堆石料剪胀特性大型三轴试验研究

基于不同初始孔隙比e0和不同固结压力σc情况下筑坝堆石料大型三轴固结排水剪切试验,分析了堆石料应力应变关系与体积应变关系以及剪胀率ζ与塑性剪应变εsp关系的变化规律。研究表明:对于颗粒级配确定的堆石料,剪胀剪缩转化关系由临界初始孔隙比(e0)crit与临界固结压力(σc)crit共同决定;堆石料破坏时的剪胀率ζf可以成为材料剪胀性的判断标准。通过试验数据非线性拟合分析,提出了堆石料应力应变关系与塑性体积应变关系的统一表达式以及初始物理力学状态依赖的应力-剪胀方程和破坏时剪胀率ζf表达式,并由此获得了堆石料剪胀剪缩转化的判断准则。     

堆石料的亚塑性边界面模型及其验证

 基于高莲士教授针对天生桥面板堆石坝灰岩所进行的堆石料在不同应力路径下的大型三轴试验结果, 提出亚塑性理论边界面模型参数的简捷确定方法。通过编制调参程序, 对堆石料典型三轴试验进行数值模拟, 与试验成果的对比表明, 模型能较好地反映各种复杂加载路径下堆石料变形性状。进而对恒应力比转折、恒应力比加卸载、平面应变试验等土体单元可能出现的复杂应力路径进行了预测, 对深入了解高围压下摩擦型材料的屈服及应力路径相关等特性具有一定意义。同时合理的模型参数为进行面板堆石坝的变形预测和数值仿真模拟奠定了基础。     

堆石料剪切过程中的颗粒破碎研究

颗粒破碎直接改变堆石料本身结构,影响土体的剪胀、内摩擦角、峰值强度、渗透系数和流变变形。但是,目前对于堆石料在剪切过程中的破碎规律尚不明确。通过室内固结排水三轴试验,研究了古水面板坝玄武岩堆石料在制样、固结和剪切过程中的颗粒破碎规律。研究结果表明:堆石料在制样过程中会产生较为显著的颗粒破碎现象;在等向固结过程几乎不产生颗粒破碎。低围压下,颗粒间的翻越和滑移受围压约束较弱,剪切过程中的颗粒破碎不明显。髙围压下,颗粒间的翻越和滑移受到限制,颗粒间的咬合力显著提高,随着剪切应变的增大,土体颗粒不断发生破碎。在颗粒破碎过程中,大粒径颗粒首先破碎,破碎的颗粒从大粒径逐渐向小粒径扩展。粒径在0.5 mm以下颗粒的含量始终随剪应变的增大而增多,且增长幅度随着围压的增大而增大。土体颗粒破碎同时受围压和剪切变形的影响,相同围压下剪切过程中的相对破碎参量Br和剪应变之间的关系可采用双曲线公式描述。     

Investigation for the key technologies of ultra-high asphalt concrete core rockfill dams

The mechanical characteristics of ultra-high asphalt concrete core rockfill dams (UACCRDs) at different periods is investigated via Rankine’s earth pressure theory, and a shear safety control standard for UACCRDs is proposed. The reasonable material parameters of the asphalt concrete core (ACC) and transition material that independently and comprehensively satisfy the shear safety control standard are back-calculated. The engineering measures that reduce the stress level (shear stress) of the ACC are given. Moreover, the engineering measures (straight asphalt concrete core rockfill dams (SACCRDs) are designed as curved asphalt concrete core rockfill dams (CACCRDs)) that reduce the tensile stress of the ACC are proposed. Based on the theory of the straight beam and curved beam on Winkler elastic foundation, the simplified mechanical models of straight asphalt concrete core (SACC) and curved asphalt concrete core (CACC) are established. The improvement effect of CACC that reduces tensile stress is also investigated. The results show that the following value ranges of the internal friction angle, cohesion of ACC and the internal friction angle of transition material for the suitable construction of UACCRDs are recommended: φ_a ≥ 30.5°, C_a ≥ 0.25 MPa and φ_t ≤ 43.5° (h = 200 m), with the growth gradient adjusted by 0.5%, 1.5% and ?0.5%/25 m. The stress level of ACC can be obviously reduced by increasing the internal friction angle and cohesion of ACC, and reducing the internal friction angle of transition material. The simplified mechanical models of SACC and CACC can estimate the force and deformation characteristic of the ACC (SACC and CACC) well. The CACC can significantly reduce tensile stress to a level approximately 42.8% lower than that of SACC.     

堆石料变形与强度特性的大型三轴试验研究

在两种堆石料饱和试样大三轴试验成果的基础上，对其变形与强度特性变化规律进行了总结。给出了其应力—应变关系、侧向应变与轴向应变关系的指数关系表达式，以及抗剪强度包络线在q—p面上的幂函数表达式；探讨了试验现象的内在机理；分析了剪胀、破碎与颗粒重排的相互关系；提出了弹性剪胀和塑性剪胀相互转化的观点。     

堆石料的颗粒破碎规律研究

粗颗粒土剪切过程中的颗粒破碎现象已被广泛认识,并且在试验和理论方面进行了大量研究。利用大型三轴仪开展了一系列不同级配、不同密度、不同围压条件下堆石料的排水剪切试验,并对试验前后的试样分别进行了颗粒分析,以探讨堆石料的颗粒破碎规律及其影响因素。试验结果表明:密度对颗粒破碎影响较小,而级配和围压的影响较大,围压越高则颗粒破碎越严重。对比试验前后的粒径分布曲线发现,颗粒破碎主要集中在粒径20 mm以上的颗粒范围内,粒径变化幅度随粒径的减小呈减小趋势。基于分形理论,建立了颗粒破碎分形维数与围压和颗粒级配之间的关系表达式,为进一步研究堆石料的强度、变形及剪胀特性提供依据。     

Behavior evaluation of geogrid-reinforced ballast-subballast interface under shear condition

The effective functioning of a railway track under operating conditions depends largely on the performance of various rail track interfaces (e.g. ballast-subballast interface, subballast-subgrade interface). In this context, a series of large-scale direct shear tests were conducted to investigate the shear behavior of unreinforced and geogrid-reinforced ballast-subballast interfaces at different normal stresses (σ_n) and rates of shearing (S_r). Fresh granite ballast and subballast having average particle size (D₅₀) of 42?mm and 3.5?mm respectively, and five geogrids with different aperture shapes and sizes were used in this study. Tests were performed at different normal stresses (σ_n) ranging from 20 to 100?kPa and shearing rates (S_r) ranging from 2.5 to 10.0?mm/min. The laboratory test results confirmed that the shear strength of ballast-subballast interface was highly influenced by the applied normal stress (σ_n) and rate of shearing (S_r). The friction angle (φ) of unreinforced ballast-subballast interface was found to decrease from 63.24° to 47.82° and dilation angle (ψ) from 14.56° to 5.23° as the values of σ_n and S_r increased from 20 to 100?kPa and 2.5–10.0?mm/min, respectively. Further, the breakage of ballast (B_g) was found to increase from 2.84 to 6.69%. However, geogrid inclusions enhanced the shear strength of the ballast-subballast interface and also reduced the extent of B_g. The results indicate that it is possible to establish a relationship between the friction angle (φ) and breakage of ballast (B_g), wherein the friction angle (φ) of both unreinforced and geogrid-reinforced interfaces reduces with the increase in breakage (B_g). The interface efficiency factor, defined as the ratio of the shear strength of the geogrid-reinforced ballast-subballast interface to the original shear strength of ballast-subballast interface varies from 1.04 to 1.22. Moreover, the current study revealed that the shear behavior of ballast-subballast interface was influenced by geogrid aperture size (A).     

Effect of shearing rate on the behavior of geogrid-reinforced railroad ballast under direct shear conditions

A series of large-scale direct shear tests were conducted to investigate the behavior of unreinforced and geogrid-reinforced ballast at different rates of shearing. Fresh granite ballast with an average particle size (D₅₀) of 42?mm and five geogrids having different aperture shapes and sizes was used in this study. Tests were performed at different normal stresses (σ_n) ranging from 35?kPa to 140?kPa and at different rates of shearing (S_r) ranging from 2.5 to 10.0?mm/min. The laboratory test results revealed that the shear strength of ballast was significantly influenced by the rate of shearing. The internal friction angle of ballast (φ) was found to decrease from 66.5° to 58° when the shearing rate (S_r) was increased from 2.5 to 10.0?mm/min. It is further observed that the interface shear strength has improved significantly when the ballast was reinforced with geogrids. The interface efficiency factor (α), defined as the ratio of the shear strength of the interface to the internal shear strength of ballast, varies from 0.83 to 1.06. The sieve analysis of samples after the testing reveals that a significant amount of particle breakage occurs during shearing. The value of breakage, evaluated in terms of Marsal's breakage index (B_g), increases from 5.12 to 13.24% with an increase in shearing rates from 2.5 to 10.0?mm/min. Moreover, the influence of aperture shape and size of geogrid on the behavior of ballast-geogrid interfaces was also examined in this study.     

Stress-Dilation of Undisturbed Sand Samples in Drained and Undrained Triaxial Shear

Stress-dilation behavior of undisturbed sand samples tested in both drained and undrained triaxial shear has been studied. The stress-dilation relation is recognized as being a basic component of the stress-strain behavior of granular materials. The dilation angle, ψ commonly used to represent the dilation characteristics of the soil, is defined clearly for plane strain conditions. However, the paper discusses confusion regarding its definition under triaxial loading conditions, and adopts the definition sin ψ=-dε_v^p/dγ_max^p. Drained triaxial tests performed on specimens obtained from undisturbed block samples of sand indicated that the undisturbed material exhibits a well defined stress-dilation relation. By referring to plastic (irrecoverable) components of strain, it was found that this relation was also compatible to results of tests in undrained triaxial shear. Demonstration of this compatibility required that the small membrane penetration effects in the undrained triaxial shear tests, resulting from changing effective confining stress, be taken into account. From the results of the present investigation, and of other studies reported in the literature, it was found that the relation between friction angle, φ′ and dilation angle, ψ, under axi-symmetric conditions, as defined above, can be reasonably expressed by the empirical expression: φ′≍0.4ψ+φ′_cv.     

Shear strength response of reinforced pond ash

Reinforced pond ash is a composite material, which can be used as an alternative construction material in the field of geotechnical engineering. To study the shear strength response of reinforced pond ash, a series of unconsolidated undrained (UU) triaxial test has been conducted on both unreinforced and reinforced pond ash. In the present investigation the effects of confining pressure (σ₃), number of geotextile layers (N), and types of geotextiles on shear strength response of pond ash are studied. The results demonstrate that normal stress at failure (σ_1f) increases with increase in confining pressure. The rate of increase of normal stress at failure (σ_1f) is maximum for three layers of reinforcement, while the corresponding percentage increase in σ_1f is around (103%), when the number of geotextile layers increases from two layers to three layers of reinforcement. With increase in confining pressure the increment in normal stress at failure, Δσ increases and attains a peak value at a certain confining pressure (threshold value) after that Δσ becomes more or less constant. The threshold value of confining pressure depends on N, dry unit weight (γ_d) of pond ash, type of geotextile, and also type of pond ash.     

胶凝粗粒料的弹塑性模型与应用研究

目前工程技术人员对胶凝粗粒料能否应用于高堆石坝等重要、永久性建筑物还存在疑虑,这主要是由于目前对胶凝粗粒料力学性质研究尚不充分。尚没有广为认可的胶凝粗粒料标准本构模型,使得胶凝粗粒料数值模拟结果可信度较低。进行了胶凝砂砾料大围压范围(7级围压,范围100～3000 kPa)的室内三轴剪切试验,试验结果表明,胶凝砂砾料力学性质具有压硬性、强度非线性、强剪胀性和应变软化性等显著特征。研究发现,三轴应力路径的胶凝砂砾料应力应变关系可用驼峰曲线较好描述,其体积剪胀性可采用Rowe剪胀方程描述,以此建立了三轴应力平面内胶凝砂砾料应力应变关系,并根据广义塑性理论中切线模量与塑性模量之间的关系,将模型拓展至三维应力空间,得到了胶凝砂砾料弹塑性本构模型。试验结果和前人多组试验结果对该模型进行了验证,均表明模型具有良好的适用性,模型简明、实用,易于数值实现。将该模型成功应用于一个高面板坝的"胶凝增模区"弹塑性分析,并从坝体、防渗体应力变形安全性方面评估了胶凝料用于高堆石坝"增模"的可行性。     

Shear strength criteria for rock,rock joints,rockfill and rock masses: Problems and some solutions

Although many intact rock types can be very strong, a critical confining pressure can eventually be reached in triaxial testing, such that the Mohr shear strength envelope becomes horizontal. This critical state has recently been better defined, and correct curvature or correct deviation from linear Mohr–Coulomb (M-C) has finally been found. Standard shear testing procedures for rock joints, using multiple testing of the same sample, in case of insufficient samples, can be shown to exaggerate apparent cohesion. Even rough joints do not have any cohesion, but instead have very high friction angles at low stress, due to strong dilation. Rock masses, implying problems of large-scale interaction with engineering structures, may have both cohesive and frictional strength components. However, it is not correct to add these, following linear M-C or nonlinear Hoek–Brown (H-B) standard routines. Cohesion is broken at small strain, while friction is mobilized at larger strain and remains to the end of the shear deformation. The criterion ‘c then σ_n tan φ’ should replace ‘c plus σ_ntan φ’ for improved fit to reality. Transformation of principal stresses to a shear plane seems to ignore mobilized dilation, and caused great experimental difficulties until understood. There seems to be plenty of room for continued research, so that errors of judgement of the last 50 years can be corrected.     

孔隙率对堆石料强度与变形的影响规律

针对拟建某300m级高土石心墙堆石坝所用堆石料,开展了不同孔隙率的大型静力三轴试验,分析了孔隙率对堆石料的强度和变形的影响。试验结果表明:随着孔隙率的增加,堆石料的峰值强度会减小,体积变形逐渐增大,剪切至峰值点时的剪切位移增大,剪切至出现最大体积变形的剪切位移也会增加,随着围压的升高,颗粒出现一定量的破碎;在邓肯–张和南水模型中表现为参数K,dn值的减小和n,dc,dR值的增大等趋势。分析其原因主要是:随着孔隙率的增加,颗粒内部空隙增多,颗粒间间距增大,颗粒间的接触点减少,在相同围压及剪应力作用下,体积变形增大,最大减缩体应变对应的偏应力也会增加;孔隙率的增加,颗粒间的间距增大,颗粒间的咬合作用减弱,导致堆石料的初始切线模量减小,颗粒抵抗外力的作用减弱,颗粒间内摩擦角与抗剪强度的减小。此外,5P(27)的含量对堆石料的强度和变形作用较为明显,孔隙率相同的情况下,峰值强度、最大体积变量、剪切至峰值强度点对应的剪切位移以及最大体积应变对应的剪切位移均会随5P(27)含量的增加会而增大。孔隙率的变化对堆石料强度和变形影响非常明显,建议堆石坝施工现场碾压时严格控制压实度以满足设计要求,确保大坝安全。     

基于细观数值试验的非饱和土石混合体力学特性研究

 从土石混合体细观结构出发,融合细观结构模型生成技术、主–从接触面模型及非饱和土渗流与强度理论,建立非饱和土石混合体的细观数值模拟方法。通过与非饱和土石混合体室内试验结果进行对比,验证所建立的细观数值模拟方法的可行性和合理性。利用该细观模拟方法,分析土–石界面接触特性、含石量及饱和度等因素对非饱和土石混合体力学特性与破坏机制的影响。结果表明：(1) 非饱和土石混合体在低围压下表现出明显的剪胀性,且受含石量和饱和度影响显著;在较高围压下基本上表现为剪缩变形,随含石量的增大其剪缩变形减小,饱和度对剪缩性的影响较小。(2) 土石混合体的峰值强度和变形模量随土–石界面摩擦因数的增大呈非线性增长,在界面摩擦因数大于0.6以后,两者基本趋于稳定值。(3) 含石量越大,非饱和土石混合体的峰值强度和变形模量越大,应变硬化特征更为显著,在含石量增加到58%后峰值强度和变形模量趋于稳定值。在低围压下剪胀变形随含石量的增加而增大;在较高围压时,剪缩变形随含石量的增大而减小。(4) 饱和度越大,基质吸力越小,非饱和土石混合体的峰值强度越低,但变形模量变化不大。     

非线性强度参数对高土石坝坝坡稳定性的影响

堆石料在高应力状态下，其抗剪强度具有明显的非线性。使用对数形式的非线性强度准则得到的坝坡计算安全系数要高于按线性强度准则得到的计算安全系数。现行规范所规定的许可安全系数，是与多年来大量使用偏低的线性强度指标所作的稳定计算相适应的，偏于保守：改用非线性强度后应作适当的提高。对采用非线性强度计算的安全系数比采用线性强度计算的安全系数提高程度进行了统计分析，给出了用非线性强度指标进行稳定分析的安全系数取值标准的建议值。     

Modelling of shear strength of rockfills used for the construction of rockfill dams

This paper presents the results of examination of shear properties of rockfills used for the construction of rockfill dams. Based on the results of a large number of experiments, mathematical model of the shear strength of the rockfill was derived, as a function of normal stress, initial compaction and uniaxial compressive strength of intact rock material. Using this model it is possible to make a realistic assessment of the shear strength of a rockfill for the Preliminary Design. The experimental data used for modelling were obtained by direct shear tests performed in the majority at the “Jaroslav Cerni” Institute for Development of Water Resources in Belgrade, Serbia.     

粗粒土强度和变形的级配影响试验研究

开展了不同细粒含量的无黏性和含黏粒粗粒土的共8组大型三轴排水剪切试验,研究了级配对粗粒土强度、变形、剪胀特性和颗粒破碎的影响。试验结果表明细颗粒含量的大小、是否含泥是粗粒土力学特性的重要影响因素;分析了无黏性粗粒土的颗粒破碎率随围压大小、级配的变化;研究了剪切峰值随围压、细颗粒含量的变化规律,讨论了不同围压、不同级配特征情况下粗粒土的剪胀特性。根据含黏粒粗颗粒土的试验结果,分析了含泥量对强度和变形特性的影响,并从机理上分析了细粒含量对无黏性和含黏粒粗粒土的力学特性影响的差异性。试验结果表明对于土石坝工程良好的坝体填筑料级配、严格控制小于0.075 mm颗粒含量,有利于提高坝体的稳定性和减小坝体沉降。     

软弱岩石峰后应变软化力学特性研究

 软弱岩石给采矿工程中巷道支护和维护带来一系列棘手的问题,深入研究软弱岩石受力变形、破坏的机制和规律,对于保证巷道围岩的安全和稳定具有十分重要的意义。通过对软弱泥岩进行常规三轴压缩试验,得到不同围压下的全应力–应变关系曲线,然后依据峰后岩石任意一点应力状态均满足Mohr-Coulomb极限破坏条件的假设,建立以广义黏聚力 和广义内摩擦角 两个状态参数来表征的软弱岩石后继屈服面模型。在此基础上,利用试验数据绘制岩石峰后不同软化状态时的几组莫尔应力圆,通过“切线法”得出莫尔强度包络线的拟合方程,进而确定出不同围压条件下的 和 值,并借助Matlab软件对广义黏聚力 、广义内摩擦角 与等效塑性剪切应变、围压之间的关系进行最小二乘曲面拟合,得出软弱岩石峰后力学参数的软化规律,结果表明：随着围压的增加,广义黏聚力 值呈快速增加的趋势,而广义内摩擦角 值则显著减小;广义黏聚力 受岩石软化程度的影响也十分明显,从岩石峰值状态到残余状态 值迅速降低,平均降低53.88%,而广义内摩擦角 值在该软化过程中则基本保持稳定。最后,将得到的广义黏聚力 和广义内摩擦角 的拟合方程嵌入到FLAC内置应变软化本构关系中,并利用FLAC3D软件对模型的正确性进行数值模拟验证,结果表明数值模拟曲线与试验曲线比较吻合。     

考虑颗粒破碎影响的粗粒土临界状态研究

粗粒土的临界状态是构建本构模型的重要工具,目前考虑颗粒破碎影响的粗粒土临界状态并不明确。通过大型三轴CD试验,研究了粗粒土在临界状态时的颗粒破碎规律和应力变形特性,探讨了颗粒破碎对粗粒土临界状态应力比和临界状态方程参数的影响。主要结论如下:①粗粒土的临界状态应力比M_c并不是定值,而是与围压之间呈现出非线性的关系;②初始孔隙比对于临界状态应力比M_c的影响则可以忽略;③初始级配相同,围压和初始孔隙比不同的粗粒土,其临界状态在e–(p/p_a)~ξ平面可以用同一条直线来描述,且直线的参数是与初始孔隙比和围压无关的常数。     

考虑围压影响的深埋圆隧围岩应变软化与剪胀特性分析

 不同围压下岩石应变软化与剪胀特性不同,若在隧洞开挖中考虑围岩塑性区域内变化围压影响,其应力–应变场求解方式将区别于既有文献中的传统方法。根据围压影响下应变软化围岩的临界塑性剪切应变变化特征,给出改进的判断围岩是否进入塑性残余区域的规则;引入考虑围压与临界塑性剪切应变的非线性剪胀模型。基于Hoek-Brown屈服准则,根据一定径向应力增量将围岩塑性软化与残余区域分层,采用有限差分法对围岩应力–应变场进行求解;为分析围压对围岩稳定性的影响,根据临界塑性剪切应变与剪胀系数变化与否,设定4种非线性力学模型,深入分析并比较4种力学模型下临界塑性剪切应变、剪胀系数与围岩变形等在塑性软化与残余区域的分布规律。研究结果表明：地质强度指标GSI较小时,考虑围压影响下的围岩应力–应变场与未考虑时差异明显;此时临界塑性剪切应变的减小对开挖边界的围岩剪胀性具一定抑制作用。