Some factors affecting the cyclic stiffness of railway formation material

Engineered pavements are a key part of the transport asset that includes flexible road pavements, railway formations and airport runways. Their design, construction and maintenance need to be economically and environmentally optimised, while meeting serviceability criteria over the design life of the pavement. The main serviceability criteria used for railway track design are the permanent deformation and track stiffness. Subgrade stiffness influences the design depth of ballast and compacted formation layers are used to improve stiffness and reduce stresses induced in the natural subgrade. The track stiffness also influences vehicle/track interaction and wear, energy required for movement, and ride quality. Knowledge of the stiffness is required for safe and economic design of the railway track structure. Several factors influence the stiffness or resilient modulus of compacted formation material including stress state, material type, and soil physical state (density and water content). A laboratory investigation was conducted on a representative subballast material, at a range of water contents, using a Cyclic Triaxial Apparatus and Cyclic Hollow Cylinder Apparatus to investigate these aspects. The resilient modulus of the clayey sand material reduced as the applied deviator stress increased, and increased under higher confining stresses and matric suction. It was found that the resilient modulus increased by a factor of six due to a reduction in water content, and thus the increase in matric suction. Principal stress rotation, representative of train induced stresses, reduced the measured resilient modulus by approximately 13% for optimum water content specimens with reducing influence for dry of optimum specimens. The results from this work emphasise the benefit of matric suction, and thus the need for well drained formation layers. They also highlight the risk of reduced stiffness and increased deformation of railway formation subject to long periods of rainfall or wetting and the potential impact of predicted climate change. Simplified stress path testing using a Cyclic Triaxial Apparatus overestimates resilient modulus, especially for saturated material subject to large number of cycles. However, the use of more advanced testing of the resilient modulus using the Cyclic Hollow Cylinder Apparatus is only suggested where high cyclic shear stresses are expected.