Railroads need ballast, the material beneath the tracks, to support the cross-ties and rails. The ballast helps to stabilize the track and assists in proper drainage. A good ballast material needs stability and strength to safely carry the weight of the train. Modern trains put a huge amount of stress on the tracks and, without the support of a firm foundation, the trains could derail.
Early railroads in the U.S. did not always use proper ballast materials. Some tracks were laid directly over a layer of earth or burnt coal cinders that was not always sturdy or stable enough to support the trains passing over it. Derailments occurred due to the poor quality of the track. Engineers discovered that the best way to solve the problem was to place a layer of permeable material under the cross-ties and rails.
Experimentation with ballast materials revealed that the best choice for a supporting layer under the track was crushed stone. Railroads started to use crushed limestone and quartz for the track underlayment. These materials proved a great advance over using earth or coal cinders as a support system as they were much stronger and more durable. Crushed stone does an excellent job of preventing any weeds from growing on the track and also drains well. Drainage remains an important part of any railway today, which is why the outside edges of the ballast slope downward and away from the track.
Beneath the ballast is another layer of material helping called the subgrade. The overall health of the track depends on the stability of the subgrade. In the past the subgrade was often crushed stone, like the layer of ballast just under the track, but more recently railroads have started using a subgrade of pavement, such as Hot Mixed Asphalt (HMA). HMA has proved superior to the more traditional granular materials as a track subgrade, according to the University of Kentucky.
When railroad ballast gets fouled -- becomes filled with other materials such as soil or coal dust -- its structure starts to deteriorate. A study done by the Transportation Research Board in 2009 showed that fouling can lead to significant weakening of the ballast. The most damaging fouling agent was coal dust, the study found. Wet coal dust produced the most damage to the ballast material. The other fouling agents tested -- clay soil and mineral filler -- also caused the ballast to lose strength, although not as much as the coal dust.
- American-Rails.com: Ballast, Supporting The Rails and Ties
- University of Kentucky; Comparisons of Railroad Track and Substructure Computer Model Predictive Stress Values and In-Situ Stress Measurements; Jerry G. ROSE
- Transportaion Research Board; Laboratory Characterization of Fouled Railroad Ballast Behavior; Hai Huang; 2009
- Photo Credit Photos.com/Photos.com/Getty Images
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