Ballast contamination due to intrusion of fine particles from subgrade is characterized as a conventional source of ballast fouling. To diminish this phenomenon, sub-ballast layer is incorporated between the subgrade soil and the ballast course of railway track structure. Present study investigates the influence of subgrade type, compaction level and gradation of sub-ballast material and particle size distribution of ballast aggregate on filtration accomplishment of sub-ballast functioned as a filter layer. For this purpose, prepared structure representing placement of sub-ballast and ballast course over subgrade soil is subjected to upward water flow by exerting considerable values of hydraulic head. Both the thickness of ballast contaminated and the percentage of fine particles infiltrated among ballast aggregate are measured. Also, the hydraulic conductivity of sub-ballast aggregate is separately measured. As expected, using granular sub-ballast comprised of broader size range along with further compaction of this filter course leads to the improvement of filtration against intrusion of fine particles from underlying layer into the upper structure. Likewise, observing insufficiency in drainage of prepared sub-ballast under this extreme condition is inevitable. Furthermore, the level of ballast contamination (especially the value of fouling ratio) is higher in the case of providing ballast aggregate with more uniform particle size distribution. Finally, the fouling indices measured evidently reveal that the ballast layer is further contaminated in the case of placement over subgrade type of silt (in comparison with clay).

由于路基上的细小颗粒侵入而引起的压载物污染是压载物结垢的常规来源。为了减少这种现象，在路基土壤和铁路轨道结构的道ast之间添加了道-层。本研究研究了路基类型，压实度和子压载物的级配以及压载骨料的粒度分布对用作过滤层的子压载物的过滤效果的影响。为了这个目的，通过施加相当大的水头压力，使准备好的代表子压载物和压载物路线在路基土壤上的位置的结构经受向上的水流。测量了压载物的厚度和压载物骨料中渗透的细颗粒的百分比。也，副压载骨料的水力传导率是单独测量的。如所预期的，使用具有更宽尺寸范围的粒状子镇流器以及该过滤器路线的进一步压实导致过滤的改进，以防止细颗粒从下面的层侵入上部结构。同样，在这种极端条件下观察已准备好的子镇流器的排水不足也是不可避免的。此外，在提供具有更均匀粒度分布的压载聚集体的情况下，压载污染物的水平（特别是结垢率的值）更高。最后，测得的结垢指数显然表明，如果放置在路基类型的淤泥上（与粘土相比），则压载层会进一步受到污染。