XiSpan™Masonry Arch Bridge Stengthening Technique

Introduction

There are in excess of 25,000 masonry arch underline bridge spans on the UK rail network. Many of these are over 100 years old, and cannot all be relied upon to carry planned increased rail traffic tonnage and increased line speeds.

Balfour Beatty Rail, in collaboration with LimitState, have developed XiSPAN, an innovative strengthening solution for masonry arch bridges. XiSPAN uses the Network Rail Approved XiTRACK® ballast strengthening product to enhance the performance of arch bridges.

LimitState carry out the analysis and design while Balfour Beatty Rail carry out the necessary installation work using XiTRACK®.

XiSPAN is a tailored strengthening solution for masonry arch bridges which involves the use of XiTRACK polyurethane, following detailed analysis and design.

The XiSPAN solution is based on the use of the XiTRACK polyurethane ballast stabilisation and reinforcement material. The polyurethane creates a strong matrix composite material with the ballast. The resulting ‘raft’ serves to distribute loads more widely and evenly over the bridge, maintaining or increasing its load bearing capacity.

XiSPAN Description

XiSPAN takes ballast strengthening further to improve bridge performance. The system is designed to improve the load distribution over the arch thereby reducing live load effects on the arch barrel. In addition, the system can enhance ‘passive zone’ restraint to the arch barrel.

The stiff, reinforced layer provided by XiSPAN acts to

disperse applied loads over a wider area.

XiTRACK®

XiTRACK is an established process for solving and preventing track problems. It is not a ballast glue and should not be confused with traditional gluing methods - it has very different properties and has seen use in a range of applications over a number of years. XiTRACK® has gone through a comprehensive development and testing programme.

It is proven both in the laboratory and in the railway environment. XiTRACK® has full Network Rail Product Acceptance and can be used without the need for further approvals.

XiTRACK® works by linking the void structure of the ballast together with a quick setting polyurethane which gives strength and support to the track, yet leaves the ballast free draining. The degree of strength needed is calculated in advance, enabling a design tailored to each site to be produced.

Benefits and qualities

The XiSPAN system is quick to install due to its simple pumped installation – this means minimal interruption to traffic and other bridge users. There will also be no noticeable change in the appearance of the bridge after the installation process.

XiSPAN increases the load bearing capacity of arch bridges yet has a minimal impact on existing services in the bridge.

The polyurethane material used exhibits long term durability and is environmentally safe. It can be applied at any time of year and can subsequently be broken out if required. In the railway environment the material operates at stresses well below its yield strength, ensuring that fatigue failure does not occur in normal operation.

The XiSPAN system leaves the fundamental mode of response of the masonry arch bridge unchanged; the arch continues to carry the load as it always has done. This is in contrast to conventional intrusive ‘strengthening’ solutions, which can involve the use of stitches, rigid anchors, and/or under-arch supporting elements. These solutions can sometimes leave parts of the masonry unstressed and other parts overstressed.

Other benefits are:No interference with bridge structureNo new failure modesNo additional stresses introduced No new inspection requirements introducedExhibits a ductile mechanism of failure XiTRACK® has also been used extensively on transitions as it can be graded to match the stiffnesses of different track types. This means that XiSPAN does not require any transitions at either end and the solution is specially designed for each particular case, leading to a ‘one fix’ solution.

Please contact us for further information:Balfour Beatty Rail LtdFitology House, Smedley St East, Derbyshire, DE4 3GH, UKTel: +44 (0)1629 760750 / Fax: +44 (0)1629 760751info.bbrt.uk@bbrail.com / www.bbrail.co.uk

Backfill over an arch bridge provides passive resistance to the arch barrel. When extended over a sufficient distance, the XiSPAN system will also help provide increased restraint to the arch, in the form of increased passive fill pressures.

Composite raft structure

Stiff XiTRACK layer

Before treatment

After treatment

Dispersion of applied load

XiTRACK polymer geo-matrix