The A4440 Worcester Southern Link Road (SLR) is an essential part of Worcestershire’s strategic road network and provides an important link between junction 7 of the M5, South and West Worcester, Great Malvern, the wider Malvern Hills district, Ledbury, Upton and Herefordshire.
An important bypass to the city centre, the SLR is used by well over 30,000 cars each weekday and provides one of only two road crossings of the River Severn in Worcester City. It therefore offers an important, alternative route to the city centre, which would otherwise suffer from congestion.
To improve capacity, and therefore reduce journey times and improve journey time reliability, the local Highways Authority, Worcestershire County Council, devised an overarching scheme to dual the busiest stretch of the SLR between the M5 and Powick. This scheme is being delivered in phases and includes a new bridge over the River Severn as the final phase of the scheme.
The scheme also called for a railway bridge, which carries the North Cotswold line between Paddington and Worcester, to be extended at Battenhall to enable dualling of this part of the SLR. Once this phase of the scheme is completed by spring 2019, two northbound lanes will then go under the original bridge and two southbound lanes under the new, extended part of the bridge.
To minimise disruption to traffic, the plan was to build the new bridge off-line a short distance from its final position, and then to move it into place using a self-propelled modular transporter (SPMT).
Planning the works
Principal contractor for the scheme was Worcestershire County Council’s term civil engineering contractor, Alun Griffiths, working in conjunction with the council’s term design consultant, Jacobs. The council’s term rail consultant, SLC Rail, was engaged to manage the project and Tony Gee and Partners brought in as the Category 3 Independent Checker for the bridge design, based on the client’s evaluation criteria that included capability, quality and financial assessments.
Specialists from SLC Rail negotiated the third party underbridge agreement with Network Rail and facilitated the design approval, reviewing and providing feedback to the designers before final submission.
The team of project managers from SLC Rail, contractors and Network Rail’s asset protection personnel carried out extensive risk assessments on sufficiency of time, manpower and machinery as well as to ensure completion within the designated blockade possession time.
Having completed the check of the bridge design, Tony Gee was engaged by Alun Griffiths to undertake a series of temporary works designs for the bridge construction. The vast majority of designs required technical approval from Network Rail, with Tony Gee also acting as the contractor’s responsible engineer for the temporary works designs.
The series of temporary works designs included the complex analysis of the existing bridge to determine the effects on the structure during construction of the new bridge. This included finite element analysis of the anticipated ground movements and their effect on the existing bridge during construction, analysis of the effects of the above movements on the existing bridge structure and the design of multiple-strand ground anchors to retain the north abutment as the railway was being rebuilt around the repositioned structure.
The concern was that the differential loading on the existing bridge, which would result from the excavation of fill to the south abutment, could lead to movement in the structure. One solution could have been to excavate the northern abutment as well, to ‘balance’ the forces and remove the risk of movement. However, it was decided that the best option was to install ground anchors, effectively ‘pinning’ the northern abutment in place and obviating the need to temporarily excavate the northern abutment.
The analysis determined both the predicted and allowable movements of the existing structure with the anchors in place. Limits were set to ensure that the structure would remain within serviceability criteria. Contingency limits were also determined, in addition to contingency measures to be implemented should limits be exceeded. Movements were predicted at various stages throughout the construction sequence for comparison against actual values during the blockade, when the movements of the existing structure were captured using an automatic total station, tilt and strain gauges with data uploaded in ‘real time’ to an online platform. This enabled the Tony Gee engineers, on site throughout the blockade, to continuously monitor the movements against the predicted figures as construction proceeded.
Throughout the blockade, the bridge movements all stayed below predicted levels and well within limits.
As the installation of the temporary works would impact the existing bridge capacity, both during and following the works, Tony Gee also undertook an assessment of the structure to demonstrate that it would still achieve the required capacity.
Closure and blockade
Prior to the blockade, the route of the new southbound carriageway was excavated and levelled, apart from one ‘embankment’ section on which the railway sat undisturbed. Once the new road alignment was effectively complete, Alun Griffiths constructed the new bridge from reinforced concrete, complete with wing walls. The finished structure was 34 metres wide overall with a single, arched 13.5-metre portal and weighed around 1,500 tonnes.
With the build of the new bridge complete and all railway critical signalling and telecommunications diverted, everything was ready for a closure of the A4440 to move the bridge into place. This was planned from Thursday 24 May to Friday 1 June, with the railway blockade running from Saturday 26 May to Thursday 31 May.
Meticulous planning went into every aspect of this project including the traffic management put into place with such a key route into and out of the county being closed for nearly a week. No stone was left unturned in the quest to reduce congestion and to encourage motorists to think ahead and take alternative routes before the formal diversion route was reached.
On the Thursday prior to the blockade, the SPMT was set up by Mammoet below the new bridge. In addition to the SPMT, three hydraulic struts were installed to cross-prop between the abutment walls. These would prevent the inwards ‘swing’ of the abutment walls once the bridge was lifted. Prior to lifting, they were preloaded and then adjusted before the bridge was set-down to return the bridge to its original geometry.
Once the SPMT was set up and the hydraulic struts pre-loaded a trial lift was undertaken. This went without issue and the team then turned its focus to the enabling works that could now be undertaken during the road closure.
On the morning of the blockade, the first priority was to remove 112 metres of double track and its remaining embankment. 22,000 tonnes of soil were removed and a portable roadway was laid to accommodate the transporters that would move the new bridge to its destination. Alun Griffiths prepared and levelled the formation below the abutment and wingwall foundations.
In addition to working on Battenhall bridge, SLC Rail is also rail advisor to Worcestershire County Council for the new Worcestershire Parkway station. To reduce costs, SLC’s project manager at Worcestershire Parkway worked with colleagues to schedule works that made the most of the Battenhall bridge rail blockade. During the closure, workers replaced sleepers and rails on the Oxford, Worcester, Wolverhampton (OWW) line, realigned track to facilitate possible twin-tracking in future and undertook the piling for the new station platform.
Back at Battenhall bridge, once everything was ready, the three-module transporter picked up the bridge using its hydraulic jacks and was slowly driven along the temporary roadway into position, ending up just 30mm away from the abutment of the original, single-carriageway bridge.
Once the transporter had set the bridge down, it was removed and the backfilling operation commenced. The void left between the existing and new bridge measured just nine metres by six metres and required infilling with 350m3 of fill. In order to lift the fill into place, Alun Griffiths utilised a conveyor – once in place it was compacted with a remotely controlled compacting plate. Between the rear of the southern abutment and benched excavation of the existing ground, approximately 2,100m3 of fill was hauled into place and compacted.
Once the structure was backfilled, the track was re-laid, which itself required 216 new sleepers, four 112-metre lengths of rail and 1,200 tonnes of new ballast. Several hundred metres of cable trough were renewed, as well as the diversion and reinstatement of all safety critical signalling and telecommunications infrastructure.
The thorough pre-planning, execution and risk management resulted in the new bridge being completed early, enabling the Southern Link Road to be reopened to traffic 36 hours ahead of schedule and the railway possession to be handed back to Network Rail at the designated time. All in all, 180 engineers were on site, working three eight-hour shifts each day, to make sure the project was completed on time.
Alun Griffiths’ rail director Shaun Thompson was pleased with the outcome. “The Battenhall Bridge project team planned, rehearsed and executed the works with meticulous precision,” he said afterwards. “The detailed productivity study and risk assessments ensured contingencies were in place for all eventualities. With input from our infrastructure colleagues on the SLR, the safe delivery of this project was an incredible success and demonstrates our multi-skilled capability. Well done to all involved.”
Communication
In addition to the bridge being moved into place successfully and ahead of time, another success of the project was the communication to local residents, road and rail users.
On the run up to, during and after the bridge move, the communications team at Worcestershire County Council kept a steady stream of content accessible to stakeholders. This was done through the more traditional updates through local media and via the website, but by far the biggest engagement came through the Council’s social media channels. A number of time-lapse videos, shown throughout the project, were collectively viewed more than 500,000 times. This meant that key groups across the county, and further afield, felt informed. It also helped to keep the congestion on the diversion routes to a minimum as a very high number of local residents and businesses were aware of the project, timelines and the potential impact on the local routes.
The project is an excellent example of a progressive Local Authority, with a clear transport infrastructure strategy, commissioning leading consultants and contractors from the private sector to deliver outstanding results for local residents and those using the local infrastructure.
Read more: Rail Engineer August 2018 – Infrastructure focus
