A Spongy and Successful Lift
Barnard’s Lock is a rail underbridge structure spanning the river Kennet, located 3.7km west of the centre of Newbury, West Berkshire. The underbridge carries the Berks and Hants iine (BHL) between Hungerford and Newbury, a section of the Reading to Taunton line, which is a major branch of the Great Western main line.
Barnard’s Lock underbridge is located in a saturated, wooded area with the nearest road access about 2km away. The area is a Site of Special Scientific Interest, often referred to as a “triple SI”. The location would attract the attention of the Environment Agency if any form of engineering work were to be proposed due to special concerns for the local population of otters, water voles and other forms of wild life in the area.
The bridge comprises three wrought-iron half-through girders with intermediate wrought-iron cross girders supporting a timber decking. It has two spans, is 40 metres in length and is skewed at an angle of 57 degrees to the channel of the river Kennet, so the brick bridge abutments are aligned with the direction of the river flow. The clearance between the bridge deck and the river is very small, so the bridge has the capacity to become a barrier to the river flow after heavy rainfall.
Well over fifteen years ago, structural inspections and assessment of the underbridge, carried out by JBA Consulting, identified significant deterioration of the timber deck and section loss of the main and cross girders throughout the structure. As a consequence, the structure was considered to be life expired and required renewal.
However, subsequent load monitoring of the structure has enabled Network Rail to maintain the bridge to ensure trains are able to run at line speed, which is 110 mph, in line with its ‘Putting Passengers First’ policy, but they knew that this could not last. The timber deck continued to deteriorate, ballast was being lost and, with the advent of electrification, and new rolling stock with different breaking and accelerating characteristics, the forces on the superstructure were becoming unacceptable. It was decided that the bridge needed to be replaced.
Tony Gee and Partners was invited by Network Rail to develop options for replacing the bridge. An emerging initial plan was to construct a new culvert structure within the railway embankment to the west of the underbridge. A new river channel would then have to be excavated, offering a permanent diversion of the river away from the current underbridge, under the new culvert.
The new river channel would be approximately 135 metres in length and would take water from the existing channel, at a point approximately 50 metres upstream of the current underbridge, and convey it northwards through the new culvert and then eastwards before connecting back into the existing river channel immediately downstream of the underbridge. This would then enable the underbridge to be demolished, and the railway embankment filled in.
It was a good plan, but close scrutiny by the Environment Agency raised serious concerns about changing the flow of the river as it could lead to a change in the flood plain which could affect local communities and also local wildlife habitats. There were too many unknowns, and the estimated cost of the work also exceeded budget allocation, so, eventually, the proposal was dropped.
Therefore, it was agreed that the old bridge would have to be replaced insitu. However, the location with its restricted access points, woodland copses and sodden spongy ground, the length of bridge spans involved, as well as the radius for lifts required, adjacent to the bridge, meant that engineers had to get their thinking caps on. Network Rail’s project engineer Matthew Roberts and programme manager Scott Pillinger told Rail Engineer what happened next.
A new plan and design emerged and the principal contractor appointed for the £4 million contract. They had sixteen weeks to get the site ready, as the essential work to replace the bridge was planned to start at 00:10 on Saturday 4 May and finish at 05:10 on Tuesday 7 May 2019. It was decided that the bridge had to be assembled on site in four sections. Tony Gee and Partners designed eight Z-type steel main girders plus cross girders, and the decking would be a reinforced concrete design.
The steelwork was fabricated in Wigan by Time DMG. This included manufacturing 1.5-metre-deep Z type girders and associated cross girders that would be transported to site and spliced together to form the four deck structures that were up to 34 metres in length. But how would they get the steelwork to site and how would they create enough space and solid ground to construct the four decks? As Matthew pointed out the “problems kept snowballing”.
Once the four elements of the superstructure were constructed on site, they would need to be lifted into position. It was calculated that the biggest lift would exceed 105 tonnes and the radius would be more than 60 metres. This was not work for a normal crane and they eventually sourced a Sarens Gottwald 1,200 tonne crane, apparently capable of lifting 160 elephants!
How would they get this crane to site and how would they ensure that it did not sink into the spongy, sodden ground adjacent to the river? Well, firstly they had to create a 2km haul road using more than 10,000 tonnes of fill material. They were able to source much of this material from three other Network Rail sites and Scott explained that the intention is for the material used to be passed on to a school project in the vicinity. Recycling at its best!
Once the crane was on site and the four superstructures constructed and ready to be lifted in place, how would they ensure that the crane would be stable when the lift commenced? To do that, they had to construct four sheet-piled cofferdams, ten metres deep and six metres square in section, and fill them with engineering fill. These cofferdams would be placed so that the load from the outriggers would be transferred to stable ground.
Whilst the cofferdams were being constructed, a substantial amount of woodland needed to be cleared to create the workspace for the bridge superstructure construction and for the site offices and accommodation, and the sixteen weeks were ticking away.
At the design stage, Tony Gee and Partners became concerned about the ability of the breaking and traction forces of the trains to transfer through to the deck to the supporting abutments. Their solution was to include fifty-six 100mm diameter, four-metre-long dowels into the design. These dowels would need to be placed into holes in the precast sill beams and then drilled into the brick abutment during the possession, after the old decks had been removed.
Matthew had calculated that it would take two hours using a diamond drill for each dowel. In a 76-hour possession, this was a significant undertaking and the golden rule was that there must not be an overrun – ‘Putting Passengers First’.
The team prepared contingency plans which would involve additional temporary speed restrictions, dependent on the number of dowels in position. They carried out trials and adjusted the resources with the intention of completing all the drilling within the possession.
The contractors would also need to have access to both sides of the river when the old superstructure was removed, so a floating pontoon, made up of interlocking plastic units, was positioned across the river. This created a walkway, working platform and support for temporary troughing for the signalling cables and other equipment.
Wind and water
A ‘Go/No-Go’ plan was drafted before the possession started. Weather forecasts were analysed for potential heavy rain. There were sluice gates that could regulate the water flow, but that could also cause problems, as outlined earlier. Wind was a fairly critical factor. The crane would only attempt the lifts required if the wind was less than 35mph. If this was exceeded, work would have to stop.
Fortunately, the weather was kind with no rain and little wind throughout the possession. The line was handed back to traffic on time with a 50mph speed restriction.
The track was reinstated by local track engineers, underpinning another Network Rail policy known as the ‘Orange Pound’. In the past, contractors have been responsible for reinstating the track after such work. Now, the view within Network Rail is that this work should be done in-house, using engineers who know the area and can maintain it on a daily basis, ensuring that not only the track is maintained properly but also the speed restriction boards, track magnets and temporary track crossings are also looked after.
Many readers would identify this approach with a similar one adopted some time ago.
So, the new bridge is in place, the myriad of engineering challenges addressed and the local environment left in peace with the water voles now admiring the new structure that has emerged from this sodden, watery part of the Berks & Hants main line.