2. 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 structure
No new failure modes
No additional stresses introduced
No new inspection requirements introduced
Exhibits 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 Ltd
Fitology House, Smedley St East, Derbyshire, DE4 3GH, UK
Tel: +44 (0)1629 760750 / Fax: +44 (0)1629 760751
info.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
