SUMMARY

The Thames Tideway Tunnel is an under-construction 25 km (16 mile) tunnel running mostly under the tidal section of the River Thames through central London, which will provide capture, storage and conveyance of the combined raw sewerage and rainwater discharges that currently overflow into the river from London. Works commenced in 2016 with a target completion of 2022.

The scope of the works included the installation of a UV liner into the Low Level One Suction Culvert. The purpose of the liner was to prevent leakage of the existing Low Level 1 Suction Culvert during the piling and excavation activities for the interception shaft for the Tideway Tunnel. This also would protect the Low Level 1 Suction Culvert from the imposed loads created by ground movements and vibrations from excavation of the proposed CSO shaft at FALPS.

Thames Tideway Central Project Outline Brief

The Thames Tideway Tunnel (TTT) is an under-construction 25 km (16 mile) tunnel running mostly under the tidal section of the River Thames through central London, which will provide capture, storage and conveyance of the combined raw sewerage and rainwater discharges that currently overflow into the river from London. Works commenced in 2016 with a target completion of 2022.

The TTT Central Contract comprises the construction of 8no. drop shafts, intervention of the Thames Water Utilities (TWUL) network and associated work sites along with 2no. principal Tunnel drives from Kirtling Street to Chambers Wharf (East Contractor Site) and Carnwath Road (West Contractor Site).

Works Description

The scope of the works included the installation of a 1200mmø x 40mtrs x 15.5mm thick UV liner into the Low Level One Suction Culvert 5mtrs deep. The purpose of the liner was to prevent leakage of the existing Low Level 1 Suction Culvert during the piling and excavation activities for the interception shaft for the Tideway Tunnel. This also would protect the Low Level 1 Suction Culvert from the imposed loads created by ground movements and vibrations from excavation of the proposed CSO shaft at FALPS. Site constraints meant that all works had to be carried through a restricted access of 900mmø manhole.

Figure 1: Demonstrates the location of Low Level 1 Suction Culvert. The red area also demonstrates the area requiring the liner.

As you will note from the design drawings the inlet was 1200mm diameter before opening up to a 2100mm diameter with a cascade after which reducing back down to a 1200mm diameter before entering into the pump chamber. This project facilitated the opportunity to improve the hydraulic characteristics of the pipe by creating greater gradient and decreased friction coefficient from the brick host to the new liner installation.

When working alongside the design team the solution was to install a 1200mm UV liner on steel ‘chairs’. This would act for 3 reasons;

  • create the new gradient of the sewer taking into account the cascade
  • create the new installation profile
  • act as anchors / structural supports when grouting around the annulus to fill the void between the liner and the 2100mm host pipe and prevent buckling.

CRITERIA

Understanding the required outcomes and the concerns of the client from conceptual stage allowed the solution to be designed to meet the client objectives. We considered the objective and provided several installation techniques all of which we would manage and integrate into the programme. The new installation materials in isolation as a solution is not itself innovation however, the complex nature and environment which it was used brought together a plethora of options to develop a single delivery channel for the sole purpose of achieving the outcome. The client had a streamlined and efficient supply chain all working to the high-quality standards. The techniques used included McAllister Bros assisting in the raised ‘chair’ frame design to house the liner as well as fully designed UV cured glass liner, direct labour installed UV lining, supported by our supply chain to deliver the ‘chair’ installation and welding. The ‘chair’ eliminated the reliance on the existing oversized host and allowed the new installation and asset re-design without a single excavation.   

Ensuring that we achieve and/or exceed the requirements of the client we believe that liner quality and testing is something that we at McAllister Bros take very seriously and as a result we have our liners tested to the highest standards. The design specification used was State III for cracked pipes with larger deformations and considerable risk to collapse in the near future from the German ATV-M127.

Sometimes true innovation is knowing when to choose the right solution(s) to overcome a set of circumstances rather than trying to change the nature of the circumstances for the sake of innovation.

  • Environmental Management

UV lining itself is a tried and tested trenchless method of increasing asset security with minimal impact on the environment however in this instance McAllister Bros developed a system for a ‘new installation’. Traditional excavation methodology would have generated considerable construction waste that would have required removal from site with a direct impact in the carbon footprint of the project as well as the commercial impact for disposal.

The environmental complexities of this project (see initial project brief) was challenging in itself, being involved in project planning at such an early stage allowed complete waste management to be considered and reduced. Due to the restrictions forced on the installation with the size of the manhole (900mm) access the steel used for the ‘chair’ construction was designed as a modular installation so could be cut and constructed off-site, dismantled, delivered to site and constructed again within the pipe, this prevented waste on site by sheet materials being delivered. This method also reduced the number of deliveries required to site as it was a single supplier.

During planning phase, a detailed survey of the site was taken, this allowed McAllister to consider the correct amount of lining materials required for the new installation without creating additional unnecessary waste. The liner was manufactured to the exact measurements which mitigated the risk of environmental impact and additional cost.

  • Community Impact & Customer Care

At project inception the impact on the Community and Customer was at the forefront of both client and contractor, which in return had a direct impact on the programme. As an example; McAllister Bros works were critical to the successful delivery of the entire project and was deemed to be critical path. To ensure that our works were not affected by the other elements of the work and that our work was 95% underground so would not have a significant impact on the community/customer, it was decided that we would work 100% out of hours with deliveries and other vehicle movements would be restricted to the beginning end of the shift.

The client erected hoarding around the site, not only to eliminating the risk of any unauthorised entry the site but also to control any residual noise that may come from the site during the day or when our works were delivered.