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.

The Lias outcrop extends continuously from Dorset to Yorkshire in England, with outlying areas in Somerset and Wales. It underlies the transport routes between a number of major UK cities. Understanding the material properties of the Lias Group is therefore important for infrastructure construction and maintenance across England and Wales.This study examines the influence of weathering on the engineering properties of the Charmouth Mudstone Formation (Lias Group) in the light of recent developments in ground investigation practice including (i) the use of modern visual weathering classifications for soils and rocks and (ii) the availability of large ground investigation datasets from the construction of the High Speed Two (HS2) railway.The variability in the undrained shear strength data was consistent with the moisture content, liquidity index and plasticity index of the samples, but they were poorer indicators than shown in previous studies. The visually-assessed weathering class and the depth below ground level were found to be more useful indicators of the undrained shear strength of the clay and mudstone samples of the Charmouth Mudstone Formation.