High-speed corridors’ critical requirement is to control track geometry deterioration to maintain different tolerances well within the defined limits.
A feature of track design, axle-load, speed, and sub-grade characteristics is the deterioration of track geometry—sub-grade improvement in low soil areas recognized as one of the most critical factors.
The factors involved and the methodologies for sub-grade enhancement to followed are listed here. In addition to discussing the various solutions available, the engineering method to solve the problems posed by badland areas is addressed, detailing their different advantages and limitations.
Interface Between Track, Sub-grade and Ground
A feature of track design, axle-load, speed, vehicle and sub-grade characteristics is the deterioration of track geometry.
The design has two separate specifications for tracks carrying mixed traffic: lightweight passenger trains at high speeds and heavily loaded freight trains at lower speeds. This results in a sub-grade requirement that can provide the required surface and alignment necessary for high-speed operation while withstanding heavy axle loads without causing rapid degradation or needing regular maintenance. It is also essential to recognize the trade-off for cant and cant deficiency between a high-speed passenger train and slow-speed heavy freight trains’ stability. Another high-speed problem is whether to have standard coaching stock with increased superelevation or a higher-speed tilting train. Instead of planning for too much differential velocity on the same track, techno-economic solutions have to be found to allow safe train running at higher speeds on traditional railways without costly alignment work.
The track system comprising Rails, Sleepers, Ballast and Sub-ballast is usually divided by a geo-textile separator layer from the sub-grade. The necessary support for the track structure is provided by sub-surface layers of track (ballast, sub-ballast and sub-grade).
The sub-grade provides the ballasted track system with a stable foundation. The track system distributes the loads from the rolling stock to a safe level. These stresses do not create excessive sub-grade strains that would cause unrecoverable deformations and gradual deterioration of the track’s geometry, affecting the safety and quality of the trip.
The nature of the ballast track system is affected by the sub-grade characteristics, particularly the modulus of resilience of the sub-grade soils. The modulus of strength has a significant impact on the ability to preserve track geometry.
Deterioration of conditions is a chronic problem in areas where sub-grade shifts from geotechnical to structural components are 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.
Common problems due to poor sub-grade
Poor subgrade may result into:
- Massive shear failure – attributable to the low shear strength of the sub-grade material
- Progressive shear failure or general sub-grade failure due to the stresses imposed by the axle loads progressively squeeze the overstressed sub-grade clays to the side.
- Attrition or local sub-grade failure where the repeated loading on the sub-grade, 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.
It is also essential to test the slope stability of embankments and cuts and preclude the risk of massive shear failure. The selected sub-grade material consists primarily of well-compacted residual soil fill material that provides a high shear strength and durability module for most projects, preventing the potential occurrence of progressive shear failure.
In compressible sub-soils, higher axle load can place higher stresses on the sub-grade, resulting in accelerated track deterioration. In the case of compressible sub-soils, sub-grade settlement can occur independently of the degree of axle load, and this can cause rail track degradation, especially if the payments are not uniform.
Excessive and uneven track deterioration can result from poor sub-base conditions. Uneven deterioration of the track results in expensive repairs and can also adversely affect the track’s safety. Besides, differential settlements will result in the soft soils’ non-uniform existence, which will contribute over time to rail track degradation.
Ground Improvement Options for Stabilization of Subgrade
Sub-grade improvement is integral to the improvement of the underlying natural ground formation and relies on it. In low soil areas, ground treatment is needed. The naturally occurring sub-soils will not sustain the embankment and rail system without exceeding the client’s design brief specifications.
Different soft-ground soil treatment methods can be generally divided into structural (rigid) and geotechnical solutions based on other factors, including fill height, soil thickness and compressibility, and time and expense.
Following methods of ground treatment can be adopted for various poor ground conditions:
- 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
Vibratory rollers use surface vibratory compaction for the densification of loose cohesionless soils. For loose sandy deposits having less than 15 per cent of fines for depths up to 10 m, deep Vibro-compaction can be achieved. Compaction is achieved by placing the probe up to the design depth of enhancement and allowing a certain period to compress the probe’s soil. Then the probe is raised by about 0.5m to compact the soil around the vibrator and the process is repeated.
Removal and Replacement
Removal of inadequate material and replacement with appropriate fill can be done for localised areas with soft soils of restricted depth and thickness. These unsuitable materials have been identified in valleys and low-lying areas and can be substituted with well-compacted fillings. Up to 5m to 6m of excavation and replacement may be carried out.
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.
Preloading may have resorted to low embankment over soft compressible soil where the poor soil is of the limited thickness (short drainage path) or is capable of rapid compression under a load of excess preload fill due to the presence of sand lenses.
Preloading of soft soils is based on the principles of consolidation, whereby; pore water is squeezed from the voids until under the loading stresses imposed by the surcharge the water content and the volume of the soil are in equilibrium.
This is generally correlated with gain in soil shear strength. To a certain extent, the primary consolidation under final loading can be achieved during construction and hence post construction settlement reduces.
Prefabricated Vertical Drains and Preloading
However, with increased soft clay thickness, where the consolidation time is too long for primary settlements to be completely consolidated, vertical drainage can be implemented in combination with preloading to speed up the settlement.
Vertical drains can be suggested in areas where the soft soil thickness is restricted to less than 10 m, and the height of the embankment is low. The anticipated primary and secondary settlements in such areas are limited.
Dynamic replacement can be used for densification of loose cohesionless soils up to 5 to 6 m deep and where the embankment height is more than 2.5 m. Dynamic replacement uses a heavy pounder, generally raised to the designed height by crane and then lowered onto the soil in a grid pattern to adequately cover the site. 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.
In areas where subsoil consists of more than about 5 m thick, soft cohesive soil, stone columns may be given. Stability and stringent considerations can not be fulfilled with traditional soft material removal/replacement. Stone columns allow the embankment to be built to its full height continuously without requiring stage construction.
Piled Embankment and Viaduct
It may be appropriate to resort to stage construction of the embankment in areas with a low safety factor against bearing capacity and slope stability. A waiting period must be formed 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 construction stages.
Besides, these berms may extend beyond the right of way and involve acquiring more land, in cases of problems with limited time and space constraints, structural solutions that need to be implemented. Embankment height in soft soil areas exceeding the pre-consolidation pressure would result in excessive settlement. This can be avoided using structural solutions such as viaduct or piled embankment. The structural solution is recommended in soft ground conditions with depths exceeding 15 m.
The structural solution is also needed where zero mm viz points and crossings/turnout in yards is the settlement requirement. If the dam’s height is greater, the pilled embankment cost will be higher, and it may be appropriate to include the viaduct. In both alternatives, the rail system supported by piles pushed through the soft soil and built on the stiffer material underlying it.
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.
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:
- 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 broadly govern the frequency of maintenance (tamping) and the possession time required for maintenance. Significant upfront investment may be necessary to reap long-term savings. General tendency to reduce initial cost (construction cost) of the project results in the adoption of methodologies, which gives initial lower price but may result in higher recurring cost.
An opposite scenario would be the demand for zero total settlement of sub-grade during operation to keep maintenance costs at the lowest possible level, resulting in a very high initial cost of construction.
A system consisting of structural solutions for zero settlement with the ballastless track provision may cost about 2 – 2.5 times that of the conventional ballasted track with geotechnical solutions of ground improvement for a given permissible total settlement. If the sub-soil conditions are low, the system’s life cycle cost can be 3 to 4 times more if no proper ground treatment is carried out.
Therefore, a trade-off between improvement cost of ground and sub-grade characteristics and maintenance cost arising out of sub-grade deterioration will reduce the life cycle cost of maintenance and renewals. However, to harness such a trade-off’s true potential, it will be necessary to provide suitable transition structures, which may permit varying methods of ground treatment.
Conclusion and recommendations
The methodology to carry out ground improvement works has to be based on the requirement of ground settlement criteria during operations while exploiting the permitted settlement to use cost-effective ground treatment option.
It is recommended to consider Optimization of Life Cycle Cost as one requirement during the definition and design development phase. Very prohibitive settlement conditions may lead to a significant increase in the Life cycle cost due to very high capital cost, although maintenance and operations cost could be substantially lower.