非饱和土强度公式的比较研究

 非饱和土力学的基本理论已经确认了基质吸力的存在有利于非饱和土的强度提高,但是对于强度提高的机制到目前为止没有统一的结论,非饱和土强度理论中存在着单应力状态变量公式和双应力状态变量公式2种基本的理论表达式,并在此基础上产生了许多实用的强度公式。通过珞珈山土样的土–水特征曲线试验和非饱和三轴剪切试验对非饱和土强度公式进行比较研究。结果表明,珞珈山土样的进气值和残余含水量分别为237.7 kPa和4%;珞珈山土样非饱和强度在低吸力范围内,与吸力基本成线性增长关系,验证了非饱和土双变量强度公式的适用性;珞珈山土样的非饱和抗剪强度参数约为20.7°;现有的非饱和土实用公式精确性较差,非饱和土抗剪强度理论的应用有待进一步的研究与完善。     

Effect of temperature on the soil–water retention characteristics in unsaturated soils: Analytical and experimental approaches

In unsaturated soil mechanics, the soil–water retention curve (SWRC) continues to play an important role, since it provides the necessary links between the properties and behaviour of unsaturated soils with a variety of engineering challenges. The temperature has been identified as the main factor influencing SWRC as compared to a variety of other parameters. The goal of this research is to describe theoretical and experimental aspects of the temperature effect on unsaturated soil water retention phenomena. Theoretically, a brief review of the constitutive laws governing the thermal-hydro-mechanical (THM) behaviour of unsaturated soils is presented, along with links between variations in suction with water content, temperature, and void ratio. It also provides a broad framework that would to be well adapted to describing many specific circumstances. Through a closed-form predictive relationship that is developed in this framework, the effect of temperature is examined. By using this relationship, the soil–water retention curve at arbitrary temperature could be determined from one at a reference temperature, therefore significantly decreasing the number of tests necessary to describe the thermo-hydro-mechanical behaviour of a soil. Besides, the SWRC of kaolinite clay was also measured at three different temperatures in an experimental program. The test findings reveal that when the temperature rises, the SWRC decreases significantly. The experimental results were then integrated with sixteen other available data sets covering a wide range of soil types, densities, and suction to create a complete verification program for analytical models. The proposed model has a good performance and reliability in forecasting the fluctuation of non-isothermal SWRC than any existing model, according to statistical assessment results. The analytical model can be used to examine the thermo-hydro-mechanical characteristics of unsaturated soils in numerical simulations.     

荆门膨胀土土–水特征曲线特征参数分析与非饱和抗剪强度预测

 为探讨荆门原状、压实、石灰改良膨胀土的非饱和抗剪强度与土–水特征曲线(SWCC)的相关关系,利用Fredlund-Xing模型对3种膨胀土的SWCC试验数据进行非线性拟合,获得相应模型参数;利用Fredlund公式(1996年)对3种膨胀土控制吸力下的固结排水剪三轴压缩试验抗剪强度成果进行相关分析。分析结果表明：(1) 使用Fredlund-Xing模型拟合3种膨胀土SWCC试验数据的效果均较好。(2) 荆门原状、压实、石灰改良膨胀土的进气值分别为210,68和18 kPa;石灰改良膨胀土的持水能力(水稳定性)最强,压实膨胀土次之,原状膨胀土最弱;3种膨胀土的残余吸力可均取为3 000 kPa。(3) 3种膨胀土的有效内摩擦角(j´)和与基质吸力相应的内摩擦角(jb)均非常数;j´与基质吸力相关,jb与基质吸力和应力状态均相关,这意味着Mohr-Coulomb破坏包面是双向弯曲的。(4) 使用Fredlund公式,能够在较大的吸力范围内较好地预测荆门膨胀土的非饱和抗剪强度,原状、压实、石灰改良膨胀土的土性参数κ分别为2.4,2.7及3.4。(5) 预测值与实测值间存在差别的主要原因是Fredlund公式本身未考虑到jb与应力状态相关,若要考虑该因素,可将应力状态对SWCC的影响引入到Fredlund公式中。     

基于饱和土总应力强度指标的非饱和土强度 理论及其应用

 实际工程中求解和测定吸力比较困难,特别在不排水条件下,吸力并非常量,从而难以确定。而饱和度相对容易确定,选择饱和度代替吸力,建立关于饱和度的非饱和土的抗剪强度公式有着重要的实用价值。提出不排水(等含水量)条件下的基于饱和土总应力强度指标的抗剪强度公式,克服了以往相关模型只适合于较小范围的缺陷,能够实现从非饱和土状态到饱和状态的平稳过渡,同时更能反映土体的工程特性;结合非饱和土的土水特征曲线试验及常含水量三轴剪切试验成果,确定适用于云南红黏土的相关拟合参数;将所得抗剪强度公式应用于地基承载力分析中,得到地基承载力随饱和度的减少接近线性增加的结论。     

非饱和土强度的吸力贡献形式研究

在三级法向应力条件下,分别对处于饱和与最优含水率状态时的4种土样进行直剪实验。所得结果表明,基质吸力会转化为土样的一种黏聚力,并以这种形式来增强土样的强度。与此同时,吸力对土样内摩擦角所造成的影响却是很微小的。分别用同种土样来进行持水特性试验,得到了每种土样处于吸湿与脱湿过程时的土–水特征曲线。对土–水特征曲线进行分析后可知,同种土样的强度特征与其所经受的水力路径是息息相关的;另一方面,与饱和土相比,促使非饱和土强度得以增加的因素是多样的,基质吸力只是占主导地位的因素。依据分析所得结果,可将Fredlund等人提出的非饱和土抗剪强度公式表述为更为直观的形式。     

非饱和土抗剪强度公式分类及总结

将众多非饱和土抗剪强度公式分为结合土-水特征曲线、数学拟合、分段函数、总应力指标及其他形式5类,分析了当前非饱和土抗剪强度公式的特点及研究不足.结果表明:有效应力抗剪强度公式和双应力状态变量抗剪强度公式的力学本质不同,吸附强度表达式的不同导致了抗剪强度公式的多样性,急需加强非饱和土真三轴试验研究,建立符合工程实际受力状况的非饱和土强度理论,完善非饱和土的理论基础,并加快非饱和土强度理论的工程实践与应用进程.     

Numerical simulations of triaxial shearing-infiltration tests

The failure of steep slopes during rainfall is commonly associated with a decrease in matric suction in the unsaturated soil zone above the water table. The shear strength characteristics of residual soil under water infiltration have been studied in the laboratory using unsaturated triaxial tests. This paper presents a development of a numerical model for simulating a triaxial shearing-infiltration test to investigate the shear strength characteristics of a compacted kaolin under infiltration condition. Both the hydraulic and mechanical responses of the compacted kaolin are modeled using the commercial software SIGMA/W and in-house software YS-Slope. The numerical analyses result and their validation against laboratory test results are presented and discussed in this paper. The results from the numerical analyses show good agreements with those from the laboratory tests, indicating that the proposed numerical model can be used to simulate the triaxial shearing-infiltration tests in laboratory.     

考虑端部效应影响的非饱和压实土三轴试验研究

 在非饱和压实土控制吸力条件三轴试验中,应用数字图像测量技术,可研究非饱和压实土试样考虑端部效应影响下的强度与变形特性。通过比较非饱和压实土控制吸力条件三轴试验中试样整体与试样中部1/3区域的试验结果,证明试样中部1/3区域受端部效应的影响较小,其试验结果更能代表非饱和土的实际受力变形特性,可以为研究非饱和土的应力–应变关系提供更为准确的试验数据。应用双应力状态变量方法分析试样的抗剪强度,结果表明,端部效应对抗剪强度方程的参数取值有较大影响。应用非饱和土Alonso弹塑性模型进一步研究端部效应对非饱和土数值计算结果的影响。     

Consolidation behavior of an unsaturated silty soil during drying and wetting

In this work, the effect of hysteresis phenomenon on the consolidation behavior of an unsaturated silty soil was investigated through a program of experimental tests. Compacted samples were prepared by the slurry method and experimental tests were carried out in a double-walled triaxial cell. Consolidation tests were conducted by the ramping method at suctions of 0, 100, 200, 250 and 300 kPa on drying and wetting paths of the soil water characteristic curve. The results show that the paths of specific water volume and specific volume are not consistent during stabilization in either condition (drying or wetting). In addition, the yield stress for the wetting path is higher than that for drying. The trend of variations of the specific water volume during loading is similar to the consolidation curves for different suction. For both conditions of drying and wetting, the slope and intercept of the virgin line due to variations of specific volume and specific water volume are a function of suction. While their values decrease with increasing suction, these values are higher for the dry path than wetting.     

南阳膨胀土变形与强度特性的三轴试验研究

对南阳膨胀土做了4种应力路径的三轴试验，即控制吸力的各向同性压缩试验、控制净平均应力的收缩试验、控制净平均应力的膨胀试验、同时控制净室压力和吸力的排水剪切试验，结果表明：膨胀土各向同性压缩屈服特性仍可用一般非饱和土的LC屈服线描述；不同围压下的收缩路径的屈服吸力随围压增大而减小，且屈服后的收缩指数接近各向同性压缩屈服后的压缩指数；非饱和状态下的内摩擦角近似等于饱和状态下的有效内摩擦角，总粘聚力随吸力增加而线性增大；在各向同性压缩试验和三轴剪切试验过程中，土样含水量随净平均应力的变化都近似为线性关系。     

Seasonal influence on cone penetration test: An unsaturated soil site example

Interpretation of electric cone penetration test (CPT) based pore water pressure measurement (CPTu) is well established for soils with behavior that follows classical soil mechanics. The literature on the interpretation of these tests performed on unsaturated tropical soils is limited, and little is known about the influence of soil suction on in situ test data. In this context, the CPT data are presented and discussed to illustrate the seasonal variability in an unsaturated tropical soil site. The test data show that soil suction significantly influenced CPT data up to a depth of 4 m at the study site. It shows the importance of considering seasonal variability in unsaturated soil sites caused by soil suction, which was related to water content through a soil-water retention curve (SWRC). It is also important to consider this aspect in the interpretation of CPT data from these soils.     

Soil-water characteristics of Gaomiaozi bentonite by vapour equilibrium technique简

Soil-water characteristics of Gaomiaozi (GMZ) Ca-bentonite at high suctions (3–287 MPa) are measured by vapour equilibrium technique. The soil-water retention curve (SWRC) of samples with the same initial compaction states is obtained in drying and wetting process. At high suctions, the hysteresis behaviour is not obvious in relationship between water content and suction, while the opposite holds between degree of saturation and suction. The suction variation can change its water retention behaviour and void ratio. Moreover, changes of void ratio can bring about changes in degree of saturation. Therefore, the total change in degree of saturation includes changes caused by suction and that by void ratio. In the space of degree of saturation and suction, the SWRC at constant void ratio shifts to the direction of higher suctions with decreasing void ratio. However, the relationship between water content and suction is less affected by changes of void ratio. The degree of saturation decreases approximately linearly with increasing void ratio at a constant suction. Moreover, the slope of the line decreases with increasing suction and they show an approximately linear relationship in semi-logarithmical scale. From this linear relationship, the variation of degree of saturation caused by the change in void ratio can be obtained. Correspondingly, SWRC at a constant void ratio can be determined from SWRC at different void ratios.     

Hydro-mechanical behaviours of highly compacted sand-bentonite mixture

Abstract： This paper presents the results of laboratory testing on a heavily compacted sand-bentonite mixture. To measure the soil-water retention curve （SWRC） of the mixture over a large range of suction, a pressure plate apparatus and filter papers were used. The obtained SWRC shows that the measurements via the two methods consistently agree with each other. By using a suction-controlled oedometer for unsaturated soils, a series of one-dimensional compression tests were performed on the unsaturated compacted sand-bentonite mixture at different constant suctions. The testing results indicate that the yield stress increases and compression index decreases with the increase of imposed suction. The results also demonstrate that the mixture wetted to saturation and subsequently dried to a certain suction level has a lower yield stress than that wetted directly to the same suction.     

干–湿循环过程中吸力与强度关系研究

针对膨胀土干–湿循环过程中的吸力变化和强度变化特征,开展试验研究。采用体积压力板仪实现脱湿和吸湿并制备不同吸力的试样,通过直剪试验测试不同吸力下的抗剪强度。试验成果显示:①土水特征曲线是不稳定的,它与土体含水率的变化路径有关;②在干湿循环过程中,相同的吸力具有不同的强度贡献。     

重塑非饱和黄土的变形、强度、屈服和水量变化特性

共做了３种应力路径的三轴试验,即：控制吸力的各向同性压缩试验、控制净平均应力的三轴收缩试验和同时控制吸力与净室压力的三轴排水剪切试验。研究结果表明：应力路径对土的变形和水量变化有较大影响;土－水特征曲线依赖于净平均应力;与净法向应力相关的摩擦角在各种吸力下近似等于饱和土的有效摩擦角;剪切强度与吸力的关系是非线性的;三轴剪切条件下的水量变化指标接近常数;屈服净平均应力和屈服偏应力随吸力增大;在净平均应力不变的条件下屈服吸力是一常数但不一定等于土样在历史上曾受过的最大吸力。根据试验资料,提出了一个新的吸力增加屈服条件,并建议了一个确定三轴剪切条件下的屈服应力的新方法。     

温控土工三轴仪的研制及其应用

为了研究温度对土的力学性质的影响,研制了一套温控土工三轴仪。该设备采用模块结构,体小轻巧;实现了三轴仪与恒温箱的巧妙结合,能经受高压高温;对温度、吸力、应力、应变既能较好地控制,也能较准确地量测;体变、排水、气压量测精度高,传压滞后小;数据自动采集处理,应用方便,造价低;能进行控制温度的饱和土与非饱和土试验。利用该设备对陶岔胀土进行了三种应力路径的控制温度的三轴试验,结果表明,温度对该土的变形、强度和土–水特征曲线有较大影响。温控土工三轴试验设备的研制成功为研究土的热力学特性提供了有力工具。     

基于NMR技术及分形理论预测SWRC

土-水特征曲线(SWRC)是非饱和土力学中的基础本构关系,在研究非饱和土强度、体变及渗透系数等方面具有重要作用。通过试验直接测量SWRC,耗时较长,且通常得到的只是离散数据点,缺乏连续性,不能完全满足非饱和土研究的需要,因此,通过间接方法快速预测SWRC的完整数学表达式具有实际意义。以无损伤的核磁共振(nuclear magnetic resonance)技术为基础,结合Young-Laplace理论,建立基质吸力ψ与T2值的关系式;利用分形理论,推导出质量含水率w与T2值的关系式,最终建立了预测SWRC的数学模型,研究结果表明其预测结果与实测值吻合较好。     

脱湿速率影响下的膨胀土工程性状与持水特征初探

通过控制恒温恒湿箱的温湿度工作参数,实现饱和膨胀土在不同脱湿速率条件下的状态变化,借助收缩试验和非饱和三轴试验,研究脱湿速率对膨胀土收缩特性、强度特性与持水特性的影响规律。试验结果表明,膨胀土的收缩系数随脱湿速率的减小而变大,并先呈现线性脱湿过程,后呈现非线性脱湿特征;在相同含水率条件下,膨胀土的吸附强度亦随脱湿速率的减小而增大,尤在含水率和围压较低时更趋明显。结合吸附强度与吸力关系的乘幂函数表达式,利用收缩系数和吸附强度反算出不同脱湿速率下的膨胀土"双线性"土水特征曲线,发现不同脱湿速率时曲线下降段接近平行趋势,脱湿速率越小,膨胀土的进气值越大,相同含水率时的吸力越高。     

单峰和双峰土水特征曲线基本参数的确定

土水特征曲线基本参数(如进气值、残余吸力值和过渡区脱湿曲线的斜率等)的确定是预测非饱和土强度、渗透性以及本构关系的基础。基于修正Fredlund和Xing拟合方程,提出单峰和双峰土水特征曲线基本参数的确定方法。首先,对试验数据点进行最优化拟合,获取拟合方程的参数,对于双峰土水特征曲线,根据试样的孔径分布特征将其分段进行拟合;其次,根据拟合方程的几何关系求出过渡区和残余区土水特征曲线的斜率及其切线方程;再根据切线方程可以准确地确定单峰和双峰土水特征曲线的进气值、残余吸力值等。最后,利用单峰和双峰土水特征曲线的试验数据,验证了此方法的可行性。     

冻融循环对非饱和粉质粘土SWCC及强度的影响

为研究季冻区非饱和粉质黏土在冻融条件下的土―水特征曲线及强度的变化,对季冻区非饱和粉质粘土的SWCC(土―水特征曲线)和强度进行了测量,观察了冻融循环次数及冻结负温对非饱和路基粉质黏土土―水特征曲线及强度的影响。研究表明:非饱和粉质黏土的土―水特征曲线用Gardner模型进行拟合效果良好;土―水特征曲线受冻融循环次数和冻结负温的影响较大,其强度随冻融循环次数的增加呈先减小后趋于稳定的趋势,冻融一次以后其强度降低最为明显,并且冻结负温为-10 ℃时其粘聚力损伤值最大。由基质吸力引起的累计粘聚力损伤值随冻融循环次数增加呈线性增加,随冻结负温减小成指数型增加。