Often referred to as Subgrade, erosion or mud pumping or wet spots, the issue has always been a problem in ballasted track, particularly on weakly cemented mudstones or over consolidated clays. These soils have a high shear strength and as such do not need a deep trackbed to support track loading, yet if unprotected the upper surface degrades easily to a slurry when exposed directly to water. The open texture of ballast allows free water to come into contact with the exposed formation/subgrade surface.
When the exposed surface contains fine grained particles, these can be readily eroded by the water accumulating in the voids, forming weak, highly mobile slurry. This slurry is then ‘pumped’ up into the overlying ballast by each passing axle load.
Why Is Subgrade Pumping An Issue?
Contamination of the clean ballast layer by the fine soil particles in the slurry very quickly reduces the load-bearing properties of the ballast and leads to loss of track alignment in the affected area. A small amount of slurry can considerably reduce ballast life. Under extreme conditions the ballast will become unmaintainable within a very short time post-installation. Areas where mud pumping occurs are often expensive to operate due to frequent maintenance intervals, particularly if the issue is not properly rectified.
As a proportion of a whole network the length of track affected by subgrade erosion may be quite small, typically 5%, but experience shows that in areas where extensive mud pumping occurs the line is expensive to operate due to frequent maintenance interventions; permanent speed restrictions (slow orders) are often required. If the cause of subgrade pumping is not identified and correctly treated, subgrade erosion will quickly re-occur.
How Does Subgrade Pumping Occur?
Subgrade pumping problem is therefor can occur on any fine grained soil, either due to a poor quality intermediate layer separating ballast and subgrade (eg. sub-ballast), or a complete absence of any separating layer. Thee particularly acute where track drainage is poor, i.e. where there is often standing water.
An example of where subgrade pumping is likely to occur is over stiff clays or weakly cemented mudstones, particularly of the trackbed is poorly drained. These soils have a high bearing capacity and as such do not need a deep trackbed to distribute track loading yet they still require some intermediate layer to prevent ballast scouring the subgrade surface. If no such layer is present the open texture of ballast allows free water to come into contact with the exposed formation/subgrade surface. Fine grained particles resulting from scouring action then combine readily with water accumulating in the voids, forming highly mobile slurry.
It is important to emphasize that the problem is not limited to fine grained subgrades; intermediate layers formed from weak or cohesive materials are also susceptible to pumping. Subgrade pumping can also occur if the intermediate layer is poorly graded.
As slurry begins to rise in the ballast it is then pumped upward under the high pressure generated by train loading. A suction force develops under the ties after the load has passed, causing the slurry to be pulled further up into the ballast section.
Mud pumping results in ballast failure, a weakening of the track modulus, and a consequential reduction in track structure bearing capacity. A small amount of slurry at the base of the ballast will have a minimal impact on track geometry, but when the slurry migrates upwards into the ballast voids, the load bearing capacity of the ballast can be severely compromised. At this stage tamping is only a short-term fix to track surface problems.
The rate at which mud pumping develops will vary, dependent upon a number of factors. Where there is a fine-grained soil present at the ballast/sub-ballast interface, a slurry will develop, but if drainage is good or the line is lightly loaded, the slurry may remain at a low level and not significantly affect ballast performance. On the other hand, if the drainage is poor and the loading is significant, mud pumping can develop very quickly.
How to Identify Subgrade Pumping?
It’s important to understand the track substructure and the different components to help identify if pumping is an issue. The primary function of the layers is to distribute the track forces to ensure the stresses in the subgrade are at a satisfactory level.
Ballast
Ballast is the free draining granular material placed at the top of the substructure layer. Its three major functions are:
1) maintain track geometry by resisting vertical, lateral and longitudinal forces;
2) distribute train loads to a larger subgrade area, thereby limiting the stresses applied to the subgrade;
3) facilitate drainage.
Subballast
Capping Layers are commonly constructed from naturally occurring sands and gravels, and crushed stone aggregates. The particle size of the materials vary extensively, from fine sand through to wellgraded coarse sand and gravel. They may also contain a proportion of clays or silts.
Subgrade (Natural ground)
The Subgrade is the upper part of the earthworks or natural ground upon which the capping layers and ballast layers are placed. The subgrade is the most inconsistent and potentially weakest component of the track, yet it is the foundation on which all other components are supported.
Trackbed Assessment Chart
Methodologies To Solve Subgrade Pumping
Adding clean ballast may help in the short term, but it is not a longterm solution. A modification of the track structure may be needed. The worst pumping problems often occur after undercutting, when the existing sub-ballast layer, which had provided adequate subgrade protection, is removed and the subgrade is exposed to direct ballast loading.
When heavy duty geotextiles first became available it was initially thought that they might replace a traditional sand blanket. However no geotextile available has a pore size sufficiently small to prevent the upwards migration of clay or silt sized particles into the ballast. See the table below for soil particle sizes and standard geotextile opening sizes. While geotextiles might be suitable for prevention of the pumping for coarser soils, they would not be appropriate for fine subgrade pumping.
An anti-pumping geocomposite has now been developed to address subgrade pumping even in the most onerous conditions. This product has undergone thorough testing and seen increasing adoption from major rail operators including; Network Rail (UK), Norfolk Southern & Union Pacific (US), ProRail (Netherlands). The anti-pumping product can be deployed to reduce track possession times and installation costs as well as operating expenditure. To find out more information please click here
Other methodology includes the use of a sand blanket between the subgrade and subballast, the thickness of the blanket is normally between 200mm and 300mm. The ‘traditional’ sand blanket is highly successful at curing subgrade erosion problems, when a sand blanket is laid, the sand flows into the surface voids and fills in any depressions in the excavated surface on which it is placed. This prevents upwards movement of slurry. Slurry generation might continue at the clay surface after the sand has been placed, but there is no process for perpetuating it. Water can move upwards through the sand, but the clay particles are retained, so the slurry will dry out in time. However this process necessitates large volumes of excavation. Additional trains are often required to transport the large amounts of spoil and import new materials. With increasing traffic on the rail network it is important to reduce possession times and the movement of materials trains.
