Network Rail's revised Code of Practice now encourages using more innovative techniques like geogrid reinforcement to improve trackbed performance over soft or unstable ground. Stiff polymer grids have been used successfully in the UK for decades to reinforce ballast on difficult sites. Recent research and monitored installations continue to show benefits, including reduced maintenance needs. The first project using the new Code involved placing geogrid under 1.5km of track near Derby known for poor geometry, restrictions, and frequent maintenance due to variable unstable ground. Initial results showed improvements.

1. 00 July-August 2005 RAILWAY STRATEGIES
R
ailway trackbed
supported on low or
variable stiffness
formation is a major
cause of poor track geometry.
Until recently the typical
response to this problem has
been frequent maintenance
involving ballast
reinstatement and recurring
line-speed restrictions. However, Network
Rail’s revised Code of Practice on ‘Formation
Treatments’ RT/CE/C/039 (Issue 2,
December 2003) now encourages
permanent way engineers to maintain and
renew such trackbed more efficiently by
using more innovative techniques.
The revised Code recognises the
structural contribution of stiff geogrid
reinforcement to trackbed performance
over soft or variable formation. This allows
the trackbed layer thickness to be reduced
difficult railway sites over
the years, but historically in
a somewhat ‘belt-and-
braces’ approach. Typical
applications have included
installations over low-lying
peaty fens on the East
Coast Main Line, where
Network Rail has reported
significantly improved
maintenance levels, enabling tamping to be
more effective and last longer, and the
formation to be more stable with a lot less
deformation than previously experienced.
Major full-scale research was carried out
by British Rail Research shortly before
privatisation in order to assess more
objectively the contribution of stiff geogrids
in trackbed reinforcement over soft
substructures. This showed clear benefits
and confirmed the experience from earlier
monitored installations. Nevertheless,
GEOTECHNICAL ENGINEERING
by up to 30 per cent without loss of
structural performance. However, the main
application for track renewals is likely to
involve placing the reinforcement below the
traditional depth of clean ballast (typically
300mm from the underside of the sleeper)
in order to significantly extend maintenance
intervals – including tamping frequency –
and to reduce the number of possessions.
Stiff polymer grids have been produced in
the UK since 1980 and have been used for
ballast reinforcement in a number of
Network Rail’s revised Code of Practice on Formation Treatments
encourages the use of alternative remedies for poor track
geometry. JOHN DIXON explains…
solidbase
fortrack
A
Trackbed geometry showing geogrid location
Ballast
Geogrid
Ballast being
placed onto
geogrid
reinforcement

2. RAILWAY STRATEGIES July-August 2005 00
recognition in the UK of the benefits of
grid-reinforced trackbed was not
widespread and the application of grids
remained sporadic prior to the revised Code
and the more recent drive for reduced
whole-life cost on rail assets.
Carillion Rail is believed to have
completed the first installation to the new
Code in 2004 on the line between Derby
and Proof House (Birmingham) using
Network Rail-approved biaxial geogrid. A
1.5km stretch of the line, which runs over
extensive coal measures, had long been
affected by a ground formation with highly
variable stiffness, with materials ranging
from sandstone to weak fireclays. This
stretch of track had a history of poor track
geometry, speed restrictions and frequent
maintenance cycles. Consultant Atkins Rail
designed a geogrid-reinforced solution, in
accordance with the new Code and, to
provide further stiffness improvements over
particularly problematic stretches, also
specified a double layer of geogrid and a
sub-ballast layer.
The ballast and formation were
excavated to 400mm below the underside
of the sleepers and polymer geogrid was
unrolled onto the formation. Over this
initial geogrid layer, a 100mm thick layer of
well-graded Type 1 sub-ballast was placed
and compacted. Then the upper layer of
geogrid was laid and covered with ballast
to the correct height. In locations where
the clay formation was exposed, the
excavation was taken 100mm deeper, and
additional sub-ballast was placed.
Approximately 15,000m2 of geogrid was
installed in a single possession.
Other recent tracked installations in
London have been carried out to mitigate
differential settlement between fixed
structures and soft approaches. By installing
geogrid below the ballast at the interfaces
between under-bridges and clay core
approach embankments, the
effects of seasonal volume changes
carried out at the University’s Railway
Testing Facility.
This is complemented by monitored
Network Rail installations, including an
800m length of poor formation with high
water table at Coppull Moor near Wigan on
the West Coast Main Line. Initial results from
the HSTRC (high speed track recording
coach) have shown significant improvements
of vertical standard deviations on the rail in
the reinforced sections compared with the
adjacent renewed, unreinforced control
sections. The main results of this research
are expected later this year and are likely to
further optimise the trackbed reinforcement
design process.
can be ameliorated.
Stiff geogrids are also available in
composite form, factory bonded to non-
woven geotextiles for instances when the
function of reinforcement needs to be
combined with that of separation – usually
when placed directly on a sand blanket.
These composites save the time of laying
two geosynthetics separately and are
installed grid-side-up to optimise interlock
with the overlying aggregate particles. They
have been used recently over significant
areas of the West Coast Main Line.
In order to fully appreciate the
mechanisms involved in geogrid
reinforcement of trackbed, further research
and development is being undertaken. The
Brian Mercer Award, administered by the
Royal Society, has been awarded on
‘Geogrid Reinforcement of Ballast’ to a
team headed by Nottingham University and
including Network Rail, Carillion Rail, Scott
Wilson Pavement Engineering and Tensar
International. The research work involves an
examination of the influence of different
grid properties, such as stiffness and
aperture size, with full-scale testing being
G John Dixon B.Sc.
C.Eng. MICE has
been with Tensar
International since
1981 and is now
special projects
engineer. John has
worked
throughout the
UK, as well as North America, Europe,
Africa and the Far East. Much of the
past five years has been spent working
on the Channel Tunnel Rail Link (CTRL)
contracts. He is currently primarily
involved with the company’s railway
application and sales developments.
Tel: 07771 978992
Email: jdixon@tensar.co.uk
Web: www.tensar-international.com
I
Diagram
showing
that
interlock
with a stiff
geogrid
prevents
both lateral
and vertical
migration of
aggregate
particles
Derby to Proof
House track renewal
showing geogrid
rolled onto prepared
formation