The failure characteristics of layered rocks are a vital issue for stability evaluations of underground engineering projects. Layered rocks in a restricted true three-dimensional (3D) stress state behave substantially differently from those under uniaxial or conventional triaxial stress. In this study, true triaxial experiments are carried out to investigate the mechanical and volumetric fracturing behaviors of layered composite sandstones (LCSs) with a change in grain size (Cgrain size) between the layers. Under true triaxial stress, the strengths of LCSs increase considerably as the Cgrain size value increases, which is first reported. The strength enhancements of LCSs with increased σ2 (intermediate principal stress) and Cgrain size values are related to the enhanced grain interlocking effect and the interface effect between the layers. As σ2 increases, the volumetric fracturing behaviors of LCSs change from 3D failure toquasi-3D failure characterized by delamination fracture. The effects of σ2 on fracture are revealed by quantifying the cracking morphologies on the layer interface. Aided by the nuclear magnetic resonance (NMR) technique, the correlations of rock porosity with the σ2 and Cgrain size values are established. This paper promotes the understanding of the failure mechanisms of LCSs with a contrast in grain size under true triaxial stress and helps interpret field observations.
Highlights: True triaxial compression experiments are performed to investigate the mechanical and volumetric fracturing behaviors of layered composite sandstones (LCSs).The effects of the contrast in grain size (Cgrain size) between the layers and intermediate principal stress (σ2) on the rock behaviors are investigated.The correlations of rock porosity with Cgrain size and σ2 are revealed with the nuclear magnetic resonance (NMR) technique.It is first reported that the strength of LCSs under true triaxial stress increases considerably as the Cgrain size value increases.The volumetric fracturing behaviors of LCSs change from 3D failure to quasi-3D failure characterized by the delamination fracture as σ2 increases.