Bridge 8 repairs start

REPAIRS TO Br. 08 CARRYING THE B4632 ROAD at STANTON.

After months of design, drawing and tender preparation and evaluation, we finally reached the point where we knew precisely what it was going to cost to do the first stage of repairs of the steelwork of this bridge to enable the traffic lights, which have been in place for over 3 years, to be removed.

The first problem was that, after over 18 months of either running no trains at all, and then running trains less than half full, we had not managed to re-build our bank balances enough to pay for it!!! Then an appeal was started, and quite incredibly the amount that was needed was donated within just 4 weeks – an absolutely fabulous outcome. As Structures Engineer, I would like to add my extremely grateful thanks to everyone who donated.

As is usually the case with projects of this scale, a complete line closure is required and so the work has been planned to run from 4^th January to 28^th February 2022, when this is the only time possible. The huge problem that this creates is that all the very detailed planning and signing up of contractors and their sub-contractors and then agreeing all the Risk Assessments and Method Statements, has to be done in the period before Christmas, when most organisations are winding down!!! The other problem is that it means that the actual site work will be done in cold weather conditions and with very short hours of day light. Fortunately, the bulk of the work is taking place underneath the bridge, giving protection from rain and maybe snow (hopefully not) but still in the cold and getting dark early.

 

Anyway, we finally made it and the Contractors duly arrived on site on Tuesday 4^th January. The bridge will have to be completely propped, as the main task is to replace badly corroded steel plates on the bottom flange of the two long cross girders, which carry over half the weight of the bridge and the traffic running over it. 

Obviously, these old 118-year-old steel (not wrought iron as proved my metallurgic testing) plates have to be removed, and removing such plates weakens those girders, almost completely.

 

The problem affecting the propping was that the track has to remain in place, which creates a very uneven base on which to erect the very extensive propping required. To remove the track, sleepers and ballast would add a week at the start and then another week at the end, plus the cost of cutting and re-welding the rails in this section of continuously welded track. We simply do not have enough time to do that. 

 

 

The solution was to cover it with a protective fabric and then bring in about 140T of ballast to create a level “plateau”. This had to be carefully laid to avoid damaging the track, and also had to be rolled to consolidate it enough to carry the loads required. This will all have to be removed and be carted to a stockpile at Stanton yard, when the repairs are completed, and will become useful for our P-way dept, when any future ballasting work is required.

 

 

 

Next, the propping system could be installed. Props are required at 10 points around each of the 2 defective cross girders, plus at 8 more locations where more localised strengthening is needed. shows the timbers under the twin posts which spread the load over the ballast. The propping is a heavy duty, aluminium system which is much lighter to handle than the usual steel type. 

 

 

Props in place, on a bed of ballast.

 

General view of props under bridge 8.

 

 

Props as they support the bridge deck above.

 

Installation started on Monday 10^th Jan, and with a team of up to 6 men, was completed, jacked up to carry the dead load and approved for use, on Friday 14^th Jan. (the pictures show the installation through to virtual completion).

 

To enable the steelwork contractor to carry out the repairs, a scaffolding platform has to be installed, approx. 1.5M below the working level. Installation started on Wednesday 12^th Jan. and will be completed by p.m. on Monday 17^th Jan. The replacement plates are almost 8M long and weigh just under 1T, each. To get them into place a loading deck, made of scaffolding will be built at the Downside, High mileage end of the bridge. For the installation, the plates will be lifted onto the platform with a 360 excavator. A similar platform has already been built on the Upside low Mileage side. This is so that the plates for the girder nearest to the Downside abutment can be rolled through to the Upside, then skewed over and rolled back to slide underneath it. 

 

 

The actual repair work requires the corroded plates to be removed first. These are attached to the rest of each girder with two rows of 7/8” diam rivets. These rivets pass through between 1 and 4 plates giving a combined thickness of up to 2”. A total of 180 rivets will have to be removed from each girder to get the plates free!! This can only be done by a combination of grinding their heads or bases off and then drilling out the shanks (up to 2” long) with a rotabroach, driven by a magnetic drill. 

Once the old plates are removed, then the new ones resting on the scaffolding will be jacked up and bolted to the bottom flange angles using high strength 20mm Torsion control bolts.

 

Whilst we have contractors on site, we are also having some timber post and rail fences installed behind the wing walls of the bridge. This is to provide safety during vegetation clearance, and any future fence and wall maintenance. This fencing work was just starting when I left the site on Friday.

We'd also like to thank all those who donated to the £200.000 appeal for this first phase of the works.

A second phase is still required in about 12 months' time, the funding for which is not yet in place.

Also a request not to visit the bridge, as it is a construction site with no way through for pedestrians. There isn't much to see from above anyway.

 

REPORT BY JOHN BALDERSTONE

STRUCTURES ENGINEER – GWSR

15.1.22.