Challenges in construction of HS2 Diaphragm Wall shafts and Portal (UK)-PPT Presentation

PPT Presentation

Pdf Summary

The presentation describes VSL’s construction of five ventilation and intervention shafts for HS2’s Chiltern Tunnels and Colne Valley Viaduct works. The wider 21.6 km scheme included a 16.04 km twin-bored tunnel and a 3.37 km viaduct. VSL’s shaft works ran from July 2020 to December 2022.<br /><br />Four shafts were built using circular diaphragm walls, each with an internal diameter of 18.16 m; the fifth, at Chesham Road, used a sunk-caisson design. Shaft construction was complicated by varied ground conditions, chalk dissolution features, groundwater sensitivity, and tunnel connections that cut into the shaft walls. Pre- and post-treatment grouting was required, with environmental controls especially important because the sites were in a groundwater protection zone near an active water supply.<br /><br />A major operational challenge was managing bentonite slurry contaminated by chalk and concrete fines. Limited site space constrained excavation, storage, and treatment capacity. The team developed a compact, vertically arranged recycling plant, used centrifuges and polymer flocculant to improve slurry treatment, and reused bentonite between sites. This reduced disposal substantially: at CSP, only about 400 m³ of bentonite was destroyed despite 6,300 m³ of excavation.<br /><br />Other challenges included variable excavation rates, coordinating plant and deliveries, developing buildable reinforcement designs, cage-supply delays, and concrete delivery rates that affected the rate of placement. Planning reusable pipe and cable layouts in advance improved site setup and efficiency.<br /><br />The presentation highlights the value of early coordination, adaptable design, careful slurry monitoring, and aligning excavation rates with treatment capacity. Shaft works recorded only minor first-aid cases, with plant movement and crane slew zones receiving particular safety attention.

Keywords

HS2

Chiltern Tunnels

Colne Valley Viaduct

ventilation shafts

intervention shafts

diaphragm walls

sunk caisson

bentonite slurry

groundwater protection

shaft construction