BACKGROUND

A pre-inspection visit was done to check the suitability of the bridge. On the visit a few photos were taken to determine the type of the bridge and any identifying numbers.

It was found that the bridge was built to carry a two track railway line over a small river. The engineers code [BGK] was found which revealed the line runs from Bethnal Green to King’s Lynn [http://www.railwaycodes.org.uk/]. Other websites have it as ‘The West Anglia Main Line’. The first line from Stratford to Broxbourne opened on 15^th September 1840, and it is likely there would have been a bridge in this location on that line. Another website has a list of details about the bridge [http://abcrailwayguide.uk/] :

· Bridge Reference: 1425

· Area: Tottenham

· Route: Anglia

· Line Reference (ELR): BGK

· Line Distance: 16 miles 1474 yards

· Type: Underline Bridge

· Description: Mill Stream

· Constructed: 1882

· Primary Construction Material: RBE- Wrought Iron

· Operational Status: Functionary

· Owner: Network Rail (CE-Struct)

The bridge type ‘underline’ signifies the bridge has been built to allow the river to pass under the tracks. The earliest OS maps (survey in 1880) show a bridge in the location. Over the years the line has been run by many different operators and the line gauge and engineering details have progressed. This section of line was one of the earlier ones to get electrified, being done under British Rail in the 1960s. Pre-covid the line was a main commuter line into London with trains every few minutes. Crossrail 2 had been proposed to run up this line although the plans for that have been suspended for the foreseeable future.

The current bridge type appears to be of plate girder construction. Using the ‘measure distance’ function on google maps it was found that the bridge is around 30m span. The construction will be further discussed in the next section.

The bridge is situation in the Lee Valley which is a popular nature area for local people and there is a café and a carpark that looks onto the bridge. Along side the bridge are 4 holiday lets and a canoe rental centre operates on the river. The hire canoes pass under the bridge on the way onto the canal which runs parallel to the railway.

CONSTRUCTION FORM AND MATERIALS

As mentioned above it appears the bridge was built in 1882, there is a plaque that appears to confer, but it is unclear and inaccessible. The bridge is of a plate girder type construction which links in with the time period the bridge was built and the length of its span.

Due to no disruptive inspection being carried out it is unknown what foundations were used for the bridge. The abutments are constructed of brick walls with concrete plinths on the brickwork to support the main beam of the bridge. On first look I thought the corners of the masonry were built from larger stone blocks, but this was actually a heavy steel (15mm+) plate, presumably to protect the wall from impacts. The wing walls are constructed from (steel) sheet piles with a concrete cap along the top edge. This would be a much more recent addition to the bridge. There is also a steel construction in the water to stop boats colliding with the abutment that isn’t protected by the path.

The bridge is single span of around 30m. No obvious expansion joint was seen but it would be assumed where the beam sits on the plinth would allow some movement. The bridge is constructed of three main (longitudinal) beams making it a triple-girder bridge. With one running down the centre of the line and one down each edge. All three appear to be of similar construction, about 1.3m in depth, although it would be assumed that the centre one would need to support the same load as the outside two. The wrought-iron girders are riveted together and have vertical web stiffeners (approximately) every 2m. The top flange is of continuous thickness, but the bottom flange gets thicker at the centre of the bridge. At the supports the bottom flange only has one thickness of metal (around 12mm) but at the centre point there are 4 thicknesses riveted together. Cables are run along the south side and a 300mm diameter pipe runs along the other. There are handrails along the outside of the top webs.

It looks like the bridge deck has been added at a later date. The secondary beams are constructed from I beams approximately 450mm apart. They are welded together as 4 beams and then each set of 4 is bolted togther. This suggests that they were fabricated off site as a set of 4 and then bolted together when in position. This width would allow them to be transported and lifted into position more easily. There is smaller stiffening beams running between longitudinally between the secondary beams. These beam for the underside of the deck, with a steel plate welded to the top of each beam. This element of the bridge is connected to the vertical web stiffeners on the primary beam.

Observations of the top side of the bridge were not taken due to obvious safety concerns.

Bridge Condition and Defects

Overall the condition of the bridge was found to be ok, although there was some areas of concern.

While there evidence of water being present of the face of the abutment masonry, there was no obvious deformation or cracks in the structure. Looking at the brickwork revealed various repairs that have been made over time, most notably the top two courses of bricks look fairly recent, although there have been many other repairs done to the wall. The worst part of the masonry wall was around the corner protector on the south corner of the abutment.

Cracks were observed on the surface of the bearing plinth on the southern corner. Due to the nature of the inspection it was not possible to determine the depth of the cracks but they were over 1mm on the face. The cracks originated down from the edge of the main girder which could signify a structural defect, although there was no signs of resulting damage to the masonry below, suggesting that the wall isn’t being overloaded by the defect.

Overall the main girders look serviceable. The outside of main stem of the girder could do with cleaning up (shot blasting or equivalent) and re-painting but there doesn’t appear to be significant pitting to the metal. It appears that the ends of the main girders have been serviced or replaced fairly recently. They have fresher looking paint with little signs of rust. Along the lower web on the inside there does appear to be significant pitting, corroding most of the way through some of the rivets, although this has occurred at a point with a significant thickness of metal so it is unlikely the pits are greater than 10% of the total metal thickness.

The cause for concern here is the vertical web stiffeners. While most are in ok condition a couple have pretty much rusted all the way through. Depending on how critical these stiffeners are for the integrity of the bridge would direct further action required.

As mentioned above it appears the main deck has been added more recently and is in overall good condition.

The handrail along the top flange is ok on the south side, but on the north side a section had rusted through and fallen away.

Causes of defects, structural behaviour

If, as we determined correctly above, the main girders of the bridge are 130 years old this would explain some deterioration in the condition of them. While this could be put down to naturally occurring damage, had a better maintenance schedule (mostly painting) been carried out the bridge would currently be in better condition.

There does appear to be an issue with draining in the deck, highlighted by the wet patch on the wall and the excessive corrosion in the area in question. It would be assumed due to this that the drainage system is not functioning properly to carry the water away from the structure. Exact construction methods were not determined but an assumption was made that the deck is waterproofed and that either there is damage to the waterproofing layer, or that the water is not finding its way to the drainage system correctly. These is a drain pipe visible but it was not clean how the drainage system was designed from underneath the bridge. This could be down to either the original design or incorrect maintenance. If vegetation had been allowed to grow it could have blocked up the drainage system. Over time this water is going to cause further damage to both the blockwork and the metalwork.

There were no visible cracks seen during the inspection, only damage to the metalwork from corrosion. This corrosion was caused by both the lack of paint and the presence of moisture caused by the drainage not functioning correctly. The damage caused by pitting to the rivet heads is also a concern.

The cracks in the bearing plinth could be caused by a range of factors. This could be overloading from the bridge (and trains), ie insufficient design capacity, it could be caused by settlement of the abutment or just by the contractor not building it to the correct specification. It doesn’t appear that the crack has moved recently so could have been caused by an initial settlement, and it is not propagating further.

Aside from the issues mentioned above the bridge elements appear to be in serviceable condition. The wing walls and abutments have not been highlighted as areas of concern. Aside from the graffiti there were no signs of vandalism or deliberate damage of the structure.

Recommendations for testing and monitoring

Due to the current condition it is recommended further testing and monitoring is carried out on the bridge. There are many types of testing available . The type of test will depend on both the material of the bridge and the required outcome of the test. A good routine of testing and monitoring bridges could end up saving the owner from expensive repairs by carrying out preventative maintenance before serious damage occurs to the structure. In the case of this bridge if the drainage had been fixed the corrosion would not be so extensive. The same goes for keeping the structure protected from water with a protective layer (generally paint). Recently 2-Pack epoxy paints have been used to provide better protection for longer than more traditional type paints.

Where there is significant corrosion on the metal work it would be recommended that the area is cleaned back to good metal and the thickness of the plate measured along with the depth of the pitting. The thickness of the metal is generally measured with an ultrasonic device and pits can simply be measured with a micrometre. Although (rusty) metal would have to be removed to perform this test it would be considered non-destructive. A specialist operator would be required to operate the ultrasonic tester.

In the areas where there rivets are subject to serious corrosion a specialist engineer would need to investigate and determine if the structure was compromised.

It is also recommended that the abutments are surveyed for line and level so their position can be re-measured after a period of time to check for movement. This is recommended due to the cracks in the bearing plinths. This would also require specialist equipment and personnel. Generally an EDM and a surveyor would perform this type of work. Aside from measuring the movement of the structure the cracks could be monitored using a simple crack measuring guide which would not require such a level of specialism to perform.

Finally once the problem with the leaking has been rectified it would be recommended that the area was monitored for moisture content to ensure the problem was resolved.

Recommendations for maintenance, repair and strengthening

By this point all of the issues have been well highlighted and discussed. In this section advice on further damage to the bridge occurring and possible solutions to the current problems will be addressed.

First of all it would be recommended that all loose metal (or rust) was cleaned from the superstructure. A lot of care would need to be taken here as the structure runs over a water course, so all dust and debris that came off would have to be contained. This would likely be done with a scaffolding tent structure. The best method for cleaning the metal is sandblasting or shotblasting but this could prove difficult due to the location. Once the metal has been cleaned it should be inspected for thickness and structural integrity by a suitably qualified engineer. If it was found that the major elements were structurally sound they should be painted with a suitable protective layer. If they were found not to have the required strength the three main girders would need replacing or reinforcing if this was possible.

It is recommended that the vertical web stiffeners are replaced in the areas where they have rusted through. It should be possible to weld reinforcement on in the required areas. This repair could be carried out with minimal disruption to the train service.

The cause of the drainage problem needs to be identified and fixed. Without further investigation it is not possible to determine the cause, but once fixed it should be monitored.

It would also be recommended that the brickwork is steam cleaned and re pointed in the areas required. Along with the corner protectors being cleaned and re-painted along with the rest of the metalwork.

After all the initial repairs have been carried out it is recommended that Network Rail pay some attention to the bridge and maintain it properly. Monitoring it to see if the damp comes back and having a proper schedule for painting maintenance rather than leaving it for years until it has gone rotten.