Strength and failure characteristics of hard rock containing a single structural plane under varied loading angles: A true triaxial investigation

Abstract

The spatial relationship between structural planes and principal stresses significantly affects the mechanical properties of deep hard rock. This paper examines the effect of the loading angle under true triaxial compression. While previous studies focused on the angle β between the maximum principal stress and the structural plane, the role of angle ω, between the intermediate principal stress and the structural plane, is often overlooked. Utilizing artificially prefabricated granite specimens with a single non-penetrating structural plane, we set the loading angle β to range from 0° to 90° across seven groups, and assigned ω values of 0° and 90° in two separate groups. The results show that the peak strength is negatively correlated with β up to 45°, beyond which it tends to stabilize. The angle ω exerts a strengthening effect on the peak strength. Deformation mainly occurs post-peak, with the strain values ε1 and ε3 reaching levels 2−3 times higher than those in intact rock. The structural plane significantly influences failure mode when ω=0°, while failure localizes near the σ3 surface of the specimens when ω=90°. The findings enhance data on structural plane rocks under triaxial compression and inform theoretical research, excavation, and support design of rock structures.

1. Introduction

The structural plane, a critical component of rock masses, plays an essential role in determining strength and failure responses [1 −6]. During engineering construction, a variety of structural failures can manifest, such as unloading cracks along structural planes, block collapse, landslides influenced by structural or stress-structure interaction, and rock bursts dictated by structural constraints [7−11]. These issues are more acute in deep environments subjected to high stress, where the impact of structural planes on rock failure is more pronounced compared to shallow-buried engineering projects [12 −14]. Consequently, comprehending the fundamental mechanical behaviors of rock masses that include structural planes is essential, especially in high-stress conditions [15, 16]. This understanding is key to managing and mitigating the risks associated with complex geological conditions in deep engineering projects.

In three-dimensional stress environments, understanding the spatial relationship between structural planes and the principal stresses is crucial for determining the failure characteristics of rock mass. These relationships are defined by the loading angle β, which is the angle between the maximum principal stress and the structural plane, and the loading angle ω, the angle between the minimum principal stress and the structural plane [17, 18], as illustrated in Figure 1(a). After tunnel excavation, this relationship becomes more complex due to the interplay between structural planes, redistributed stress, and the tunnel’s empty face [19 −22]. In certain spatial configurations, these factors can lead to various types of structural rock mass disasters [23−26]. Early scholars who conducted research on this topic include JAEGER [27, 28], DONATH [29, 30], CHENEVERT and GATLIN [31], MCLAMORE and GRAY [32], and POMEROY et al [33]. Under uniaxial and conventional triaxial compression conditions, the strength of layered rocks exhibits “U-shaped”, “wavy”, and “shoulder-shaped” patterns [34] as the loading angle β changes. Moreover, the maximum strength typically occurs when β is either 0° or 90°, with β=90° being the most frequent. The minimum strength, on the other hand, often occurs between β =20° and 45° , most often occurring near β=30°. Additionally, the variation in rock specimen strength with β is influenced by confining pressure, where an increase in confining pressure typically leads to a decrease in strength anisotropy [35]. However, since the stress state achieved under conventional loading conditions is relatively simple, it cannot fully reflect the high true triaxial stress state encountered in deep engineering projects, nor can it adequately investigate the effect of loading angle ω. Therefore, its applicability to field problems is limited [17, 36, 37].