Stabilization of Subgrade Soils using Ground Improvement Techniques
Controlling the degradation of track geometry is a must for High Speed corridors in order to keep various tolerances well within the prescribed limits. Track geometry degradation is influenced by Track Design, Axle Load, Speed, and Sub-Grade features. One of the most important factors is the improvement of the sub-grade in low-lying areas.improvement on the ground Techniques for Ground Improvement Here are the factors to examine as well as the strategies to use for sub-grade improvement. The engineering procedure for overcoming the challenges caused by bad ground regions is described, as well as an evaluation of the many choices available, including their benefits and drawbacks.
Interface Between Track, Sub-grade and Ground
The effects of Track Design, Axle-Load, Speed, Vehicle, and Sub-Grade factors on track geometry degradation. The design of the mixed-traffic railway has two requirements: light-weight passenger trains travelling at high speeds and heavily loaded freight trains travelling at slower speeds. This necessitates the use of sub-grade, which can provide the essential surface and alignment for high-speed service while also enduring strong axle loads without deterioration or the need for frequent repair. The trade-off between high-speed passenger trains and the stability of slow-moving heavy freight trains for cant and cant deficiency must also be considered.Another question concerning high speed is whether to use normal coaching stock with greater super elevation or a tilting train.
Rather than designing for too much differential speed on the same track, techno-economical solutions must be developed to permit safe train operation at higher speeds on standard railway track without costly alignment work. A layer of geo-textile separator separates the track system, which consists of Rails, Sleepers, Ballast, and Sub-ballast, from the sub-grade. The track structure is supported by the track sub-surface layers (ballast, sub-ballast, and sub-grade).
The ballasted rail structure has a stable platform thanks to the sub-grade. The track system distributes the loads from the rolling stock to a safe level, ensuring that these stresses do not induce undue strains in the sub-grade, resulting in non-recoverable deformations and progressive track geometry degradation, compromising safety and reliability.
The properties of the sub-grade, particularly the resilience modulus of the sub-grade soils, influence the design of the ballasted rail system. The capacity to preserve track geometry is influenced by the resilience modulus.The degradation of conditions at areas where the sub-grade transitions from a geotechnical to a structural element is a persistent issue. The rate of track deterioration in these places may be unusually high, necessitating 8–10 times more maintenance. Transition structures are required in these regions. Improvements in the sub-grade result in a slower rate of track geometry deterioration and lower maintenance costs.
Common problems due to poor sub-grade
It’s possible that a poor subgrade will lead to:Massive shear failure — caused by the sub-grade material’s low shear strength. The strains exerted by the axle loads squeeze the overstressed sub-grade clays to the side, causing progressive shear failure or general sub-grade failure. Attrition or local sub-grade failure occurs when repetitive loading on the sub-grade, particularly in the presence of water, turns it into slurry that can “pump” to the surface. Consolidation, moisture content changes, or gradual deformation owing to repeated traffic pressures can all produce sub-grade settlement. The slope stability of embankments and cuts must also be examined, as well as the likelihood of major shear collapse. For the most part, well-compacted sub-grade material is used for most projects.
Higher axle loads might put more stress on the sub-grade, resulting in faster track deterioration. In the case of compressible sub-soils, settlement of the sub-grade can occur regardless of axle load, causing rail track degradation, especially if the settlements are not uniform. Excessive and uneven track degradation can be caused by poor sub-base conditions. Uneven track degradation necessitates expensive maintenance and may jeopardise track safety. Furthermore, due to the non-uniform nature of the soft soils, differential settlements will occur, resulting in rail track damage over time.
Ground Improvement Options for Stabilization of Subgrade
The development of the sub-grade is inextricably linked to and reliant on the improvement of the natural ground formation beneath it. Because naturally occurring sub-soils may be unable to maintain the embankment and rail system without exceeding the criteria of the client’s design brief, ground treatment is essential in poor ground regions. Based on several parameters, such as the height of fill, thickness and compressibility of the soil, as well as time and cost, various techniques of ground treatment for soft ground can be roughly divided into structural (rigid) and geotechnical solutions.
Deep Vibro-compaction and Vibratory Surface Using a vibratory roller, surface vibratory compaction is used to densify loose cohesionless soils.For loose sandy deposits with less than 15% particles, deep vibro-compaction can be done to depths of up to 10 m. Compaction is accomplished by inserting the probe up to the desired improvement depth and letting the soil surrounding the probe to compact for a set period of time. The probe is then lifted by roughly 0.5m to compact the soil around the vibrator before repeating the operation. Replacement and Removal of unsuitable material and replacement with suitable fill may be carried out in localised regions with soft soils of limited depth and thickness. In valleys and gorges, these inappropriate materials were discovered.
Preloading may be used for low embankments over soft compressible soil with a restricted thickness (short drainage channel) or the ability to compress rapidly under the pressure of extra preload fill due to the presence of sand lenses. Preloading of soft soils is based on the concept of consolidation, in which pore water is squeezed from the voids until the water content and volume of the soil are in balance under the loading forces given by the surcharge.This is frequently followed by an increase in soil shear strength. Primary consolidation under final loading can be performed to some extent during construction, reducing post-construction settlement.
Vertical Drains and Preloading Prefabricated Vertical Drains and Preloading However, where the consolidation period is too long for full consolidation of primary settlements due to increased thickness of the soft clay, vertical drainage may be used in conjunction with preloading to speed up the settlement. In regions where the thickness of soft soils is less than 10 m and embankment height is modest, vertical drains may be considered. In such settings, primary and secondary settlements are expected to be limited.
Dynamic replacement can be used to densify loose cohesionless soils that are up to 6 metres deep and have a height of more than 2.5 metres. Dynamic replacement involves the use of a large pounder that is lifted to a predetermined height by crane and then placed onto the soil in a grid pattern to cover the entire site.The pounder creates craters, which are then filled with sand or aggregate and compacted. This method is only acceptable for places distant from settlement-sensitive structures because to the strong vibrations caused by the lowering of the pounder. Stone Pillars In regions where the subsoil consists of more than around 5 m deep soft cohesive soil and where stability and stringent considerations are not possible, stone columns may be used.
Piled Embankment and Viaduct
In places where the factor of safety against bearing capacity and slope stability is low, stage construction of the embankment may be required, with a waiting period between stages to allow for consolidation and strength growth. Stability berms must be used to reduce the number of construction stages when the required construction period exceeds the time frame available. Furthermore, these berms may extend beyond the right of way, necessitating the acquisition of additional land. In situations where there is a lack of time or space, a structural solution may be required.Excessive settling will occur in soft soil areas if the embankment height exceeds the pre-consolidation pressure. This can be avoided by the use of.
Chemical stabilization of subgrade soils can improve the short-term strength
Soil stabilization is the process which is used to improve the engineering properties of the.
Pavement construction tends to be relatively expensive in areas where subgrade soil is poor.