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1.
本文利用应力控制式全自动GDS静三轴仪对含水率相同、干密度相同重塑网纹红土进行分级加载条件下不同恒应力比路径、不同基质吸力的三轴固结排水试验,研究了非饱和网纹红土比例加载特性。研究结果表明：在比例加载条件下,非饱和网纹红土的基质吸力平衡过程中,反压排水量有一个明显的转折点,此转折点可作为判断基质吸力是否到达平衡的时间点;通过比较不同基质吸力下的轴向应变和时间的关系曲线发现,基质吸力的增加引起轴向应变增加;在偏应力保持不变的条件下,基质吸力的增加没有引起轴向应变的明显增加;不同基质吸力下的偏应力和净平均应力呈直线关系,其斜率近似为恒应力比;提出一种可以预测非饱和网纹红土比例加载条件下的径向应变的方法。 相似文献
2.
为了模拟实际工程中黄土在不排水条件下的破坏,在控制含水率不变条件下,利用非饱和土真三轴仪对不同初始吸力非饱和原状黄土进行了等向固结试验和不同中主应力参数b值的剪切试验,研究了非饱和原状黄土的力学特性。研究结果表明：真三轴等向压缩屈服应力随着初始吸力的增大而增大,吸力随着净平均应力的增大而减小,净平均应力对吸力的影响程度随着初始吸力的增大而增大。不同 值条件下,广义剪应力和净平均应力破坏状态线近似呈平行直线关系,广义剪应力和有效净平均球应力试验点可归一为饱和土的排水剪切破坏状态线;剪切破坏时吸力随着b值的增大基本呈线性增大趋势;孔隙比和净平均应力对数临界状态线近似呈线性平行关系,其斜率比饱和土大,且大于等向压缩屈服前的斜率;非饱和土孔隙比和有效净平均球应力对数临界状态线与饱和土的临界状态线近似呈线性平行关系;不同净围压条件下,不同 值非饱和土与饱和土的破坏状态孔隙比的比值与气体饱和度的试验点分布于1.0～1.2的直线带内,但同一净有效球应力条件下可以归一为一条非线性曲线。 相似文献
3.
Rocha Breno Padovezi Rodrigues Roger Augusto Giacheti Heraldo Luiz 《Geotechnical and Geological Engineering》2021,39(8):5957-5969
The site characterization of unsaturated soils is well stablished based on laboratory tests, which are expensive and time-consuming. In-situ testing methods, such as the flat dilatometer test (DMT), are an alternative to the traditional approach of drilling, sampling, and laboratory testing. The literature on DMT interpretation is well established on saturated and well-behaved soils. Only few studies deal with DMT interpretation in unusual soils, and little is known about the influence of soil suction on this test. This paper presents and discusses the influence of soil suction on four DMT campaigns carried out in an unsaturated tropical soil site, also incorporating the soil suction influence on the DMT interpretation. Soil suction was estimated by the soil–water characteristic curve (SWCC) and water content profiles. The water content profiles range from 11.3 to 19.7% which corresponds to a suction range estimated by SWCCs mostly between 6 and 200 kPa. Soil suction significantly influenced DMT data up to 5 m depth at the studied site (the unsaturated active zone) increasing the intermediate DMT parameters. The average horizontal stress index (KD) was equal to about 1.7 and the average dilatometer modulus (ED) was about 4.7 MPa in the active zone and practically doubled their values due to in situ soil suction. The estimated peak friction angle (?) was 20–30% higher due to soil suction influence on DMT assuming the soil behaves as a sand like material. Soil suction must be considered to assess the behavior of the investigated soil by the DMT. The suction influence should be incorporated in the effective stress and this approach considerably improved the site characterization of the studied site.
相似文献4.
为了研究复杂应力条件下非饱和原状黄土的非线性本构模型,通过考虑真三轴条件下中主应力和吸力对大主应变的影响,提出了一个考虑中主应力影响的非饱和原状黄土的非线性本构模型。该模型包含12个参数,均可由常含水率非饱和土真三轴试验确定。研究了中主应力对土体变形和强度的影响,探讨了模型参数的变化规律。初始吸力一定时,土体的黏聚力随着中主应力的增大而增大,而内摩擦角随着中主应力的增大呈线性减小趋势。当吸力小于162 kPa时,吸力摩擦角等于21.8°;当吸力大于162 kPa时,吸力摩擦角等于5.5°。初始吸力相同时,不同中主应力条件下土体的体积模量可取为常数。利用该模型预估了常含水率真三轴等向固结试验的吸力变化规律,理论结果与试验结果比较接近。但是对于真三轴剪切试验吸力的预估还需要进行深入探讨。 相似文献
5.
This paper involves an evaluation of a relationship describing the evolution in yield stress of unsaturated soils during hydraulic hysteresis, and an application of this relationship in an elasto-plastic framework to predict the compression curves of unsaturated soils under drained (free outflow of air and water with constant suction) or undrained (constant water content with no outflow of water and varying suction) conditions. The yield stress was quantified as the apparent mean effective preconsolidation stress obtained from compression tests reported in the literature on specimens that had experienced different hydraulic paths. It was observed that the preconsolidation stress does not follow a hysteretic path when plotted as a function of matric suction, but does when plotted as a function of the degree of saturation. Accordingly, an existing logarithmic relationship between the preconsolidation stress and matric suction normalized by the air entry suction was found to match the experimental preconsolidation stress results. This same relationship was also able to satisfactorily predict the trends in preconsolidation stress with degree of saturation by substituting the hysteretic soil–water retention curve (SWRC) into the place of the matric suction. The relationship between preconsolidation stress and suction was combined with an elasto-plastic framework to predict the compression curves of soils during drained compression, while the wetting-path relationship between preconsolidation stress and degree of saturation was combined with the framework to predict the compression curves of soils during undrained (constant water content) compression. A good match was obtained with experimental data from the literature, indicating the relevance of considering the hysteretic SWRC and preconsolidation relationships when simulating the behavior of unsaturated soils following different hydro-mechanical paths. 相似文献
6.
This paper presents a three‐dimensional elastoplastic constitutive model for predicting the hydraulic and mechanical behaviour of unsaturated soils. It is based on experimental results obtained from a series of controlled‐suction triaxial tests on unsaturated compacted clay with different initial densities. Hydraulic hysteresis in the water‐retention behaviour is modelled as an elastoplastic process, with the elastic part modelled by a series of scanning curves and the elastoplastic part modelled by the main drying and wetting curves. The effect of void ratio on the water‐retention behaviour is studied using data obtained from controlled‐suction wetting–drying cyclic tests on unsaturated compacted clay with different initial densities. The effect of the degree of saturation on the stress–strain‐strength behaviour and the effect of void ratio on the water‐retention behaviour are considered in the model, as is the effect of suction on the hydraulic and mechanical behaviour. The initial density dependency of the compacted soil behaviour is modelled by experimental relationships between the initial density and the corresponding yield stress and, thereby, between the initial density and the normal compression line. The model is generalized to three‐dimensional stress states by assuming that the shapes of the failure and yield surfaces in the deviatoric stress plane are given by the Matsuoka–Nakai criterion. Model predictions of the stress–strain and water‐retention behaviour are compared with those obtained from triaxial tests with different initial densities under isotropic compression, triaxial compression and triaxial extension, with or without variation in suction. The comparisons indicate that the model accurately predicts the hydraulic and mechanical behaviour of unsaturated compacted soils with different initial densities using the same material constant. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
7.
The Barcelona basic model cannot predict the mechanical behaviour of unsaturated expansive soils, whereas the Barcelona expansive model (BExM) can only predict the stress–strain behaviour of unsaturated expansive soils without the water‐retention behaviour being incorporated. Moreover, the micro‐parameters and the coupling function between micro‐structural and macro‐structural strains in the BExM are difficult to determine. Experimental data show that the compression curves for non‐expansive soils under constant suctions are shifted towards higher void ratios with increasing suction, whereas the opposite is true for expansive soils. According to the observed water‐retention behaviour of unsaturated expansive soils, the air‐entry value increases with density, and the relationship between the degree of saturation and void ratio is linear at constant suction. According to the above observation, an elastoplastic constitutive model is developed for predicting the hydraulic and mechanical behaviour of unsaturated expansive soils, based on the existing hydro‐mechanical model for non‐expansive unsaturated soil. The model takes into consideration the effect of degree of saturation on the mechanical behaviour and that of void ratio on the water‐retention behaviour. The concept of equivalent void ratio curve is introduced to distinguish the plastic potential curve from the yield curve. The model predictions are compared with the test results of an unsaturated expansive soil, including swelling tests under constant net stress, isotropic compression tests and triaxial shear tests under constant suction. The comparison indicates that the model offers great potential for quantitatively predicting the hydraulic and mechanical behaviour of unsaturated expansive soils. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
8.
以三峡库区黄土坡滑坡临江I号崩滑体的滑带土为研究对象,研究基质吸力对非饱和滑带土的强度与变形等力学性质的影响。利用GDS非饱和反压直剪仪对滑带土进行了不同净法向应力和基质吸力组合下的直剪试验。试验结果表明:相同的基质吸力作用下,滑带土抗剪强度随着净法向应力的增大而增大。当滑带土试样中基质吸力较小时(50kPa),试样剪切过程中的应力-应变曲线,随着净法向应力的增大表现为应变硬化型。之后随着基质吸力的增加,当吸力大于净法向应力时,剪切应力-应变曲线表现为较明显的达到峰值后软化。反之,应力-应变曲线表现为应变硬化型。相同净法向应力下,抗剪强度随着吸力的增大而增大,剪切应力-应变曲线由低法向应力下的达到峰值后软化向高法向应力下的应变硬化转变。剪切过程的轴向位移表现为:当吸力大于净法向应力时,表现为剪胀; 反之,表现为剪缩。滑带土固结排水剪切条件下得到的黏聚力随着基质吸力呈线性增长关系。得到了滑带土的吸力摩擦角b为13.7,有效内摩擦角有较小增长,平均有效内摩擦角15.9。 相似文献
9.
Unsaturated soil behaviour, such as volume change, shear strength and yield stress, is usually interpreted and modelled in terms of stress and suction. This approach is consistent with laboratory tests where suction is a controllable variable. However, it also suffers some limitations. This paper (Parts I and II) presents an alternative approach for interpreting unsaturated soil behaviour, which is built in the space of stress versus degree of saturation. In Part I, a new volume change equation is proposed in terms of stress and degree of saturation, to give a better explanation to the non-linear change of soil compressibility under constant suctions. The soil compression index is assumed to be a function of the effective degree of saturation and is interpolated from the known compressibility at the fully saturated state and that at a dry state. An alternative approach to simulate hydraulic hysteresis and hydro-mechanical interaction is then introduced, which enables the calculation of the effective degree of saturation under complex stress and suction paths. The proposed volume change equation and the approach to describe saturation variation, which are two fundamental aspects to establish constitutive laws for unsaturated soils, are validated against a variety of experimental data in literature. 相似文献
10.
目前主动土压力计算方法多仅针对土体处于饱和或干燥状态,忽略了其从非饱和到局部饱和,或饱和到非饱和的渐变过程,进而导致计算结果失真。鉴于此,首先开展了一系列刚性挡墙非饱和砂土主动土压力模型试验,揭示了墙后土体的破坏规律：（1）墙后土体顶部出现了近似竖直裂缝,且其发展深度随墙面粗糙度和含水率的增加而变大;（2）墙土界面摩擦对塑性区形状影响较小,且在挡墙移动过程中,墙后土体塑性区形状始终近似保持为平面。在试验基础上,引入广义有效应力原理,基于极限平衡分析建立了考虑吸应力效应的非饱和土主动土压力计算方法,理论与试验实测值表明,所提方法相比其他方法,更接近于试验值。分析了各主要因素对非饱和土压力分布规律的影响,结果表明：主动土压力随有效内摩擦角值增加而减少,而界面摩擦角对其分布影响较小;相比于无吸力情况,考虑吸力时主动土压力更小;随着进气值增加,吸应力对主动土压力的贡献减少,最终趋于恒定。 相似文献
11.
非饱和黄土强度参数的试验研究 总被引：1，自引：0，他引：1
非饱和土强度理论的研究是非饱和土力学的核心问题。目前公认的非饱和土强度理论是Bishop的单变量理论和Fredlund的双变量理论,二者都引入了基质吸力这一应力参数,只是前者将其归到有效应力中,后者作为独立变量。要得到这两个公式中相关参数需利用控制基质吸力的非饱和土三轴仪或直剪仪测定,控制基质吸力是通过同步增加气压和孔隙水压力保持吸力不变实现的,即所谓的轴平移技术,该项技术一直面临测试周期长的困难和合理性的质疑。鉴于此,本文采用不同含水率的常规三轴CU试验,测定了有效稳态强度参数,得出了吸应力和体积含水率的关系曲线,非饱和强度直接用吸应力函数表达,该强度公式回避了测定基质吸力的问题,便于在工程实际中推广。为了进一步和Bishop及Fredlund的强度公式进行对比,又用张力计测定了同一试样的水土特征曲线,获得了其强度参数。理论上,这3种强度理论对机理的解释不同,但在数学上可以互相转化。从工程应用的角度,基于吸应力的抗剪强度更便于工程应用。 相似文献
12.
非饱和土强度参数的确定是其能够用于工程实际的关键。目前公认的非饱和土强度理论有Bishop的单变量理论、Fredlund的双变量理论以及Lu Ning的吸应力强度理论。其中吸应力强度理论基于吸应力特征曲线,可直接由常规三轴试验得出,回避了测定基质吸力的问题,便于在工程实际中推广。本文通过不同含水率的三轴CD试验,测得陕西泾阳原状Q2（L5）黄土的有效强度参数。结果表明,该黄土不同含水率的有效内摩擦角基本接近;有效黏聚力随含水率的增大呈指数递减,当含水率超过塑限时,黏聚力趋于定值。根据有效黏聚力和吸应力的关系,可以获得吸应力和含水率的关系,即吸应力特征曲线,并通过拟合得出吸应力函数,以及非饱和土强度表达式。 相似文献
13.
非饱和渗透系数是土体渗流分析的基础,成都黏土作为一种典型的非饱和膨胀土,具有吸水膨胀、失水收缩的特性,在受侧限的浸水过程中,土颗粒的膨胀致使孔隙体积减小,渗透性降低,使得直接对其进行非饱和渗透试验十分困难。根据瞬时剖面法的原理,利用EC-5土壤水分传感器测含水率、MPS-2电介质水势传感器直接同步测量同一位置的基质吸力,通过水平渗透试验研究了非饱和成都黏土在侧限条件下的渗透性。含水率和基质吸力的同步测量,保证了其测试条件的一致性,避免了采用其他土水特征曲线的影响。试验表明,试样的非饱和渗透系数为（1.33×10-11~3.14×10-9）m·s-1,非饱和渗透系数与基质吸力并非单调线性关系。基质吸力较高时,受膨胀土颗粒吸水膨胀的影响,渗透系数未出现明显变化,基质吸力降低到一定程度后,渗透系数快速增大。试验结束时土体已接近饱和,土中气体排出较慢,过水断面增加缓慢,促使渗透系数仍然持续增大。采用VG模型拟合k-s曲线,拟合参数α=0.048 kPa-1,n=1.79,m=0.48,试验结果可以用于成都黏土地区的渗流分析。 相似文献
14.
The quality of a numerical modeling solution of moisture flow through unsaturated soil, in part, depends on properly described
unsaturated soil properties. The variability of the Soil Water Characteristic Curve, SWCC, is attributed to hysteresis and
reproducibility of measurement. Because the unsaturated conductivity function is rarely directly measured, the variability
of the unsaturated soil hydraulic conductivity function is attributed to the uncertainty associated with the estimation of
this parameter with currently available fitting functions, and hence a range of reasonable variation was considered. One-dimensional
modeling of expansive soil under dry initial conditions (suction of 1,500 kPa) was performed; both potential evaporation and
infiltration boundary conditions were considered. It was found that small variations in the unsaturated soil hydraulic conductivity
function result in significantly different modeling outputs, as expected, while substential variation in SWCC alone (assuming
the same unsaturated soil hydraulic conductivity for all SWCCs) produced almost identical soil response in terms of soil suction
when the slope of the SWCC is similiar. Thus, proper characterization of the slope of the SWCC is important to proper suction
profile determination. 相似文献
15.
In the last decades, a number of hydro-mechanical elastoplastic constitutive models for unsaturated soils have been proposed. Those models couple the hydraulic and mechanical behaviour of unsaturated soils, and take into account the effects of the degree of saturation on the stress–strain behaviour and the effects of deformation on the soil–water characteristic response with a simple reversible part for the hysteresis. In addition, the influence of the suction on the stress–strain behaviour is considered. However, until now, few models predict the stress–strain and soil–water characteristic responses of unsaturated soils in a fully three-dimensional Finite Element code. This paper presents the predictions of an unsaturated soil model in a Three-dimensional Framework, and develops a study on the effect of partial saturation on the stability of shallow foundation resting on unsaturated silty soil. Qualitative predictions of the constitutive model show that incorporating a special formulation for the effective stress into an elastoplastic coupled hydro-mechanical model opens a full range of possibilities in modelling unsaturated soil behaviour. 相似文献
16.
Yongfu Xu 《Geotechnical and Geological Engineering》2004,22(4):611-625
It is difficult to determine the bearing capacity of a foundation in unsaturated expansive soil, although this is most important. The bearing capacity of unsaturated expansive soil is related to the drying and wetting environment. Swelling pressure occurs when the soil volume change is constrained as an expansive soil is inundated. The expansive lateral pressure, induced by the swelling pressure is similar to the passive earth pressure. By considering the effect of the expansive lateral pressure in Terzaghi's bearing capacity formula, the bearing capacity of unsaturated expansive soil is derived. Because it is very difficult to measure suction in situ, the bearing capacity is expressed using the expansive lateral pressure offers a feasible approach to calculate the bearing capacity of a foundation in unsaturated expansive soil, when suction is not measured. Plate load tests to measure the bearing capacity in situ were performed for the foundation in natural soil and saturated soil immersed by water. The verification of the bearing capacity formulae presented in this paper is conducted by comparing the predicted results with the results of the plate load tests on unsaturated expansive soils in Handan and Bingxia, China. 相似文献
17.
18.
A new constitutive model is developed for the mechanical behaviour of unsaturated soils based on the theory of hypoplasticity and the effective stress principle. The governing constitutive relations are presented and their application is demonstrated using several experimental data from the literature. Attention is given to the stiffening effect of suction on the mechanical response of unsaturated soils and the phenomenon of wetting‐induced collapse. All model parameters have direct physical interpretation, procedures for their quantification from test data are highlighted. Quantitative predictions of the model are presented for wetting, drying and constant suction tests. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
19.
A thermodynamically consistent extension of the constitutive equations of saturated soils to unsaturated conditions is often worked out through the use of a unique ‘effective’ interstitial pressure, accounting equivalently for the pressures of the saturating fluids acting separately on the internal solid walls of the pore network. The natural candidate for this effective interstitial pressure is the space averaged interstitial pressure. In contrast experimental observations have revealed that, at least, a pair of stress state variables was needed for a suitable framework to describe stress–strain–strength behaviour of unsaturated soils. The thermodynamics analysis presented here shows that the most general approach to the behaviour of unsaturated soils actually requires three stress state variables: the suction, which is required to describe the invasion of the soil by the liquid water phase through the retention curve; two effective stresses, which are required to describe the soil deformation at water saturation held constant. However a simple assumption related to the plastic flow rule leads to the final need of only a Bishop-like effective stress to formulate the stress–strain constitutive equation describing the soil deformation, while the retention properties still involve the suction and possibly the deformation. Commonly accepted models for unsaturated soils, that is the Barcelona Basic Model and any approach based on the use of an effective averaged interstitial pressure, appear as special extreme cases of the thermodynamic formulation proposed here. 相似文献
20.
Yanbo Feng Dongyan Liu Dongsheng Li Qianlei Zhao 《Geotechnical and Geological Engineering》2017,35(4):1357-1367
Micro-test approaches like XRF, XRD and SEM and routine soil tests were applied to analyze the chemical composition, mineral composition, microstructure and basic physical property indicators of three kinds of red mud, which were the Bayer red mud, sintering red mud as well as the mixed red mud prepared by sintering red mud and the Bayer red mud at the mixture ratio of 1:1. On this basis, in the dehumidification-moisture absorption process, the relationship among matric suction, hydraulic conductivity and moisture content of the three types of red mud was studied by transient hydraulic characteristic cyclic test of unsaturated mud. The results indicated: physicochemical properties of the three kinds of red mud, such as mineral composition, microstructure and size grading varied remarkably; on the condition of the same moisture content, SWCC and HCFC of the three kinds of red mud demonstrated obvious hysteresis effect, which is the most significant on the Bayer mud, less on the sintering red mud and the least on the mixed mud; the Bayer red mud possess smaller hydraulic conductivity, worse drainage and stability. 相似文献