Changing climatic conditions worldwide are causing changes in moisture conditions of railway formations and slopes, thereby either strengthening or weakening them. Current railway formation design methods do not take into account the changing moisture conditions over the predicted life of a railway formation. These changes in soil strength are due to the influence of soil suctions. The basic principles of unsaturated soil mechanics are well established in the field of geotechnics, and this study joins an international body of work that seeks to apply unsaturated soil theory to the field of railway and pavement formation materials.
This study comprised box testing of subgrade and subballast formation materials at different moisture contents to demonstrate the behaviour of these two materials relative to each other. The formation models were instrumented with tensiometers to monitor the effect of cyclic loading and loading frequency on material behaviour and soil suction under typical heavy haul loading (26 tonne/axle). The study also investigated the evolution of suctions present in railway formation materials as well as practical aspects relating to suction measurement under cyclic loading conditions.
Suctions were successfully measured in both the subgrade and subballast materials subjected to cyclic loading, demonstrating the different suction magnitudes generated by the materials under typical railway formation conditions. The well-drained subballast material was found to generate low magnitudes of suction over a large range of degrees of saturation (1 kPa – 15 kPa), while the less well-drained subgrade material was found to generate a greater range of suctions (1 kPa – 95 kPa) with a smaller variation in the degree of saturation.
The deformability of the subgrade material depended to a greater on the moisture state and soil suctions compared to the subballast material and showed a large variation in deformability as a function of its moisture state. At its wettest, the subgrade material experienced deformation exceeding the allowable failure limit, while the subgrade material deformed less than the subballast material at its driest. The subballast deformation was significantly affected by the loading frequency and less by its moisture state due to the low suctions present.
The findings of this study have practical implications for both in-situ and laboratory testing of railway formation materials and emphasized the importance of the moisture state of the formation during the railway service life.