Fine-grained clay and silt particles in the ballast voids can be derived from several sources including ballast breakdown, infiltration from surface sources, underlying granular layers or the subgrade soil, and sleeper wear. The presence of fine-grained clay in the ballast reduces its permeability and the ballast layer can become saturated. Under train loadings, excess fluid pressures develop in the saturated ballast layer. This pressure is dissipated by mud pumping up to the surface.
The contaminated ballast cannot resist the forces applied to the sleepers and track geometry problems result. In addition to pumping mud problems, fouled ballast can also contribute to subgrade failures. Dry, clean ballast will distribute the sleeper loads such that the stresses on the subgrade are greatly reduced. Fouling and saturation cause the ballast layer to lose its load spreading ability and high stresses are transmitted to the subgrade soil through water pressure.
This failure mode occurs in fine-grained clay and silt subgrades. Initiation of progressive shear failure is often the result of inadequate ballast thickness. The failure develops at the subgrade surface as the soil is sheared and remolded due to repeated overstressing. The soil moves outward and upward and cross-level develops in the track. This failure mode is usually apparent from soil heave along the track shoulders. The rotational movement of the subgrade and addition of ballast during resurfacing create a depression known as a ballast pocket below the track.
Water becomes trapped in the ballast pocket causing a further strength reduction in the subgrade soil, and the situation worsens. Excessive plastic deformation ballast pockets also develop from permanent cumulative strain of soft, fine-grained subgrade soil as it consolidates or compacts when subjected to repeated loading. Vertical displacements occur as the soil compresses and addition of ballast is required to maintain track grade. Water becomes trapped in the ballast pocket reducing the shear strength of the surrounding soil.
Slope failures occur by processes that increase shear stresses in the slope or that decrease the shear strength of the soil mass. An example of an activity that increases shear stresses in a slope is excavation at the toe.
However, the most common cause of slope instability is an increase in soil pore-water pressures caused by heavy rainfall. As the slope soils become saturated, the pore-water pressures increase causing a decrease in the soil shear strength. Movement of the soil mass is resisted by the shear strength along potential failure surfaces. If the shear strength is reduced sufficiently, the shear stresses will exceed the shear strength and sliding will occur. Cracks can develop and fill with water. Water in the cracks exerts hydrostatic pressure on the slide mass causing additional sliding.
Mud pumping can also occur from subgrade attrition. Attrition can occur when the subgrade consists of a soft rock or a hard clay layer that is in direct contact with the ballast and water is present above the contact between the two materials. During loading, the ballast is pushed into the hard layer causing local subgrade failure. Mud slurry is produced from failure of the fine-grained subgrade. The mud migrates upward during subsequent loading cycles fouling the ballast and eventually pumping at the ground surface.
Fouled ballast problems can be treated by removing the degraded ballast and replacing it with a more durable material. Placing a protective blanket of subballast, or even better: a resilient mat layer, between the hard subgrade layer and the ballast can often control subgrade attrition.
Several methods are in use for treating progressive shear failure and ballast pocket problems. These methods include, but are not limited to, increasing the ballast thickness, placing an asphalt layer between the ballast and subgrade, and installing geosynthetics or resilient confinement layers over the subgrade. Another method involves pressure injection of various mixes (e.g. lime or cement based) to fill ballast pocket voids and/or chemically stabilize the subgrade soils.
Each of the above methods may be appropriate under certain circumstances; however, each has its limitations. Undercutting to remove fouled ballast is expensive, costing up to 30.000 EUR/km for a single track.
Increasing the ballast thickness to treat progressive shear and plastic deformation problems may be severely limited by clearance constraints. A possible remedy is the use of resilient subballast mats as previously mentioned. In addition, the ballast pockets should still be drained or the use of resilient mats alone may not solve the problem.
Installing asphalt layers or geosynthetic products requires removal of the entire track structure. Slurry injection methods can be used without removing the track; however, their effectiveness is highly susceptible to mix design, subgrade soil properties, injection patterns and methods. Furthermore, proper implementation of these methods requires a thorough understanding of the subsurface conditions. Subsurface explorations and laboratory testing should be performed and proper design procedures used when available.
Slope stabilization methods commonly used by the railroads include removal and replacement of the slide mass, toe buttresses, various retaining wall schemes, pile driving, excavation to unload the head of the slide, and subsurface drainage techniques. The appropriate stabilization method will depend on site constraints and subsurface conditions. In many cases, several methods may be appropriate for a particular site and the method selected is based solely on cost.