Analysis Of Two Trackbed Failure Mechanisms

The ballast is formed of igneous rock which is single sized granular material – nominally 50mm, which has a good resistance to crushing. These aggregate forms the trackbed which then supports the track. The ballast are angular particles and are irregular to stop them from sliding over each which helps drainage, this prevents pooling caused from rainwater also stopping vegetation growth from occurring, which could destabilise the track and cause major problems. The ballast is placed on a sand blanket/ fine grained materials and other protective layers (to enhance the properties of the sub-grade) known as the formation, underneath and in between the sleepers (the members laid perpendicularly underneath the rail) and filled to the side (known as a ‘shoulder’) to provide vertical and lateral support.

Dirty Ballast Failure

Over a period of time the ballast rubs against each other as there is a high level of compaction from the vibration of the train running at high speed with a substantial load, this creates the ballast to rub against each other which creates fine pieces of granite which fill the voids, this is known as ‘Dirty Ballast’. The aggregate becomes rounded so therefore loses the useful properties, as the fines mixes with the water they stick together making it harder for drainage. The ballast is consequently less flexible which reduces the tracks ability to restrain the train.

Dirty ballast failure is mostly common in areas with rail irregularities due to increased dynamic forces, such as joints. Further break down (of ballast) can arise from external sourced fines from aged componentry such as joints, welds, environmental such wind-blown materials and from freight vehicles (i. e. open topped coal) add to the fines within the ballast. Reduced void content (from fines) lowers the permeability and allows negative pore water pressure to develop. As further breakdown starts to develop as the void size continues to decrease, positive pore water pressure starts to build up which increases further material breakdown. Fines then become mobilised within the retained water which forms a slurry.

This failure mechanism is relatively straight forward to fix, as re-ballasting would significantly improve the track quality. An inclusion of a permeable separator geotextile should be included when replacing the ballast at the base of the new ballast as this will potentially prolong the tracked life by preventing intermixing from the fines of the remaining subgrade.

Subgrade Erosion (Pumping/ Intermixing Failure)

As well as dirty ballast failure, subgrade erosion (pumping failure) is a common failure mechanism seen on the rail. This can be separated into two types of failure: intermixing and pumping; due to subgrade or formation erosion. The migration of fines is similar as dirty ballast failure with deterioration in track quality and some presences of wet spots. Pumping occurs as slurry rises into the ballast due to dynamic action (from train loading) as the blanket (a layer/ several layers laid over the subgrade to give the formation its desired properties, layers include: granular material; to act as a filter, or synthetic layer) is either: absent, inadequate or susceptible to erosion.

This mechanism erodes underlying fine-grained subgrade which are typically clays/ silts or fine sand, however weakly-cemented sedimentary rocks and inadequate blanket layers can also be susceptible. A small amount of slurry that is at the base of ballast has an insignificant effect on the track performance however, this becomes severe as the slurry starts to travel to the base of the sleepers thus effecting the load bearing of the sleeper. The slurry acts as a lubricant between the interlocking of the ballast which results in a deterioration in track quality, which may require speed restrictions.

The most severe types of pumping problem occur shortly after re-ballasting where no action was taken from tracks already affected from pumping, or from the formation being removed from track renewal exposing the erosion susceptible subgrade. Intermixing is where the layers between the ballast and the formation mixes together which appears to be indistinct between the two. Intermixing is not as severe as pumping as this process self-limits itself as the ballast is relative to the grading of the fines which will form a natural filter over time.

Both of these failures (Pumping and Intermixing) and dirty ballast failure should be clearly categorised as both failure mechanism are treated with different method.