Introduction
HB Tunnelling supported Balfour Beatty by installing a drainage pipe from the new underpass at the modified Mytongate Junction, linking it to the pump station shaft. The original plan to use a micro-bore TBM for a 1200mm ID pipe-jacked tunnel proved unfeasible due to bridge works preventing TBM removal. Instead, we were commissioned to design a 730mm ID auger-bored connection, including reinstatement of tunnel eyes at the shaft and diaphragm wall, and a headwall to support the pipe opening.
To complete the connection, several challenges had to be overcome. The 42.9m tunnel passed beneath the live A63 carriageway and through the shaft wall. Ground investigations revealed weak alluvial clays and silts at tunnel level, with granular alluvium and glacial till beneath.
Augerboring
Objective
Preparatory works included draining the shaft, installing a temporary concrete base, and designing and installing grout blocks outside the structures to provide soft eyes to launch and retrieve the auger through. This was required due to the soft nature of the ground behind the walls and the concern that, when the tunnel eyes were removed, the surrounding material could surge.
The Challenge
Tunnel eyes were cored out and stabilised with shotcrete. A thrust block and auger bore rig were installed in the shaft, and the bore was completed in three phases: pilot rod, 419mm intermediate auger, and final 762mm steel pipe. Pipes were installed in 6m sections, fully welded and inspected. At the reception end, the opening was modified and strengthened with dowels before final pipe installation, monitored throughout to ensure tolerances.
Seals and puddle flanges were fitted, and the annulus grouted with high-strength, non-shrink grout. A 20.5mm UV liner, designed for a 120-year service life, was installed, cured with UV light, and sealed. Minor weld seepage was patched prior to liner installation.
The temporary shaft base was then removed, the shaft cleaned, and benching installed by HBT. HBT managed all shaft works, including daily checks and permit compliance under Balfour Beatty’s system.
The works commenced in September 2025 and concluded in December 2025.
“This was our first time using trenchless technique &
HB Tunnelling, we have heard great things about HBT but I can confirm that
they are very professional & organised company & we will be looking at the
next scheme to use this type of works”
- Persimmon
