Stabilization of Subgrade Soils using Ground Improvement Techniques
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.
The considerations involved and the methodologies to be adopted for sub-grade improvement are described here. 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.
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.
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.
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.
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 well-compacted 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.
Ground Improvement Options for Stabilization of Subgrade
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 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:
- 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.
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.
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 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.
Prefabricated Vertical Drains and Preloading
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.
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.
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.
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 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.
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 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 long-term 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 sub-grade 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.
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.
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.
Mohan Tiwari, Director (Projects), IRCON International Ltd