REPAIRS TO the Docklands Light Railway extensions south tunnel are likely to follow the model set following the Heathrow Express collapse, it emerged this week.
Engineers are still assessing damage to the tunnel, blown apart in a violent compressed air explosion last week (NCE last week) and are considering two options for repair.
But the most likely method will follow the technique used to recover the Heathrow Express tunnels which collapsed in October 1994 (NCE 21 December 1995). This would mean exposing the tunnel from the surface and replacing the damaged section with a cast insitu concrete box.
The other option being considered is to replace the damaged concrete lining from inside the tunnel with cast iron rings. This is likely to be discarded on safety grounds.
Mowlem project manager Bill Shepherd said it was still too dangerous to enter the tunnel to confirm the damage but that so far it appeared that at least two of the 1.2m long rings had been completely destroyed. Access in the future, he said, was also likely to be difficult.
The bulkhead on the north side is too close to the damaged area to enter safely, and the bulkhead on the Greenwich side is not equipped with an access, said Shepherd.
Work will certainly be hampered by groundwater flowing into the tunnel from highly permeable Thames Gravels. This is estimated at 5 litre/s and means that if engineers opt to repair the tunnel from the surface they will have to create a sealed access shaft. Specialist subcontractor Keller Ground Engineering claimed that this is likely to mean completely flooding the tunnel.
Keller project director Charles McAnally confirmed: If the tunnel was flooded we would be working in a static environment so it would be much easier to achieve a seal against the undamaged tunnel lining with piles or grouting.
Keller has been pressure grouting all week to stem the flow of groundwater and to stabilise ground around the damaged tunnel. It has also probed the blast area to establish the extent of damage. Engineers expect to decide the method of repairs by the end of the week.