I am wondering whether they will go for grouting the mines under the tunnel invert before digging out the track, sorting the drainage (with effluent treatment of the ochre) and relaying ballasted track, or whether they might try slab track to reinforce the invert and hope that what is underneath will stay there and isn't the railway's problem?
The problem with this is that I believe that UK "law" is that the owner of a culvert is responsible for what comes out of it, even if it's a sealed / water-tight tube and they haven't contributed at all.
Grouting is pretty much an inevitability at this point, slabtrack needs something to support it and it sounds like these voids are extensive.
I'd imagine the programme of works will be;
- Additional surveys to understand the extent of the voiding. (8-12 weeks)
- Design works to grout up the voids (this will be difficult as it sounds like there is a tidal element and pumping grout into the sea won't be well received - additionally the voids have potentially allowed track drainage, once grouted flooding in the tunnel may be greater. The tunnel will restrict the size of plant that could be used which will make working more difficult, expensive and longer duration) (30-50 weeks)
- Track / Drainage works (3 weeks)
Whether the ochre problem is sorted at the same time is likely being discussed at a high level - as alluded to above there is no money at the moment (or for the forseeable) meaning this will be paid for at the expense of other projects already programmed in.
I'd assume the tunnel will be closed for 12 months.