Introduction
As part of the planned major improvement works to the A3 carriageway, HB Tunnelling was appointed to carry out the shaft and tunnelling works. This also included obtaining all necessary approvals from National Highways in accordance with CD622, as well as delivering the full design scope.
The under road crossing (URX) was required to facilitate the diversion of a 630mm medium pressure gas pipeline.
The proposed URX consists of a 1200mm internal diameter (nominal) tunnel beneath the A3 carriageway.
Microtunnelling - 1200mm
Objective
The tunnel was installed using a state-of-the-art closed-face Micro Tunnel Boring Machine (MTBM).
HB Tunnelling installed a 8.2m internal diameter launch shaft, 15 metres deep, incorporating a 2m x 1.55m deep jacking collar. The reception shaft measured 6m in diameter and 12 metres deep, with a 1.5m x 1.5m jacking collar.
The launch shaft was positioned adjacent to the A3 southbound carriageway, while the reception shaft was located beside the northbound carriageway. Both shafts were constructed through Made Ground and the Bagshot Formation. Ground conditions posed a challenge due to a high groundwater table, which resulted in persistently wet excavations during shaft installation.
Ground conditions posed a challenge due to a high groundwater table, which resulted in persistently wet excavations during shaft installation. Excavation of both launch and reception shafts was carried out using HB Tunnelling’s Hitachi 350 telescopic clam shell, completing the dig to formation level.
The Challenge
Excavation of both launch and reception shafts was carried out using HB Tunnelling’s Hitachi 350 telescopic clam shell, completing the dig to formation level.
Once formation was reached, a 3.2m deep anti-flotation plug was installed using a tremie pour. After curing, a 500mm reinforced concrete capping slab was cast to seal and stabilise the shaft in preparation for tunnelling operations. The next phase included the installation of launch eyes and setup of the MTBM within the launch shaft.
For this crossing, HB Tunnelling selected the Iseki 1200 TCC — a full-face tunnelling machine capable of precise microtunnelling.
Tunnelling progressed at an average rate of 9 metres per shift and was completed on programme, leaving a 1200mm internal diameter concrete sleeve ready for the installation of a double-coated, 610mm heavy-walled pipe to facilitate the highpressure gas diversion.
“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
For more information on shafts and tunnelling, please contact us on 0845 388 3325.
