Experimental investigation on mechanical behavior and properties of cemented paste backfill under biaxial loading condition

Abstract

Cemented paste backfill (CPB) is widely used in underground mines such as Kidd Creek Mine (Ontario) and Red Lake Operations (Ontario) for tailings disposal and ground support. However, its mechanical response is commonly evaluated using uniaxial compression or axisymmetric triaxial tests, which do not fully represent the non-axisymmetric stress conditions acting on exposed backfill structures. This thesis experimentally investigates the mechanical behavior and properties of CPB under biaxial loading conditions, with emphasis on the effects of curing time, cement content, and intermediate principal stress σ2. A custom biaxial compression testing system was developed to independently apply the major principal stress σ1 and intermediate principal stress σ2, while maintaining the minor principal stress as σ3=0. A total of 81 biaxial compression tests were conducted on cubic CPB specimens prepared with cement contents of 2.25%, 4.5%, and 9.0%, and cured for 7, 28, and 90 days. Different σ2 levels were applied to evaluate the influence of biaxial confinement on stress-strain response, volumetric deformation, stiffness, normalized strength, p-q strength envelopes, crack initiation stress, crack damage stress, and failure patterns. The results show that curing time and cement content significantly increased stiffness and peak axial stress, reflecting progressive hydration and improved cementation. The elastic modulus increased from about 4 to 172 MPa. Increasing σ2 enhanced peak strength, reduced post-peak softening, delayed crack development, and shifted failure from brittle tensile-shear cracking toward more ductile shear-dominated deformation. Normalized strength increased with the intermediate principal stress parameter b, especially for weaker and early-age CPB. The p-q strength envelopes shifted upward and rightward with increasing cement content, curing time, and confinement, indicating pressure-dependent strength behavior. Overall, this study provides new experimental evidence for the biaxial mechanical response of CPB and offers useful parameters for constitutive modelling and stability assessment of exposed backfill structures.