The final pour and the dismantle of CEMEX’s Lee Tunnel concrete plant have taken place, having produced 220,000 cu metres of concrete for this part of London’s largest sewer upgrade scheme by Thames Water. The new £635million, 4 mile long tunnel will handle around 39 million tonnes of sewage a year.
From concrete mix design to delivery and placement CEMEX has worked with joint venture MVB, Morgan Sindall, Vinci Construction Grands Projects and Bachy Soletanche to provide a number of bespoke concrete mix designs.
“This is one of the most complicated infrastructure projects of recent years. Even before work began on site in autumn 2010 we had to look at every eventuality with the concrete and how performance might be affected.
“We carried out trial mixes at our National Technical Centre and on site reviewing amongst others, cement designation and content, maximum aggregate size and admixture combinations. We had to be sure the mixes would be usable and retain consistence even when they had been skipped down the shaft or pumped onto the concrete train,” explains Richard Kershaw, National Technical Manager, CEMEX UK Readymix.
Starting with the diaphragm walls of the outer shaft, the concrete used was a C50/60 grade mix incorporating 70% ground granulated blast furnace slag (GGBS)as a cement replacement. Within this outer shaft the main inner lining was cast as a 750mm thick concrete cylinder. The outstanding aspect was a 29 day
In the tunnel itself CEMEX worked with MVB to develop a solution to the scheme’s requirements for the secondary lining. The primary lining was a precast system whilst the secondary lining was a 300mm thick steel fibre reinforced lining cast in-situ within two 30.6 metre shutters reducing the previously bored 7.8m internal diameter to 7.2m. The lining was built along the complete 7km tunnel.
For this part of the project, after months of trials the team developed a concrete that would retain consistence for up to six hours, contained Dramix 5D steel fibres at 40Kg/m3, develop early compressive strength and flow for 15m without any segregation.
“Lee Tunnel was a challenging project in which the design and technology of the concrete was pushed to the limit. The concrete was essential to help build an exceptional structure needed by the people of London”, concludes Richard.