and local residents. During the extreme conditions, to mitigate the potential for damage, the joint decision was made to dismantle non-critical protection (but still ensuring that traffic, cyclists and pedestrians remained safe at all times) and carryout more frequent checks and maintenance on the remaining critical protection. We ensured that a presence was on site 24/7 to continue with barrier system checks and maintenance.     

Our collaborative approach to the works showed real value when demobilisation commenced. Demobilisation started on Saturday morning, when the flow stoppers were removed further structural deformity with the existing sewer was identified. Our one team approach allowed for a seamless reactive response whereby McAllister site team notified the client (via project manager) to the issue prior to the dismantling of the complex flow management and as a result a repair team was onsite within hours to carry out the repair. The expedient nature of the response allowed for the existing flow management to remain in place thus reducing the need for further costly installations at a later date along with further disruption to local residents.

Outcome:

Now the project is complete, the client has a structurally secure asset with an operational life in excess of 50 years.

Working collaboratively allowed the project team to maximise the strengths of each organisation and as such were in total control of all aspects of delivery. Any changes to the programme were identified at an early stage, the project team was able to respond to the changes quickly thus having little or no impact to the overall programme and being able to maintain project completion.