GROUND IMPROVEMENT
FOR HIGH SPEED CORRIDOR
Mohan Tiwari, Director (Projects), IRCON International Ltd
Abstract
Essential prerequisite for High Speed corridor is to have control on the
degradation of track geometry so as to keep various tolerances well within the
specified limits. Degradation of track geometry is a function of Track Design,
Axle-Load, Speed, and Sub-Grade characteristics. Improvement of sub-grade
in poor ground areas is recognized as one of the most significant factor.
This paper describes the considerations involved and the methodologies to be
adopted for sub-grade improvement. The engineering process to overcome
the problems presented by poor ground areas is discussed along with the
examination of the various options available, outlining their various
advantages and limitations.
1. Interface Between Track, Sub-grade and Ground
Degradation of track geometry is a function of Track Design, Axle-Load,
Speed, Vehicle and Sub-Grade characteristics.
For the track carrying mixed traffic, design has two differing requirements;
light weight passenger train at high speeds and heavily loaded freight train at
lower speed. This leads to the requirement of sub-grade, which can provide
the necessary surface and alignment required for high-speed service, at the
same time withstand heavy axle load without resulting in rapid deterioration or
requiring frequent maintenance. Trade-off for cant and cant deficiency
between high-speed passenger train and stability of slow speed heavy freight
trains also needs to be considered.
Another issue related to high speed is whether to have conventional coaching
stock with increased super elevation or have tilting train for higher speed.
Techno-economical solutions have to be sought to enable safe running of
train at higher speeds on conventional railway track without expensive
alignment work rather than to design for too much differential speed on the
same track.
The track system comprising of Rails, Sleepers, Ballast and Sub-ballast is
normally separated from the sub-grade by a layer of geo-textile separator.
Track sub-surface layers (ballast, sub-ballast and sub-grade) provide the
required support to track structure.
The sub-grade provides a stable platform for the ballasted track structure. The
track system distributes the loads from the rolling stock to a safe level such
that these stresses do not produce undue strains in the sub-grade that would
cause non-recoverable deformations and progressive degradation of the track
geometry, affecting the safety and ride quality.
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The design of the ballasted track system is influenced by the characteristics of
the sub-grade, in particular the resilience modulus of the sub-grade soils.
Resilience modulus has significant influence on ability to maintain track
geometry. Condition deterioration at locations where sub-grade changes from
geo-technical to structural element is a chronic problem. Track deterioration in
these areas could be abnormally high and may require 8 – 10 times more
maintenance. These areas require transition structures.
Improvement in sub-grade results in the reduction in the rate of track
geometry degradation and measurable lower maintenance cost.
2. Common problems due to poor sub-grade
Poor sub-grade may result into:
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Massive shear failure – attributable to the low shear strength of the subgrade material
Progressive shear failure or general sub-grade failure due to the stresses
imposed by the axle loads progressively squeeze the overstressed subgrade clays to the side.
Attrition or local sub-grade failure where the repeated loading on the subgrade, especially in the presence of water reduces the sub-grade to slurry
which can “pump” to the surface.
Sub-grade settlement that can be caused by consolidation, moisture
content changes or progressive deformation due to repeated traffic
stresses.
Slope stability of embankments and cuts also need to be assessed and the
possibility of massive shear failure has also to be precluded. For most
projects the chosen sub-grade material predominantly consists of wellcompacted residual soil fill material that offers a high shear strength and
modulus of resilience, precluding the possible occurrence of progressive
shear failure.
Higher axle load can impose higher stresses on sub-grade, which
consequently gives rise to accelerated track deterioration Settlement of the
sub-grade can occur independent of extent of axle-load in the case of
compressible sub-soils and this can cause degradation of the rail track
particularly if the settlements are not uniform.
Poor sub-base conditions can result in excessive and uneven track
degradation. Uneven track degradation results in costly maintenance and may
even adversely affect the track safety. Further, the non-uniform nature of the
soft soils will result in differential settlements, which will lead to rail track
degradation over time.
3. Ground Improvement Options
Improvement of the sub-grade is integral with and dependent on the
improvement of the underlying natural ground formation. Ground treatment is
required at poor ground areas, as the naturally occurring sub-soils may be
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unable to support the embankment and rail system without exceeding the
requirements of the client’s design brief.
Various methods of ground treatment for soft ground can be broadly
categorized into the structural (rigid) and the geotechnical solutions based on
various considerations, which included the height of fill, thickness and
compressibility of the soil as well as time and cost. Following methods of
ground treatment can be adopted for various poor ground conditions:
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3.1
Vibratory surface compaction and Deep vibro-compaction
Removal and replacement of soft cohesive deposits of limited thickness
Preloading of existing soft/loose fill
Preloading with vertical drains.
Dynamic Replacement.
Stone Column
Piled Embankments in areas having soft soil to large depths
Viaduct for high embankments on ground having very deep soft soils with
organic deposits.
Vibratory surface and Deep Vibro-compaction
Surface vibratory compaction is used for densification of loose cohesionless
soils using vibratory roller.
Deep vibro-compaction can be done for the loose sandy deposits having less
than 15% of fines for depths up to 10 m. Compaction is carried out by
inserting the probe up to the design depth of improvement and allowing the
soil around the probe to get compacted for certain time interval. Then the
probe is raised by about 0.5m to compact the soil around the vibrator and the
process is repeated.
3.2
Removal and Replacement
For localised areas with soft soils of limited depth and thickness, removal of
unsuitable material and replacement with suitable fill may be carried out.
These unsuitable materials were encountered in valleys and low-lying areas
and may be replaced with well-compacted suitable fill. Excavation and
replacement could be carried out up to 5m to 6m.
The removal and replacement may be required to be carried out even in
cutting areas where the naturally occurring soils were found to be of a low
shear strength and high moisture content. Subsurface drainage may have to
be introduced in most of these areas.
3.3
Preloading
For low embankment over soft compressible soil where the poor ground is of
limited thickness (short drainage path) or is capable of compressing rapidly
under load of excess preload fill due to presence of sand lenses, preloading
may be resorted. Preloading of soft soils is based on the consolidation
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concepts, whereby; pore water is squeezed from the voids until the water
content and the volume of the soil are in equilibrium under the loading
stresses imposed by the surcharge. This is usually accompanied by gain in
shear strength of soil. To a certain extent, the primary consolidation under
final loading can be achieved during construction and hence post construction
settlement reduces.
3.4
Prefabricated Vertical Drains and Pre-loading
However, with increased thickness of the soft clay where the consolidation
period is too long for full consolidation of primary settlements, vertical
drainage may be incorporated in conjunction with preloading in order to
accelerate the settlement. Vertical drains may be proposed in the areas where
the thickness of soft soils is limited to less than 10 m and embankment height
are low. The anticipated primary and secondary settlements in such areas
are limited.
3.5
Dynamic replacement
Dynamic replacement may be used for densification of loose cohesionless
soils which are up to 5 to 6 m deep and where height of embankment is more
than 2.5 m. Dynamic replacement utilizes a heavy pounder, usually lifted by
crane to designed height and then dropped onto the soil, in a grid pattern
such that the site is adequately covered. Craters formed by the pounder are
filled with sand or aggregate and compacted. Due to large vibrations induced
by the dropping of the pounder, this method is only suitable at locations away
from settlement-sensitive structures.
3.6
Stone Columns
Stone columns may be provided in areas where subsoil consists of more than
about 5 m thick soft cohesive soil and where stability and stringent
considerations cannot be satisfied with conventional removal / replacement of
soft material. Stone columns enable the embankment to be constructed to its
full height continuously without requiring stage construction.
3.7
Piled Embankment and Viaduct
In the areas having low factor of safety against bearing capacity and slope
stability; stage construction of the embankment may have to be resorted to, in
which waiting period have to be introduced between stages to allow for
consolidation and strength gain. When the required construction period
extends beyond the limited time frame available, stability berms need to be
introduced to reduce the number of construction stages. Moreover, these
berms may extend beyond the right of way and require more land to be
acquired. In cases of problems of limited time and space constraints it may
be necessary to adopt structural solution.
In soft soil areas, embankment height exceeding the pre-consolidation
pressure will give rise to excessive settlement. This can be avoided by
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means of structural solutions such as viaduct or piled embankment.
Structural solution is recommended in soft ground conditions with depths
exceeding 15 m. Structural solution is also required where settlement
requirement is Zero mm viz Points and crossings / turnout in yards. Where
height of embankment is more, cost of pilled embankment may be higher and
Viaduct may have to be provided. In both alternatives, the rail system is
supported on piles driven through the soft soil and founded within the
underlying stiffer material.
The trade off option between viaduct and piled embankment is governed by
the embankment height. Economical analysis indicates that viaduct is more
feasible for embankment in excess of about 6m, below which piled
embankment is favourable.
4.
Transition Structures
Transition structures will be required to be provided at all locations having
abrupt change in the sub-grade resilience. Following type of transitions may
be required:
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5.
At the transition between the vertical drain treatment area, which will
undergo residual primary consolidation plus secondary settlement in
the long term and the rigid viaduct, transition structure consisting of
piled slab followed by an approach slab.
Flexible approach slab as a transition between viaduct and dynamic
replacement area.
At all other locations transition structures in form of a mechanical hinge
or approach slabs after additional preloading at the interface before
construction of the piled embankment to avoid differential settlement
between the rigid structures and settling fill.
Overall Cost economy
The type of ground treatment will greatly govern the frequency of
maintenance (tamping) and the possession time that is required for
maintenance. Significant up front investment may be required to reap longterm savings. General tendency to reduce initial cost (construction cost) of the
project is resulting into adoption of methodologies, which gives initial lower
cost but may result in higher recurring cost.
An opposite scenario would be the demand for zero total settlement of subgrade during operation to keep costs of maintenance at lowest possible level,
resulting into very high initial cost of construction. System consisting of
structural solutions for zero settlement with provision of ballastless track may
cost about 2 – 2.5 times that of conventional ballasted track with geotechnical
solutions of ground improvement for a given permissible total settlement. If
the sub-soil conditions are poor, the life cycle cost of the system can be 3 to 4
times more in case no proper ground treatment is carried out.
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Therefore, a trade-off between improvement cost of ground and sub-grade
characteristics and maintenance cost arising out of sub-grade deterioration
will enable reduction in life cycle cost of maintenance and renewals. However,
to harness the true potential of such a trade off, it will be necessary to provide
suitable transition structures, which may permit varying methods of ground
treatment.
6.
Conclusion and recommendations
The methodology to carry out improvement works to ground and sub grade
has to be based on requirement of settlement criteria during operations, at the
same time to exploit the permitted settlement to use cost effective ground
treatment option.
It is recommended to consider Optimization of Life Cycle Cost as one of the
requirement during definition and Design development phase. Very prohibitive
settlement conditions may lead to significant increase in the Life cycle cost
due to very high capital cost, although maintenance and operations cost could
be substantially lower.
In case of “Design and Build” contract, a longer maintenance period could be
specified to discourage short-term gain by Design & Build Contractor.
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