Shear strength of an unsaturated weakly expansive soil

To study the weakly expansive clay obtained from a slope along Wuhan—Shiyan expressway in Hubei Province,soil-water property tests and some unsaturated triaxial tests with suction control were conducted,and the soil-water retention curve（SWRC） and unsaturated shear strength of this soil were obtained.Results show that the air-entry suction and the residual degree of saturation of the tested soil are 106 kPa and 8%,respectively.The boundary effect zone and the transition zone can be identified on the desorption curve,but the residual zone is not so obvious.The unsaturated shear strength increases as suction increases within the range of controlled suction in the test,and friction angle,b,in the triaxial shear test is 17.6°.Based on the results,constitutive models for predicting the unsaturated shear strength using the SWRC were evaluated,and comparisons between prediction and measurement were made.It is concluded that for engineering purpose,the constitutive model should be carefully selected based on soil properties when predicting the unsaturated shear strength using the SWRC.

Fractional derivative statistical damage model of unsaturated expansive soil based on unified hydraulic effect

Unsaturated expansive soil is widely distributed in China and has complex engineering properties.This paper proposes the unified hydraulic effect shear strength theory of unsaturated expansive soil based on the effective stress principle,swelling force principle,and soil–water characteristics.Considering the viscoelasticity and structural damage of unsaturated expansive soil during loading,a fractional hardening–damage model of unsaturated expansive soil was established.The model parameters were established on the basis of the proposed calculation method of shear strength and the triaxial shear experiment on unsaturated expansive soil.The proposed model was verified by the experimental data and a traditional damage model.The proposed model can satisfactorily describe the entire process of the strain-hardening law of unsaturated expansive soil.Finally,by investigating the damage variables of the proposed model,it was found that:(a)when the values of confining pressure and matric suction are close,the coupling of confining pressure and matric suction contributes more to the shear strength;(b)there is a damage threshold for unsaturated expansive soil,and is mainly reflected by strength criterion of infinitesimal body;(c)the strain hardening law of unsaturated expansive soil is mainly reflected by fractional derivative operator.

A Study on the Shear Strength Characteristic of Unsaturated Red Clay

The red clay in Chenzhou, Hunan province is mostly in unsaturated state. Simply applying the mechanical properties that derived from classic saturated soil mechanics often leads to slope failures in this region. In order to study the shear strength characteristic of unsaturated red clay in Chenzhou and to explore a shear strength equation that can be easily applied in engineering practice, a series of triaxial tests of saturated and unsaturated red clay samples were performed using the regular triaxial testing apparatus. The testing results show that the peak strength of red clay drops slightly before the moisture content of 30% but decreases sharply after that. The friction angle of red clay under unsaturated state is basically equal to the effective friction angle under saturated state, while the cohesion of unsaturated red clay is far much bigger than that of saturated one, which indicates that the matric suction makes a great contribution to the cohesion. By fitting the testing results with appropriate curves, the relationships between total strength parameters and with moisture content were obtained. The total increases logarithmically before the moisture content of 35% then decreases linearly, while decreases cubically with increasing moisture content.

Slope Stability Analyses of Outang Landslide Based on the Peak and Residual Shear Strength Behavior

Most of the natural and compacted fine-grained soil slopes that are in saturated or unsaturated condition undergo a large deformation prior to reaching failure conditions.Such slopes should be designed taking account of their strain-softening behavior using the residual shear strength (RSS) parameters.In this paper,the slope stability of a recently reactivated Outang landslide near the Three Gorges Dam in China is analyzed based on the RSS parameters of unsaturated soils.In addition,comparisons are provided in the FOS values of slope using both the peak shear strength (PSS) and RSS parameters.Firstly,a series of site investigations of the hydrologic and geologic conditions,ground surface displacements and cracks were described.The PSS and RSS behaviors of the sliding soils derived from a series of direct shear test results performed on saturated and unsaturated soil specimens are summarized.Secondly,a series of slope stability analysis were conducted considering the precipitation and Yangtze River water level variation within a representative period of 7 months,based on the PSS and the RSS properties.In this study,three different scenarios were considered,which include: i) considering only the precipitation with a constant water level;ii) considering only the decrease in water level without rainfall;iii) considering the combination of precipitation and decrease in water level.In each scenario,four steps were included to calculate the values of factor of safety (FOS) at different times.1) A steady-state seepage analysis was conducted with a constant total head at 525 m on the left boundary and 175 m on the slope surface below the Yangtze River water level.The initial pore water pressures were simulated in the slope under no precipitation and variation of water level.2) A specific boundary condition was applied on the slope surface to model the precipitation and Yangtze River water level variation.A transient seepage analysis was conducted to calculate pore water pressures at different times based on the initial pore water pressures.3) The FOS values at different times were calculated by the Morgenstern-Price method taking account of the variation of pore water pressures at different times,using the peak shear strength (PSS) parameters.4) The last step was repeated replacing PSS parameters with RSS parameters.The RSS parameters were lower than the peak values from laboratory’s direct shear test results for the soils in the sliding zones.The reduction in shear strength from peak to residual state under unsaturated soil condition was greater than that for a saturated soil.The FOS decreased almost linearly with time for the scenario in which only the influence of rainfall infiltration was considered.However,the total reduction in the FOS was relatively small.The FOS decreased rapidly at a linear rate with respect to time with a decrease in water level for the scenario in which Yangtze River water level decrease was considered.The FOS reached to a relatively constant value after Yangtze River water level reached the lowest value.The decrease in Yangtze River water level was the dominant factor that contributed to a reduction in the FOS.The FOS was strongly dependent on the development of the phreatic line after the Yangtze River water level reached the lowest value.The FOS calculated by RSS (i.e.FOSR) is less than unity;they were approximately 16% lower in comparison to that calculated by PSS (FOSP).If PSS parameters were used,the slope would still be stable even under the combined influence of precipitation and Yangtze River water level decrease.These results are inconsistent with the field observations.For this reason,the RSS parameters should be taken into account to evaluate reliably the slope stability of the Outang landslide.

非饱和黏性土的广义有效应力三剪弹塑性本构模型

将非饱和土广义有效应力原理与三剪强度准则相结合,提出了非饱和土广义有效应力三剪强度准则。将所提准则作为破坏准则,分别采用等量代换法和坐标平移法推导出新的破坏应力比,并将其与非饱和土修正剑桥模型相结合得到了新的屈服函数。相比于原来修正剑桥模型中的破坏应力比为定值,新的屈服函数可以更好地反应土体全应力状态、中间主应力效应和拉压不等效应。在弹塑性理论的框架下,建立了非饱和土的广义有效应力三剪弹塑性本构模型。以江西正常固结非饱和重塑红黏土作为试验研究对象,进行室内土工试验、土水特征曲线试验、压缩回弹试验、非饱和土常规三轴固结排水试验。将该本构模型计算结果与非饱和土三轴固结排水试验结果进行对比验证。结果表明:数值模拟结果与试验结果吻合较好,验证了该本构模型的正确性。在轴向应变较小时,等量代换法和坐标平移法模拟结果比较接近,随着轴向应变增大直至偏应力达到平稳状态的过程中,等量代换法计算结果要大于坐标平移法计算结果,且更接近于试验值。真三轴计算预测结果表明:在固结排水条件下,初始压实度、净围压、基质吸力相同,中间主应力影响系数越大,则剪应力和体应变越大。b值相同的情况下,轴向应变较小时,等量代换法计算结果和坐标平移法计算结果比较接近,随着轴向应变的增大,二者之间的差值也越来越大。

不同吸力下伊犁河谷原状非饱和黄土强度特性试验研究

西风带影响下的伊犁河谷黄土分布广泛,黄土滑坡等灾害十分发育。研究不同吸力下伊犁河谷的非饱和黄土抗剪强度及变形特性,对地质灾害防治具有重要意义。本研究以新疆伊犁河谷的原状黄土为研究对象,开展矿物成分分析和物理性质测试,并利用非饱和土应力-应变控制式三轴仪,对黄土的原状样进行了控制净围压和吸力为常数的固结不排水剪切试验,分析非饱和黄土的强度特性。研究结果表明:(1)在试验测定的基质吸力范围内,净围压越高,试样应力-应变曲线的硬化特征越明显,试样在剪切过程中基本表现为鼓胀变形;在高基质吸力条件下,应力-应变曲线趋向于软化型,试样易出现剪切破坏。(2)试样体应变的变化受净围压和基质吸力的影响。随净围压的增大,体应变趋向于剪缩状态;在较高的基质吸力条件下,体应变的峰值剪胀量越大,剪胀特征也越强。(3)基质吸力影响非饱和黄土的抗剪强度参数。试样黏聚力随基质吸力的增大,表现出良好的线性增长趋势。而试验中基质吸力对内摩擦角的影响程度相对较低,随基质吸力的增大,粉质黏土土样内摩擦角的增长速率逐渐减小至0,内摩擦角值趋于稳定;而粉土土样内摩擦角无明显变化。本研究可为伊犁河谷黄土滑坡等灾害的研究提供力学参数和理论依据。

不同饱和度下膨胀土原位孔内剪切试验及强度响应特征

膨胀土强度随饱和度变化显著,非饱和膨胀土的抗剪强度一直是研究膨胀土工程问题的关键。选取南阳市某高速路沿线边坡膨胀土为研究对象,进行了天然条件下处于不同饱和度状态的膨胀土原位孔内剪切试验,获得了自然条件下膨胀土的抗剪强度随饱和度的变化特征。膨胀土的抗剪强度、黏聚力、内摩擦角随着膨胀土饱和度的增大而逐渐减小。结合现场试验数据和土-水特征曲线,提出了以法向应力、饱和度为变量和以法向应力、基质吸力为变量的两种非饱和膨胀土的抗剪强度计算模型,并结合试验数据对计算模型的适用性进行了验证。与之前常用的非饱和土抗剪强度计算模型相比较,提出的两种计算模型所需的拟合参数少,适用于更宽吸力范围、不同法向应力条件下非饱和膨胀土抗剪强度的计算,且结果更为准确。

非饱和砂土抗剪强度变化规律与预测模型试验:以南昌地区砂土为例

为了研究非饱和砂土抗剪强度变化规律并建立有效的预测模型,以南昌地区砂土为代表,探讨了含水率、干密度等因素对非饱和砂土抗剪强度影响机理,结果表明:不同含水率和干密度工况下的非饱和砂土剪应力-剪切位移曲线均为软化型,表现出四阶段非线性变化规律,并随含水率和干密度变化呈现出递变性。非饱和砂土抗剪强度随含水率和饱和度的增大呈减小变化,随干密度增大呈增大趋势。非饱和砂土黏聚力随含水率增大呈先减小后增大的变化规律,随干密度增大近似呈线性增大,内摩擦角随含水率增大呈先增大后减小的变化趋势,由干密度变化导致的内摩擦因数变化幅度较小,约在25%以内。基于莫尔-库伦强度准则,构建了考虑含水率和干密度的非饱和砂土抗剪强度非线性回归模型,可为工程地质参数选用和预测提供参考。

基于考虑含水率对土体抗剪强度参数影响的边坡稳定性分析

降雨是诱发滑坡的关键因素,一般认为其致灾机理主要在于导致土体重度增加、孔隙水压力提高及土的抗剪强度降低。然而在水的作用下,土体抗剪强度参数还存在一定的弱化现象,这对边坡的稳定性有一定的影响。该文以东岭信滑坡为研究对象,基于对滑带重塑土的直剪试验,研究含水率对土体抗剪强度参数的影响,并基于GeoStudio软件二次开发实现考虑含水率对土体抗剪强度参数影响的边坡稳定性分析。结果表明,抗剪强度参数随含水率的增加而逐渐减小,滑带土的黏聚力和内摩擦角整体上均可与含水率拟合成线性关系。忽略水对岩土体抗剪强度参数的弱化作用会高估边坡的稳定性。研究结果对于进一步认识降雨诱发滑坡机理具有一定理论意义。

控制吸力下低应力条件的合肥膨胀土抗剪强度试验

膨胀土边坡在变饱和条件下可能发生浅层滑塌失稳。为了研究非饱和膨胀土在低应力条件下的强度特性,采用非饱和三轴仪设置多组不同基质吸力和应力条件,进行合肥膨胀土的三轴剪切试验,对试验数据进行分析。试验结果表明:当膨胀土所受净围压应力不断降低时,其应力-应变曲线会由应变硬化向应变软化过渡;非饱和膨胀土强度随基质吸力增大而增大;浅层膨胀土的抗剪强度会呈现明显降低,具有非线性特点;非饱和膨胀土的黏聚力对抗剪强度影响显著,低应力条件下黏聚力会显著降低,内摩擦角明显增大;净围压应力对低应力段土体强度影响明显,而基质吸力对低应力段土体强度影响较弱。

双孔隙结构重塑非饱和膨胀土的抗剪强度特性

针对具有双孔隙结构(即集聚体间和集聚体内孔隙)及双峰持水曲线的内乡膨胀土压实样,进行了一系列宽广吸力范围内非饱和土三轴剪切试验及峰值强度演化规律的研究。试验结果表明:在净应力相同的条件下,中低吸力下的应力应变关系为应变硬化,伴随明显的剪缩变形;高吸力下为峰值后软化,在经历了1%~3%的体积收缩变形之后开始出现剪胀。试样的脆性随吸力的增加而增长,在中低吸力下呈桶形或中心鼓形的延性破坏模式,在高吸力下发生应变局部化现象,伴随着明显的剪切带出现。此外,脆性增加了峰后软化的幅度,表现为峰值偏应力与残余破坏状态之间的差值增大。基于区分毛细和吸附作用的双峰持水曲线(SWRC)模型,针对内乡膨胀土与其它具有双孔隙结构及双峰SWRC的土体在毛细吸应力空间进行了峰值强度分析,将吸力从孔隙应力尺度乘以毛细饱和度变为骨架应力尺度时,其呈现的强度包络线为双折线特征。理论分析表明,低吸力范围内,双孔隙结构非饱和土的抗剪强度由饱和强度与毛细吸应力贡献;高吸力范围内,抗剪强度应由饱和强度、毛细吸应力与胶结作用提供。

干密度与含水率对非饱和土强度影响的试验研究

影响非饱和土强度的因素很多,在众多的因素中,干密度和含水率的变化影响无疑是巨大的,不同地区的土干密度和含水率对强度影响是没有一定的规律,有些地区是只影响到土的黏聚力,有些地区在影响到土的黏聚力的同时也影响到土的内摩擦角,本文通过对绍兴地区2种不同黏粒含量的土在不同干密度下强度特性对比分析,得出随着干密度的增加,粉质黏土和粉土土强度增加,同时进行了一定干密度下土体随着含水率的变化强度变化,通过试验结论分析得出土的强度变化,随着含水率的增加,土的黏聚力有先增大后减小的趋势。而内摩擦角随着含水率增加逐渐减小的趋势。强度的变化量与最优含水率有一定的关系,并结合三相的赋存状态分析强度指标的变化原因,为加强研究应用提供依据。

广吸力范围内一种改进的非饱和土抗剪强度模型

不同类型的非饱和土在自然界中广泛存在。由于不同类型土的颗粒级配和矿物组成成分的不同,其强度特性存在较大的差异。为了建立一种适用于不同类型非饱和土的强度预测统一模型,本研究系统地分析了土体类型对非饱和土强度演化趋势的影响,在综合考虑两种常用模型优越性的基础上,提出了一种改进的非饱和土抗剪强度模型,同时,利用现有常用的强度模型,对不同类型土在广吸力范围内的强度特性进行了预测,并对拟合结果进行了对比分析。研究结果表明:(1)在广吸力范围内,不同类型土的强度演化规律可分为两类,山峰型和增长稳定型;(2)改进模型可以较为准确地预测不同类型土在广吸力范围内的强度特性;(3)明确了改进模型中各参数的物理意义,并初步给出了改进模型在应用于不同类型土时各参数取值的建议范围。

宽饱和度范围非饱和土抗剪强度指标的演化模型

非饱和土的抗剪强度指标随饱和度变化的规律错综复杂,为了用数学模型描述这些规律,对现有文献中非饱和土的黏聚力、内摩擦角、剪胀角随饱和度变化的规律进行分析。分析结果表明:(1)当土体饱和度降低,土体从毛细作用主导逐步过渡到吸附作用主导时,土体内摩擦角会增大,其变化规律存在一个转折点,可以定义为临界饱和度Src。当饱和度Sr大于临界饱和度Src时,内摩擦角可视为常数;当饱和度Sr小于临界饱和度Src时,内摩擦角随着饱和度的减小线性增加,传统扩展摩尔-库仑准则等模型不再适用。(2)黏聚力随饱和度的变化过程存在山峰效应,即黏聚力先随饱和度Sr的降低呈指数函数关系增加,当饱和度Sr达到非饱和土强度极值对应的饱和度Srv时达到最大,随后有所降低,这也是导致非饱和土强度存在山峰效应的主要原因。(3)剪胀角随饱和度的降低和垂直压力的降低而增大,可采用线性模型描述。基于以上规律,提出了一个宽饱和度范围内的非饱和抗剪强度演化模型。通过与文献中的试验数据及现有强度模型拟合结果进行对比发现,新模型在描述宽饱和度范围内的土体强度时具有更好的适用性。

干湿循环下煤系土抗剪强度与微观结构参数关联度研究

通过直剪试验和微观扫描电子显微镜试验,研究了干湿循环作用下非饱和煤系土宏微观参数随含水率和干湿循环次数的变化规律,通过灰色关联分析得到了显著影响强度参数的微观结构参数。研究表明:煤系土试样抗剪强度及总黏聚力随含水率增加而逐渐减小,随干湿循环次数增加发生劣化,经历1次干湿循环后劣化程度较高,经历3次干湿循环后趋于稳定;含水率和干湿循环次数对总内摩擦角变化影响较小;随含水率和干湿循环次数增加,煤系土大颗粒平均直径和数量减小、大孔隙平均直径和数量增大、颗粒形态趋于简单、孔隙形状趋向圆形、颗粒排列更加有序;干湿循环过程中宏观总黏聚力与微观结构参数中孔隙丰度、颗粒分形维数、颗粒定向概率熵、颗粒平均直径密切相关。总之,干湿循环造成的煤系土颗粒崩解、破碎、胶结连接弱化是导致煤系土抗剪强度劣化的主要原因。

常吸力下非饱和粗粒土率相关力学特性试验研究

为研究山区高填方路基的时效变形特性,针对路基粗颗粒填料,开展了粗颗粒填料持水特性和一系列常吸力作用下的率相关力学特性试验,采用三种不同剪切速率对试样进行剪切,分析围压、基质吸力及剪切速率对土体强度和变形特征的影响。试验结果表明:粗颗粒填料的土水特征曲线未出现残余吸力值;不同围压下粗颗粒填料的峰值强度可表示为基质吸力的函数,基质吸力越大,非饱和粗粒土的剪切强度越高,且体积剪缩变形越小;非饱和粗粒土的强度与体积变形均受剪切速率的影响,且变化趋势表现出高度一致性,剪切速率越大,试样强度越高,对应的体积变形越小。

聚丙烯纤维加筋绍兴非饱和黏土的剪切特性研究

为了研究纤维加筋非饱和土的剪切特性,以绍兴地区广泛分布的非饱和黏土为研究对象,聚丙烯纤维为加筋材料,通过一系列非饱和直剪试验,探讨了纤维长度对加筋土剪切变形特性、抗剪强度及其指标的影响规律,并简要分析了聚丙烯纤维的增强机理,最后得出了0.2%掺量下补强效果最佳的纤维长度.研究结果表明:随着净法向应力的增大,土体的剪应力-剪切位移曲线由软化型逐渐向硬化型转化,纤维长度L为12 mm的加筋土样表现出的剪切硬化特性最明显;不同净法向应力下,纤维加筋非饱和土的抗剪强度均高于素土;随着纤维长度的增加,纤维加筋土的黏聚力呈先增加后减小,内摩擦角先增加后趋于平缓;当纤维长度L为12 mm时,聚丙烯纤维对土体抗剪强度指标的补强效果能够最大程度得到发挥.

冀北地区重塑膨胀土抗剪强度影响因素分析

为研究冀北地区重塑膨胀土的抗剪强度特性,对不同含水率、不同干密度的重塑膨胀土进行了快剪实验,分析了法向应力、含水率、干密度对膨胀土抗剪强度的影响,并采用灰色关联理论分析了各因素对土体抗剪强度及残余强度的影响程度。研究结果表明:含水率、干密度及法向应力对抗剪强度均有较大影响,法向应力及干密度的增加均会造成抗剪强度的增大,而随着含水率的增加,抗剪强度先增后减,且在最优含水率处达到峰值;从应力应变曲线可以看出,随着干密度的增大,膨胀土的剪应力-应变特性由应变硬化型转化为应变软化型;由灰色关联理论计算结果可以得出干密度对重塑膨胀土抗剪强度的影响最大,法向应力对重塑膨胀土残余强度的影响最大。

宽广吸力范围非饱和土剪切强度试验研究及其预测

以弱膨胀土为试验材料,开展了宽广吸力范围内非饱和土的持水和强度特性试验研究,并提出适用于宽广吸力范围的非饱和土强度模型。结果表明,在广吸力范围内,应力−应变曲线随吸力增加而增高,在低吸力范围呈现应变硬化,在高吸力范围呈现应变软化。低吸力范围,试样呈现剪缩变形;高吸力范围,试样在轴向应变2%处开始呈现剪胀趋势。此外,提出一种区分吸附水和毛细水的土−水特征曲线模型,并假设吸力引起的强度增强部分主要由毛细水决定。将有效应力系数用毛细水饱和度替代,并代入Bishop非饱和土强度公式,利用不同类型土剪切强度的实测数据和文献中剪切强度模型对比,表明该强度模型可以很好地描述宽广吸力范围内的非饱和土强度。

宽饱和度范围非饱和粗细混合土的强度演化规律:试验现象

粗细混合土是自然界和工程中普遍存在的土体类型,其强度对边坡、基坑、挡土墙等岩土工程稳定性至关重要。然而粗细混合土的强度在宽饱和度范围内的演化规律不明,尤其是缺少低饱和度时的试验数据和演化规律。为了阐明粗细混合土强度随饱和度变化的一般规律,采用室内直剪试验研究了7种不同细粒含量的粗细混合土的强度与变形特性。试验结果表明:①在宽饱和度范围内,由于毛细作用和吸附作用对抗剪强度贡献不同,非饱和粗细混合土的抗剪强度随饱和度的变化普遍存在“山峰效应”,即在非饱和残余区内出现了强度极大值,其强度随饱和度或吸力的演化规律可以划分为缓慢增长区、快速上升区和峰后下降区3个阶段。②当饱和度低于残余饱和度时,非饱和土的剪切破坏还出现了典型的脆性破坏特征。③粗细混合土的强度变形特征与细粒含量密切相关,随着细粒含量的增加,土骨架由粗颗粒控制逐渐转变为细粒控制,非饱和强度最大值所对应的细粒含量为50%~80%。