Experimental Study on Mechanical Properties of Rockfill Materials under Constant p and Constant b Stress Paths

[Objective] Under the actual filling conditions of high rockfill dams, rockfill materials are typically subjected to complex three-dimensional stress states. This study aims to investigate the effects of spherical stress (p), intermediate principal stress coefficient (b), and initial dry density ( ρ0) on the stress-strain relationships, strength characteristics, and non-coaxiality of stress-strain increment directions on the π-plane of rockfill materials. [Methods] Consolidated-drained true triaxial shear tests were conducted on typical rockfill materials for dam construction with constant p and constant b stress paths under three-dimensional conditions involving different p, b, and ρ0. Based on experimentally measured data, conventional strength criteria considering intermediate principal stress effects were comparatively analyzed for their applicability to rockfill material strength. [Results] The results showed that: (1) as p increased, the q- ε1 relationship curves exhibited a progressively steeper strain-hardening trend. The initial shear modulus (Ei) increased correspondingly, and peak shear strength (qmax) significantly enhanced. Shear contraction grew while dilation development was suppressed, and the failure stress ratio (Mb) gradually decreased as p increased. (2) With increasing b, both Ei and qmax progressively declined. Reduced shear contraction made the rockfill materials transition more rapidly into dilation, with increasingly significant dilatancy. Mb demonstrated a downward trend. (3) As ρ0 increased, both Ei and qmax increased markedly. Rockfill materials entered dilation earlier with progressively greater dilatancy, and Mb exhibited an upward trend. [Conclusion] The conclusions are as follows: (1) an increase in p significantly enhances the shear strength of rockfill materials, and the influence of b on its strength progressively diminishes. As ρ0 rises, both the initial shear modulus and shear strength increase markedly, highlighting the necessity for strict quality control during dam construction. (2) The Lade-Duncan strength criterion effectively characterizes the nonlinear strength characteristics of rockfill materials, while the Mohr-Coulomb criterion yields conservative predictions due to its neglect of intermediate principal stress effects. (3) Non-coaxiality between stress and strain increment directions is observed on the π-plane during testing. This non-coaxial behavior is most pronounced during the initial shear stage, and it gradually transitions toward coaxiality as the specimen approaches instability failure. (4) The non-coaxiality of rockfill materials initially increases and then decreases with increasing b. It gradually weakens with increasing p, with the weakening rate diminishing, and it intensifies with higher ρ0 with a progressively faster intensifying rate.