扰动岩体裂隙网络分形性质分析

应用相似材料试验模型,研究扰动岩体裂隙网络的演化特征和规律,借助分形几何方法计算分析了扰动岩体裂隙网络的分形维数和分形演化特征,展望了研究覆岩力学性质与裂隙网络演化的依赖关系,对评价采动覆岩变形过程、力学行为和渗流性质具有重要意义.     

Interactive roles of geometrical distribution and geomechanical deformation of fracture networks in fluid flow through fractured geological media

In this study,the combined effects of geometrical distribution and geomechanical deformation of fracture networks on fluid flow through fractured geological media are investigated numerically.We consider a finite-sized model domain in which the geometry of fracture systems follows a power-law length scaling.The geomechanical response of the fractured rock is simulated using a hybrid finitediscrete element model,which can capture the deformation of intact rocks,the interaction of matrix blocks,the displacement of discrete fractures and the propagation of new cracks.Under far-field stress loading,the locally variable stress distribution in the fractured rock leads to a stress-dependent variable aperture field controlled by compression-induced closure and shear-induced dilatancy of rough fractures.The equivalent permeability of the deformed fractured rock is calculated by solving for the fracture-matrix flow considering the cubic relationship between fracture aperture and flow rate at each local fracture segment.We report that the geometrical connectivity of fracture networks plays a critical role in the hydromechanical processes in fractured rocks.A well-connected fracture system under a high stress ratio condition exhibits intense frictional sliding and large fracture dilation/opening,leading to greater rock mass permeability.However,a disconnected fracture network accommodates much less fracture shearing and opening,and has much lower bulk permeability.We further propose an analytical solution for the relationship between the equivalent permeability of fractured rocks and the connectivity metric(i.e.percolation parameter) of fracture networks,which yields an excellent match to the numerical results.We infer that fluid flow through a well-connected system is governed by traversing channels(forming an"in parallel" architecture) and thus equivalent permeability is sensitive to stress loading(due to stress-dependent fracture permeability),whilst fluid flow through a disconnected system is more ruled by matrix(linking isolated clusters"in series") and has much less stress dependency.     

应变敏感的裂隙及裂隙岩体水力传导特性研究

通过将岩体单裂隙视为非关联理想弹塑性体，导出单裂隙在压剪荷载作用下，其机械开度和水力传导度的解析模型，并采用已有相关试验研究成果对解析模型进行验证。在此基础上，通过将岩体概化为含一组或多组优势裂隙的等效连续介质，给出一种描述裂隙岩体在复杂加载条件下考虑非线性变形特征及滑动剪胀特性的等效非关联理想弹塑性本构模型。基于该模型，给出裂隙岩体在扰动条件下应变敏感的渗透张量的计算方法，该计算方法不仅考虑裂隙的法向压缩变形，而且反映材料非线性及峰后剪胀效应对裂隙岩体渗透特性的影响。该模型通过引入滑动剪胀角和非关联理想塑性，较为逼真地反映了真实裂隙及裂隙岩体峰后的剪胀特性、变形行为和水力传导度变化特征。通过数值算例，研究了裂隙岩体在力学加载及开挖条件下渗透特性的演化规律。     

循环荷载下岩体结构面本构关系与积分算法研究

地下结构中广泛存在的各种不连续面是影响岩体工程力学特性的重要因素,研究结构面在循环荷载作用下的力学行为具有重要的工程意义。基于非关联塑性理论,同时定义加载剪胀段和反向剪缩段,建立了岩体结构面剪胀与塑性耦合本构关系;从结构面的基本损伤机制出发,基于拉、剪分离的思路,建立了一类基于能量原理的岩体结构面拉、剪损伤本构模型,该模型基于有效应力空间塑性力学基本原理,定义了岩体结构面拉、剪塑性Helmholtz自由能分量及损伤能释放率,建立了岩体结构面拉、剪损伤破坏准则。针对结构面在循环荷载作用下的强非线性问题,引入算子分解的思想,将弹塑性演化与损伤演化过程分开进行求解,提出了结构面弹塑性损伤本构关系的混合积分算法。分别进行了岩体结构面直剪和循环剪切试验的数值仿真,计算结果与模型试验基本一致,表明该模型在模拟非连续岩体复杂变形方面是合理有效的。     

Development of a hydro-mechanical local degradation approach and its application to modelling fluid flow during progressive fracturing of heterogeneous rocks

A hydro-mechanical local degradation approach is developed and used to investigate progressive damage and associated flow behaviour in heterogeneous rock. The approach is based on the establishment of an elemental scale hydro-mechanical constitutive model that incorporates simple idealised representations of degradation of strength and stiffness, confining pressure-dependent dilatancy, and deformation-dependent permeability. Through the use of such an idealised elemental behaviour, together with hydro-mechanical heterogeneity at the elemental scale, a finite difference numerical formulation allows both the mechanical and hydraulic behaviour of heterogeneous rocks to be simulated.

A series of numerical experiments that replicate laboratory-scale rock specimens have been performed to simulate fracture development and the associated evolution of fluid flow. Two independent statistical distributions are used to represent mechanical and hydraulic heterogeneity, and an uncoupled hydro-mechanical algorithm is used to investigate local variations of permeability induced by mechanical damage, and how these influence macroscopic flow patterns. The model is shown to be capable of reproducing, and allowing visualisation of, a range of hydro-mechanical responses of rock that, until now, have only been monitored in the laboratory.     

脆性岩石损伤与热传导特性的细观力学模型

 基于均匀化方法给出低孔隙率脆性岩石在热–力耦合荷载作用条件下的各向异性损伤模型和有效热传导特性模型。其中,损伤模型可考虑非等温条件下裂纹的法向压缩变形、刚度恢复以及裂纹的滑动剪胀特性,热传导特性模型可反映损伤过程中细观结构的演化以及裂纹形态、孔隙率和饱和度变化对岩石有效热传导特性的影响。讨论低孔隙率结晶岩裂纹形态和饱和度对其有效热传导特性的影响;采用瑞典Äspö闪长岩在三轴压缩条件下的应力–应变曲线验证损伤模型的有效性,并分析岩石在损伤演化过程中裂纹体积率、密度、形态、饱和度和有效热传导特性的演化规律。研究成果对于深部岩体的热–力耦合特性研究具有一定参考意义。     

三轴卸荷条件下大理岩扩容与能量特征分析

扩容现象是岩石变形破坏过程中的重要特征。基于MTS815 Flex Test GT岩石力学试验平台,采用室内三轴卸荷试验和塑性力学理论分析,揭示了大理岩在卸荷条件下的扩容特征及能量变化特征。结果表明,随着围压的增大,岩样的各特征应力随之增大,其扩容特征随之减弱;岩样的扩容参数——扩容指标以及剪胀角均具有围压效应,即扩容指标与围压呈良好的指数型分布,剪胀角与应力比呈线性分布;岩样的卸荷破坏过程中能量特征为初始时以可释放应变能为主到破坏时的耗散能为主,其间的转折点为初始损伤扩容点,同时卸荷条件下的特征能量值与围压具有良好的指数类型关系;在峰值点与残余点处,岩样的能量损伤值与剪胀角以及能量特征值与扩容指标均存在着较好的指数类型关系。     

砂土中锚板的抗拔机理与承载力计算模型研究

锚板上拔过程中板周土体变形破坏机理的研究对锚板极限承载力的可靠预测至关重要。通过室内模型试验,采用数字图像关联技术对锚板上拔过程中锚板周围土体的变形场进行了研究。分析结果表明：在锚板上拔过程中锚板周围土体伴随着剪胀现象,其应力水平、峰值摩擦角和剪胀角控制着破坏面的形状,进而影响着锚板的极限承载力。在此基础上,建立了锚板承载力计算模型。通过引入Bolton理论所建立的剪胀角、相对密实度和应力水平之间的关系,得到了锚板极限承载力的理论计算公式。该理论公式考虑了埋深率、剪切摩擦角、剪胀和应力水平等影响因素,可对不同密实度砂土中锚板的极限承载力进行预测。理论公式与其他学者的试验结果对比表明该理论模型计算结果与其他学者的试验结果有较好的一致性,验证了该理论模型的合理性。     

软岩蠕变损伤特性的试验与理论研究

 在高地应力软岩隧道修建过程中,由于围岩蠕变损伤导致其变形具有明显的时效性和非线性。岩石的蠕变损伤与其内部微裂纹的延伸和扩展密切相关,宏观表现为蠕变过程中的体积扩容,通过对岩石扩容过程中损伤耗散能变化规律的分析,建立蠕变损伤演化方程,通过引入蠕变损伤因子对ABAQUS软件自带的蠕变模型进行修正,得到非线性蠕变损伤模型。应用所建立的蠕变损伤模型,对宜巴高速泥质红砂岩的三轴蠕变试验结果进行反演,结果表明,该模型可以很好地反映高地应力软岩在蠕变中的减速蠕变、等速蠕变和加速蠕变3个阶段。     

Considerations of the dilatancy angle in rocks and rock masses

The present study responds to the poor treatment given to dilatancy in classical rock mechanics post-failure problems such as tunnel or mine pillar design. A comprehensive review of the literature and observations in regard to published test results would indicate that dilatancy is highly dependent both on the plasticity already experienced by the material and confining stress; moreover, it also appears that scale may play a non-negligible role. In our article, we provide a detailed analysis of published test data with a view to proposing a sufficiently significant but conveniently simple formulation of the dilatancy angle that reflects these dependencies and that can be readily implemented in numerical codes. The model is then tested, demonstrating that it is capable of representing rock sample strain behaviour in compressive tests. Finally, the model is applied to the resolution of ground reaction curves for tunnels in poor-to-average-quality rock masses, showing a good correlation with results obtained using practical rock engineering techniques.     

基于离散元法的真三轴应力状态下砂土破碎行为研究

基于可破碎三维离散颗粒模型模拟了一系列常规三轴试验与真三轴试验,研究了砂土在真三轴应力状态下的破碎行为。数值调查主要关注试样的应力应变特性、级配及相对破碎率的演化。随着围压增大,颗粒破碎率增大,试样应变软化特性和剪胀性逐渐减弱,而超过临界高围压后,由于固结中颗粒大量破碎,试样剪胀性反而增强。真三轴试验中,试样偏应力比峰值均随中主应力参数b值增大而减小。由于破碎随b值增加而明显增大,试样剪胀性随b值增大而逐渐减弱。试样内摩擦角φ随围压增大而减小,其演化关系基本满足对数关系;内摩擦角随b值增大先增大后减小,Lade-Duncan准则较为适合描述其变化规律。此外,试样相对破碎率增大的速率随围压和轴向应变增大而逐渐降低,暗示试样最优终极级配的存在,且相对破碎率与试验输入能量之间存在唯一的双曲线关系。     

Concrete-filled circular steel tubes subjected to local bearing force: Finite element analysis

This paper presents a finite element analysis (FEA) of circular concrete filled steel tubular (CFST) members under local bearing forces applied either perpendicularly to the member or at an angle of 45°. The established FEA modeling was verified by the experimental results that have been published in an international journal. The FEA modeling was then used to perform analysis on the typical failure modes and full-range load-deformation relations of CFST subjected to local bearing forces. Finally, simplified formulae for calculating the strength of CFST under local bearing forces was presented. Practical guidance was also given to avoid premature local buckling of brace member and premature fracture of chord member.     

Fatigue behavior of damaged steel beams repaired with CFRP strips

This paper presents the flexural behavior of damaged steel beams repaired with carbon fiber reinforced polymer (CFRP) strips. The damage is intentionally created by notching the tension flange of the beams. Six beams are tested to evaluate the static and fatigue performance of the repaired beams with emphasis on local plasticity and the CFRP-steel interface. A three-dimensional finite element analysis (FEA) is conducted to predict the experimental behavior. A modeling approach is proposed to simulate the fatigue response of the repaired beams, based on the strain-life method and cumulative damage theory. CFRP-repair results in a recovery of static load-carrying capacity of the damaged beam to that of an undamaged beam. The stress range at the damage influences the fatigue life, damage propagation, and plastic strain development of the repaired beams. Fatigue-crack propagation across the web of the beams is not significant up to 50% of their fatigue life, whereas brittle web fracture follows beyond the threshold. A bilinear fatigue response is observed at the CFRP-steel interface, whose magnitudes are dependent upon the number of fatigue cycles and the applied stress range. An empirical model is proposed to predict the fatigue behavior of the interface.     

岩石应变软化及渗透率演化模型和试验验证

为预测承载岩石的应变软化和渗透率演化,基于Gebdykes白云岩的三轴试验结果,分析了围压对岩石弹性模量、破坏应变、峰值强度、强度退化过程、残余强度和剪胀扩容的影响规律。将岩石变形全过程简化为3阶段,使用强度退化指数、脆性模量系数和扩容指数改进FLAC中的SS模型,建立了考虑围压影响的岩石应变软化模型。基于淮南潘一矿煤、凝灰岩、巴里坤砂岩、山西安家岭泥岩的渗透率与体积应变实验数据,建立了基于体积应变增透率的岩石渗透率演化模型,与改进SS模型结合,建立了考虑围压影响的岩石应变软化和渗透率演化模型。利用本文模型分别模拟了安家岭泥岩和Gebdykes白云岩的三轴压缩、渗透率演化和体积扩容过程,结果表明:1体积应变渗透率演化模型能较好地描述体积应变与渗透率的关系;2本文模型能较好地模拟围压对岩石残余强度、峰后强度退化过程和剪胀扩容的影响,能较准确预测承载岩石的渗透率演化。     

砂土的剪胀理论及其本构模型的发展

经典的应力剪胀理论被广泛应用于土体弹塑性本构模型的建立,较好地模拟了黏性土的变形特性。最新的研究表明:由于应力剪胀理论中忽略了土体内部状态参量对剪胀的影响,而使得该理论应用于砂土时发生了困难。近来一些学者纷纷将土的密度作为变量引入剪胀方程,提出了状态相关剪胀理论,成功地模拟了砂土的各种变形特性。本文就砂土的状态及其描述、剪胀理论的发展、砂土本构模拟等方面进行介绍。     

Mesoscale fracture behavior of Longmaxi outcrop shale with different bedding angles:Experimental and numerical investigations

The mechanical properties and fracturing mechanism of shale containing beddings are critically important in shale gas exploitation and wellbore stability.To investigate the effects of shale bedding on crack behavior and fracturing mechanism,scanning electron microscope(SEM)with a loading system was employed to carry out three-point bending tests on Longmaxi outcrop shale.The crack initiation and propagation of Longmaxi shale were observed and recorded by taking photos during loading.The cracking paths were extracted to calculate the crack length through a MATLAB program.The peak load,fracture toughness and fracture energy all increase with the bedding angle from 0°to 90°.The crack length and energy were also found to increase with the bedding angle in the range of 0°-600 and then drop slightly.The fracturing mechanism of shale includes the main crack affected by the bedding angle and disturbed by randomly distributed particles.The main cracking path was accompanied by several microcrack branches which could form an interconnected crack system.When the main crack encounters larger sedimentary particles,it will deflect around the particles and then restore to the initial direction.A numerical technique using extended finite element method(XFEM)coupled with anisotropic cohesive damage criteria was developed,which is able to capture the dependence of crack propagations on bedding angle and sedimentary particles.This study sheds light on understanding and predicting mesoscale fracture behavior of shale with different bedding angles.     

Analysis of fracture propagation behavior and fracture geometry using a tri-axial fracturing system in naturally fractured reservoirs

Hydraulic fracture propagation behavior and fracture geometry in naturally fractured reservoirs are studied through a series of servo-controlled tri-axial fracturing experiments. Three interaction types between induced fractures with pre-fracture were observed. Difference of horizontal stress, angle of approach and shear strength of pre-fracture were considered to control the fracture propagation behavior. With increasing shear strength of these pre-fractures, the area of arrested behavior also increased. The geometry of hydraulic fracture is mainly controlled by in-situ stress and natural fractures in natural reservoir. Two hydraulic fracture patterns were observed in different stress regimes. In a normal stress regime, it was found to cause fractures, with interacting branches because of pre-existing fracture. Tortuous fractures were found along the direction of fracture height when one of the horizontal stresses is the maximum principle stress.     

Modeling in-plane and out-of-plane displacement fields in pull-off tests on FRP strips

The present paper deals with modeling FRP strips bonded to concrete blocks and tested in pull-off. The investigation starts from the experimental observations obtained by means of an optical image-correlation measurement system which is able to monitor the 3D displacement components of a fine mesh of points on the surface of both the FRP strip and concrete block. Thus, refined measurements of both in-plane and out-of-plane displacements of the FRP strips are available.A brief overview of the key contributions available in the scientific literature on modeling the bond behavior of FRP strips glued to a concrete substrate is firstly proposed. Then, a novel model based on simulating the behavior of the FRP strip as a Bernoulli beam on a layer of springs is formulated. It is aimed at determining the 2D displacement field of the FRP strip during a pull-off test up to debonding which actually occurs in a mixed fracture mode. The model is firstly formulated within the linear range by assuming elastic behavior for the above mentioned springs. The nonlinear behavior due to the cracking of concrete beneath the adhesive interface is then introduced for simulating the above mentioned experimental results. In particular, a bilinear relationship is assumed between interface slips and shear stresses, as is generally accepted within the scientific literature. Furthermore, a damage model is considered for reducing the stiffness of the transverse springs and simulating the crack propagation at the adhesive-concrete interface. Although this is a simplified way of modeling the nonlinear behavior of concrete in shear/tension, it results in rather accurate simulations of the available experimental results. In fact, it can simulate accurately the overall behavior observed in three experimental tests on specimens characterized by significantly different mechanical properties of the strip. Since the model assumes a small set of mechanical parameters for describing the mechanical behavior of the adhesive FRP-concrete interface and results in a reasonable small set of equations, it can be easily employed for identifying the above mentioned mechanical behavior indirectly. Other numerical models already available in the scientific literature (especially those based on the theory of finite elements) for simulating the 2D displacement field in the debonding stage are generally based on much more equations and require a much higher computational effort which makes impractical their use in an indirect identification procedure like the one presented in this paper. In fact, one of the main results of this study consists in determining the distribution of normal (peeling) stresses throughout the adhesive-to-concrete interface. This stress component (Mode I) can neither be directly measured during the tests nor determined by the theoretical models usually adopted for simulating the fracturing behavior of FRP-to-concrete joints in the so-called fracture process in “mode II”. Finally, it was found that the shear stresses are significantly higher than the peeling ones and control the crack propagation process.     

考虑多尺度结构的贯通节理岩体损伤摩擦耦合模型

基于贯通节理岩体结构的多尺度特征,采用两步均匀化方法,给出了节理岩体在复杂荷载作用下的自由焓表达式,建立了节理岩体损伤摩擦耦合本构模型。模型可同时考虑岩块损伤扩展、微裂纹滑移剪胀、法向刚度恢复,节理面多阶凸起体滑移磨损、剪胀演化以及节理与岩块相互作用等特征,较好地反映岩体内部微裂纹、节理等不同尺度微结构变化对其力学特性的影响。采用Lac du Bonnet花岗岩三轴压缩试验、花岗岩节理剪切试验以及不同节理倾角与不同围压下Martinsburg板岩三轴压缩强度试验等成果对模型进行了验证,模型预测值与实测值相当吻合,论证了模型的准确性。