Effect of Moulding Moisture Content on the Mechanical Behaviour of Unsaturated Rockfill Material

In rockfill dams and high-fill engineering projects, rockfill materials are compacted under different moisture conditions and remain unsaturated during construction and service, yet the influence of the moulding moisture content on their subsequent mechanical behaviour has not been systematically clarified. In particular, it remains unclear whether the observed moisture-dependent response results only from the moisture condition. To address this issue, large-scale consolidated-drained triaxial tests were conducted on unsaturated rockfill specimens with different moulding moisture contents ranging from 0% to 3%, initial porosities of 19% and 21%, and fines contents (the content of particles smaller than 5 mm) of 19% and 35%. The results show that increasing the moulding moisture content reduced peak deviatoric stress, while the final strength weakening coefficient stabilised within 0.87–0.91. In contrast, the initial modulus first decreased and then recovered, and may even have exceeded that of the dry specimen. Higher moisture contents suppressed dilation under low confining pressures and promoted particle breakage and volumetric compression under high confining pressures, with both the content of particles smaller than 5 mm and the peak volumetric compression increasing markedly. The moisture content effect also depended on the initial density and fines content. Specimens with lower fines contents were more sensitive to moisture variation, whereas higher porosity weakened this effect. Distinct mechanical responses persisted after saturation, indicating that the moulding moisture content acts not only through suction during shearing but also through particle rearrangement and contact states formed during compaction. Therefore, moulding moisture content should be regarded as an important parameter in field construction and laboratory testing.