常压至高压下中砂剪切特性及应力–剪胀关系

为研究高围压范围内砂土相对密实度和围压对土体强度和变形特性的影响,对3种不同相对密实度砂土试样在常至高围压下进行常规三轴固结排水剪切试验,获得偏应力–轴向应变–体应变关系曲线,同时进行颗粒破碎分析。结果表明:在常至中压范围(0.8 MPa≤σ_3≤2 MPa),应力–应变曲线均表现出不同程度的应变软化,其剪胀性随相对密实度增加和围压的降低而增强;当进入高围压范围时(σ_32 MPa),应力–应变曲线逐渐向应变硬化型转变,试样体积逐渐趋于剪缩。颗粒破碎程度随着围压和密实度的增大而增大,在高围压时由于中密和密砂剪切后期出现了明显的颗粒破碎,导致剪切过程中出现了二次相变。不同密实度土体的破坏内摩擦角和对数围压表现良好的线性关系,拟合确定了破坏内摩擦角随对数围压增加的衰减率,同时基于Bolton应力–剪胀关系拟合确定了试验砂土的临界状态内摩擦角,建立了剪胀指标与初始相对密实度及平均有效应力的关系式,为高压情况下砂土地基稳定性分析等提供强度参数。     

基于材料状态相关临界状态理论的砂土双屈服面模型

分析现有砂土临界状态单屈服面模型存在的缺陷,基于临界状态理论和材料状态相关剪胀性理论,建立了一个适用于不同初始密实度的双屈服面砂土本构模型。该模型考虑了剪切变形和压缩变形机理,克服了单屈服面在一维压缩等试验条件下只有弹性应变没有塑性应变的不足,模型只用一组参数便可描述不同围压不同密实度砂土的应力应变关系,对剪切试验过程中表现出来的剪胀剪缩特性也可以进行很好的追踪。通过对Toyoura砂和Hostun砂一维压缩及三轴排水和不排水条件下的试验结果进行模拟,表明该模型具有较好的预测能力和较广的适用性。     

土的基本特性及本构关系与强度理论

土的基本特性及本构关系与强度理论是土力学及岩土工程学科的重要理论基础之一。本 文针对饱和黏土、砂土及堆石料等粗粒土,总结了这三类土在基本力学特性及本构强度理论方 面的研究现状和发展趋势。饱和黏土部分主要包括压缩特性、剪切特性—临界状态及剪胀/剪缩 、结构性及其破坏、中主应力影响、各向异性及主应力偏转效应、不排水抗剪强度、流变特性 、微观力学解析模型等;砂土部分包括临界状态概念与剪胀性、砂土各向异性、应变局部化等 ;堆石料等粗粒土部分则以颗粒破碎对堆石料等粗粒土的力学性质的影响为主线,重点介绍了 颗粒破碎的度量方法、颗粒破碎对剪胀性、临界状态线的影响以及相应的本构模型、宏观-微观 的力学分析方法。     

砂土的变形特性与临界状态(英文)

通过一系列室内三轴压缩试验研究了砂土的变形特性与临界状态。试验结果显示砂土在剪切过程中会产生剪胀或剪缩 ,其胀缩性由该砂土的当前状态所决定 ,而砂土的当前状态取决于本身的密度和所施加的有效平均正应力。土的密度越大 ,有效平均正应力越低 ,土的剪胀性就越大。试验还发现当剪应变水平较高时土样都出现了临界状态 ,而所观测到的临界状态线与排水条件无关。     

砂土剪胀剪缩特性及其对抗剪强度的影响

为了研究饱和砂土的剪胀剪缩特性及其对抗剪强度的影响,选取滹沱河细砂,利用空心圆柱扭剪仪较系统地开展了一系列不同初始密度、不同固结压力条件下的排水与不排水纯扭剪试验研究,在总应力保持不变的情况下研究了砂土的剪胀剪缩特性,着重探讨了在排水与不排水试验中,不同密度和不同有效围压的砂土在单调剪切荷载作用下的应力-应变关系、硬化与软化、土体的剪胀剪缩以及强度等特性。结果表明:砂土密度和固结压力对砂土剪胀剪缩特性具有显著的影响;砂土的剪胀剪缩特性对砂土的排水、不排水强度以及应力-应变关系产生显著的影响;由于剪胀剪缩特性的影响,砂土的不排水抗剪强度甚至可能高于排水抗剪强度;研究成果可为今后砂土的本构模型和数值模拟提供试验资料。     

冻结砂土三轴试验中颗粒破碎研究

压力作用下颗粒发生破碎是引起砂土力学特性变化的重要因素之一,冻结砂土也是如此。对冻结砂土进行了不同温度和围压下的三轴剪切试验,并筛分得到三轴试验前后的颗粒大小分布曲线。通过引入Hardin定义的颗粒破碎率Br,分析了围压与颗粒破碎的关系及颗粒破碎对冻土抗剪强度的影响。结果表明:在温度为-0.5℃,-1℃,-2℃,-5℃和围压为0.5,2,5,10 MPa的条件下,三轴剪切过程中会产生较为可观的颗粒破碎;颗粒破碎率Br随围压增大,到达一定围压后Br不再随着围压的增大发生明显变化,即存在一个颗粒不再发生明显破碎的临界围压σr。结合前人研究发现,-5℃下一般工程关心的围压范围内压融对冻土力学特性没有显著影响,而颗粒破碎起控制性作用。分析表明:-5℃条件下在不同的围压范围颗粒破碎对抗剪强度具有不同的影响。试验所采用的围压范围内,随着围压的增大,颗粒破碎率增大使得冻土的抗剪强度降低;破碎率达到极限以后,由于破碎的颗粒重排列又导致抗剪强度有所提高。     

颗粒介质剪胀、软化特性的直剪试验研究

颗粒介质的宏观力学行为特征与其微观组构演化密切相关,涉及孔隙比、颗粒形态、颗粒破碎以及各向异性等因素的交叠影响。采用DRS-I型超高压直剪试验系统对3种不同粒径的钢球进行0.4～12 MPa法向压力下的直剪试验,避免颗粒形态、颗粒破碎及各向异性的交叠影响,进而单独分析峰值强度、残余强度与孔隙比演化的关联性,以期为揭示颗粒介质的细观力学机制提供参考。结果表明:1)落雨法制得的圆球颗粒介质普遍呈现剪切软化特性,剪切过程中存在明显的峰值应力和残余应力;2)峰值强度随围压增大呈现出一定程度的减小趋势,与孔隙比变化斜率的极值相对应;3)残余强度基本不受颗粒粒径、围压的影响,对应着孔隙比的稳定值即临界状态;4)采用Rowe剪胀方程能够较好地描述颗粒介质的强度特性与体变特性的关联性。     

反压对土体强度特性的影响试验研究及其影响机理分析

为提高试样饱和度,室内试验普遍采用反压饱和法,但现行规范中对其反压大小未做具体规定。该文利用竖向-扭转双向耦合剪切仪及全自动真三轴仪,针对福建标准砂(相对密实度30%、58%、70%)及细砂,进行了不同反压下单调排水与不排水剪切试验,分析不同反压下砂土的应力-应变关系、超静孔隙水压力发展规律。试验结果表明：反压对剪胀性砂土的固结不排水剪切强度具有显著影响,且剪胀性越大,其影响更显著;反压对剪缩性砂土的不排水抗剪强度影响不显著;反压对砂土固结排水抗剪强度的影响相对较小;反压对砂土有效抗剪强度指标基本没有影响。本文认为反压对剪胀性砂土的固结不排水抗剪强度产生影响的原因是：反压不同时,土体中的气液混合体的体积模量并不相同,因此相同的体胀趋势或相同的体积回弹下所引起的超静孔隙水压力不同,进而直接影响有效围压及其砂土抗剪强度;而土体发生剪缩时,气液混合体越接近刚体,其体积模量对反压的依赖性越小,因此,相同的剪缩趋势或相同的体积压缩量下,所产生的超静孔隙水压力与反压的大小没有显著的依赖关系,因此在剪缩状态下,反压对砂土的应力-应变关系及抗剪强度没有显著的影响。在固结排水试验中无论反压设定多大,不存在超静孔隙水压力,有效围压与反压大小无关,因此反压对砂土排水抗剪强度没有显著影响。     

土石混合体三轴剪切试验及三维多重剪切边界面模型

土石混合体是介于离散和连续介质之间的特殊地质材料,其颗粒分布特征和力学特性是控制土石混合体高填方工程稳定性的重要因素。以白鹤滩水电站库区象鼻岭移民安置点防护堤工程为依托,利用大型三轴试验仪,对两种土石混合体填筑料在3种不同围压条件下进行固结排水剪切试验,分析了土石混合体的偏差应力和体积应变随轴向应变的变化规律以及剪缩和剪胀特性。在此基础上,根据粒状土的临界状态和边界面弹塑性理论,引入了适合于土石混合体的临界黏聚力和状态参数,并基于空间随机分布微观剪切结构,建立了土石混合体状态相关三维多重剪切边界面模型。通过模型模拟结果与三轴剪切试验结果的比较,验证了该模型能够合理地描述土石混合体在低围压下的应变软化和剪胀特性,以及在高围压下的应变硬化和剪缩特性。     

非饱和含黏砂土强度、变形和水量变化特性研究

通过3种应力路径的三轴试验,即同时控制吸力和净围压的12个三轴排水剪切试验、控制净平均应力的3个三轴收缩试验和控制吸力的3个各向同性压缩试验.对非饱和含黏砂土的强度、变形、屈服和水量变化特性进行了研究,得出如下结论:相同干密度,相同净围压作用下,吸力越大试样的剪切强度越大,剪胀性越强,应力应变曲线逐渐由应变硬化型向应变...     

Investigation of unsaturated frozen soil behavior: Phase transformation state,post-peak strength,and dilatancy

In this study, a database of triaxial compressive tests on unsaturated frozen soils is compiled to investigate the mechanical behavior that has not been considered in previous studies. The results for the stress-strain volume changes are presented first. Then, the physical mechanisms that might control the deformation of unsaturated frozen soils, namely, volumetric compression and frictional sliding, are used to interpret the changes in volume and deviator stress during the tests. The relationship between the compression rate due to the mean stress and the dilation rate due to shearing determines the changes in sample volume and shear stress with an increasing axial strain. The test results indicate that confining pressure and temperature significantly affect the phase transformation state, the post-peak strength, and the maximum dilation ratio. A higher post-peak strength ratio is observed in tests performed at higher confining pressure or under a higher temperature. As the confining pressure is increased, the shear stress at the phase transformation state initially increases and then decreases or stabilizes. However, the maximum dilation ratio decreases considerably and tends to reach zero after a certain confining pressure is reached. Both the deviator stress at the phase transformation state and the maximum dilation ratio are higher under a lower temperature given constant confining pressure. Ice cementation and pressure melting are attributed to the specific features of frozen soils compared to those of unfrozen soils. This paper provides new insights into the mechanical behavior of frozen soils.     

Undrained Monotonic and Cyclic Shear Behaviour of Sand Under Low and High Confining Stresses

Monotonic and cyclic loading tests have been carried out on a silica sand over a wide range of stresses in order to compare non-crushing and crushing behaviour. Samples were sieved at several stages of testing to theoretically determine the increase in particle surface area and hence degree of crushing. Undrained shear behaviour was compared for crushing and non-crushing regions above 3 MPa. Samples consolidated to 0.1 MPa demonstrated strong dilative behaviour, while above the yield stress of 3 MPa dilation was suppressed and considerable particle crushing occurred. Shearing caused a marked increase in particle crushing particularly after the phase transformation point. Crushing at the steady state was similar for isotropic and anisotropically consolidated sands. At high confining pressures the cyclic strength curves were similar to those for loose sands except for confining pressure dependency due to particle crushing. For low confining stresses cyclic strength increased with initial stress ratio, while for high confining stresses it decreased with initial shear stress ratio. In the cyclic tests there was no significant crushing for 0.1 MPa. Crushing was seen to increase rapidly after the phase transformation point, where high strains developed and where particle rotation and translation contributed to the crushing process.     

低应力条件下海洋粉土的不排水强度特性及其在海底浅层滑坡分析中的应用

 针对浅层滑坡等海底地质灾害,设计并实施一系列的十字板试验和三轴试验,对低应力条件下典型海洋粉土的不排水强度特性进行研究探讨。试验结果显示：固结度对粉土的十字板强度有明显影响,十字板强度随着超孔隙水压力的升高而降低。剪切应变速率对粉土的固结不排水三轴强度影响很小,而超固结比和剪切围压则影响较大;三轴试验中各个试样都能够达到临界状态,临界状态强度随着超固结比和剪切围压的增大而增大。分析波浪作用下海床土剪切滑动的渐进特性,指出临界状态是滑坡发生时滑带土所能达到的最终状态。采用平均有效应力,对十字板强度和三轴临界状态强度进行归一化处理,两种强度显示较好的一致性,得到低应力条件下海洋粉土的临界强度线。最后,应用所得的临界强度线,对风暴潮作用下黄河水下三角洲粉土海床浅层滑坡及“复活”进行很好地分析和解释。     

路用土石混合料的剪切特性试验研究

路用土石混合料是一种不同于土和岩石的高度非均质材料。混合材料的变形和剪切强度参数主要受含石率、块石形状和土体性质的影响。通过现场取样,综合考虑三个因素的影响,开展室内大型直剪试验。研究结果表明:应力应变关系曲线表现出三个阶段;碎石与土组成的混合料抗剪强度大于同等混合比例的卵石与土组成的混合料;含石率为40%~60%时,混合料抗剪强度较好;粘土与块石的混合料含石率较低时,低围压下表现为剪胀,高围压下表现为剪缩,含石率较高时,低围压和高围压均表现为剪胀;砂土与块石的混合料在低围压和高围压下均表现为剪胀。为快速评测现场土石混合料的粘聚力和内摩擦角,本文还提出了一种实用、简便的估算方法。     

RELATIVE DENSITY EFFECTS ON DRAINED SAND BEHAVIOR AT HIGH PRESSURES

A high-pressure triaxial compression and extension apparatus was employed to investigate the effects of initial relative density on the drained behavior of sand. Experiments were performed on cylindrical specimens of Cambria sand with initial relative densities of 30% and 60%, and compared with previously published results for 90%. Confining pressures in the range of 0.25 to 60MPa were employed to obtain the characteristic behavior of the sand over a large range of pressures. As the confining pressure increased particle crushing increasingly influenced the stress-strain and volume change relations, producing increasing strains to failure and increasingly contractive volume changes. Beyond certain high values of mean normal stress at failure, increasing with initial relative density, the volumetric contraction and strains to failure began to decrease. The Mohr-Coulomb failure envelopes show the greatest curvature for the highest density, but the failure envelopes merge at high pressures where, due to particle crushing, the relations between void ratio and isotropic pressure are described by a single curve. The experiments show clearly that the rates of dilation at failure and the friction angles are directly related to each other, as has been proposed by many investigators. The friction angles are higher in extension than in compression for lower confining pressures, but a crossover occurs and the friction angles are higher in compression than in extension for higher confining pressures.     

南京砂的稳态特征研究

以南京砂的固结不排水试验为基础,对其稳态特征进行了研究。结果表明：在低围压下稳态线的曲线表达更符合实际情况,松散南京砂在低围压下广义剪应力终值接近于 0 而表现出不稳定;稳态是土体固有属性,稳态内摩擦角不仅可以通过稳定状态下的关系得到,也可通过同一围压下不同密实度南京砂的有效应力路径末端拟合得到,本研究得到南京砂稳态内摩擦角为 35.2 °;峰值强度高估了流滑中土体的强度,土体实际发挥的是残余强度,研究中建立了南京砂峰值强度和残余强度的关系;脆性指数能较好地反映南京砂应变软化程度和流滑的可能性;孔隙比的微小变化可导致南京砂软化程度的较大变化。     

循环三轴应力路径下钙质砂颗粒破碎演化规律

钙质砂广泛分布于中国南海区域,是吹填造陆的重要材料。钙质砂颗粒容易破碎,使得其力学特性相比于普通的陆源硅质砂有显著差异。对取自中国南海西沙群岛某岛礁的钙质砂开展了三轴排水循环剪切试验,研究了围压、循环应力比、循环振次对钙质砂颗粒破碎发展过程的影响。在试验所采用的围压范围内,钙质砂在固结过程中产生的颗粒破碎较少,但是在随后的循环剪切过程中产生了显著的颗粒破碎。在循环剪切作用下,钙质砂的颗粒破碎形式主要是尖角的磨损,剪切后试样的颗粒中出现了一些碎屑和微细颗粒,大颗粒的棱角有一定程度的磨圆,但粒径无明显减小。在常围压下的等幅循环剪切中,颗粒破碎程度随着循环剪切次数的增大而增加,增长速率逐渐降低,可以采用对数曲线来描述相对破碎指数的发展过程。再考虑围压和循环应力比的影响规律,初步建立了一个描述颗粒破碎演化过程的数学模型。     

Geotechnical Properties of a Pumice Sand

The pumice sand found in the North Island of New Zealand has properties which lie beyond those usually associated with cohesionless soils. The grains are very soft and the sand has a high void ratio, thus forming a compressible material. This paper presents the results of a series of drained triaxial and K₀ tests on dry pumice sand. The tests were conducted to evaluate the geotechnical properties, particularly the critical state parameters, of the sand and also to provide background information for interpretation of cone penetration tests in the material. The K₀ tests were used to evaluate the compression envelope under conditions of no lateral strain and to determine values of constrained modulus. Significant grain crushing was found to occur during testing even at low confining stress, in fact the stress-strain-strength behaviour of the material is dominated by particle crushing. Routine soil testing techniques were found to be inadequate for the evaluation of the specific gravity of pumice sand particles and a different technique was used for this purpose. The angle of friction of pumice sand was found to be larger than that of quartz sands, however pumice sand required very large shear strains to mobilise the peak and critical state shear strength and, for several tests, critical state conditions were not reached.     

粗粒料颗粒破碎三轴试验研究

随着粗粒料在高土石坝等工程中广泛应用,颗粒破碎逐渐成为粗粒料工程特性研究的一个重要方面。通过三轴颗粒破碎试验,研究了某粗粒料等压固结、峰值以及不同应力水平下的颗粒破碎规律,分析了干湿状态的影响。结果表明:等压固结颗粒破碎率与围压之间以及峰值内摩擦角与颗粒破碎率之间均呈幂函数关系;相同围压下颗粒破碎率随应力水平增加而加速增加;相同应力水平下颗粒破碎率随围压增加而增加;不同围压下的颗粒破碎率与应力水平之间可进行归一化处理;同应力状态下湿态颗粒破碎率较干态大,二者差异的决定因素为材料软化系数。同时,对以上结论进行了理论分析,提出了一个颗粒破碎的估算方法,探讨了干湿状态对颗粒破碎影响与材料软化系数之间关系。