As usual there is a pdf version available.
This bridge at Finsbury Park now carries a foot and cycle track called the Parkway. It has obviously been messed about more than a little in it’s life but a look at the location is multiply interesting.
My attention was drawn to it by a help request for Archie-M. A question about analysing a bridge with ring separation. Since so many such cases are not truly ring separation. I requested photographs and once I had seen one had to go at the next opportunity.
One thing that isn’t at all obvious, but which explains some details is there was once a station on the bridge. You can see it here. A lot of buildings have gone including the timber cantilevered platforms and the wooden station building that once obscured the right hand arch. There is an intermediate state pictured here.
If you look at the map link above you will see that there is another railway directly underneath. The visible road and railway cross more or less at right angles but from the top we can see another railway below ground and it passes directly under this main arch at 45o. It is just visible in this picture. The dates are interesting because the Tottenham and Hampstead Railway at the bottom opened in July 1868 and the Edgeware, Highgate and London on top in August 1867. So it is at least possible that the tunnel for the T&H was driven under the bridge of the EH&L.
There is also a nasty crack visible in the parapet. And notice how the track bed drops steeply off the end of the bridge.
This is indeed ring separation but perhaps we need to think a bit more about it.
Just a little closer and it is surely clear that the right-hand half of this arch is pretty much undisturbed. The ring separation is a symptom of a hinge roughly at the top of the arch backing. There is a reverse hinge at the crown but that doesn’t cause anything like the same distress because the rings cannot drop away from there. And look, it is the same at the other end.
It was quite hard to get an angle on this but the string course dips away dramatically. The abutment has dropped by some distance.
Here you can see the sweep in the beds of the spandrel wall, but the articulation of the parapet is even more pronounced.
The layout of those props is quite strange.
What are they supported on?
In truth, very little. They are capable of holding up bricks and stopping them falling, but not of holding the bridge together.
And it looks as though the embankment has been homesteaded and turned into a garden.
Not only that, but to do it, the ground has been levelled across, dramatically reducing the support of the wing wall
Interestingly, the other end, though it looks the same from a distance, is only propped at the edges
So what about diagnosis? We have seen the symptoms, what about the cause?
Well, I suspect that the bridge below is providing very solid support for the piers and the abutments have inadequate foundations. It would be good to put an instrument over it and see just how far out of level it is but I think I took enough photos to build a model ad when there is time and space to do that we may be able to see more.
Having got this far we managed to slot a model build into an already busy programme. It is by no means perfect, produced from hand held pics from a pocket camera (admittedly one that cost £1000 with a big 20Mb sensor). But access was tricky and get good photos more so. Most were from much further away than we would normally do so the outcome is somewhat blurred. The Sketchfab Model here is even less sharp than the one in our viewer. However, we can (from the viewer), get a square shot of one of the side span spandrels which shows us a lot.
Two vertical cracks in the spandrel/parapet for a start. A closer look at those is in order.
One is roughly at the quarter point, probably starting from where the top of backing meets the arch. It is, on the whole, wider at the top than the bottom, though there is a possible slip plane at the evel of the white line, meaning the crack closes again there. The second crack, perhaps half way from the first to mid span is definitely tight at the top and more open at the bottom. Everything to the right of this crack has surely been pushed further right to allow this movement. The white line (approximately horizontal) shows the bed to be level to the left and then to slope between the two cracks.
The model these images come from is much higher quality, but you might like to explore a little for yourself so here is one in Sketchfab:
And because it is already late and I am very busy, that will have to do. Though it must be obvious there is much more to see here. I have something special in mind for the year end but I think I may return here in January.