Determining the soil-water retention curve using mercury intrusion porosimetry test in consideration of soil volume change

It is well-known that a close link exists between soil-water retention curve(SWRC)and pore size distribution(PSD).Theoretically,mercury intrusion porosimetry(MIP)test simulates a soil drying path and the test results can be used to deduce the SWRC(termed SWRCMIP).However,SWRCMIP does not include the effect of volume change,compared with the conventional SWRC that is directly determined by suction measurement or suction control techniques.For deformable soils,there is a significant difference between conventional SWRC and SWRCMIP.In this study,drying test was carried out on a reconstituted silty soil,and the volume change,suction,and PSD were measured on samples with different water contents.The change in the deduced SWRCMIP and its relationship with the conventional SWRC were analyzed.The results showed that the volume change of soil is the main reason accounting for the difference between conventional SWRC and SWRCMIP.Based on the test results,a transformation model was then proposed for conventional SWRC and SWRCMIP,for which the soil state with no volume change is taken as a reference.Comparison between the experimental and predicted SWRCs showed that the proposed model can well consider the influence of soil volume change on its water retention property.

Influence of Dry Density on Soil-Water Retention Curve of Unsaturated Soils and Its Mechanism Based on Mercury Intrusion Porosimetry

The soil-water retention curve(SWRC)can be used to evaluate the ability of unsaturated soils to attract water at various water contents and suctions. In this study, drying SWRCs for a kind of sandy soil were obtained in the laboratory by using self-modified SWRC apparatus. In addition, the porosity and the pore size distribution of the samples were investigated by a mercury porosimetry test in order to analyze the effect of dry density. Results showed that the soil-water retention of the soil specimens was strongly dependent on the dry density. Under zero suction, soil specimens with a higher dry density exhibited lower initial volumetric water content. The higher the dry density of soil, the more slowly the volumetric water content decreased with the increase of suction. There was a general and consistent trend for a soil specimen to possess a larger air-entry value and residual suction, while smaller slope of SWRC when it had a higher density. This was probably attributed to the presence of smaller interconnected pores in the soil specimen with a higher dry density. The proportion of large diameter pores decreased in comparison to pores with small diameters in the soil tested. The measured total pore volume of the soil specimen, which had a larger dry density, was lower than that of the relatively loose specimens.

Determination of strain-dependent soil water retention characteristics from gradation curve

The importance of soil water retention characteristics in modelling the hydro-mechanical response of unsaturated soils has been well recognised by many investigators in recent years.Determination of strain-dependent soil water retention curve(SWRC)is likely to be extraordinarily difficult.The first two authors have recently shown that SWRC can be computed from the gradation curve and the calculation result is consistent with the experimental results obtained from pressure plate tests.In this paper,based on a hypothesis related to change in the pore size distribution(POSD)due to volumetric strain of soil skeleton,a method to compute strain-dependent SWRC is presented.It is found that at initial degrees of saturation higher than 0.8,the influence of volumetric strain may be marginal whilst at initial degrees of saturation lower than 0.8,its influence is likely to be substantial.In all cases,the gradation curve of the soil affects the SWRC.

Soil-water characteristics of Gaomiaozi bentonite by vapour equilibrium technique

Soil-water characteristics of Gaomiaozi(GMZ)Ca-bentonite at high suctions(3–287MPa)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.

Assessment of Super Absorbent Polymer (SAP) on Plant Available Water (PAW) in Dry Lands

One of the ways of overcoming the cost of irrigation is through in-situ water harvesting at the plant roots. Super absorbent polymer (SAP) can facilitate water harvesting at the plant roots. This study attempted to assess the effect of SAP on plant available water (PAW) of different soils. In this study, SAP was sequentially added at the rate of 0.2%, 0.3% and 0.5% of the soil weight and its impact assessed in clay, sandy clay and sandy loam soils. The moisture retention characteristics of the original and SAP treated soils were studied using soil water retention curves (SWRC) and results modelled using Gardner model. PAW was estimated from SWRC as the difference between moisture content at 1.5 and 3 bar in all soils. The difference in PAW between original and treated soils was assessed at 5% level of significance. The WRC of all the samples was adequately found to be described by the Gardner model (Coefficient of determination R2 ≥ 98% and residual standard error (RSE) ≤ 0.04). SWRC changed with increase in SAP percentage in clay, sandy clay and sandy loam soils. Clay had a higher change in water retention then sandy clay and lastly sandy loam. Plant available water content (PAW) in all soils increased. In clay soil it increased with increase in SAP from 0.3291 at zero SAP to 0.6223 at 0.5% SAP. Sandy clay soil increased in PAW from 0.2721 at zero SAP to 0.5335 at 0.5% SAP and Sandy loam soils from 0.1691 at zero SAP to 0.3461 at 0.5% SAP. Hence, from the study SAP can be used to conserve irrigation water in the plant roots and therefore reducing the cost since PAW has been increased.

采用土壤孔隙表面分形维数预测土壤水分特征曲线

采用土壤颗粒体积累积曲线按照不同的粒径分级方法计算表面分形维数,避免了原始计算过程中颗粒密度不变的假设,改进了计算中的缺陷。结合土壤水分特征曲线分形模型—de Gennes模型,预测试样的SWRC。结果发现:不同颗粒分级只会影响表面分形维数的大小;相同颗粒分级情况下,计算得出的分形维数随着土壤粘粒含量降低而减小;采用D2(最大半径为1mm)的分级情况计算出的表面分形维数,预测得出的结果与实测值相差偏大,RMSE≥2.11E-02cm3/cm3只适合粗略估计田间土壤的水分特征曲线;通过D1(最大半径为0.1mm)计算出的表面分形维数,结合de Gennes模型预测水分特征曲线结果与实测值非常接近,RMSE≤0.0105cm3/cm3。研究表明采用土壤颗粒体积累积曲线计算表面分形维数,并预测土壤水分特征曲线是合理的,具有较高的预测精度。

一种改进的土壤水分特征曲线模型及其验证

土壤水分特征曲线是土壤水分与溶质运移模型必需的输入参数。本文在传统的van Genuchten模型和Campbell模型的基础上,提出了一种基于自适应权重的土壤水分特征曲线改进模型。验证发现,改进模型适用性更强,且能获得更好的模拟效果。特别是,改进模型为一个连续可微函数,能够满足非饱和土壤水流Richards方程数值求解的前提,因而具有较大的推广应用潜力。

Incorporation of a nanotechnology-based additive in cementitious products for clay stabilisation

The mechanical performances and water retention characteristics of clays,stabilised by partial substitution of cement with by-products and inclusion of a nanotechnology-based additive called RoadCem(RC),are studied in this research.The unconfined compression tests and one-dimensional oedometer swelling were performed after 7 d of curing to understand the influence of addition of 1%of RC material in the stabilised soils with the cement partially replaced by 49%,59%and 69%of ground granulated blast furnace slag(GBBS)or pulverised fuel ash(PFA).The moisture retention capacity of the stabilised clays was also explored using the soil-water retention curve(SWRC)from the measured suctions.Results confirmed an obvious effect of the use of RC with the obtained strength and swell properties of the stabilised clays suitable for road application at 50%replacement of cement.This outcome is associated with the in-depth and penetrating hydration of the cementitious materials by the RC and water which results in the production of needle-like matrix with interlocking filaments e a phenomenon referred to as the‘wrapping’effect.On the other hand,the SWRC used to describe the water holding capacity and corresponding swell mechanism of clays stabilised by a proportion of RC showed a satisfactory response.The moisture retention of the RC-modified clays was initially higher but reduced subsequently as the saturation level increased with decreasing suction.This phenomenon confirmed that clays stabilised by including the RC are water-proof in nature,thus ensuring reduced porosity and suction even at reduced water content.Overall,the stabilised clays with the combination of cement,GGBS and RC showed a better performance compared to those with the PFA included.

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.

Hydro-mechanical behaviours of highly compacted sand-bentonite mixture

： 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.

基于SFG模型的非饱和沉积土本构模型

为使修正后的模型能够更好地反映非饱和沉积土的持水和力学性质,对SFG (ShengFredlund-Gens)模型进行修正.用平均骨架应力代替净应力作应力状态变量,考虑饱和度对非饱和土变形的影响.通过分析泥浆固结土(沉积土)在等吸力条件下等向压缩和三轴压缩试验数据,得出饱和度和孔隙比的关系,并把变形考虑到持水曲线模型中.由修正后的体变方程以及持水曲线方程推导出加载湿陷屈服面方程,在修正剑桥模型基础上推导出三轴应力状态下的本构方程,并采用能统一描述剪缩剪胀性的硬化参数.基于同一模型参数,模型可以统一预测吸力控制下各向等压和三轴剪切路径下非饱和沉积土的持水和力学特性.

On the thermo-mechanical properties of unsaturated soils

The establishment of energy balance equation is necessary to study the thermo-mechanical properties of unsaturated soils.To solve this equation,the determination of two fundamental parameters as volumetric specific parameter and thermal conductivity coefficient is essential.In this paper,the effective thermal conductivity coefficient of dry soil grain is analyzed for soils with different compositions,and the thermo-mechanical properties of porous media with water and gas are studied by considering the soil water retention curve（SWRC）.Different methods,i.e.volumetric average method,self-consistent method,Hashin-Strikman method,are employed to calculate thermal conductivity coefficients,and a new method is proposed to determine the thermo-mechanical parameters.Comparison of the results obtained by different methods shows that the proposed method is in a good agreement with the experimental results and is suitable for describing the main properties of the thermo-mechanical behaviors of soils.The relationship between the SWRC and the seepage curve is further studied by the natural proportional rule.The characteristics of the SWRC,its differential coefficient and the seepage curve,are investigated by considering the physico-mechanical mechanism;the limit scopes of the indices of the SWRC and the seepage curve are also given.

Matrix Suction of Unsaturated Colluvium Slope Influenced by Rainfall and Plant Condition:A Case of Taiwan Huafan University

This study investigates the variation of matrix suction, water content and ground water level before and after the rainfall for the unsaturated colluvium slope in the campus of Fuafan University. The measuring devices including electrical matrix suction, water content and ground water level were set up in different surface of planting condition for each depth in real-time. It is observed that the matrix suction in the time from July to September is higher; however, when heavy rains caused by typhoon happened, matrix suction will drop rapidly. This variation is obvious in short grass zone and less clear in the broadleaftree zone. The maximum value in short grass zone, long grass zone and broadieaf tree zone at 2 m depth are 90.3, 68.2, 18.5 kPa, respectively. These results are expected to serve as a reference for the study of slope stability mechanisms.

利用混合改进的遗传算法反演F-X模型参数

构建混合改进的遗传算法(GA),实现F-X模型参数反演,以期为非饱和渗流机制研究提供指导。以F-X模型中的系数a、m、n以及饱和体积含水量θ_(s)作为模型参数,以简单十进制遗传算法为基础,融入优体克隆操作等算子,形成混合改进的遗传算法;采用已有数据进行算法验证,对不同干密度土样的F-X模型参数进行反演,明确模型参数与干密度间的相关关系。研究发现:1)混合改进的遗传算法反演F-X模型参数精度较高,模型拟合结果与实测值均方差均小于0.03;2)随着土样干密度的增大,F-X模型的参数a是控制土—水特征曲线(SWRC)的主要因素,而参数m、n变化不显著;3)建立的F-X模型参数与干密度间的关系也可广泛适用于其他土样的SWRC,大大简化了模型参数的获取过程。