Saturation and its effect on the resilient modulus of a pavement formation material

This paper reports the effects of changing degree of saturation on the stiffness of a typical railway formation material. Material dynamically compacted to a target dry density over a range of water contents was cyclically loaded in triaxial and hollow cylinder apparatus. The results of both test types show the large effect of the degree of saturation on stiffness. In both the cyclic triaxial apparatus and the equivalent stress path in the cyclic hollow cylinder apparatus, the resilient modulus (MR) increased considerably (from ∼36 MPa to ∼467 MPa) as the degree of saturation fell. MR at a water content of 7% (optimum under the 2·5 kg rammer) was approximately 1·5 times the near-saturated value (at w = 8%). MR at the driest state tested (w = 4%) was approximately 6 times that measured for the near-saturated specimen. In the hollow cylinder apparatus, these trends were seen regardless of whether principal stress rotation was applied. The increase in MR associated with decreasing water content is thought to result from an increase in matric suction. Increased scatter in MR at higher suction may be the result of a more variable distribution of water within drier specimens. Comparison of suction stresses derived from the soil water retention curve with values back-calculated from cyclic triaxial testing suggests that useful estimates of railway formation resilient modulus, MR, may be made on the basis of measurements of matric suction.