03-05-2011, 02:10 PM
ABSTRACT
Over the past years, considerable advances have been made into the understanding of the behaviour
of using geosynthetics to improve the performance of shallow embankments. Detailed
investigations have been performed using small scale and large scale models to evaluate their
performance and to develop rational methods for design. This document provides a guide to
geotechnical engineers who wish to analyse the performances of geogrid reinforcements when
installed in different locations of the track structure.
Keywords: railways; embankments; geosynthetics; reinforcement; geogrid;
improvement; settlement; strain gauge; f.e.m.; empirical results
1. Introduction
Railways embankments on soft soil require proper ground improvement and good soil compaction.
A railway line shall be considered as a multi-layer composite system comprising of natural ground,
fill soil, reinforcement layers, rail track system and wheel train loads. The purpose of the track
components is to convert the wheel load to relatively uniform stresses on the sub-grade. The track
sub-structure layers (ballast, sub-ballast and sub-grade) have significant influence on the railways
performance: all the stresses and thus settlements occur in these layers and may be due to several
different causes including short and long term settlements due to static and dynamic loadings.
Geosynthetics have been proved to be suitable as reinforcement for both the embankment bottom
and fill at the subgrade and sub-ballast level.
The geogrid reinforcements perform the following functions:
1. create a stiff platform where horizontal shear strains and vertical settlements are controlled
and minimised;
2. increase the bearing capacity and the load distribution by enlarging the foundation slip
failure line, thus reducing vertical stresses;
3. increase the fill soil stiffness by enhancing the soil compaction, providing inner tensile
strength and an apparent long term cohesion of the fill soil even under high dynamic loads
approaching the critical speed;
4. increase the dumping efficiency of the embankment fill thus allowing higher railways speed;
5. reinforce, filter and separate the soil components.
Therefore geogrid reinforcements are frequently used for the rehabilitation of existing railway
embankments and for design of new lines as discussed below.
2 Rehabilitation of existing railway: two instrumented cases
In recent years, due to the exceptional increase of traffic, speed and train axle load, several existing
railways lines are showing signs of distress, instability and settlements. These phenomena have
serious influence on the safety and efficiency (speed restriction) of train operation. The irregularity
of the rail level under the train passage becomes rapidly worst with additional passages. The
solution of improving the top layer of the railway track by providing suitable designed sub-ballast
layer is essential to withstand higher stresses. However installing a thick sub-ballast under running
traffic is extremely difficult and expensive and sometimes impractical. The reinforcement of the
sub-ballast by means of a geogrid allows the reduction of the depth of the excavated soil and at the
same time assures higher long-term performances.
Hereby is presented the design for the rehabilitation of the Foligno-Terontola railway line, founded
on a very old embankment (second half of the XIX century) subject to continuous and differential
settlements. The design solution was determined by F.E.M. analysis and comparing empirical
results obtained by other Authors. The solution required the reinforcement of the sub-ballast by
means of a double geogrid-geotextile geocomposite layer and the excavation and replacement of the
first 0.70 m of sub-ballast with free-drainage granular fill soil to avoid swelling and desiccation
within the silty embankment, as shown in figure below.
Download full report
http://www.tenaxgeosynthetics/tech-doc/g...kments.pdf
Over the past years, considerable advances have been made into the understanding of the behaviour
of using geosynthetics to improve the performance of shallow embankments. Detailed
investigations have been performed using small scale and large scale models to evaluate their
performance and to develop rational methods for design. This document provides a guide to
geotechnical engineers who wish to analyse the performances of geogrid reinforcements when
installed in different locations of the track structure.
Keywords: railways; embankments; geosynthetics; reinforcement; geogrid;
improvement; settlement; strain gauge; f.e.m.; empirical results
1. Introduction
Railways embankments on soft soil require proper ground improvement and good soil compaction.
A railway line shall be considered as a multi-layer composite system comprising of natural ground,
fill soil, reinforcement layers, rail track system and wheel train loads. The purpose of the track
components is to convert the wheel load to relatively uniform stresses on the sub-grade. The track
sub-structure layers (ballast, sub-ballast and sub-grade) have significant influence on the railways
performance: all the stresses and thus settlements occur in these layers and may be due to several
different causes including short and long term settlements due to static and dynamic loadings.
Geosynthetics have been proved to be suitable as reinforcement for both the embankment bottom
and fill at the subgrade and sub-ballast level.
The geogrid reinforcements perform the following functions:
1. create a stiff platform where horizontal shear strains and vertical settlements are controlled
and minimised;
2. increase the bearing capacity and the load distribution by enlarging the foundation slip
failure line, thus reducing vertical stresses;
3. increase the fill soil stiffness by enhancing the soil compaction, providing inner tensile
strength and an apparent long term cohesion of the fill soil even under high dynamic loads
approaching the critical speed;
4. increase the dumping efficiency of the embankment fill thus allowing higher railways speed;
5. reinforce, filter and separate the soil components.
Therefore geogrid reinforcements are frequently used for the rehabilitation of existing railway
embankments and for design of new lines as discussed below.
2 Rehabilitation of existing railway: two instrumented cases
In recent years, due to the exceptional increase of traffic, speed and train axle load, several existing
railways lines are showing signs of distress, instability and settlements. These phenomena have
serious influence on the safety and efficiency (speed restriction) of train operation. The irregularity
of the rail level under the train passage becomes rapidly worst with additional passages. The
solution of improving the top layer of the railway track by providing suitable designed sub-ballast
layer is essential to withstand higher stresses. However installing a thick sub-ballast under running
traffic is extremely difficult and expensive and sometimes impractical. The reinforcement of the
sub-ballast by means of a geogrid allows the reduction of the depth of the excavated soil and at the
same time assures higher long-term performances.
Hereby is presented the design for the rehabilitation of the Foligno-Terontola railway line, founded
on a very old embankment (second half of the XIX century) subject to continuous and differential
settlements. The design solution was determined by F.E.M. analysis and comparing empirical
results obtained by other Authors. The solution required the reinforcement of the sub-ballast by
means of a double geogrid-geotextile geocomposite layer and the excavation and replacement of the
first 0.70 m of sub-ballast with free-drainage granular fill soil to avoid swelling and desiccation
within the silty embankment, as shown in figure below.
Download full report
http://www.tenaxgeosynthetics/tech-doc/g...kments.pdf