Manual TBM for Southampton Link Main: design and constructability notes for engineers
Reviewed by Joe Ashwell
First reported on The Construction Index
30 Second Briefing
A 6,500kg manually operated tunnel boring machine, Ancasta, is driving a 1.5m-diameter tunnel at depths starting around 14m for Southern Water’s 19km Southampton Link Main between Otterbourne, Yew Hill reservoir and Rownhams water supply works. MGjv, the Galliford Try–M Group Water joint venture, is using two manual TBMs alongside conventional trenching to thread the pipeline beneath major roads, rail lines and sensitive ecological sites, with tunnelling running from September 2025 to May 2026. The £100m-plus scheme, spanning over three years, is aimed at boosting network resilience and reducing abstraction pressure on the Test and Itchen rivers.
Technical Brief
- Manual TBM Ancasta weighs 6,500 kg and is only 2.7 m long, suiting tight launch shafts.
- Machine is cutting a 1.5 m internal diameter drive, implying limited working space for face crews.
- Pipeline alignment links Otterbourne to Yew Hill reservoir then south to Rownhams water supply works.
- MGjv (Galliford Try–M Group Water JV) is delivering the scheme for Southern Water as principal contractor.
- Trenchless sections are specifically targeted beneath major roads, rail corridors and designated ecological sites to avoid surface disturbance.
- Remaining route uses open-cut trenching with subsequent backfill, indicating mixed construction logistics and reinstatement regimes.
- Overall programme exceeds three years with total scheme cost quoted above £100 million.
- Southern Water frames the link main within a wider Hampshire resilience portfolio for customers and river ecosystems.
Our Take
With a project cost above £100m and a duration beyond three years, Southampton Link Main sits at the larger end of UK water-infrastructure schemes in our Infrastructure database, signalling that Southern Water is committing to long-lived resilience works rather than short-term patch repairs.
Driving a 1.5 m diameter tunnel at depths starting around 14 m using two manual TBMs suggests MGjv and Galliford Try are optimising for precise control in constrained ground conditions around the Test and Itchen, where settlement and environmental impact on sensitive chalk streams are likely key design risks.
Among the 732 Infrastructure stories in our coverage, relatively few focus on deep potable-water transfer pipelines in southern England, so this Hampshire scheme will be a useful benchmark for future regulatory and permitting expectations on strategic water-grid upgrades in the United Kingdom.
Prepared by collating external sources, AI-assisted tools, and Geomechanics.io’s proprietary mining database, then reviewed for technical accuracy & edited by our geotechnical team.
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