Protocol for Testing Fouled Railway Ballast in Large-Scale Cyclic Triaxial Equipment

Abstract

A testing protocol was developed to measure accumulation of vertical plastic deformation in fouled railway ballast under cyclic traffic loading. Large-scale cyclic triaxial (LSCT) test equipment was constructed to test fouled ballast under various stress conditions. A method of introducing fouling material and moisture to ballast specimens was critical to the resulting deformational behavior. Specimens for LSCT testing should be prepared by mixing ballast with relatively dry fouling material (moisture <5 % for granular fouling and <15 % for clay-based fouling) prior to compaction to prevent a heterogeneous distribution of fouling within the specimen. A full-scale track model experiment (FSTME) was built to determine a representative state of stress (RSS) for railway ballast for use in the LSCT testing. The RSS allows for testing of ballast under consistent stresses to compare the effect of material characteristics of fouling such as fouling content and moisture content. A RSS of 90 kPa (confining) and 300 kPa (deviator) for a train axle load of 264 kN was suggested based on the FSTME results. Measured deformation of fouled ballast using the proposed testing protocol was compared with a published vertical deformation of railway track in a track test section. Results of this study indicate that the proposed testing protocol can simulate the vertical plastic deformation of railway ballast at a specified stress level.

Deformational behavior of fouled railway ballast

“…3. Loading frequency is dependent on train speed and shape of the stress distribution at different depths of ballast and varies widely as described by Ebrahimi et al (2012). In this study, cyclic triaxial testing was performed at a loading frequency of 5 Hz to simulate freight train speed of 64 km/h.…”

“…The N = 2 × 10 5 was selected in our preliminary tests to capture the effect of fouling conditions on long-term accumulation of plastic strain of ballast under traffic load. A protocol for testing fouled railway ballast under cyclic loading is presented by Ebrahimi et al (2012).…”

“…The concept of contamination of contact points of ballast was also visually verified by cutting several cross sections of frozen fouled ballast, as presented in Ebrahimi et al (2012). To evaluate the presence of fouling at the contact points of ballast particles, the macro void space created by coarse ballast particles in clean and fouled ballast is compared.…”

“…Clean ballast was compacted to a maximum dry unit weight of 15.8 ± 0.3 kN/m 3 . When preparing a mixture of fouled ballast, the method of introducing fines is critical, as discussed in Ebrahimi et al (2012). To prepare fouled ballast specimens for LSCT testing, the specimens were prepared by mixing clean ballast with relatively dry fouling materials (water content <5% for noncohesive and <15% for cohesive fouling) prior to compaction to prevent layered mixtures and to simulate the tamping activities in the track (Ebrahimi et al 2012).…”

“…Moisture was then added from the top of specimens to study deformational behavior of ballast at a target water content. Ebrahimi et al (2012) examined several methods for preparing fouled ballast specimens. A combination of tamping and vibration was found to provide the best result with respect to uniformity and homogeneity of the specimens.…”