Engineers were this week ramping up efforts to head off a worsening structural problem on a section of London Undereground’s Jubilee Line, NCE can exclusively reveal.
Final testing is underway to prove the concrete lining replacement that London Underground (LU) is to use to repair a damaged section of the southbound tunnel.
The focus of the work is on replacing a 200m section of concrete lining between Baker Street and Bond Street stations dating from the 1970s.
LU has built a specially designed train to carry out the replacement work and the overrun tunnel at Charing Cross has been used for testing and training before the main work starts.
The repairs will be undertaken during engineering hours and 31 day closures of the line over the next 18 months, Transport for London announced at the end of April.
The primary cause of the damage was clay shrinkage in the ground around the tunnel, LU principal tunnel engineer Keith Bowers told NCE. Other factors, including probable issues with the quality of design and construction, are thought to have worsened the problem.
Bowers said that there was evidence that during construction of this section there was a failure to properly align joints between the 22-segment expanded concrete rings. This meant they were stepped in places.
“About 10 years ago we started to notice concrete spalling and a few years later tension cracks was recorded,” he said.
Inspection routines were stepped up, monitoring was installed and remedial repairs involving the use of steel straps and the removal of loose concrete was undertaken to allow the section to remain in use.
“The cause is complicated,” he added. “For three decades there were no problems, but the damage has increased in the last 10 years.
“The depth of the tunnel is a contributing factor and means that the ground loads are high – up to 220t/m – which is one a half times the load of other zone 1 tunnels.
“The design of the segments with a curved edge means that the joints are vulnerable unless they are perfectly aligned. The [mass concrete] segments were made as small as possible to avoid the need for using reinforcement. If not well aligned, the stress concentration moves to the edge, which has caused the spalling.”
The affected section of tunnel was built in the 1970s, using a mix of cast iron and concrete segments. According to Bowers, the decision to use concrete on this section was driven by the need to avoid the use of more costly and time consuming cast iron bolted segments. Variable ground conditions were another influencing factor.
“The alignment of the southbound tunnel was driven by the need to pass under the Bakerloo Line and as result this section of tunnel on the Jubilee Line is one of the deepest in zone 1, reaching depths of up to 36m,” said Bowers.
“This depth takes the tunnel out of the London Clay and into the Lambeth Group, which is highly variable with sand beds within the clay. It is also faulted.”
According to Bowers, the same design has been used elsewhere on the Tube network but both tunnels – one in an overrun tunnel at Charing Cross and another at Heathrow airport – are in London Clay and at shallower depths.
“It is the lining design in combination with the depth and construction issues that have contributed to the problems on the Jubilee section,” he said.
“The real trigger for the problems has been the shrinkage of the clay surrounding the tunnel. The lining is permeable and the process of driving trains through the tunnel over the last 40 years has warmed the ground.
Cores taken show that the first 500mm beyond the tunnel is quite desiccated.
“This is a problem that is only going to get worse, which is why we are taking action to replace the concrete with new cast iron segments.
“We started testing the solution by hand in 2010 and built the train wagon to speed up the work and reduce the manual handling.”
The selection of cast iron to replace the damaged sections has been driven by the need for a modular solution with plenty of redundant capacity to cope if the ground conditions continue to deteriorate with further clay shrinkage.
Training in the repair method is continuing but enabling work on the section between Baker Street and Bond Street section has started. The first lining is expected to be replaced in late June.
While work to replace the concrete linings is underway, London Underground will also carry out repairs to cast iron linings in an adjacent section of the same tunnel to the south.
“We will be using a gel grout to create an airtight seal between the flanges of the cast iron lining to prevent air leaking from the tunnel and reacting with the naturally occurring groundwater and pyrite in the surrounding ground,” said Bower.
“The reaction has resulted in the formation of sulphuric acid that has caused damage to the linings and the gel grout will prevent the need for the linings to be replaced.”