压实花岗土的Yasufuku模型研究

利用花岗土实施多种应力路径试验,并用实测结果比较研究了Yasufuku本构模型的适用性。结果表明,用Yasufuku本构关系预测的应变与实测值较为一致,只是在提高围压的同时增大偏应力显现出较为明显的差异。     

Deformational characteristics of weak sandstone and impact to tunnel deformation

In northern Taiwan, a tunnel under construction along a segment where weak sandstone, the Mushan sandstone, was encountered and an excess crown settlement (14–30 cm) has been reported. This paper studies the deformational characteristics of Mushan sandstone and its impact on tunnel deformation. To distinguish the volumetric and the shear deformation of the sandstone, experiments with controlled stress paths, including hydrostatic compression, pure shearing and conventional triaxial compression, were conducted. The measured deformations were then decomposed into elastic and plastic components further exploring the stress–strain behavior of weak sandstone. The results indicate that, similar to other soil-like geo-materials, this sandstone has plastic strain before the stress path reaches the failure envelope and significant shear dilation is induced, especially when approaching the failure envelope. Meanwhile, the distinct features of deformation have also been highlighted by comparing the experimental results to the prediction, derived from existing constitutive models that were originally developed for other geo-materials. These features include significant plastic volumetric strain at low levels of confining stress, suppression of plastic volumetric strain at higher levels of confining stress, and the fact that the actual amount of shear compression is less than that predicted by the model. Numerical analysis indicates that the weak rock leads to the greatest inward displacement, which results from the shear dilation prior to failure state.     

Mechanical behavior of different sedimentary rocks in the Brazilian test

Mechanical behavior of the rock in tensile stress environment remains an unresolved problem in the underground mining, where surrounding rocks commonly experience tensile failure. In the present study, tensile failure behavior of three types of sedimentary rocks is investigated experimentally and numerically. The deformation response, fracture propagation, and splitting pattern as well as energy conversion of the rocks are examined in the Brazilian test with a testing machine, high-speed camera, and image scanning system. The tested rocks tend to show elastic-brittle-plastic deformation behavior in the biaxial stress state. Tensile strengths of the coal, mudstone, and sandstone are 1.2, 3.9, and 13.4 MPa, respectively. The coal and mudstone split in a static mode while the sandstone in dynamic mode. The splitting fracture initiates at the disk center in the coal and sandstone, and it emerges at the loading point in the mudstone. The fracture propagates more quickly in the rock with larger strength. It uses 1200.0, 5.8, and 0.4 ms, to break out sample surface of the coal, mudstone, and sandstone, respectively. The joint roughness coefficient (JRC) of the splitting fracture is largest (20.1) in the mudstone, followed by that in the coal (17.7), and it is smallest in the sandstone (15.3). A continuous-discontinuous coupling model, which is capable of analyzing the stress distribution and fracture propagation synergistically, is developed and calibrated against the experimental data. The numerical model accurately reproduces mechanical behavior of the tested rocks observed in the Brazilian test. The splitting fracture propagates along the maximum tensile stress plane in the sandstone. The propagation direction is locally influenced by the grain boundary with small tensile strength in the coal while, in the mudstone, the fracture is mainly formed along the grain boundary. Such differences in the fracture propagation path lead to the increasing trend from the JRC of the splitting surface in the sandstone to that in the coal and finally to that in the mudstone.

     

Non-linear, stress- and strain-dependent behavior of soft rocks under cyclic triaxial conditions

Four saturated Miocene soft rocks of Japan i.e. Ohya tuff, Yokohama siltstone, Kobe mudstone and sandstone were tested under consolidated-undrained, cyclic loading conditions. The deformation behavior, pore pressure changes and strength mobilization, and contraction/dilation characteristics of these rocks were studied with emphasis on the non-linearity, stress- and strain-dependence of their mechanical properties. A procedure was introduced to locate the yield point, to estimate Young's modulus and to study the deformation behavior of these soft rocks. Deformation modulus was found to depend on plastic straining according to an exponential equation, whose coefficients in turn, depend on confining pressure. In addition, it also depends on the deviatoric stress levels causing hysteresis to occur. Patterns of pore pressure changes and associated failure modes were interpreted in terms of the initiation/propagation of micro- and macro-cracks inside the specimens. Characteristic stress levels were found, based on which the “true” rock strength components — cohesion and internal friction angle — were determined and their mobilization with respect to stress and strain levels was analyzed using a new interpretation method. Volume change tendencies were inferred from pore pressure changes whose rate was related to the contraction/dilation behavior of soft rocks. On that basis the dilation angle was estimated and its variation and stress- and strain-dependence was analyzed. A simple equation was introduced to characterize the variation of both internal friction and dilation angles with plastic straining, whereas their stress-dependence was studied through the variation of fit coefficients with confining pressure.     

常规三轴压缩下花岗岩断裂表面的分形研究

用花岗岩试件做了常规三轴压缩试验,对在不同围压作用下的岩石断裂表面进行了分形研究。其结果表明,随着围压的增加,沿破坏而剪切错动方向分形维数增加,而垂直于该方向的分形维数则基本不变。用光学显微镜及扫描电镜观察岩石断口的分形特征,分析了岩石断裂机理。结果表明,分形维数是一个能反映岩石损伤断裂机理的特征参数。     

Application of a local degradation model to the analysis of brittle fracture of laboratory scale rock specimens under triaxial conditions

Development of brittle fracture and the associated macroscopic behaviour of rock specimens in laboratory tests are simulated using a local degradation model for brittle fracture in heterogeneous rocks. We examine this subject because laboratory uniaxial and triaxial tests are widely used in the rock mechanics community to both characterise rock behaviour and to interpret fracture phenomena observed in natural rock, such as the Earth's crust, and developed around rock engineering structures. In addition, numerous historical efforts related to the detailed study of this subject have made available a great deal of information for use both as model input data and comparison results.A series of numerical experiments have been performed to investigate the influence of a number of parameters on rock fracture. In particular, rock fracture under various confining stresses has been explored. The results show that the degradation algorithm is capable of reproducing many characteristics associated with brittle fracture in heterogeneous rocks, including: the development of fracture from the elemental scale to the macroscopic scale; fracture pattern as a function of confining pressure; variation of fracture plane angle with respect to confining pressure; the complete stress–strain curve and corresponding strain energy dissipation characteristics; dependence of the stress–strain curve on confining pressure; and loading–unloading hysteresis loops.Independent investigations into the effect on rock fracture of (i) the degradation parameter embodied with the model and (ii) the Weibull shape parameter used to introduce heterogeneity distribution are described. The results indicate that the degradation parameter controls the degree of degradation relative to confining pressure. As this parameter increases, and the elemental degradation decreases, the number of failed sites generated prior to the formation of macroscopic fracture plane increases, and both the peak and ultimate strengths of the model increase.The Weibull parameter influences the formation of the final fracture plane. As this parameter increases, reducing the heterogeneity, the number of diffused failed sites and the angle of the eventual fracture plane to the major principal stress tends to decrease, and the brittleness of the resulting stress–strain curve increases. It is suggested that values in the range 2–4 are appropriate for this parameter in representing elemental strength distribution of rock materials.     

水力压裂体积张开度模型

水力压裂体积张开度是指水力作用引起的单位体积岩石内部张开体积大小,包括孔隙体积张开度和裂缝体积张开度,是进行水力压裂效果预测和评价的重要指标。为了建立孔隙–裂缝融合一体的水力压裂体积张开度模型,提出一个三阶段水力压裂体积张开演化过程模型,利用孔隙弹性力学理论和黏聚断裂损伤演化模型,建立三阶段的体积张开度演化表达式。通过将裂缝体积张开度与经典平行板裂缝张开宽度的演化规律进行匹配拟合,将断裂传播机制对裂缝张开度的影响纳入到体积张开度模型。该模型在孔裂隙的弹性张开网络中引入平行板裂缝,实现经典平行板裂缝理论与孔隙弹性理论的有机融合。作为实例,阐明在4种极限断裂传播机制下,如何利用该模型对水力压裂张开度进行评价和预测。     

密度对砂土应力应变强度特性影响试验研究

针对某级配砂土,对不同密度试样进行常规三轴固结排水剪切试验,探讨了密度对砂土应力应变特性、强度特性的影响。试验结果表明:密度对砂土应力应变的形态影响不大,对试样破坏时的轴向应变有显著影响;砂土试样发生相变时的体积应变受密度影响显著。通过研究密度和围压对变形的影响,定量分析了相变体积应变随相对密实度和围压的变化关系,同时,发现砂土密度对其变形模量影响较大;密度对试样破坏偏应力比影响较大。试样砂土的强度指标总体上随相对密实度增大而增大,但在试验范围内影响有限。对非线性指标φ_0,Δφ,随试样相对密实度变化均可近似用直线表示;D_r每增大0.1,内摩擦角φ增大0.66°。     

Self-sealing of fractures in indurated claystones measured by water and gas flow

Self-sealing of fractures in the indurated Callovo-Oxfordian (COX) and Opalinus (OPA) claystones, which are considered as host rocks for disposal of radioactive waste, was investigated on artificially fractured samples. The samples were extracted from four lithological facies relatively rich in clay mineral, carbonate and quartz, respectively. The self-sealing of fractures was measured by fracture closure, water permeability variation, gas penetration, and recovery of gas-induced pathways. Most of the fractured samples exhibited a dramatic reduction in water permeability to low levels that is close to that of intact rock, depending on their mineralogical composition, fracture intensity, confining stress, and load duration. The self-sealing capacity of the clay-rich samples is higher than that of the carbonate-rich and sandy ones. Significant effects of sample size and fracture intensity were identified. The sealed fractures become gas-tight for certain injection pressures. However, the measured gas breakthrough pressures are still lower than the confining stresses. The gas-induced pathways can recover when contacting water. These important findings imply that fractures in such indurated claystones can effectively recover to hinder water transport but allow gas release under relatively low pressures without compromising the rock integrity.     

Effect of fatigue loading-confining stress unloading rate on marble mechanical behaviors: An insight into fracture evolution analyses

Rocks in underground works usually experience rather complex stress disturbance. For this, their fracture mechanism is significantly different from rocks subjected to conventional triaxial compression conditions. The effects of stress disturbances on rock geomechanical behaviors under fatigue loading conditions and triaxial unloading conditions have been reported in previous studies. However, little is known about the dependence of the unloading rate on fatigue loading and confining stress unloading (FL-CSU) conditions that influence rock failure. In this paper, we aimed at investigating the fracture behaviors of marble under FL-CSU conditions using the post-test X-ray computed tomography (CT) scanning technique and the GCTS RTR 2000 rock mechanics system. Results show that damage accumulation at the fatigue stage can influence the final fracture behaviors of marble. The stored elastic energy for rock samples under FL-CSU tests is relatively larger compared to those under conventional triaxial tests, and the dissipated energy used to drive damage evolution and crack propagation is larger for FL-CSU tests. In FL-CSU tests, as the unloading rate increases, the dissipated energy grows and elastic energy reduces. CT scanning after the test reveals the impacts of the unloading rate on the crack pattern and a fracture degree index is therein defined in this context to represent the crack dimension. It shows that the crack pattern after FL-CSU tests depends on the unloading rate, and the fracture degree is in agreement with the analysis of both the energy dissipation and the amount of energy released. The effect of unloading rate on fracture evolution characteristics of marble is revealed by a series of FL-CSU tests.     

Sealing of fractures in claystone

The sealing behavior of fractures in clay rocks for deep disposal of radioactive waste has been comprehensively investigated at the GRS laboratory. Various sealing experiments were performed on strongly cracked samples of different sizes from the Callovo-Oxfordian argillite and the Opalinus clay under relevant repository conditions. The fractured samples were compacted and flowed through with gas or synthetic pore-water under confining stresses up to 18 MPa and elevated temperatures from 20 °C to 90 °C. Sealing of fractures was quantified by measurements of their closure and permeability. Under the applied thermo-hydro-mechanical (THM) conditions, significant fracture closure and permeability decrease to very low levels of 10^?19 to 10^?21 m² were observed within time periods of months to years. The properties of the resealed claystones are comparable with those of the intact rock mass. All test results suggest high sealing potentials of the studied claystones.     

不等侧压下煤岩力学变形特性及声发射时空特征

通过煤岩的无侧限压缩试验测得其力学参数后,采用颗粒流软件和编制的Fish程序获得煤岩细观力学参数,而后进行双向不等侧压煤岩试验,分析研究煤岩变形、声发射和损伤特征,获得以下结论:随着侧压比增加,煤岩刚度角略增大,脆度角增大,强化角保持一定值,软化角先增大后减小;侧压比对煤岩的线性体缩阶段没有影响,对线性扩容阶段影响明显,对非线性扩容阶段影响显著。在非线性扩容阶段,低侧压比仍保持线性扩容,而高侧压比煤岩则急剧扩容失稳破坏;声发射最大强度与峰值应力并不对应,具有一定的滞后性,侧压比对声发射的滞后效应基本没有影响,声发射最大强度对应应力与峰值应力之比值大致在0.92左右;侧压比小时煤岩沿着单一剪切面发生剪切破坏,侧压比大时煤岩呈现X型共轭剪切破坏,且随着侧压比的增加,煤岩破坏的主控破裂面宽度加大,并逐渐形成破碎带,破碎带内煤岩呈粉碎性破坏状态。     

Numerical study on static and dynamic fracture evolution around rock cavities

In this paper, a numerical code, RFPA^2D (rock failure process analysis), was used to simulate the initiation and propagation of fractures around a pre-existing single cavity and multiple cavities in brittle rocks. Both static and dynamic loads were applied to the rock specimens to investigate the mechanism of fracture evolution around the cavities for different lateral pressure coefficients. In addition, characteristics of acoustic emission (AE) associated with fracture evolution were simulated. Finally, the evolution and interaction of fractures between multiple cavities were investigated with consideration of stress redistribution and transference in compressive and tensile stress fields. The numerically simulated results reproduced primary tensile, remote, and shear crack fractures, which are in agreement with the experimental results. Moreover, numerical results suggested that both compressive and tensile waves could influence the propagation of tensile cracks; in particular, the reflected tensile wave accelerated the propagation of tensile cracks.     

Numerical evaluation of strength and deformability of fractured rocks

Knowledge of the strength and deformability of fractured rocks is important for design, construction and stability evaluation of slopes, foundations and underground excavations in civil and mining engineering. However, laboratory tests of intact rock samples cannot provide information about the strength and deformation behaviors of fractured rock masses that include many fractures of varying sizes, orientations and locations. On the other hand, large-scale in situ tests of fractured rock masses are economically costly and often not practical in reality at present. Therefore, numerical modeling becomes necessary. Numerical predicting using discrete element methods(DEM) is a suitable approach for such modeling because of their advantages of explicit representations of both fractures system geometry and their constitutive behaviors of fractures, besides that of intact rock matrix. In this study, to generically determine the compressive strength of fractured rock masses, a series of numerical experiments were performed on two-dimensional discrete fracture network models based on the realistic geometrical and mechanical data of fracture systems from feld mapping. We used the UDEC code and a numerical servo-controlled program for controlling the progressive compressive loading process to avoid sudden violent failure of the models. The two loading conditions applied are similar to the standard laboratory testing for intact rock samples in order to check possible differences caused by such loading conditions. Numerical results show that the strength of fractured rocks increases with the increasing confning pressure, and that deformation behavior of fractured rocks follows elasto-plastic model with a trend of strain hardening. The stresses and strains obtained from these numerical experiments were used to ft the well-known Mohr-Coulomb(MC) and Hoek-Brown(H-B) failure criteria, represented by equivalent material properties defning these two criteria. The results show that both criteria can provide fair estimates of the co     

堆石蠕变与颗粒破碎特性三轴试验

为分析堆石的蠕变规律和研究颗粒破碎与蠕变的关系,对某堆石料进行了不同应力水平和围压下的大型三轴排水蠕变试验.蠕变试验后通过筛分试验测量了堆石的颗粒破碎程度.结果表明堆石的蠕变变形与应力水平和围压有关;轴向蠕变和体积蠕变随应力水平的增大而增大,应力水平相同时则随围压增大而增大.堆石的应变-时间关系可用幂函数表示,轴向应变...     

卸荷条件下花岗岩力学特性试验研究

 基于岩石试件的卸荷试验,研究卸荷条件下岩石的变形、参数及破裂特征。研究结果表明：(1) 卸荷过程中岩石向卸荷方向回弹变形较为强烈、扩容显著,脆性破坏特征明显。(2) 卸荷过程中岩石的变形模量E逐渐减小,泊松比m逐渐增大,E减小了5%～27%,而m增大了50%～335%,变化均随初始围压的增大和卸荷强度的增强而增大,两者均与体积应变相关。(3) 相对于加载试验,卸荷岩石的c减小而j增大,且卸荷强度愈强,c减小得越多,j增大的程度越小。峰值c减小了33.2%～47.8%,而残余c为正常值的65.3%～77.6%,峰值j增加了14.7%～33.2%,而残余j增大了5.9%～9.4%。(4) 卸荷条件下岩石破坏具有较强的张性破裂特征,各种级别的张裂隙发育,双向卸荷时甚至在次卸荷方向上也可能出现张拉裂隙,剪性破裂面一般追随张拉裂隙发展。     

三向受力条件下冻结岩石力学特性试验研究

 随着寒区或特殊施工环境条件下基础设施建设的需要,越来越有必要对冻结岩石力学问题进行深入的研究。以陕西彬长矿区胡家河煤矿冻结立井为背景,从现场采集的煤岩和砂岩为代表,进行常温(+20 ℃)和不同冻结温度(－5 ℃,－10 ℃,－20 ℃)条件下的岩石在不同围压下的三轴压缩试验。分别探讨了围压对于冻结岩石三轴强度特性的影响和冻结温度对于冻结岩石三轴强度特性的影响规律,分析煤岩及砂岩在相同围压不同温度条件下及相同温度不同围压条件下的强度特性,并对2种岩样冻结温度的同一性和差异性进行比较研究。煤岩和砂岩在冻结的效应方面有着明显的差异性,主要原因是其岩石内部结构性的差异。富水砂岩冻结后对温度的敏感程度要高于煤岩。强度随温度降低而增大的主要原因是温度降低时,岩石冻结时的矿物收缩和冰本身的强度及冻胀力使得富水冻结岩石的峰值强度得到提高。为低温条件下岩石力学特性和煤矿冻结立井设计施工提供参考。     

Rock rupture phenomenon and pillar failure in tuffs in the Cappadocia region (Turkey)

The aim of this study is to investigate the relation between the critical failure stresses for rupture and pillar instabilities observed in tuffs in Cappadocia region and stresses triggering the early crack propagation stages such as crack initiation, systematic cracking and crack coalescence thresholds determined from Brazilian and uniaxial compression tests on rock samples obtained from these instabilities. These failures in the region have been occurring as a result of strength reduction of rocks through time, and long-term strengths of these rocks are achieved by own weight of the rock mass. In this study, these failures were evaluated in order to estimate long-term failure strength of rocks by classical short-term laboratory tests. For this purpose, some cases on rock rupture phenomenon and a pillar failure case were picked from different parts of the region. During the selection of these instabilities, the ones with relatively regular geometries were preferred. Rock block samples were collected from each case, and uniaxial compressive and indirect tensile (Brazilian) tests were carried out on dry and saturated specimens. In these tests, axial deformations and acoustic emission (AE) activity were also recorded in order to identify thresholds of the crack propagation and deformation stages of rock samples. The simplified back-analysis of each case was conducted, and the tensile stresses on rupture surfaces and uniaxial compressive stress on pillar were calculated. Then, stress levels at the thresholds of crack propagation and deformation stages determined by laboratory tests and stress levels determined by back-analyses were compared. Finally, it is concluded that crack initiation and/or systematic cracking thresholds may be a good indicator to determine lower bound of the in-situ strength of these tuffs and can be used as an estimator of their long-term strengths.     

Stress-dependent shear wave splitting and permeability in fractured porous rock

It is well known that shear wave propagates slower across than parallel to a fracture, and as a result, a travelling shear wave splits into two directions when it encounters a fracture. Shear wave splitting and permeability of porous rock core samples having single fracture were experimentally investigated using a high-pressure triaxial cell, which can measure seismic shear wave velocities in two directions mutually perpendicular to the sample axis in addition to the longitudinal compressive wave velocity. A single fracture was created in the samples using a modified Brazilian split test device, where the cylindrical sample edges were loaded on two diametrically opposite lines by sharp guillotines along the sample length. Based on tilt tests and fracture surface profilometry, the method of artificially induced tensile fracture in the sample was found to create repeatable fracture surfaces and morphologies. Seismic velocities of the fractured samples were determined under different levels of stress confinement and fracture shear displacement or mismatch. The effective confining stress was varied from 0.5 MPa to 55 MPa, while the fractures were mismatched by 0 mm, 0.45 mm and 1 mm. The degree of matching of the fracture surfaces in the core samples was evaluated using the joint matching coefficient (JMC). Shear wave splitting, as measured by the difference in the magnitudes of shear wave velocities parallel (V_S1) and perpendicular (V_S2) to the fracture, is found to be insensitive to the degree of mismatching of the fracture joint surfaces at 2 MPa, and decreased and approached zero as the effective stress was increased. Simple models for the stress- and JMC-dependent shear wave splitting and fractured rock permeability were developed based on the experimental observations. The effects of the joint wall compressive strength (JCS), JMC and stress on the stress dependency of joint aperture were discussed in terms of hydro-mechanical response. Finally, a useful relationship between fractured rock permeability and shear wave splitting was found after normalization by using JMC.     

Dynamic notched semi-circle bend(NSCB) method for measuring fracture properties of rocks:Fundamentals and applications

Rocks are increasingly used in extreme environments characterised by high loading rates and high confining pressures.Thus the fracture properties of rocks under dynamic loading and confinements are critical in various rock mechanics and rock engineering problems.Due to the transient nature of dynamic loading,the dynamic fracture tests of rocks are much more challenging than their static counterparts.Understanding the dynamic fracture behaviour of geomaterials relies significantly on suitable and reliable dynamic fracture testing methods.One of such methods is the notched semi-circle bend(NSCB) test combined with the advanced split Hopkinson pressure bar(SHPB) system,which has been recommended by the International Society for Rock Mechanics and Rock Engineering(ISRM) as the standard method for the determination of dynamic fracture toughness.The dynamic NSCB-SHPB method can provide detailed insights into dynamic fracture properties including initiation fracture toughness,fracture energy,propagation fracture toughness and fracture velocity.This review aims to fully describe the detailed principles and state-of-the-art applications of dynamic NSCB-SHPB techniques.The history and principles of dynamic NSCB-SHPB tests for rocks are outlined,and then the applications of dynanic NSCB-SHPB method(including the measurements of initiation and propagation fracture toughnesses and the limiting fracture velocity,the size effect and the digital image correlation(DIC) experiments) are discussed.Further,other applications of dynamic NSCB-SHPB techniques(i.e.the thermal,moisture and anisotropy effects on the dynamic fracture properties of geomaterials,and dynamic fracture toughness of geomaterials under pre-loading and hydrostatic pressures) are presented.