Reliable estimation of shear strength of rock mass is important for design of tunnels or underground structures. Especially, the assessment of shear strength of a rock joint is critical because the rock joint is one of the weakest points of rock masses due to its discontinuity. Until now, numerous works have been performed through experimental tests, but it is hard to find a microscale study focusing on asperity geometry or asperity size of a rock joint and its spatial distribution. This thesis centered on the shear behavior of rock joints with respect to micro-scale asperities. The main objectives were to identify the shear mechanism of asperities, to explore the effects of microscale characteristics of asperities on joint shear behavior, and to suggest the theoretical model for prediction of shear behavior of rock joints.
First, two failure modes of a rectangular asperity are identified depending on its shape and critical aspect ratio: one mode is a dilative failure with a failure plane of $45-φ_f/2$ and the other is a non-dilative failure with shearing of asperity. The critical aspect ratio, which is used as a failure mode criterion of a rectangular asperity, is a function of peak friction angle, cohesion, and normal stress, and is the most sensitive to peak friction angle.
Two shear mechanisms of triangular asperities are reviewed: one is a sliding mode and the other is a shearing mode. The critical inclination angle, which is used as a failure mode criterion of a triangular asperity, is a function of peak friction angle, basic friction angle, cohesion, and normal stress.
The shear strength of rectangular and triangular asperities can be determined with peak friction angle, basic friction angle, cohesion, normal stress, and geometric condition of an asperity (e.g., aspect ratio α, inclination angle θ). And also the shear behavior of rectangular and triangular asperities can be predicted with the shear modulus and sliding stiffness of an intact material.
Upon this basic analysis, the effect of asperity size distribution, asperity shape distribution, asperity strength degradation, normal stress, and specimen size are discussed on the shear behavior of idealized rock joints. Asperity size distribution has a significant influence on the shear behavior of rock joints, altering peak shear strength and shear displacement at peak. Numerical analysis results show that the joint of different asperity sizes experiences progressive failure in order of asperity size, while the joint of an identical asperity size approaches failure simultaneously. Asperity size variation renders smaller shear strength and more ductile behavior of joints. However, shear strength approaches a constant value after variation of asperity increases up to a certain level (i.e., coefficient of variation = 80 %). Asperity shape distribution also affects the shear strength. Although joints have the same average aspect ratio or inclination angle, their shear strength can vary depending on the variation of asperity shape. Numerical simulation shows that surface roughness alone may not be appropriate for reliable estimation of rock joint shear strength. Thus, a parameter for characterization of joint surface roughness should be able to reflect the effect of asperity shape distribution. As the strength of asperities is degraded, the shear strength of a joint becomes smaller. Especially, it shows a remarkable effect at the non-dilative failure of rectangular asperity and shearing mode of triangular asperity. Thus, the weathering effect should be considered on the rock surface when the asperity strength is determined.
The increase of normal stress renders stiffer slopes of curves because the shear modulus of an intact material generally increases with the increase of confining stress. This effect is more prominent as the surface of rock joint is much rougher so the asperities are sheared with breaking rather than sliding. Smaller-sized asperities contribute to peak shear strength at lower normal stresses while larger-sized asperities do at higher normal stresses. Thus, the increase of normal stress alters the evolution of rock joint shear behavior due to the progressive failure of joint asperities and makes the joint shear behavior ductile (or deformation-hardening). The mechanism of hardening behavior of a rock joint with the increase of normal stress is related to reduction of cohesion effect relative to normal stress. Decrease of peak shear strength due to increase of normal stress means the shear failure envelope should be curvature. Smaller specimens cause much bigger variation of shear strength and their shear behaviors become uniform when the number of asperity approaches or is greater than 100. Therefore, when the rock joint specimen is sampled for reliable estimation of shear strength, the specimen size has to be longer than a minimum required length (i.e., 100 times greater than mean asperity size).
For the purpose of application to a natural rock joint, the decomposition of rough profile into small-scale and large-scale roughness is effective to estimate the shear strength of undulating rock joints. Since the undulation is constrained to the regime of sliding mechanism and large displacement, the shearing effect of small-scale asperities is well captured in the overall shear behavior. The proposed model gives an adequate prediction of the shear behavior and the shear strength for natural rock joints, and especially provides a good estimation for a granite rock joint with relatively smooth roughness.
터널 및 지하 구조물의 설계 시 암반의 전단강도에 대한 정확한 평가가 필수적이며 특히 불연속면은 암반에서 가장 약한 부분이므로 절리면의 전단거동이 근본적으로 이해되어야 한다. 본 논문에서는 미시적인 관점에서 절리면의 전단 거동에 대하여 연구를 하였으며, 미소 에스퍼리티의 근본적인 전단 거동과 미소 에스퍼리트들의 특성이 전단거동에 미치는 영향을 규명하고 실제 절리면의 전단거동을 예측하기 위한 이론적인 모델을 제시하는 것을 목표로 한다.
우선 사각형과 삼각형 에스퍼리티의 전단거동이 규명되었으며, 각각의 거동 특성에 따른 전단 강도와 전단 거동을 이론적인 분석을 통해 예측하였다. 이러한 근본적인 전단거동을 바탕으로 미소 에스퍼리티들의 특성들(크기, 모양, 강도), 수직응력 그리고 시편의 크기가 전체적인 전단거동에 미치는 영향에 대하여 이론적이고 수치적인 접근을 통하여 알아보았다.
마지막으로 실제 암석의 절리면을 대상으로 전단강도와 거동을 예측해 보았다. 대상 시편의 거친면 프로파일을 획득한 후, 스케일에 따라 분해 후 각각의 거동을 분석하였다. 그 결과를 바탕으로 이론적으로 예측한 전단 거동과 실험을 통해 얻은 실제 전단거동의 결과가 매우 잘 일치하였다. 이로써 본 논문에서 미시적 관점에서의 에스퍼리티 특성을 고려한 전단거동을 규명하였으며, 제시된 이론을 바탕으로 실제 절리면의 전단 강도와 전단 거동을 잘 예측할 수 있음을 확인하였다.