Tony Archer from Cemex UK on creating the concrete walls for the tunnel shafts
What was the problem?
The £635m Lee Tunnel is a 4.5 mile-long tunnel designed to prevent more than 16 million tonnes of sewage mixed with rainwater from overflowing into the River Lee, then on into the River Thames each year. When building the widest of five 90m deep shafts concrete teams working for Morgan Sindall, Vinci and Bachy Soletanche and Cemex UK faced the challenge of creating the inner wall in one continuous concrete pour to avoid the creation of joints that would compromise the shaft’s structural integrity.
What was your approach?
Using slipforming techniques, the form was gradually raised at a rate of 100mm to 150mm per hour. Three cranes delivered the C50/60 concrete to three skips inside the shaft, which poured 120m3 per linear metre rise and a total of 11,000m3 of concrete over 29 days, making it the longest continuous slipform concrete pour ever completed.
The slipform rises faster the higher up the shaft it goes, so we had to create nine different concrete blends incorporating different admixtures including Isoflex superplasticiser MR 800D retarder, designed to allow the concrete to cure at different rates. We also wanted the option to slow the curing process down by pouring concrete with a high level of retardation should problems occur, such as cranes being winded off. The concrete mix contained 500 tonnes of steel fibres as reinforcement.
Slipformed concrete needs to be workable enough to be placed into the form and consolidated via vibration, but quick-setting enough to emerge from the form with strength. On this job, the concrete’s workability in its plastic state was very consistent throughout, which is extremely difficult to achieve.
How do you staff a 29-day pour?
It creates a lot of logistical problems, but jobs like this are only as good as the people employed to do them. The permanent site team was fantastic and some employees on other jobs even volunteered to come in to provide relief to those doing long shifts.
