HS2 Burton Green Tunnel – South Portal

The entrance and exit of high-speed trains from tunnels creates significant issues of air pressure dispersal. The solution adopted by the designers for the Burton Green Tunnel South Portal was to have a fixed height porous portal section of varying width with pressure relief vents of varying sizes.

Whilst the overall design was issued to ABM by BBV, the 178 precast roof panels encompassed 96 unique dimensional combinations and presented significant challenges to efficient manufacture as did the presence of the vent ‘chimney’ of varying size on many of the units.

ABM developed an innovative but simple mechanical formwork solution which allowed several ‘standard’ moulds to be rapidly and accurately altered to each of the 96 unique designs during the production cycle.

Whilst the units could have been manufactured monolithically by casting inverted, this solution to the ‘chimney’ issue would have been impractical as rebar projecting from the upper surface of the units would have led to expensive formwork or rebar coupler solutions and significant challenges in turning the units over. Instead, ABM chose to cast the ‘chimneys’ as a secondary operation using a set of variable sized internal/external formwork.

ABM also championed the use of alternate detail for the critical construction joints in the ends of each panel where they would connect to the in-situ portal walls. Traditionally, this detail would use a secondary surface preparation to ensure an acceptable connection between the precast and in-situ elements of the works.

By agreeing the innovative use of a cast, indented construction joint detail (in accordance with BS EN 1992-1-1; clause 6.2.5) production efficiency was improved by elimination of the secondary surface preparation process.

178 precast units weighing up to 30 tonnes were produced to high dimensional tolerances in an efficient and cost-effective manner.

The project benefited from the efficient production arising from ABM’s innovative approach to the manufacture of these complex pre-designed units.