I promised something special for the close of year 9. I have been trying to get to Crowland for many a year but never found the time even for a side excursion. This year, Hamish was asked to build a model for Lincolnshire County Council and we agreed to shared ownership of the model from the start so here we go.
As usual, there is a pdf version here.
Crowland is a small, once market, town in the very south of Lincolnshire, only 20 minutes from Peterborough by bus, I found. It is graced by an Abbey, much of which is in ruins but still well worth a visit. Unusually, only the south aisle remains whole but the scale of it betokens the magnificence of the original. Of course, this is deep fen country and in Medieval times much of the area would have been underwater through the winter, so it was a remote fastness, entirely suitable for an abbey.
The real deal, though is Trinity Bridge. There can be very few like it and none of such antiquity. What is special about it? Well zoom in on that map for a start and then explore the model here.
The first, obvious, question is why would you build such a complex structure over land? Because a river used to run through here and split under the bridge. The rivers moved away when the fens were drained and the bridge now stands at a crossroads in the middle of town.
But why build something so complex anyway? A few metres down stream and it would have been possible to build two simple bridges end to end and achieve all the access desired. Perhaps it was a proud mason wanting to show off. The obvious grandeur of the Abbey hints at that underlying reason. Let’s begin with a quick look around, using a higher quality model than is immediately available on the web.
From the North East
From the North West
From the South
There is a lot to see even in those three views. The smaller stones at the base in the first image are surely foundation stones that would have been buried. Post drainage, the ground sank from around the bridge. The higher levels of the walls are of different stone from the lower and are almost certainly later rebuild. The bridge surely had longer approaches, though not as much longer than first appears, remembering that the ground has sunk. The ramps have been cut off or turned down the centuries to allow continued access while reducing the land take.
A plan view is interesting, showing the different nature of the three approaches. Top right was perhaps once two ramps, or one very wide one. Top left the ground may have remained level or been made up so that only 4 steps were needed. Bottom centre the steps are very steep and, forming a spiral, especially so at the left here. That must be a result of truncating the bridge to allow the road past.
The upward view shows an outline where there are edges pointing slightly downwards but we can usefully zoom in somewhat.
This closer view shows the bridge to divide the circle in three. That is an easy construction and would present no challenge to a master mason. If we assume that the ribs were built on light centres and the webs filled in behind, we get an interesting question about what is the minimum amount of centring that could be used. My thoughts so far: It would have been possible to erect two adjoining outer ribs and the opposite central one. With the little horizontal struct in place, they would have been stable and the centre could then move round 120degrees. Otherwise I think the whole set of ribs must have been erected together.
I wonder whether that collection of stone at the bottom of the rib is, original foundation or later patching. Given that it is well behind the main rib soffit, the foundation idea probably wins. This view is square to a rib with everything in front removed.
Removing the front face lets us see the central rib in elevation.
Cropping that and zooming in makes the profile clearer. Certainly, a circle originally.
Foundations exposed again here but look at that copper plate and its effect on the stone.
Looking closer you can see how the splash zone is killing off the lichen.
Notice in this high-resolution model, even the fine print is almost legible. Compare that to the Sketchfab model linked above.
The steepness of these stairs is terrifying from above.
The model allows us to check that this is the same stone inside and outside.
Bit of an overhang there. I wonder what that is about?
Taking a section radial to the arch shows the complexity of the profile, especially at the edge. That circular groove each side was carved in two parts. We didn’t have that opportunity with photography but managed to get the whole surface into the model.
This corner is particularly interesting because much of it cannot normally be seen. The fence has three of those hooped pales closing the gap between the bridge and the shop. Probably to stop people walking through and over the edge of the step.
The shadow is behind the litter bin which can be seen in Google Streetview below.
This illustrates another advantage on the model, you can cut away anything you don’t want to see provided you have good photography behind. The closer you have to get to the surface, the more detail you get but the more photographs you need. 50% minimum overlap must be maintained, which is why there is a white patch at floor level behind the fence.
One last view. If you can work out the curve of the underside of the arches, it is possible to recompute every point in the model and unroll the soffit. Doing that on a normal bridge has more value, but the effect at Trinity bridge is striking. View the model here.
Being able to examine the soffits like this is useful, though.
In the first of those the black streak looks rather like a patch. Look closer and it looks less so.
And in high resolution (not unrolled) even less so. It is a water mark.
A final comment.
This report occupied most of a 4.5 hr journey but still wasn’t finished. By the time it has been formatted and published it will be a day’s work. Nothing I have mentioned is insignificant in terms of the condition of the bridge, and even as I close, I am aware of things I haven’t picked up and commented upon.
I don’t think this bridge is unusually demanding in terms of understanding, fault spotting and reporting, and I had the clear benefit of a very high-resolution model that I could explore at leisure. The aim of doing this month by month is to show people what is necessary in bridge inspection and assessment. How many such studies are priced for this sort of result? More importantly, are the reports being produced under current cost regimes worth doing at all? Will real faults be picked up? How many get overlooked by young engineers working under pressure with little or no support?
This is a problem that sits squarely at the door of bridge owners. By letting work at lowest bid and not properly auditing them, they have encouraged a steady decline in quality from some (not all!) sources.
The drivers of damage in masonry bridges are many and various. Loads increase in scale and frequency. It is rash to assume that even changing the pattern of loads is anything other than more damaging. There is a wide range of environmental processes that can change in an hour in such a way that they do rapid damage. What is most needed is a good, well trained, pair of eyes and a mind open to reinterpret what is seen when the results are unexpected. We also need engineers engaged with the task who will at the very least alert the owner if a different bridge creates a revised view that material affects a previous assessment.
Very soon, all the masonry bridges left will be design life expired. Most will survive for many years yet but every year a few are destroyed or reach a condition where no engineer is prepared to sign off for them to be used. If we are to find those few before they reach that stage, we need a different approach.