At a meeting to provide an update on the remediation of the troubled Ninham Viaduct rail bridge in Oxford shire, Network Rail capital delivery director Stuart Calvert has confirmed that the structure will re-open by 10 June as the final stages of stabilization work get underway. The bridge, which crosses the River Thames near Abingdon in Oxford shire, was initially closed on 3 April when stability issues with its southern abutment were identified, with Network Rail monitoring equipment at the site showing increasing movement in the structure. Nonheme Viaduct is a bowstring truss bridge and its current structure opened in 1907 with a new arched brick-built northern abutment constructed in 1929. However, the southern abutment was not rebuilt and was the recent focal point of the destabilization that led to the structure being closed to rail traffic last month. The bridge has been monitored since 2018, with extra equipment installed earlier this year. Since 2019 the design solution has been to strengthen the ground using various techniques, with the marked acceleration in groundwork movement in recent months prompting the current remedial project. During the press briefing yesterday Calvert said: “We are confident that we will complete the work by 10 June. This kind of work would typically take north of two years if we’d planned it, but we’ve been able to complete it in weeks.” Calvert said the final stages of the remediation work will roll out over the next couple of weeks, following the removal of 167-year-old abutment by engineers led by Balfour Beatty on 24 May. He confirmed that, at the time of the meeting, the last cell beam to support the bridge structure was being installed, with piling to be complete by 27 May. This would be followed by work to reinstate the steel work of the bridge deck, followed by the reinstatement of the embankment, then reinstallation of trackwork and signaling. In recent weeks a total 24 piles were driven 10 to 15m into the riverbed to hold up a temporary propping system, which was moved into place by a 750t crane to hold up the bridge’s southern span while the old abutment was removed. About 3,000m3 of material was also removed from the embankment to create space to build a new abutment, with eight piles driven up to 20m into the embankment to support the new structure.
When The Stability Issues First Became Apparent, Polymer Grout.
was injected into the soil in the surrounding area to stabilize it, but this proved unsuccessful and the later-than-planned deadline of 10 June was established after the scale of the work was identified. Calvert confirmed that the issues affecting the bridge were not to do with the structure itself, but instead to do with ground conditions, with the ground under the Nonheme Viaduct described as “like porridge”. However, he added that the team had not yet reached firm conclusions over why the destabilization of the ground had dramatically accelerated in recent times and that it was not yet clear whether the addition of polymer grout had slowed, had no effect on or sped up the destabilization. He said that geotechnical analysis was underway to find answers to these questions. He confirmed, however, that no other “bridges of concern” facing similar problems had been identified by Network Rail. Calvert said: “This project has been a challenge for the team. We have very good people, but this has been an unusual project, and for many on the team the completion of the work will be a career-defining moment. “Some of the critical stages include bringing in the first piles, creating a temporary structure, and jacking up the bridge. We’ve got all those ‘nasties’ out of the way and that has been a massive logistical operation.” Calvert said the construction costs for the bridge’s remediation, along with the financial losses due to rail traffic disruptions, had not yet been calculated.