Railroad maintenance crews in the U.K. have been using sprayed polymer to strengthen track ballast for about a decade. One contractor now aims to go one step further and use strengthened ballast as a non-intrusive reinforcement for some of the country’s 25,000 aging brick-and-stone arch bridges.
Balfour Beatty Rail Ltd., Redhill, is offering a method to treat a bridge and its polymer-coated ballast as a composite structure. The resulting enhanced strength of the arch can eliminate the need for concrete or steel reinforcement, says Andy Curzon, BBR’s head of technical services.
The technique, called XiSPAN, would preserve the appearance of arch bridges, many of which are over a century old, says Curzon. “We think it’s cheaper than a structural solution. But obviously it depends on the specific bridge.” The contractor and the national railroad owner Network Rail Ltd. will soon select a single-arch bridge for a full-scale test, he adds.
From a construction point of view, XiSPAN will be identical to the polymer-coated ballast technique now used by BBR and other contractors. A two-component, fast-acting polymer is sprayed to penetrate the ballast to form a reinforcing cage that effectively converts loose pieces of rock into a flexible slab, amenable to engineering analysis.
The patented XiTrack ballast system has been developed over the last decade by geotechnical engineering firm 2Ei Ltd., a company spun off by Heriot-Watt University, Edinburgh, and the polymer supplier Dow Hyperlast Ltd., High Peak.
For about 10 years, BBR has used XiTrack on ballast that is subject to high loading or that sits on weak or variable foundations. Last year, the contractor recruited the Sheffield-based engineering firm LimitState Ltd. to provide analysis and design skills to extend the technique to bridge strengthening.
LimitState has developed software that models the behavior of both a masonry arch and the materials above it. Traditional methods do not account for the composite behavior of the arch and its soil and are less accurate, says Matthew Gilbert, LimitState’s managing director.
By also including the treated ballast in the calculations, “we are talking about the potential for a completely new approach to strengthening masonry bridges,” says Gilbert. But, he suggests, until full-scale trials are done, XiSPAN will remain only a theoretical prospect.