Some repair work has been carried out, and currently there is a 5mph speed restriction for east-bound buses.
When I travelled back from Cambridge on Sunday, my west-bound bus was still faced with poor track at this location.
These two photos show the the point near Histon where one of the westbound dips is. It's difficult to capture in a photo but the slight change of alignment can be detected on the right hand edge of the track.
Link to Cambridge News Article here includes a slideshow with close-ups of the faulty track.
Comment from Stephen Lawrence sent to me by email:
ReplyDeleteI travelled the length of the northern busway [a few days ago] - the HIston "dip" has been fixed, and a 5mph limit imposed past the scene. Interesting that no such speed reduction was in place when the "dip" was there.
There appear to be two other locations where "dips" have occurred in the Histon-A14 stretch - marked with white arrows and a certain amount of evidence that the ballast has been dug out in the past.
There's also a dip at the start of the guideway ("B" service, by Orchard park West) and another at the south end, just south of the Shelford Rd overbridge (latter may have been fixed - but it wasn't marked as 'noted' with arrows when I last went past)
WIth the ballast dug out, we can see that there are two types of beam - a thick one, with one central support pad, and a thin one, with 2 supports. It was a thin beam that was affected at Histon. Becasue they are thin, these beams have quite a lot of rubber block holding them up to the same height as the thick beam (next door). Consequently, when the rubber block worked its way loose (as they seem to do in various locations) there is quite a long way for the beam to fall - which caused the rather spectacular dip.
The rest of my day was spent assessing the overal ride quality in relation to additional subsidence more generally. It is certainly increasing. Ride on the lower deck of a double-decker is now noticeably bumpy, more like what it used to be upstairs. Upstairs can (in various stretches) be really quite bad. I suspect reading would be unpleasant. (Cup of Coffee? - you must be joking!) Single-deckers do not pick up the 2Hz bump at beam ends, but do pick up the subsidence which translates into sideways and vertical lurches.
If this is where we are after 2 years, where will we be in 10? Not possible to extrapolate simply....but, a potential concern.
Maybe the tracks can't the strain from over filled buses. People standing on top deck is not allowed but the drivers don't care.
ReplyDeleteJames, I did wonder about extra weight of passengers being critical at this location - 15 passengers = 1 tonne, so while the weight is significant, is it really confined to this final stretch? I think it more likely to be all the way from Lonstanton / St Ives, so in that case subsidence would worse in the whole section, equally. No, I think ground conditions are just worse in the Histon area - viz the pools of water in the dug-out section where the repair took place, which I saw. Are they still there now it is a bit drier? If so, then I think that's our answer.
ReplyDeleteI went today (Tuesday) and the water was very much still there - tho' only at the bottom of the deepest trench. I'd say this suggests this is where the water table comes up to. I think it might explain the greater subsidence problems over this stretch. Incidentally, there is still a bit of a 'dip' left, and the speed limit is not always observed. When I was there, one bus went thru at 5mph, another at about 25mph.
ReplyDeleteThanks for your update, Stephen.
ReplyDelete