lMetropolitan Line The King's Cross to Farringdon section of the Metropolitan Line has suffered persistent water infiltration from leaking water mains for years.
Leaks were so intense that they were dubbed the 'P-way shower', dousing track maintenance gangs working on the line.
The labyrinthine layout of decaying pipework above the tunnel has made a clear diagnosis of the origin of leaks tricky. 'It is such a complex system of pipes, and because of the way the ground falls, water could be coming from other places, ' notes McKenzie.
Last summer Metronet, working with Thames Water, was initially unable to locate the source of the P-way shower. It tried to resolve the problem by installing a temporary diversion pipeline, only to find the source of the leak the day installation was completed.
Now, still using the original mains supply, Thames Water has partially reduced water ingress to the Metropolitan Line by repairing the severest leaks. Plans to replace the mains entirely are under discussion. 'In the meantime we are trying to improve impermeability by grouting the crown of the tunnel and fixing leaks as and when, ' says Sockett.
Long term, Metronet plans to mitigate the effects of water leakage by renewing ballast and replacing existing unperforated clay pipe underlying the track with a perforated one.
'The interface between signals and water here is less obvious.
But it is an issue, because if you do have a failure it is a very heavily trafficked route and can cause severe disruption, ' says McKenzie.
l District Line Flooding at the southern end of Southfields Tunnel on the District Line has been causing disruption to rail services for years but Metronet suspects it could be a link to regular signal failures in the area between Wimbledon and East Putney.
A temporary pumping system installed in 1999 has failed to cope with storms of a once a year intensity. Metronet is in the process of conducting a drainage survey.
'We keep finding more drains, ' says Sockett. 'Existing drawings are not complete and we've found many blocked drains which have to be cleaned to be surveyed.' In addition, London Underground and Network Rail service patterns mean just one engineering hour is available, on weekday nights only. 'There is a desire to find a permanent solution but it's like working with one leg and one arm tied behind your back, ' says Sockett.
Metronet hopes to replace the supplementary pump system that has been installed under Operation Sponge with a permanent, pump-free track drainage system. It hopes discovery of a 225mm diameter connection - albeit in poor condition - between the catchpit in Southfields tunnel and a Thames Water storm sewer might make this easier.