Cardiff Arena Sewer Diversion and Enabling Works

LOCATION:

Cardiff Bay

Infrastructure

Water Utilities

A £5.5m project for Live Nation and Robertson Property Development to undertake the diversion of a DCWW (Welsh Water) 1200mm diameter live storm water drain. This diversion was required to realign the storm drainage to enable the commencement of the construction phase for the new Cardiff Arena development.

Key elements of the contract

Translocation of orchids to an area approximately 500m away.
Excavated and installed a lightweight concrete infill to reduce the potential risk of delay due to poor ground conditions and obstructions from affecting the pipework installation works.
Tunnel boring of approximately 165m new 1200mm diameter jacked concrete pipework at 8m depth to invert.
Installation of three, three metre diameter 8m deep manholes to adoptable standard.
Installation of a new pedestrian footpath to the perimeter of the new development.

A Tunnel Boring Machine (TBM), also known as a “mole” or “worm”, is a specialised machine used to excavate tunnels through various types of soil and rock, often used in large infrastructure projects.

These works will create a corridor for all future utilities diversions while also undertaking a comprehensive UXO survey to inform the wider development and enable progression through the final design stage.

Challenges

Installation of large diameter drainage in an infilled dock feeder at 8m depth with a high-water table and pre-contract ground investigation report suggesting potential for ‘fragments’ of wood and brick only at varying depths.

When excavated for installation of the first tunnel driving pit, the extent of obstructions was significantly greater than the investigation report suggested and not possible for the TBM to operate without repeated blockages and breakdowns, which would significantly impact the time and cost of the project and the ability to give uncertainty to the client.

Solution

After exploring options, we were only left with a single solution, which was to excavate the corridor for the TBM to its full depth and width plus 500mm. Install trench support boxes and infill with a lightweight flowable concrete.

This enabled us to clear obstructions and create a consistent material suitable for retaining the remaining ground and allowing the TBM to install the required pipework.

Outcome

TBM commenced the first of two drives in November and completed with extraction just prior to the Christmas period. The second drive was then commenced January 2025 and completed mid-March. DCWW conducted their CCTV and visual inspections one week after completion and signed off with no recommendations for snagging.