Interim work has just been completed on Bridge 8 at Stanton, taking the B4632 over the railway, to re-grade the verges and tarmaccing them. This is designed to stop, or greatly reduce the rainwater penetration of the verge areas, by tipping the water into the road. See before and after photos below:
|North side - before.|
|North side - after adding the tarmac verge.|
|South side - before.|
|South side - after adding tarmac.|
During the regular examinations done on this bridge, it was noticed that the deck and supporting steelwork below the actual road width was almost completely dry, whereas that below both verge areas was constantly dripping. The steelwork below the road was in much better condition and the paint applied in 2007 was generally in very good condition. We therefore deduced that the ¾” asphalt waterproofing, which was shown on the original drawings that we have, had probably been damaged by whoever carried out the installation of gas and water mains. These pipes have been changed, over the years, to put larger ones in place, which may have caused further damage. In addition, the entire road was modified (possibly in the 60’s) and it has been raised by app 300 mm and given a cross fall. The road modification work could also have added to the waterproofing damage. The effect of the thicker road construction is also to add app. 100T of extra weight for the bridge to carry!!
In addition, B.R. (or maybe earlier) carried out a repair to a down side cross girder, which can be seen by the bolts, in the splice plate just visible in the attached photo.
This may be due to steam loco emissions and a leaking deck.
Consequently, in October 2016 we started a contract to expose the deck and replace the waterproofing. We tackled the Low Mileage (LM) (Broadway end) verge first and quickly exposed a 150mm plastic water main and a 9”, probably steel gas main (see photo below), the latter sitting right on the deck.
We also discovered the original granite sett kerb, just below the water pipe (see close-up photo below).
|Water pipe laid on top of original setts.|
To make the problem worse, the space between the granite kerb and the outer girder was entirely filled with concrete. We knew that the 200mm water main in the H.M. verge was concrete covered. (see photo of broken concrete below)
To attempt to get underneath that could have caused the main to fracture. Consequently, the 2016 contract was abandoned, and the LM excavation backfilled with stone.
The steelwork to this bridge has suffered a lot of corrosion, over its app. 120 years life, and in 2007 we spent over £100,000 on major repairs. This was primarily to one short section of the downside end of the first short cross girder, which lies under the LM verge (see before and after photos) and one small section of another cross girder.
14 years on, the current single lane restriction is now required because the thickness of the bottom flange plates of the two long cross girders (see 2007 photo below showing location),
|Cross girders, photographed in 2007.|
has been substantially reduced by corrosion and delamination of the steel, which has considerably reduced its carrying capacity. The one advantage of a steel bridge is that poor parts can be removed, and new replacements can be installed. That is what we intend to do, this time.
To prepare the site to carry out the work, the track will have to be lifted and the ballast moved out, to create a flat level surface for heavy duty propping to be installed and make room for handling the steel that is to be removed and replaced. Then localised blast cleaning will have to be done to the existing steel to remove rust and scale before the steel repair work can be started.
Obviously, to allow the traffic to keep running, the rest of the bridge must be propped up so that we can do the repairs. Back in 2007, we had to prop 3 points and a 4-leg tower was used (see photo below).
This time we will have to prop 20 points, so there will not be a lot of space down there for removing and replacing steel plates, app. 9M long and weighing about 1 ½ T.
The weakest part of the bridge is now the corroded bottom flange plates (up to 4 plates ½” thick, around the centre)
which has reduced their thickness by a large amount. The repair proposed involves removing the corroded plates by drilling out app. 160 7/8” diameter rivets per girder, passing through steel plates between 1” and 2 ½” thick. The new plates will comprise two thicker plates, and these will be attached to the bottom flange connection angles with 22mm Torsion Control bolts. All the new plates will probably have to be drilled on site, as rivet spacing can be slightly variable.
In addition to this bottom flange plate replacement work, the vertical web part of the cross girders has also suffered considerable loss of thickness due to corrosion (see photos below)
and these web plates need thickening up by the welding in of additional plates. All welds will have to be tested, especially as early steel, made almost 120 years ago, is not as good as what we will now be putting back, and so welding will have to be very carefully done.
The other main work required is where the cross girders connect to the outer main girders (See photo)
The upper parts of the cross girders are enclosed in the Jack-arch brickwork, as can be seen in the photos, and so this will have to be cut away locally, to expose the steel angles which are rivetted to the vertical web of the cross girder. Again, drilling out of rivets is required, but this location only involves just 15 7/8” diameter rivets, but working in a very confined space.
Once the 2 new web connection angles are installed, the brickwork will have to be made good, possibly with concrete, as we did in 2007 (see photo below).
When all the steel repairs are completed, all new replacement steel, and probably some other, poor condition steel parts, will be cleaned and painted.
The one good thing about the work now required is that it is all below the deck and so will be sheltered from the weather, although cold temperatures, and short hours of daylight can restrict and delay certain works.
Depending on the costs, once we get tenders back, we hope to do the same repairs to the ends of the 2 short cross girders, which was not done in 2007. As before this will involve installing 6 props for each short cross girder.
There are other minor works to be done, including replacing missing, or damaged Jack-arch bricks, and possibly some small repairs to other steel looking in poor condition.
Finally, the ballast and track will have to be replaced and made safe to run trains again.
As mentioned in the recent V.A. the repair scheme for that work is in hand. This involves designing the changes to the steelwork, preparing detailed drawings and schedules of work so that we can go out for competitive tenders. We hope to be getting tenders returned sometime in September. This will then show us how much money will be needed, and how long the work will take.
The amount of work required to bring this bridge completely up to anything like the standard achieved on the Bridges to Broadway contract done in 2014, is likely to take far too long to be done in one shut-down period, and so it will almost certainly need to be split into 3 stages. Stage 1 will see the removal of the traffic lights, at last.
I hope that this is of interest, and understandable!
John Balderstone – Structures Engineer - 29.7.21