Lee Tunnel tapering rebate wall requires design experience
Project: Lee Tunnel
The Lee Tunnel forms part of the Thames Tideway Scheme and is designed for the storage and conveyance of sewage mixed with rainwater, running from Abbey Mills Pumping Station down to Beckton Sewage Treatment Works.
Members of the team here at PDMA worked at different times on different parts of the scheme for several different contractors.
- Permanent and temporary works design on the same project
- Designs for three different contractors
- All elements of temporary and permanent works design were challenging
Adaptation key to success
Initially the team’s involvement was with Slipform International, who required support for the lining of a 40m diameter shaft. The standard Slipform equipment had to be adapted for a single-sided shaft lining with a tapering rebate.
Following construction of the tunnels, the standard Slipform equipment was adapted a second time to form a tapering wall, keyed into the rebate at each side of the shaft.
To enable the formwork to taper, the normal fixed joints between members, on which the rig relies for stability, had to be pinned.
Alternative load paths were introduced to achieve this, with a sophisticated computer analysis undertaken to help convince the client that the finished rig would be stable.
The analysis used out-of-balance load cases in accordance with Eurocode 0, which describes the basis and general principles for structural safety, serviceability design and durability. Although a challenge to demonstrate the rig was stable under this loading, it was ultimately successful.
Dredged channel requires retaining wall design
The outfall from the Lee Tunnel was constructed as an immersed tube installed in a dredged channel.
It was members of our team responsible for designing the free-flow sheet pile retaining walls required to support the ground adjacent to the channel and to allow the 3m diameter tubes to be floated into position.
Part of the new outfall and retaining walls had to be installed under an existing jetty which carried services to a docking facility.
It was not permitted to disturb the jetty, so a scheme was devised to lower pre-cast beams under the jetty, which would then be sunk in place like a caisson whilst the ground was removed under the jetty.
This challenging project required the innovation and expertise of our highly experienced engineer, Mike Tindale, who commented: “To avoid settlement of the existing jetty foundations, it was essential to avoid loss of ground during installation of the temporary works.
“A further complication was that all the necessary work would have to be carried out under water.
“Our scheme was carefully detailed so that the components fitted together perfectly to avoid any paths where ground might enter the excavation. We predicted settlement and this was monitored throughout the construction period, ensuring it was kept within tolerable levels.”
At the end of the outfall there is a reinforced concrete headwall, which is below low water level.
We were asked to design this by the contractor, and we designed the permanent works so that the formwork could be used as a bulkhead to enable the headwall to be floated into position and installed at the end of the free-flow retaining walls.
The outfall itself is heavily skewed, which required finite element analysis in accordance with Eurocode 7, to ensure the design achieved the required solution.