充填节理破坏机理及实验

充填节理的破坏主要有充填物破坏和充填物与节理接触面破坏两种形式,其抗剪强度与最小抗剪部分的强度相同.当充填物破坏时,单独对充填物部分进行受力分析,得到此时的节理抗剪强度公式.讨论了充填节理强度随法向压应力及充填厚度的变化规律:随着法向压应力的增加先增大后减小,但随充填厚度的变化并不十分明显.对不同厚度的砂浆充填节理进行直剪试验.结果表明:其剪切强度与破坏模式有关;在同一破坏模式下,不同充填厚度的节理其剪切强度变化不大.      

Numerical Implementation of Anisotropic Continuum Model for Rock Masses

In this technical note, rock mass has been regarded as an equivalent anisotropic continuum. Constitutive relationships for the rock masses have been derived. The approach aims to capture the overall behavior of the rock mass based on the constitutive characteristics of intact rock and rock joints including their orientation, spacing, roughness (waviness), number of joint sets, block size, and normal and shear stiffness, etc. This model has been applied to analyze a tunnel using a software package developed for the purpose, with satisfactory results.     

Field and Laboratory Testing of St. Peter Sandstone

The purpose of this project was to evaluate mechanical properties of St. Peter sandstone by in situ testing, and to compare the field data with laboratory results. Direct shear tests were conducted to evaluate the strength-dilatancy behavior, and thin-section microscopy was used to help explain the significant friction angles associated with the material. St. Peter sandstone is nearly cohesionless, but it possesses a friction angle of 57–63° at low confinement. The large angle of internal friction at failure may be due to locking of sand particles or to postdepositional quartz overgrowths. Tests on pulverized densely packed sand and loosely packed sand were conducted in the same manner as the intact specimens and yielded friction angles of about 42 and 34°. Pressuremeter tests were performed in situ and the results were interpreted using an elasto-plastic analysis in terms of total stresses. By appropriate consideration of system stiffness, Young’s modulus was found to be about 0.5 GPa, slightly lower than the laboratory value, although unload-reload cycles were not attempted. Assuming associative behavior, the friction angle was estimated to be at least 56°.     

Comparison between Models of Rock Discontinuity Strength and Deformation

One important component in the design of tunnels in urban areas is a correct assessment of the interaction between the underground excavation with other structures in the vicinity. In this sense a correct stress-strain response by the model representing the rock mass behavior is essential. The shear and normal displacement of rock discontinuities and their shear and normal stiffness control the distribution of stress and displacement within a discontinuous rock mass. In conditions where an equivalent continuum based approach is not applicable, the joint material model should be able to describe important mechanisms such as asperity sliding and shearing, post-peak behavior, asperity deformation, and the effect of soft infilling. The distinct element code UDEC was used to simulate the direct shear tests on a natural joint profile, and the prediction of two existing models of discontinuity strength and deformation were then compared with a new soil-infilled joint model and with experimental data for clean and soil-infilled rock joints. A numerical modeling of a cavern excavated in a jointed medium is also presented to illustrate the response of different models. The proposed soil-infilled joint model described more comprehensively the occurrence of dilation and compression with lateral displacements and also better represented the double peak shearing in relation to the adopted squeezing mechanism that could not be captured by the two existing models.     

Residual Strength and Fracture Energy from Plane-Strain Testing

Field observations indicate that failure in soft rock is often associated with a slip surface or shear band, where deformation is concentrated in a narrow zone; displacements occur with decreasing stress within the shear band, whereas outside the band the material appears to be intact. In examining the propagation of the shear band, it is useful to establish the relation between shear stress and slip displacement. This was accomplished within a laboratory setting with a plane-strain compression apparatus, developed to study localized failure under controlled conditions. Tests on a soft rock, a sandstone with a uniaxial compressive strength of 10 MPa and a modulus of 2 GPa, were conducted to estimate fracture energy GIIC, a quantity used to evaluate energy dissipation of the failure process. GIIC was found to decrease by a factor of 3 when considering the actual displacements, rather than assuming tangential displacement only, that is, no displacement normal to the shear band. The experiments showed that the shear band was not completely formed until after peak strength and that sliding along the band during softening was associated with compaction; residual behavior exhibited virtually no volume change. The shear strength at peak stress was nonlinearly related to the normal stress, but the shear strength at the residual state displayed a linear relationship. For normal stresses less than the uniaxial strength, those typical of civil engineering practice, the response can be described as cohesion softening, with friction remaining constant in going from the peak to the residual stress states.     

Plasticity Model for Stress-Release Induced Damage

Stress-release damage in rocks may result in a permanent alteration of the rock properties, which has to be quantified and modeled in order to describe the in situ behavior of the rock based on tests from core specimens. A dual yield surface elastoplastic model is introduced to describe stress-release damage in a synthetic sandstone manufactured as an analogue to a reservoir sandstone. The synthetic sandstone, composed of sand and quartz cement, is cemented under in situ stress conditions. This process allows the quantification of damage during a stress-release that simulates the coring process. In the model, one yield surface describes the behavior of the sand matrix and the other the behavior of the cement, while the total stresses are given as in mixtures theories. The model is calibrated on synthetic sandstone test results and used for back analysis of the experiments.     

不同受力维度下红砂岩失稳评判指标研究

为了研究不同受力维度下岩石峰值应力、峰值应变与失稳的关系,分别对红砂岩进行一维、二维、三维受力条件下的单轴抗压试验、变角板剪切试验和直接剪切试验以及三轴压缩试验。试验结果显示：单轴压缩试验中,峰值应力和峰值应变变异系数分别为6.61%和9.36%;变角板剪切试验中,峰值应力和峰值应变平均变异系数分别为5.69%和19.81%;直接剪切试验中,峰值应力和峰值应变平均变异系数分别为4.32%和14.74%;三轴压缩试验中,峰值应力和峰值应变平均变异系数分别为6.03%和7.44%。上述研究表明：当红砂岩处于一维和三维受力状态时,峰值应力和峰值应变均可以作为评判红砂岩是否失稳的指标,但峰值应力比峰值应变作为评判指标的可靠性更高;当红砂岩处于二维受力状态时,只有峰值应力能作为评判红砂岩是否失稳的指标。上述研究规律不仅对评判室内试验中岩石是否失稳具有重要意义,而且对评判岩土工程中岩石是否处于安全状态提供理论依据。     

Practical Equivalent Continuum Model for Simulation of Jointed Rock Mass Using FLAC3D

A simple practical equivalent continuum numerical model for simulating the behavior of jointed rock mass has been extended to three-dimensional using FLAC3D. This model estimates the properties of jointed rock mass from the properties of intact rock and a joint factor (Jf), which is the integration of the properties of joints to take care of the effects of frequency, orientation, and strength of joint. A new FISH function has been written in FLAC3D specifically for modeling jointed rocks using the Duncan and Chang hyperbolic model. This model has been validated first with simple element tests at different confining pressures for different rocks with different joint configurations. Explicit modeling of the joints has also been done in element tests and results obtained compare well with the results of equivalent continuum model and also with experimental results. Further, this has been applied for a case study of a large underground power house cavern in the Himalayas. The analysis has been done under various stages of excavation, assigning a null model available in FLAC3D for simulating the excavation.     

Shear Banding in Sandstone: Physical and Numerical Studies

The development of a localized damage zone or shear band in rock was studied through numerical and physical experiments. A laboratory biaxial (plane strain) compression test was conducted on a prismatic specimen of Berea sandstone. The shear band initiation and propagation were monitored by locations of acoustic emission and examination of thin sections of the failed specimen. Numerical analysis using a discrete element model was conducted to simulate the laboratory results. The model was able to capture many details of the damage zone observed in the physical experiment.     

铜银合金线材载流摩擦磨损行为研究

在改造过的MS-T3000摩擦磨损试验机上,以黄铜为摩擦副对热型连铸技术制备的铜银合金线材进行载流摩擦磨损试验,研究了电流对铜银合金导线载流摩擦磨损行为的影响.结果表明,电流强度对铜银合金干摩擦磨损行为有显著影响.电流在0～15 A范围内,随着电流的增加,摩擦系数与磨损率变化基本一致,呈现先减少后增加的趋势.电流较小时,接触电阻也比较小且较稳定;电流高时,接触电阻比较大,波动剧烈,而且有电弧出现.铜银合金导线在带电条件下的主要磨损形式为磨粒磨损、粘着磨损以及以电化学作用为主的氧化磨损或腐蚀磨损.     

Shear Strength in Preexisting Landslides

Drained residual shear strength is used for the analysis of slopes containing preexisting shear surfaces. Some recent research suggests that preexisting shear surfaces in prior landslides can gain strength with time. Torsional ring and direct shear tests performed during this study show that the recovered shear strength measured in the laboratory is only noticeably greater than the drained residual strength at effective normal stress of 100 kPa or less. The test results also show that the recovered strength even at effective normal stresses of 100 kPa or less is lost after a small shear displacement, i.e., slope movement. An effective normal stress of 100 kPa corresponds to a shallow depth so the observed strength gain has little, if any, impact on the analysis of deep landslides. This paper describes the laboratory strength recovery testing and the results for soils with different plasticities at various rest periods and effective normal stresses.     

Simple Model for the Cyclic Behavior of Smooth Sand-Steel Interfaces

The object of this note is the formulation of a simple elastoplastic model for the behavior of smooth sand-steel interfaces. The model is derived from a series of constant normal stiffness direct shear tests between a siliceous sand and a smooth steel plate. These tests highlight the importance of the shear stress degradation on the final value of the shear resistance and can be seen as the elementary mechanism that models in the laboratory the pile shaft-soil interaction. The goal of the presented model is to represent the observed behavior in a very simple way by using a reduced number of constitutive parameters.     

Development of New Peak Shear-Strength Criterion for Anisotropic Rock Joints

The roughness of a natural rock joint was measured in different directions using a laser profilometer. Two stationary roughness parameters and a nonstationary roughness parameter (all fractal based) were used to quantify anisotropic roughness. A plaster of Paris based model material was used to make model material replicas of the natural rock joint. Direct shear tests were performed at five different normal stresses, in each of the directions that were used for the roughness measurements, to measure the anisotropic peak shear strength of the model joint. Required observations and experiments were conducted to estimate (1) the asperity shear area as a proportion of the total surface area of the joint, for each tested joint; (2) the basic friction angle of the model material; and (3) the joint compressive strength. Tests were also conducted to develop a peak shear-strength criterion for the intact model material. Part of the direct shear test data was used to develop a new peak shear-strength criterion for joints including the aforementioned parameters. The other part of the data was used for model validation.     

Shear Strengthening of RCT-Joints Using CFRP Composites

The rehabilitation of RC columns jacketed with carbon fiber-reinforced plastic (FRP) composites for improving shear strength, confinement, and ductility has received considerable attention. However, research for improving the shear capacity of beam-column T-joints using FRP composite materials is still in the early stages. The present paper describes the experimental results of 14 1∕3-scale tests of concrete beam-column joints. The variables considered were the composite system, the fiber orientation, and the surface preparation. The tests have demonstrated the viability of carbon FRP composites for their use in improving the shear capacity of the joints as evidenced by the experimental results. Based on these experimental results, a design aid was developed for T-joints with inadequate confinement and shear reinforcement.     

Behavior of FRP-Retrofitted Joints Built with Plain Bars and Low-Strength Concrete

Two series of tests on eight full-scale exterior beam-column joint subassemblages built with plain bars and low-strength concrete were conducted. No transverse reinforcement was present in the joint cores. In the first series of tests, which included three specimens, the behavior of joints before fiber-reinforced polymer (FRP) retrofitting was investigated. In the second series, which included five specimens, the behavior of the FRP-retrofitted joints was investigated. The six specimens consisted of a column, an in-plane beam, a transverse beam, and a slab part, and two specimens were plane members without transverse beams and slabs. The utilized retrofitting scheme is easily applicable for actual exterior beam-column joints, even in the presence of a transverse beam and a slab. Two types of strength limitation were observed for specimens in the first series. The strength of the specimen with beam longitudinal bars sufficiently anchored to the joint core was limited by the shear strength of the joint. The strengths of the other two specimens were limited by the slip of the beams’ longitudinal bars at their anchorages. In the second series of tests, significantly better performance was obtained both in terms of shear strength and ductility, provided that the slip of the beam bars was prevented. Furthermore, by using a simple theoretical algorithm based on truss analogy, the strength and deformability characteristics of the tested reference and FRP-retrofitted joints are predicted with reasonable accuracy. The same algorithm is used for predicting the joint shear strength of specimens tested by other researchers, and satisfactory agreement is obtained between the predictions and test results.     

Three-Dimensional Joint/Interface Element for Rough Undulating Major Discontinuities in Rock Masses

Major civil engineering structures are being constructed now a days in complex geological environment with faults, shear zones, and other major discontinuities. These major discontinuities can cause a variety of problems in both surface and underground constructions. Unfavorably dipping major discontinuities may create unstable conditions in underground openings and contribute to the deformations of a rock mass under external static loading. Hence, rock–structure interaction analysis should simulate arbitrarily oriented rough and undulating major discontinuities within the rock mass, as well as the undulating interface along the structure and the rock mass such as dam foundations and underground excavations intersected by fault/shear zones. Realistic simulation of the mechanical behavior of rock joints is a prerequisite for successful numerical modeling of discontinuous rocks. When joint modeling is designed to include different degrees of joint roughness, dilation, and aperture, then realistic response depends upon the appropriate constitutive models and the way these parameters interact with stress change. Due to low values of the normal and tangential module, a unique characteristic of a rock discontinuity is that dilation may occur as soon as relative slip takes place and this may significantly alter the stress distribution, particularly around an underground excavation. In view of these practical requirements, a generalized formulation of a three-dimensional joint/interface element has been proposed here to account for dilatancy, roughness, and undulating surface of discontinuities.     

红砂岩中矿物颗粒的塑性应变分析

为定量研究砂岩中矿物颗粒在载荷作用下的塑性应变和应力,揭示岩石内部赤铁矿颗粒的变形行为,基于弹塑性力学理论和张量分析,采用X射线CT对砂岩三维结构进行扫描（空间分辨率为4.6 μm）,分析岩石内部矿物颗粒的运移规律,并提出了岩石矿物颗粒变形梯度张量计算的方法研究其塑性应变。首先对砂岩矿物颗粒位移进行提取,并采用Non Local Means滤波算法对砂岩三维数字图像去噪;然后基于砂岩三维结构,对不同矿物组分进行分割,通过构造主轴应变的变形张量,计算砂岩矿物颗粒的变形梯度和应力、应变分量。结果表明：该滤波算法对砂岩CT图像和射束硬化现象具有显著改善效果。此外,基于X射线CT的砂岩原位测试结果显示,砂岩内部存在较复杂的变形行为和应力响应,且在断裂带和非断裂区域变形行为有着显著差异;岩石内部颗粒在Z轴方向受到压应力,而在XY平面受到拉伸应力的作用,同时矿物颗粒内部存在较大的塑性应变,且试样内部颗粒的应变和应力要远远大于试样宏观应变和应力。该方法对于揭示岩体内部结构和应力、应变状态演化过程具有重要作用。     

Proposal for Standardization of Pull Tests on Rock Joints

This note intends to approach the mechanical characterization of rock joints, emphasizing the application of pull tests, which are an economic and simple type of shear experiments. Explanation is given on the drawbacks resulting from the classical testing procedures which do not control the sample’s middle plane position, leading to normal scale effects (represented by negative exponential regressions), where small sample results are misleading in terms of safety. In effect, pull tests allow a very easy measurement of several joint properties, such as the so-called joint roughness coefficient, the slope of joint roughness, the roughness anisotropy, and joint strengths under low normal stresses. Some rules for their standardization are suggested by pointing out the systematic leveling of the middle joint sample plane, thus rehabilitating the interest on small samples, because of the inverse scale effects on strength that are obtained, represented by positive exponential regressions.     

Laboratory Measurement of Two-Phase Flow Parameters in Rock Joints Based on High Pressure Triaxial Testing

The accurate flow measurement of each individual phase is important in unsaturated flow through rock joints, where both air and water phases flow together. An increase in the quantity of one fluid phase decreases the relative permeability of the other phase. The relative permeability is important in numerical models to analyze the risk of ground-water inundation and uncontrollable gas flows in underground excavations in jointed rock. A new apparatus, the High Pressure Two-Phase Triaxial Apparatus (HPTPTA), has been designed for examining the strength and coefficient of permeability characteristics of fractured and intact rocks under two-phase flows. In single-phase triaxial equipment, the rock specimen is subjected to a single fluid flow (either water, oil, or gas) through the fractures. In the HPTPTA, two fluids (e.g., water + air, water + oil, and oil + air) can be forced to flow through the specimen, and the flow rates of the fluids can be measured independently. The scope of tests that can be carried out in this apparatus is wide, including the evaluation of (1) stress-strain behavior subject to internal fluid flow; (2) relative permeability of each fluid phase under different degrees of saturation; and (3) the associated volume change of the specimen. In this paper, the design concepts of the HPTPTA and the results based on the testing of fractured rock specimens are discussed. The laboratory results are compared to a simplified mathematical model developed by the writers. Based on the laboratory results, it is shown that the well-known Darcy's law can be modified for estimating the two-phase flow rates using the relative permeability concept.     

Shear Strength of Municipal Solid Waste

A comprehensive large-scale laboratory testing program using direct shear (DS), triaxial (TX), and simple shear tests was performed on municipal solid waste (MSW) retrieved from a landfill in the San Francisco Bay area to develop insights about and a framework for interpretation of the shear strength of MSW. Stability analyses of MSW landfills require characterization of the shear strength of MSW. Although MSW is variable and a difficult material to test, its shear strength can be evaluated rationally to develop reasonable estimates. The effects of waste composition, fibrous particle orientation, confining stress, rate of loading, stress path, stress-strain compatibility, and unit weight on the shear strength of MSW were evaluated in the testing program described herein. The results of this testing program indicate that the DS test is appropriate to evaluate the shear strength of MSW along its weakest orientation (i.e., on a plane parallel to the preferred orientation of the larger fibrous particles within MSW). These laboratory results and the results of more than 100 large-scale laboratory tests from other studies indicate that the DS static shear strength of MSW is best characterized by a cohesion of 15?kPa and a friction angle of 36° at normal stress of 1?atm with the friction angle decreasing by 5° for every log cycle increase in normal stress. Other shearing modes that engage the fibrous materials within MSW (e.g., TX) produce higher friction angles. The dynamic shear strength of MSW can be estimated conservatively to be 20% greater than its static strength. These recommendations are based on tests of MSW with a moisture content below its field capacity; therefore, cyclic degradation due to pore pressure generation has not been considered in its development.