The influence of cyclic loading on the response of soft subgrade soil in relation to heavy haul railways

The design of rail tracks is often challenged by the highly compressible behaviour of soft estuarine clays over which they must pass. It is prudent that the realistic long-term behaviour of subgrade materials under repeated loading applied by fast moving heavy haul trains is properly understood. One feature that should not be ignored when estimating the long-term performance of track foundations is the continuous principal stress rotation (PSR) induced by moving wheel loads. The main purpose of this research is to combine the traditional cyclic triaxial test results (fixed axes, no PSR) with those obtained from the dynamic hollow cylinder apparatus (allowing PSR) to examine the relative influence of cyclic stress ratio (CSR) and frequency on the behaviour of soft subgrade subjected to simulated heavy haul train loading. Employing these two types of equipment applying contrasting stress path regimes, a series of cyclic undrained laboratory tests was conducted on reconstituted sandy clay specimens at varying frequencies (f = 0.1-1 Hz) and cyclic stress ratios (CSR = 0.2-0.3). The hollow cylinder test results have shown that higher CSR values and lower frequencies induce greater permanent deformations and excess pore water pressures at a given number of loading cycles (N). For CSR = 0.2, pore pressures and axial strains were found to increase even after a large number of cycles (N = 50,000). However, when the higher CSR value of 0.3 was imposed, the soil failed in less than 300 cycles by reaching 5% of axial strain. Undoubtedly, PSR adversely affected the accumulation of axial strains and soil degradation, whereas in contrast, the development of pore water pressure was less influenced by PSR.

Automated summary

The research combines traditional cyclic triaxial test results with dynamic hollow cylinder apparatus results to examine the relative influence of cyclic stress ratio and frequency on the behavior of soft subgrade. It was found that higher CSR values and lower frequencies induce greater permanent deformations and excess pore water pressures in the soil. PSR adversely affected the accumulation of axial strains and soil degradation, while pore water pressure development was less influenced by PSR.