Performance evaluation of shock mats and synthetic grids in the improvement of rail ballast
In Australia, railways offer the most prominent transportation mode in terms of traffic tonnage serving the needs of bulk freight and passenger movement. Ballast is an essential constituent of conventional rail infrastructure governing track stability and performance. However, in recent time, high traffic induced stresses due to dramatically increased train speeds and heavier axle loads cause excessive plastic deformations and degradation of ballast. This seriously hampers safety and efficiency of express tracks, for instance, enforcing speed restrictions and effecting more frequent track maintenance. The problem becomes severe under impact loading because it accelerates ballast breakage. Therefore, understanding the complex mechanisms involved with the transfer of impact loads on the substructure and their effect on ballast breakage and degradation are essential for predicting the desirable track maintenance cycle as well as improving new track design. The measurement of track settlement is well established practice in conventional track monitoring systems, however, it is also important to monitor lateral deformations (parallel to sleepers) that affect track stability especially in the absence of sufficient confinement. Therefore, a field trial was conducted on a section of rail track in the town of Bulli (north of Wollongong City) to measure deformations and cyclic stresses. It was demonstrated that in the case of wheel flats, extremely high stresses would be transmitted to the ballast bed. Installing layers of synthetic materials such as rubber pads (shock mats) in rail tracks can lead to the attenuation of high impact forces and thereby mitigate ballast degradation. In order to evaluate the effectiveness of shock mats, a series of laboratory tests using a high capacity drop-weight impact testing equipment was carried out. The field trial further proved that the moderately-graded recycled ballast when used with a geo-composite layer was found to perform better in comparison to traditionally utilized highly uniform fresh ballast, with clear implications on reduced track maintenance costs and longevity. The results of large-scale direct shear tests also revealed that the appropriate application of geogrids significantly improved the performance of ballast. This keynote paper describes in detail, the results of large-scale laboratory testing of ballast and the observations from a full-scale instrumented field trial characterising the behaviour of rail ballast improved by shock mats and synthetic grids.