非饱和土双应力变量广义土水特征曲线理论模型构建

土水特征曲线(soil-water characteristic curve,SWCC)方程是非饱和土力学中最重要的土性表征手段之一。该文评价当前经典的SWCC方程,指出其未具有包容复杂因素的能力,具有灵活性的优点但却同时具有对试验数据量依赖性高的缺点,不能处理多孔隙尺度集群土体固-液-气共同运动及作用的水力-力学耦合效应问题。建立双应力变量广义SWCC概念图示并定义相对体积含水比,基于Fredlund双应力变量理论及van Genuchten土-水表征方程,构建考虑土体变形及多孔隙分布形态的双应力状态变量的广义SWCC方程。相较于2个参数的Brooks等的方程、3个参数的van Genuchten方程以及4个参数的Fredlund等的方程,广义SWCC方程仅3个参数,其中2个参数在双对数坐标系的"相对体积含水比-吸力"平面中进行最小二乘法线性拟合得到,仅1个参数需非线性最小二乘法拟合得到。该模型可利用不同应力状态下的至少3个土水试验数据点,绘制出1条具有适宜精度的单峰SWCC;方程考虑了多峰孔隙概率密度函数分布及土体变形因素,实现了从应力历史推广到应力状态的广义情况,为定量描述不同孔隙结构土体双应力状态下的持水特性、渗透特性和强度特性提供了一条途径。     

Zusammenhang zwischen hydraulischer und mechanischer Bodenstabilität in Abhängigkeit von der Belastungsdauer

Interaction between mechanically and hydraulically affected soil strength depending on time of loading Soil‐deformation analysis often only considers the direct effects of mechanical stress on changes in void ratio or pore functions while the interaction between hydraulic and mechanical processes is seldomly mentioned. Thus, we analyzed the effect of mechanical stress and time of soil settlement on changes in soil strength and the corresponding interactions between stress‐dependent changes in pore water pressure on precompression stress for a clayey silt. Disturbed samples with a bulk density of 1.4 g cm^–3 and a water content of 25 g (100 g)^–1 were compressed for four time steps (10–240 min) at eight stresses (20–400 kPa) with four replications. During the experiments, the changes of pore water pressure and void ratio were registered. With increasing time of stress application, we determined an increased soil strain. The higher the stress‐application time, the smaller gets the void ratio and the precompression stress value. Parallel to these variations in settlement, we also found changes in the pore‐water‐pressure values. This is a consequence of decreasing pore diameter while the water saturation increases. Thus, the proportion of neutral stresses on total stress increases which coincides with a change of water suction (= unsaturated) conditions up to even positive pore‐water‐pressure values (from less negative to positive pore water pressure values). From our experiments, we can conclude that the changes in pore‐water‐pressure values already occur at normal stress values smaller than the precompression stress. This underlines the increasing sensitivity of soil deformation processes close to the internal soil strength. The results support the idea, that in order to quantify the mechanical strength of structured unsaturated soils, we always have to determine the changes in pore‐water‐pressure values, too.     

基于三轴UU试验的土体含水率对根土复合体强度特性的影响

为研究自然边坡其强度特性随含水率的变化规律。通过设计不同体积含水率(20%,27%,35%,40%,45%)和2种含根量(RAR=0.1%,0.3%)的重塑土进行不固结不排水三轴试验,以研究含水率对土体破坏方式、抗剪强度和初始切线模量的影响。结果表明：(1)试样破坏模式多为剪切变形和剪胀变形,在含水率为20%,27%时,出现明显剪切贯穿面破裂带,其余含水率下表现为剪胀变形;(2)含水率对根土复合体抗剪强度主要体现在黏聚力上,含水率从20%升至45%时,2种含根量的根土复合体黏聚力分别降低67%,72%;含水率低于35%时,可用对数关系表示,随含水率升高,黏聚力明显降低;含水率达到35%以上时,可用线性关系表示,黏聚力下降速度减缓;含根量增加,抗剪强度提高;(3)不同含根量和围压下根土复合体初始切线模量与含水率关系可以用线性关系拟合,含水率从20%升至45%时,2种含根量的根土复合体初始切线模量平均分别降低43%和47%。综合考虑土体弹塑性和植物因素对土体位移或变形的影响,可提高边坡稳定性计算精度。研究结果可为植物边坡以及水土保持植被建设治理提供科学参考,丰富根系固土领域的科学理论。     

Einfluss des Porenwasserdruckes auf die Zugfestigkeit von Bodenproben

Effect of pore water pressure on tensile strength Direct tensile testing with measurements of the pore water suction was used to investigate the relationship between tensile strength and suction. The tests were conducted on a till and a clayey soil, both homogenized. A closer view is focused on the relationship between material strain and the development of suction. Beyond, the factor χ of the effective stress equation for unsaturated soils by Bishop (1959), which was calculated by the data of tensile strength and corresponding matric suction is compared to the volumetric χ of the tested soil specimens. It could be shown, that the pore water pressure changes with strain. Therefore, not the initial suction of a soil is relevant for its failure but the actual one that can be measured in the failure zone at the moment of fracture. In addition the application of the volumetric χ in the effective stress equation compared to the χ derived from tensile testing leads to an 1.6 to 2.8 fold overestimation of the contribution of matric suction to soil tensile strength.     

Variation of the critical-state boundaries of an agricultural soil

The critical-state theory can be applied profitably to analyse the mechanical behaviour of agricultural soil. Critical-state parameters and other soil properties are affected by the microstructure and unsaturated nature of agricultural soils. We determined the critical-state boundaries of an agricultural soil in both saturated and unsaturated triaxial tests and examined the effects of matric suction and initial structure on critical-state boundaries. On the compression plane, the presence of air and matric suction in the pores of unsaturated soil significantly affected critical-state boundaries by increasing compressibility, λ On the deviatoric stress-mean net stress plane, the strength increased with matric suction. On this plane, the critical-state lines for the unsaturated tests had non-zero intercepts. For a given soil structure, the frictional parameter M remained fairly constant with matric suction change. However, a change in the initial microstructure resulted in a change in M, causing the position of the critical-state line to ‘pivot’ in state space.     

不同土壤水吸力与耕作方式对土壤压缩—回弹特性的影响

[目的]合理耕作方式是缓解土壤压实、提升土壤生产能力的有效措施,而土壤水分是影响土壤机械物理性能的重要因素,直接影响土壤耕作质量.通过研究不同土壤水吸力和耕作方式下土壤压缩曲线及模型拟合效果,分析土壤回弹—再压缩曲线变化及机械力学参数(预固结压力、压缩指数和回弹指数)差异,以期为农田土壤耕作和培肥提供科学依据.[方法]...     

非饱和土双应力变量广义土水特征曲线模型验证

利用基于轴平移技术的Geo-Expert高级型应力相关土-水特征曲线压力板仪研究不同覆土压力(0、40、100、200 k Pa)对南阳膨胀土土水特征曲线(soil-water characteristic curve,SWCC)的影响;并对提出的考虑土体变形及多孔隙分布形态的双应力变量广义SWCC表征方程进行如下试验验证:1)不同覆土压力下微多孔隙分布形态的南阳膨胀土侧限固结试验及脱湿试验SWCC验证;2)零净法向应力状态双孔隙尺度硅藻土双峰SWCC试验验证;3)不同净围压状态下单孔隙尺度韩国残积土SWCC试验验证;4)多应力路径下法国Bapaume黄土,不同初始干密度下日本Edosaki砂土在脱湿-吸湿过程SWCC试验验证;并比较分析新方程与van Genuchten方程及Fredlund等方程的差异性。结果表明:1)覆土压力会显著改变膨胀土结构及孔隙通道,进而改变SWCC边界效应区、过渡区形态;也改变了双重孔隙尺度土壤的进气值;第1个波峰SWCC进气值均在1 k Pa左右;相比于零覆土压力试样,40、100、200 k Pa覆土压力试样第2个波峰SWCC进气值分别高4.74、17.58、67.23 k Pa;2)未考虑净法向应力影响的单应力状态多峰SWCC、考虑侧限双应力状态多峰SWCC、各向同性净法向应力单峰SWCC、不同脱湿-吸湿路径SWCC及不同初始干密度的SWCC试验拟合曲线均表明,双应力广义SWCC具有包容复杂影响因素的能力;3)新方程能够利用至少3个土-水数据即可拟合出具有较高的精度的整条SWCC。研究为定量描述土壤的持水、渗透及强度特性提供参考。     

基于土-水特征曲线的植物边坡抗剪强度研究

土-水特征曲线通常用于估计非饱和土壤的抗剪强度,但是由于植物根系的吸水作用和加筋效应,使得植物边坡的抗剪强度不能直接利用基于基质吸力的非饱和土抗剪强度计算公式进行计算.以裸露边坡和植被边坡为研究对象,采用张力计现场测试和室内剪切试验相结合的方法,拟合水土特征曲线关系.结果表明:(1)植物根系的植入有效地提高了边坡土体的...     

紫色土埂坎根-土复合体土水特性与抗剪强度

研究根-土复合体土-水特征曲线与抗剪强度的关系,可为紫色土埂坎根-土复合体强化机理的揭示与埂坎稳定性的维持提供科学依据。选取三峡库区典型紫色土坡耕地埂坎草本植物根—土复合体为研究对象,结合Hyprop2土壤水分特征曲线测量仪、滤纸法与直剪试验,拟合土-水特征曲线,揭示基质吸力对根-土复合体抗剪强度的影响。结果表明:(1)根-土复合体土-水特征曲线明显分为边界效应区、过渡区与非饱和残余区,3种常用模型(B-C、VG、F-X)中F-X模型拟合该曲线效果最好,根-土复合体饱和含水率、进气吸力、残余区含水率以及相同体积含水率下的基质吸力均高于素土。(2)随着体积含水率降低,根-土复合体黏聚力先增大后减小,试验范围内黏聚力最大值51.25 kPa出现在体积含水率约23%时,内摩擦角则线性增大。相同体积含水率下,根-土复合体黏聚力较素土最大增加50%,内摩擦角提升不大。(3)基质吸力对根-土复合体抗剪强度的增强作用具有阶段性特征,各阶段临界吸力值与土-水特征曲线一致,过渡区(基质吸力为3~500 kPa)土体抗剪强度提高明显,进入非饱和残余区后(基质吸力>500 kPa)由于黏聚力下降,土体抗剪强度增速减慢,根-土复合体抗剪强度随基质吸力增大而提升的幅度大于素土。通过建立埂坎根-土复合体土-水特征曲线和抗剪强度的关系,可估测实际工况下的埂坎土体抗剪强度,进而为坡耕地埂坎的建设、维护管理以及坡耕地侵蚀阻控提供理论依据。     

Die Stabilität von Sandböden in Abhängigkeit vom Gehalt an organischer Substanz und deren Humifizierungsgrad

The Stability of Sandy Soils and its Dependence on Humification Degree and Content of Organic Matter The influence of organic matter on the strength parameters cohesion and friction angle was investigated with mixtures of a fine sand and 0-8% peat of different humification degrees - determined by using a thermobalance - and under different soil water suction. The results show that the strength parameters cohesion and friction angle increase with increasing organic matter content, but the increase is greater with lower humification degree. Especially if the organic matter content is lower than 5%, cohesion and friction angle depend on soil water suction and have a maximum in the range of about 50 hPa soil water suction. A simplified model based on the capillary theory for the calculation of cohesion was proposed to explain the influence of content and humification degree of organic matter on cohesion. One possible reason for the lower cohesion with higher humification degree is its lower wettability. The dependence of friction angle on soil water suction and humification degree of organic matter needs further examinations.     

Estimation of density—Moisture—Stress functions from uniaxial compression of unsaturated, structured soils

A quasi-theoretical soil compaction model is presented which gives good estimates (r²=0.980−0.995, P<0.001) of the complete density-stress compression line for soils of variable initial moisture content under static loads from 0 to 1.0 MPa. The 3 unknown parameter coefficients of the generalized model equation are shown to be highly correlated to several soil properties such as moisture content, pre-compression (initial) void ratio, texture, organic matter content ans the Atterberg consistenct limits. A 3-tier classification for partition consisteny limits. A-tier claasificaton for pationing soils into groupings of response to compressive stress is proposed for soil compaction modeling. This categorization is based on soil plasticity and the existence of a “compaction threshold“ sensitivity threshold” in most soils.     

Changes of soil water suction,conductivity and dry strength during deformation of wet undisturbed samples

Deformation (without failure along local shear-planes) of wet clayey soil during tillage and traffic operations can cause significant undesirable changes in soil physical properties, even though it is not generally accompanied by severe compaction. Field conditions were simulated by deforming wet, dense, unsaturated, cylindrical field samples in quick (or undrained) triaxial tests. In these tests, soil samples were compressed to half their initial height, which was accompanied by large lateral expansion. Soil water suction (at constant water content) typically increased from 5 to 30 kPa and air and water transmission characteristics dramatically decreased with deformation. The soil water suction increase occurred at a nearly constant degree of saturation, which may explain the often observed phenomenon of soil adhering to tool and tyre surfaces. Deformation doubled tensile strength after drying. Deformability was characterized by a coefficient of viscosity and a threshold shear stress, typical values being 7·10⁶ mPa.s and 6·10⁷ mPa, respectively.     

A STOCHASTIC MODEL FOR THE GROWTH OF ROOTS IN TILLED SOIL

A method of defining soil macrostructure, based on observations of the distribution of aggregates and voids along horizontal transects, is combined with a model for the probability of a root either penetrating, or being deflected by an aggregate, after passing through a void. The latter is based on the strength of the aggregate, the angle of incidence, the length of the preceding void, and root properties. Effects of soil macro-structure, aggregate strength and plant species are discussed. A good correlation was obtained between the predicted nutrient uptake per unit length of root and the measured uptake per plant from beds of different-sized aggregates. The inclusion of the effect of aggregate strength on root elongation rate removed all effects of aggregate size on the rate of uptake of non-mobile nutrients. In order to reconcile this result with the correlation mentioned above, it is hypothesized that greater root branching occurs in beds of smaller aggregates so that the nutrient uptake remains the same as it would have been in the absence of rate effects. It is shown that an optimum' soil macro-structure exists for the maximum growth of roots of a given plant species. This optimum structure depends on the strength of the soil aggregates.     

秸秆覆盖对三峡库区坡面侵蚀的影响

[目的]分析农作物秸秆不同覆盖处理对坡面产流、产沙影响,为三峡库区坡耕地水土保持措施优化提供技术支持。[方法]利用人工模拟降雨技术,分析了5种不同雨强下不同秸秆覆盖处理的径流、泥沙过程。[结果]坡面覆盖秸秆能推迟坡面初始产流。在小雨强时,秸秆覆盖推迟初始产流显著,但随着雨强的增加,推迟初始产流的作用降低。在坡面下坡段覆盖秸秆推迟初始产流效果优于在坡面上坡段覆盖秸秆。对于产流产沙而言,在坡面上坡段覆盖秸秆与无覆盖差异不显著,但在坡面下坡段覆盖秸秆时径流模数和侵蚀模数为无覆盖的61.57%~69.1%和14.54%~18.31%,且减少地表径流和泥沙流失效果优于在坡面上坡段覆盖。秸秆覆盖后土壤流失比(SLR)介于0.022~0.611之间,且随着覆盖率的增加而降低,在坡面下坡段覆盖秸秆SLR值在上坡段的1/4。[结论]合理选择农作物秸秆在坡面覆盖的坡位及覆盖度,能有效降低坡面产流与侵蚀产沙,产生较好的水土保持效应。     

Changes in penetration resistance of Ultisols from southern China as affected by shearing

Shear stresses and soil properties modified due to stress play an important role during formation of seals in a series of rainfall events and during tillage. The objectives of the study were to evaluate the effects of the penetrometer geometry on the penetration resistance as affected by shearing under different initial soil conditions and to use the information on soil strength to elucidate shearing process. Nine homogenous air-dried soils (<2 mm) were sprayed and stored so as to obtain equilibrium soil water contents. The moist soils were sheared by horizontal displacement of layers of soil particles/aggregates in between hands in one direction. The soil cores were prepared with comparable bulk density before the measurement of maximum penetration resistance (P_max) with a small flat tip and a cone tip penetrometers. At a wide range from 0.05 to 6.2 MPa, P_max was linearly correlated between the small flat tip and the cone tip penetrometers. The conversion ratio was higher under the saturation condition irrespective of the shearing effect. The penetrometer with the small flat tip was more sensitive for the weak soils. Shearing generally increased P_max in most cases, but it decreased P_max for some sandy soils under both saturated and unsaturated conditions and for a clayey soil under the saturated condition. The soil consisting of swelling clay exerted a decrease in P_max. Rearrangement and/or sliding of particles/aggregates and increase in soil suction during shearing were attributed to the increase in P_max. Increase in porosity due to the aggregation during shearing was ascribed to the decrease in P_max. In addition, it was shown that agricultural cultivation resulted in a reduction in soil strength.     

Physical and mechanical properties of washed sediment mixed with organic matter

Soil from cleaning and washing of sugar beet during processing is collected and decanted in tanks each year over a period of several months. Instead of spreading it on agricultural land, another option is to reuse the sediment for crop growth. The physical and mechanical properties of the non-structured washed soil (WS) and the efficiency of added organic matter (peat and green waste compost) were evaluated by comparison with an arable silt loam soil (AS). Water retention data were expressed in a double-exponential function which characterized soil structural and matrix pore space. The effects on saturated hydraulic conductivity and pore space morphology from applying loads of 60 and 200 kPa on two initial volumetric water contents (12 and 25%) were investigated using image analysis. WS was a silt loam with no plasticity, and its void ratio and water retention were higher than the AS before compression. However, WS had a very small amount of structural pore space and despite its higher void ratio, its hydraulic conductivity was always lower than AS after compression. Organic matter improved all the WS properties by increasing structural porosity and vertical stress resistance. Organic matter created elongated and tortuous pores and increased K _s values by changing pore size distribution. During compression large pores with a radius >1500 μm disappeared in WS mixtures but were still observed in AS and were maintained by aggregate stability.     

Physical and chemical variations within prisms of a saline polder clay in relation to seasonal changes

The prismatic structure and soil behaviour were studied on a saline clayey soil in the polder of the Marais de ?Quest (France). Three profiles with different histories were examined, and the structure under wet and dry conditions, ionic concentrations, and changes in void ratio and water suction were measured on prisms sampled between 5 and 40 cm deep, in four places (base, centre, top and side). Seasonal dynamics were accompanied by reorganization within the prismatic structure, changing both the size and distinctiveness of the blocky substructure. Chemical and physical characteristics were closely related to the sharpness of these two imbricated structures. In the absence of blocky substructure the salinity increased horizontally from the centres of the prisms to their sides (the latter acting as exchange surface between the soil and the atmosphere), and no physical gradient was apparent. These results emphasized the permanence of lateral hydraulic continuity in individual prisms. On the other hand, where the blocky substructure was well developed there was no lateral variation of salinity within the prisms, whereas the swelling potential varied from the centres to the sides: the sharpness of the substructure therefore caused a decrease in lateral water transfer. The internal structure of the prisms appeared to determine the seasonal behaviour of the soil.     

含水率对小麦粮堆弹塑性力学特性的影响

不同地区粮仓中粮堆的含水率会有较大的差异,为了明确含水率对粮仓设计参数的影响,通过三轴试验研究了含水率对小麦粮堆非线性强度、临界状态和模量等的影响规律。结果表明：不同含水率下小麦粮堆的峰值强度和残余强度符合非线性强度指标的Mohr-Coulomb强度准则;参考压力（100 kPa）下峰值内摩擦角和残余内摩擦角随着含水率的增大呈线性增大,含水率每增加1%,峰值内摩擦角和残余内摩擦角分别增大0.22°和0.30°。小麦粮堆的临界状态特性符合剑桥弹塑性理论,偏应力随着平均法向应力的增大呈线性增大;峰值应力比和临界状态应力比随着含水率的增大呈线性增大;含水率每增加1%时,峰值应力比和临界状态应力比分别增大0.012和0.014。不同含水率下初始模量、割线模量与围压间可采用幂函数模型表示;参考压力下初始模量和割线模量均随着含水率的增大呈线性降低;含水率每增加1%,初始模量和割线模量分别降低0.98和0.25 MPa。     

Effect of temperature on water retention phenomena in deformable soils: theoretical and experimental aspects

In this paper, a theoretical and experimental investigation of the effect of temperature on water retention phenomena in deformable soils is presented. A general law expressing the change in suction with water content, temperature and void ratio is proposed theoretically. This law accounts for the influence of density and temperature on water retention. It also provides a general framework which appears to be well-adapted to describe many particular cases. The effect of temperature is studied through a predictive relationship which is established in this framework. This relationship allows us to obtain the water retention curve at any temperature from that at a reference temperature, thus reducing strongly the number of tests required to characterize the thermo-hydraulic behaviour of a soil. The relevance of this relationship was experimentally verified from new tests as well as the results previously reported in the literature. The new tests were performed on two model media, namely, a terracotta ceramic and a clayey-silty sand. The tests taken from the literature concerned two different clays. Comparison between theoretical prediction and experimental data was particularly promising and shows the capability of the model to cover a wide range of soils.     

基于应力路径试验的小麦粮堆力学特性和应力应变关系模型

粮仓中粮堆在装卸时存在着复杂的应力路径,为了得出复杂应力路径对粮堆模量和临界状态特性的影响规律,以及粮堆应力应变关系模型,该文在侧向应力50~300 k Pa下,进行了常规三轴压缩(conventional triaxial compression,CTC)、等p压缩(constant mean normal stress compression,CMS)、三轴主动压缩(reduced triaxial compression,RTC)三轴应力路径试验,分析了应力路径和侧向压力对模量的影响和粮堆临界状态特性;修正岩土体三次曲线模型,建立了适于描述仓内小麦粮堆应力应变的模型,并通过应力路径试验结果和文献试验结果对模型的适用性进行验证。研究结果表明:各应力路径下初始模量、割线模量E50均随着侧向应力呈幂函数增长;CTC、CMS试验的割线模量E50比初始模量发生较大的降低,而RTC试验没有明显降低。在参考压力(大气压力)下,对于初始模量,CTC试验的结果最大,RTC试验的结果最小;对于割线模量E50,CTC试验的结果最小,RTC试验的结果最大。CTC试验的初始模量、割线模量均随着侧向应力增长最慢,而RTC试验的结果均随着侧向应力增长最快。不同应力路径和侧向应力下,试验的破坏点均落于同一临界状态线上,小麦粮堆临界状态应力比为0.976。修正三次曲线模型反映了粮堆强度、峰度系数和峰值应变等特性,并通过8个参数进行计算;通过应力路径试验结果和文献试验结果对该模型进行了验证。研究结果可为粮仓装卸料压力、变形的计算提供更符合实际应力路径条件的参量,建立的修正三次曲线模型可用于粮堆应力和变形的数值模拟,为粮仓的设计提供参考。