Performance of Tire Cell Foundation as a Subballast Capping Layer under Cyclic Train Loading

This paper presents laboratory and field test results on the use of tire cell track foundation (TCTF) consisting of an assembly of infilled rubber tires to reinforce capping material below the ballast layer. Large-scale cubical triaxial tests were carried out with two different infill materials (crushed basalt rockfill and recycled spent ballast) and they were subjected to varying cyclic loading magnitudes and frequencies. A multistage cyclic loading was performed with and without the inclusion of tire cell reinforcement, whereby the cyclic loading was applied in four different stages with 25,000 loading cycles in each stage. In the first two stages, the frequency was increased from 10 to 15 Hz for an equivalent axle load of 25 t. For the third stage, the axle loading was increased to 35 t with a frequency of 10 Hz, which was then increased to 15 Hz in the final stage. The results showed that the TCTF could reduce the vertical stress transmitted to the subgrade layer as well as curtail the vertical and lateral displacement of the ballast layer. The TCTF further stabilized the track without any significant reduction of the resilient modulus of the overlying ballast as the loading and frequency increased. Compared to a traditional track, the TCTF showed a reduction of 40.1% and 28.3% in the breakage index for the crushed latite basalt and spent ballast (i.e., recycled from ballast tips) infilling the tire cells, respectively. Test results confirm that the TCTF can significantly improve the overall track performance, and this could be mainly attributed to the increased confining pressure provided by the tire cell assembly, as well as the enhanced damping properties of the rubber tire inclusions. In addition, the concept of TCTF was tested using a fully instrumented track (20 m long) subjected to the passage of a 22-t locomotive with two fully loaded carriages. The trial section was constructed within a maintenance yard for heavy haul rolling stock located in a western suburb of Sydney, Australia. Field measurements revealed that, compared to the standard track, the TCTF significantly reduces stress transfer to the subgrade soil. This ultimately mitigates excessive deformation and subgrade failure, making TCTF a sustainable solution for soft and weak subgrade soils despite initial settlement.