Experimental Study on Source Failure Mechanism and Acoustic Emission Characteristics of Red Sandstone Under Uniaxial Compression

In order to study the rock fracture mechanism and precursor characteristics, uniaxial compression experiments of red sandstone were carried out. Using acoustic emission technology and digital speckle correlation method as experimental observation means, the evolution characteristics of deformation field and acoustic emission index during rock deformation were studied. The results show that : (1) The deformation concentration of rock deformation localization zone is the main cause of nonlinear evolution of rock stress-strain curve. (2) The volume parameters of different types of cracks in rock acoustic emission change with the relative displacement rate and dislocation rate of deformation localization zone. (3) In terms of failure types, there are more high-frequency components of tensile fracture main frequency, more low-frequency components of shear fracture main frequency, and wider distribution of mixed fracture main frequency. In the time sequence, the spectrum distribution of acoustic emission signals is wide and the amplitude is small at the sudden change time. At the sudden change time, the spectrum distribution of acoustic emission signals becomes narrow, the amplitude increases, and the spectrum distribution of peak points is greatly narrowed. Therefore, it is considered that the spectrum distribution is greatly narrowed can be used as an early warning precursor.

Deformation Characteristic of a Supported Deep Excavation System: A Case Study in Red Sandstone Stratum

A complete case record of a deep foundation pit with pile-anchor retaining structure excavated in red sandstone stratum is presented in this study. The horizontal displacement of pile top, the horizontal displacement at various depths, the axial force of anchor cable, and ground settlement during construction are measured. A three-dimensional numerical model is established to analyze the additional stress and deformation induced by the excavation and the accuracy of the FEM model is verified by comparing with field measured results. Both the measured and numerical simulation results show that the deformation of the pile-anchor supported deep excavation is significantly affected by the spatial effect. The results show that the deformation in the middle of the foundation pit is greater than the pit angle and that the deformation of the long side is greater than that of the short side and gradually decreases from the middle to the pit angle. The deformation and stress in the middle of the long side of the foundation pit are the largest, which is the most unfavorable part. With the increase of vertical excavation depth, the spatial effects tend to increase, and the influence scope of spatial effects is about five times the vertical excavation depth in the red sandstone stratum. The ground settlement outside the pit is mainly distributed in a groove shape, and the maximum settlement occurs about 8.5 m away from the pit edge. Finally, parametric studies of reinforcement parameters indicated that 1.5–2.0 times the initial elastic modulus and cohesive force of soil should be used for reinforcement. It is recommended that the ranges for pile diameter, pile spacing, anchor cable prestressing and inclination angle should be selected as 0.8–1.2 m, 1.4–2.0 m, 100–150 kN, and 10°–20°, respectively.

Fracture Morphology Analysis of Frozen Red Sandstone under Impact

The deformation and failure characteristics of red sandstone under subzero temperature were studied by the split Hopkinson pressure bar (SHPB) dynamic impact test. The effects of different subzero temperatures on rock strength properties, fractal dimension, and dissipated energy were analyzed combined with microfracture morphology. The reasons for rock dynamic mechanical property deterioration under lower subzero temperatures were revealed. The research shows that low subzero temperature will cause “frostbite” of red sandstone. Under high strain rate loading, the rock will quickly lose its bearing capacity, and its dynamic mechanical strength will drop sharply. The dissipated energy W L of the frozen rock specimen is positively correlated with the fractal dimension D and closely related to the macroscopic failure characteristics. It could be concluded that greater dissipation energy leads to more serious damage of rock and accordingly results in a larger fractal dimension. Fracture morphology analysis shows that the lower subzero temperature generated remarkable cracks in the material interface of the red sandstone. The damage of the red sandstone could be explained by the fact that the crack tip had low plastic deformation ability under high strain rate loading and the composition of cement was vulnerable to the subzero temperature effect.

Nonlinear Young’s Modulus of New Red Sandstone: Experimental Studies

Young’s modulus of New Red Sandstone was investigated experimentally to gain insight into its nonlinear nature. A large experimental programme was carried out by applying a controllable quasi-static and dynamic uniaxial loading to 286 dry sandstone samples of four different sizes. The static and dynamic tests, similar to those aiming at determining the uniaxial compressive strength, were conducted using the state-of-the-art experimental facilities at the University of Aberdeen including a custom-built small experimental rig for inducing a dynamic uniaxial compressive load via a piezoelectric transducer. The obtained results have confirmed a complex nature of Young’s modulus of sandstone. Specifically, under a harmonic dynamic loading, it shows strongly nonlinear behaviour, which is hardening and softening with respect to frequency and amplitude of the dynamic loading, respectively.