Over-consolidation effect on shear behavior of rock joints

Although many researchers have studied the normal and shear behavior of fractures under stresses, the over-consolidation effect on the slip/shear behavior of discontinuities has not been considered. The over-consolidation behavior of non-planar rock fractures should be considered when deposition–consolidation–erosion (or excavation) sequences occur. Plaster replicas of representative natural rock joint surfaces were prepared for this study. In this case, the surface roughness and other geometrical properties remain constant during the laboratory direct shear tests. It was observed that the shear strength within a large range of roughness, joint wall strength and normal stress values significantly increases with increasing over-consolidation ratio. According to the test results, a new model is developed as an extended form of Barton's shear failure criterion for rock joints. This model considers the effect of various paths of normal loading/unloading before shearing and over-consolidation ratio in a fracture. A new joint over-closure (JOC) parameter is also introduced as the ratio of closure in over-closed to normally closed conditions.     

Effect of roughness on the shear behavior of rock joints subjected to impact loading

The shear behavior is regarded as the dominant property of rock joints and is dramatically affected by the joint surface roughness. To date, the effect of surface roughness on the shear behavior of rock joints under static or cyclic loading conditions has been extensively studied, but such effect under impact loading conditions keeps unclear. To address this issue, a series of impact shear tests was performed using a novel-designed dynamic experimental system combined with the digital image correlation (DIC) technique. The dynamic shear strength, deformability and failure mode of the jointed specimens with various joint roughness coefficients (JRC) are comprehensively analyzed. Results show that the shear strength and shear displacement characteristics of the rock joint under the impact loading keep consistent with those under static loading conditions. However, the temporal variations of shear stress, slip displacement and normal displacement under the impact loading conditions show obviously different behaviors. An elastic rebound of the slip displacement occurs during the impact shearing and its value increases with increasing joint roughness. Two identifiable stages (i.e. compression and dilation) are observed in the normal displacement curves for the rougher rock joints, whereas the joints with small roughness only manifest normal compression displacement. Besides, as the roughness increases, the maximum compression tends to decrease, while the maximum dilation gradually increases. Moreover, the microstructural analysis based on scanning electron microscope (SEM) suggests that the roughness significantly affects the characteristics of the shear fractured zone enclosing the joint surface.     

Evaluating damage during shear tests of rock joints using acoustic emissions

The acoustic emission (AE) signals generated during direct shear test were evaluated on different types of joints (rock–rock, rock–concrete and concrete–concrete). Several boreholes were cored from a dam body, rock mass and interface between dam and rock mass, and the samples were prepared and tested under direct shear test. A laser profilometer scanner was used for scanning the joint surfaces in order to assess surface roughness. By correlating the AE signals with the shear graphs one can predict the starting point of shearing during direct shear test. Count and energy parameters were analyzed in two different methods to monitor the shear behavior of the joints: a graph of the count and energy rates, and a graph of cumulative count and energy. Four separated periods were observed for bonded and non-bonded joints: linear pre-peak period, non-linear pre-peak period, post peak period and residual period. This study showed that AE has enough accuracy to monitor the shear behavior of the joints and it can be used in site confidently.     

Numerical study of shear behavior of intermittent rock joints with different geometrical parameters

The purpose of this paper is to investigate shear behavior of rock specimens containing several intermittent joints with different geometrical parameters. Through three series of direct shear tests based on rock failure process analysis (RFPA^2D) code, the whole shear failure process is visually observed and the failure patterns in reasonable accordance with other experimental results are obtained. In general, the failure pattern is mostly influenced by the geometrical parameters of rock joints while shear strength is closely related to the failure pattern and its failure mechanism. It is observed that the propagation of wing cracks depends on the joint separation and the joint azimuth angle, and the connection of wing cracks dominates the eventual failure pattern and determines the peak shear load of the rock specimens. The results also show that macro shear fracture is due to the mesoscopic tensile damage of a large number of elements. This numerical study has made a more reasonable theoretical explanation for the shear mechanism of jointed rock due to the inner damages that are difficult to observe in an experimental testing.     

Experimental study of the hydro-mechanical behavior of rock joints using a parallel-plate model containing contact areas and artificial fractures

In recent years, geological disposal of radioactive wastes is considered to be the most promising option, which requires the understanding of the coupled mechanical, hydraulic and thermal properties of the host rock masses and rock fractures. The hydro-mechanical behavior and properties of rock fractures are usually determined by laboratory experiments on fracture specimens that serve as the basic building block of the constitutive models of fractured rock masses.Laboratory testing of rock fractures involve a number of technical issues that may have significant impacts on the reliability and applicability of the testing results, chief among them are the quantitative estimation of the evolutions of hydraulic transmissivity fields of fractures during shear under different normal constraint conditions, and the sealing techniques when fluid flow during shear is involved. In this study, a new shear-flow testing apparatus with specially designed fluid sealing techniques for rock fractures were developed, under constant normal load (CNL) or constant normal stiffness (CNS) constraint. The topographical data of all fracture specimens were measured before testing to constitute the geometrical models for simulating the change of mechanical aperture distributions during shearing. A number of shear-flow coupling tests were carried out on three kinds of rock fracture specimens to evaluate the influence of morphological properties of rock fractures on their hydro-mechanical behaviour. Some empirical relations were proposed to evaluate the effects of contact area and surface roughness on the behavior of fluid flow through rock fractures.     

Influence of asperity degradation on the mechanical behavior of rough rock joints under cyclic shear loading

A cyclic shear testing system was established to investigate the mechanical behavior of rough rock joints under cyclic loading conditions. Laboratory cyclic shear tests were conducted for two joint types of Hwangdeung granite and Yeosan marble: saw-cut and split tensile joints. Prior to test, the roughness of each specimen was characterized by measuring the surface topography using a laser profilometer. Several important aspects of cyclic joint behavior, such as high peak shear strength and non-linear dilation in the first loading cycle, different frictional resistance for the reversed shear loading direction, and anisotropic shear behavior and its dependence on the normal stress level were identified from the cyclic shear test results. These features and their variations in the subsequent loading cycles are mainly due to the effect of second order asperities and strength of rock material. It was also observed from experimental results that degradation of asperities under cyclic shear loading also followed the exponential degradation laws for asperity angle and the mechanism for asperity degradation would be different depending upon the shearing direction and the type of asperities. Based on the experimental results an elasto-plastic constitutive model, which can consider the degradation of second order asperities, was proposed. Numerical simulations for the monotonic and cyclic shear loading indicated agreement with the laboratory test results.     

Breakage and shear behaviour of intermittent rock joints

The breakage and shear behaviour of intermittent rock joints have been investigated in a series of direct shear tests with a new shear device, specifically designed for this purpose. The tests have been performed on specimens of rock-like material or hard rock, respectively, incorporating idealized non-persistent joints, made up of a number of short cracks in an en-échelon arrangement along the central shear axis.The shear behaviour of such a joint constellation has been found to be composed of different phases. The first phase of shearing is that of the actual rupture, initiated by the formation of wing cracks, starting from the existing cracks and growing into the material bridges, and concluded by the generation of additional new fractures connecting the initial cracks in the zone between the wing cracks. The second phase of shearing is characterized by friction processes and volume increase in the then continuous shear zone. Finally, the third phase of shearing, reached after large shear displacements, is determined by sliding processes inside the strongly fractured shear zone.In a large number of shear tests the geometrical parameters of the discontinuous joints as well as the loading conditions have been found to influence the activated shear resistance in each phase of shearing to a noticeably different extent. The orientation of the initial cracks and the normal stress, however, have been identified as the most influential parameters. Depending on the test conditions, an initially discontinuous rock joint can activate the largest shear resistance not just before rupture but in one of the two subsequent phases of shearing as well.The mechanisms which govern the different shear phases could be identified as (1) tensile rupturing, (2) rolling and sliding friction of dilatant joint zones and (3) sliding within the joint filling composed of brecciated material.     

非贯通节理岩体直剪贯通模型和强度研究

基于Lajtai和Jennings理论,研究了直剪应力状态下共面闭合非贯通节理岩体的受力特点,将非贯通节理岩体破坏分为三种模型;分析了各种模型的破坏机理;通过修正Lajtai理论,建立了非贯通节理岩体的贯通破坏强度准则。重点推导了岩桥以拉剪复合破坏时,非贯通节理岩体的峰值强度公式。通过与前人的试验比较,表明提出的破坏机理能较好地解释试验现象,理论计算的峰值强度与试验实测值吻合较好。     

A theoretical model for rock joints subjected to constant normal stiffness direct shear

The authors have conducted an investigation into the behaviour of rock joints subjected to direct shear. Both concrete/rock and rock/rock joints were investigated. The behaviour of rock/rock joints is important for the assessment of stability issues involving rock masses (e.g. rock slope stability). Concrete/rock joints are vital to the assessment of performance of concrete piles socketed into rock, rock anchors and concrete dam foundations.This investigation included an extensive series of direct shear tests under a range of stress boundary conditions. The rock used for the tests was a soft artificial siltstone, called Johnstone. The results from the tests on concrete/Johnstone joints have been presented in Seidel and Haberfield (Geotech. Testing J. (2002), accepted for publication) and on Johnstone/Johnstone joints in Fleuter (MEngSc Dissertation, Department of Civil Engineering, Monash University, Australia, 1997) and Pearce (Ph.D. dissertation, Department of Civil Engineering, Monash University, Australia, 2001, in preparation). This paper describes the theoretical models developed to simulate the observed behaviour, including asperity sliding, asperity shearing, post-peak behaviour, asperity deformation and distribution of stresses on the interface. These models have been combined into a micro-mechanical simulation of joint shear. Comparisons between program predictions and measured performance are presented and discussed.     

一种考虑剪切速率的粗糙结构面剪切强度准则

动荷载(不同剪切速率)下的节理岩体结构稳定性是目前工程建设亟待解决的问题之一。以黄岛国家石油储备库地下水封石油洞库工程为工程背景,采用JAW-600岩石剪切流变–渗流耦合试验机对4种粗糙度下的类岩石粗糙结构面进行不同剪切速率下的常规剪切试验,对结构面的强度特性进行了基础性研究。研究结果表明:剪切速率和粗糙度对结构面剪切强度都有较为明显的影响;同一结构面的剪切强度随着剪切速率的增大而减小,并不受粗糙度系数的影响;同时结构面剪切强度随粗糙度系数增加呈现出线性增加的趋势,但不受剪切速率的影响。最后,基于试验结果与Barton模型,提出考虑剪切速率的粗糙结构面剪切强度模型。Barton标准粗糙剖面线结构面及粗糙结构面的试验结果与新模型预测值误差保持在20%之内,平均误差为10.539%,能够较好的预测粗糙结构面动荷载条件下剪切强度。     

Effect of opening on the shear behavior of a rock joint

Irregularity of rock joints is an important parameter which can enhance the stability of potentially removable rock blocks or wedges around an underground opening. When irregular-sided blocks or wedges are bounded by joints, additional shear strength develops as a consequence of the confinement. With open joints, the interlock effect is reduced or lost and the shear strength drops, possibly to its residual value. A joint opening model has been developed that considers the effect of strength loss along the joint as a result of opening. The model relates the geometric parameters and stress–displacement and can easily be incorporated into existing joint models. Several numerical simulations of direct shear tests are presented to demonstrate the performance of the joint opening model.     

节理的剪切力学性质与含三维形貌参数的剪切强度准则比较研究

利用水泥砂浆材料浇注3组不同表面形貌的节理试件,由常法向应力下的直剪试验研究节理的剪切力学性质,并分析法向应力、三维形貌特征对抗剪强度的影响。直剪试验结果表明：峰值剪胀角与法向应力成反变化关系,与粗糙程度呈正变化关系;峰值抗剪强度与法向应力、粗糙程度均呈正变化关系。分析了JRC-JCS准则计算值偏低于试验值的原因,根据试验现象建议采用三维形貌参数、抗拉强度描述节理的剪切强度。对比分析了含三维形貌参数的峰值抗剪强度准则,建议低法向应力水平下采用双曲线形式的峰值剪切强度准则估算岩石节理的峰值抗剪强度。     

Numerical simulation of a direct shear test on a rock joint using a bonded-particle model

Rock joints were numerically simulated, and an extensive series of direct shear tests were carried out using the code PFC3D. The feasibility of reproducing a rock joint using the contact bond model was demonstrated, and the effects of the geometrical features and the micro-properties of a joint on its shear behavior were examined. Asperity failure was observed from the micro-cracks and contact force distribution, as well as the stresses and displacements in shear and normal directions. A rough joint with a joint roughness coefficient (JRC) value ranging from 10 to 20 was produced in an intact sample by defining the joint-contacts along a predefined joint surface. To simulate a decrease in joint wall strength (JCS) caused by weathering and alterations, the bond strength between particles involved in the joint-contacts was reduced by up to 70%. The shear behavior and failure progress at a given stress corresponded well to those observed in laboratory tests. The friction coefficient was the most important factor governing the shear strength and dilation angle. The variation in joint roughness and contact bond strength had a larger effect on the cohesion than peak friction angle. In addition, a new approach to represent JRC and JCS values of a joint was proposed for practical use. A numerical 3D-profile scanning technique was developed to evaluate the actual JRC of the simulated joint, and the relationship between the JCS and the contact bond strength was investigated.     

剪切荷载下贯通结构面应变能演化机制研究

 开展贯通结构面剪切荷载下应变能演化规律研究,有利于更好地理解岩体复杂的力学行为。以重庆武隆鸡尾山滑坡岩体结构面为例,综合利用三维激光扫描、红外热成像以及离散元数值模拟等技术,开展岩体结构面室内与数值试验,获取剪切破坏全过程应变能演化规律。研究结果显示：(1) 贯通结构面剪切破坏时,能量集聚发生在岩体结构面表面,其中面向剪切方向的部位,能量集聚相对较多,证明这些部位在剪切破坏过程中起到主要抗剪作用;(2) 贯通结构面上下盘接触部位在剪切破坏前,发生能量集聚现象,弹性应变能增加,当增加到能量极限时,接触部位岩体被剪断破坏,能量释放,应变能减小。并且不同部位的应变能演化规律不一样,结构面中部位置剪切破坏时上下盘可能发生多次摩擦,能量演化也交替出现集聚与释放现象;(3) 红外热成像试验温度分布结果与数值模拟弹性应变能结果具有较好的一致性,说明数值模拟数据真实可靠。该研究成果对完善岩体不同荷载下的能量演化机制做出了一定贡献。     

Experimental study on the shear performance of quasi-NPR steel bolted rock joints

Quasi-NPR (negative Poisson’s ratio) steel is a new type of super bolt material with high strength, high ductility, and a micro-negative Poisson’s effect. This material overcomes the contrasting characteristics of the high strength and high ductility of steel and it has significant energy-absorbing characteristics, which is of high value in deep rock and soil support engineering. However, research on the shear resistance of quasi-NPR steel has not been carried out. To study the shear performance of quasi-NPR steel bolted rock joints, indoor shear tests of bolted rock joints under different normal stress conditions were carried out. Q235 steel and #45 steel, two representative ordinary bolt steels, were set up as a control group for comparative tests to compare and analyze the shear strength, deformation and instability mode, shear energy absorption characteristics, and bolting contribution of different types of bolts. The results show that the jointed rock masses without bolt reinforcement undergo brittle failure under shear load, while the bolted jointed rock masses show obvious ductile failure characteristics. The shear deformation capacity of quasi-NPR steel is more than 3.5 times that of Q235 steel and #45 steel. No fracture occurs in the quasi-NPR steel during large shear deformation and it can provide stable shear resistance. However, the other two types of control bolts become fractured under the same conditions. Quasi-NPR steel has significant energy-absorbing characteristics under shear load and has obvious advantages in terms of absorbing the energy released by shear deformation of jointed rock masses as compared with ordinary steel. In particular, the shear force plays a major role in resisting the shear deformation of Q235 steel and #45 steel, therefore, fracture failure occurs under small bolt deformation. However, the axial force of quasi-NPR steel can be fully exerted when resisting joint shear deformation; the steel itself does not break when large shear deformation occurs, and the supporting effect of the jointed rock mass is effectively guaranteed.