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.     

Stress relaxation of five different soil samples when uniaxially compacted at different water contents

The average size of rainfed and irrigated agricultural farms in Spain has grown steadily over the past two decades. This has called for the use of machinery of higher field capacity and greater weight that in turn requires a high drawbar power. All this has resulted in soil changes such as an increased compaction and compactibility. The confined uniaxial compression test was used to assess compaction and viscoelastic behavior of five soil samples from different agricultural areas of Spain. The bulk density–compression stress line was fitted to a three-parameter multiplicative compaction model and viscoelastic behavior was evaluated by means of stress-relaxation tests. The objectives were to determine to what extent the parameter coefficients of the compaction model equation and the relaxation of the stress induced in the compacted soil were influenced by the type of soil, its water content and the compression stress applied. Gravimetric water contents of 5, 10, 15, 20 and 25% were considered, and maximum normal stresses of 50, 100, 200 and 400 kPa were applied to the soils in a universal testing machine. The soil samples considered differed in texture, sandy loam (SL), sandy clay loam (SCL), loam (L), clay (C) and silt-loam (SiL), and organic matter content.

The slope of the bulk density-compression stress line at zero normal stress was strongly dependent on soil water content and plasticity index; whereas the slope of the curve at high applied normal stresses was influenced by soil moisture but not by soil plasticity. The viscoelastic behavior of the soils compared was dictated by their water content and plasticity index, as well as by the compression stress applied. The stress relaxation rate at time t=0 was scarcely influenced by water content. In fact, the rate remained constant over the water content range from 10 to 20% (w/w) at values that were higher than those obtained at 5 and 25% (w/w), which in turn were identical to each other. The stress-relaxation rate was also found to increase linearly with the logarithm of the compression stress. On the other hand, the residual stress decreased linearly with increasing water content. However, the latter increased linearly with compression stress. Increasing soil plasticity resulted in decreasing relaxation rate and increasing residual stress. Therefore, the more plastic the soil was the lower was the rate at which stress relaxation started and the smaller was the amount of stress dissipated.     

铁矿尾砂配施有机物料对褐土压缩及回弹特性的影响

铁矿尾砂作为工业废弃物已经应用于农业生产，可以改善土壤结构；农业机械作业造成的土壤压实、破坏土壤结构是影响作物产量的主要原因之一。论文旨在探讨铁矿尾砂配施有机物料对褐土压缩—回弹特性的影响，将混有铁矿尾砂和有机物料的土壤以18%含水率培养一昼夜，按1.25 g/cm3容重装入土工试验专用环刀，采用快速固结试验方法，进行单轴压缩试验。结果表明，随铁矿尾砂施用量增加，在低应力时，土壤孔隙比减小量（?e）变大；在高应力时，土壤 ?e 变小。预固结压力值（Pc）和压缩指数（Cc）均随铁矿尾砂施用量增加而降低，Pc和Cc变化范围分别为72.91～119.30 kPa、0.445～0.720，二者均与有机质含量呈极显著正相关关系；与砂粒含量呈极显著负相关关系（P<0.01）。回弹指数（Cs）变化范围为0.0109～0.0169，与有机质及砂粒含量均无显著相关关系，有机物料是影响土壤回弹指数的主要因素。较对照相比，20%铁矿尾砂配施有机物料处理使压缩指数降低12.77%，预固结压力值和回弹指数分别提高6.93%和22.14%，降低压实风险。     

有机质与黏粒含量对黑土压缩-回弹特性的影响

为探明有机质和黏粒对黑土压缩-回弹行为的影响,以典型黑土区耕作土壤为研究对象,通过人工添加腐植酸、人工分离-提取-添加黏粒、恒温恒湿培养的方法各配制3个梯度的重塑土。采用室内固结的方法,通过压缩系数、压缩指数及回弹指数的测定与分析,研究了2种含水量条件下黑土压缩与回弹对有机质和黏粒含量变化的响应行为。结果表明:(1)压缩指数均随有机质含量的增加而增大,且在高含水量时二者呈极显著正相关,有机质含量最高时压缩指数为0.246 3,但有机质含量对回弹无显著影响。(2)无论含水量高低,压缩指数均与黏粒含量呈极显著正相关,而回弹指数随着黏粒含量的增加逐渐降低,且在低含水量时二者呈显著负相关。(3)含水量不同,有机质与黏粒对黑土压缩-回弹特性的影响亦不同;黏粒对黑土压缩-回弹行为的影响更为显著。     

Wheel traffic impact on soil conditions as influenced by tillage system in Central Anatolia

The increased limiting effects of soil compaction on Central Anatolian soils in the recent years demonstrate the need for a detailed analysis of tillage system impacts. This study was undertaken to ascertain the effects of seven different tillage systems and subsequent wheel traffic on the physical and mechanical properties of typical Central Anatolian medium textured clay loam soil (Cambisol), south of Ankara, Turkey. Both tillage and field traffic influenced soil bulk density, porosity, air voids and strength significantly except the insignificant effect of traffic on moisture content. Traffic affected the soil properties mostly down to 20 cm. However, no excessive compaction was detected in 0–20 cm soil depth. The increases of bulk density following wheel traffic varied between 10–20% at 0–5 cm and 6–12% at 10–15 cm depth. In additions, traffic increased the penetration resistance by 30–74% at 0–10 cm and 7–33% at 10–20 cm. Less wheel traffic-induced effects were found on chisel tilled plots, compared to ploughed plots. Soil stress during wheel passage was highly correlated with soil strength. Also, both tillage and traffic-induced differences were observed in mean soil aggregate sizes, especially for mouldboard ploughed plots. The obtained data imply that chisel+cultivator-tooth harrow combination provides more desirable soil conditions for resisting further soil compaction.     

Field compaction at different soil-water status: effects on pore size distribution and soil water characteristics of a Rhodic Ferralsol in Western Cuba

The level of compaction induced on cultivated fields through trafficking is strongly influenced by the prevailing soil-water status and, depending on the attendant soil degradation, vital soil hydraulic processes could be affected. Therefore, understanding the relationship between field soil-water status and the corresponding level of induced compaction for a given load is considered an imperative step toward a better control of the occurrence of traffic-induced field soil compaction. Pore size distribution, a fundamental and highly degradable soil property, was measured in a Rhodic Ferralsol, the most productive and extensively distributed soil in Western Cuba, to study the effects of three levels of soil compaction on soil water characteristic parameters. Soil bulk density and cone penetration index were used to measure compaction levels established by seven passes of a 10 Mg tractor at three soil-water statuses corresponding to the plastic (Fs), friable (Fc) and relatively dry soil (Ds) consistency states. Pore size distribution calculated from soil water characteristic curves was classified into three pore size categories on the basis of their hydraulic functioning: >50 μm (f_>50 μm), 50–0.5 μm (f_{50–0.5 μm}) and <0.5 μm (f_<0.5 μm). The greatest compaction levels were attained in the Fs and Fc soil water treatments, and a significant contribution to compaction was attributed to the existing soil water states under which the soil compaction was accomplished. Average cone index (CI) values in the range of 2.93–3.70 MPa reflected the accumulation of f_<0.5 μm pores, and incurred severe reductions in the volume of f_>50 μm pores in the Fs and Fc treatments, while an average CI value of 1.69 MPa indicated increments in the volume of f_{50–0.5 μm} in the Ds treatment. Despite the differential effects of soil compaction on the distribution of the different pore size categories, soil total porosity (f_Total) was not effective in reflecting treatment effects. Soil water desorption at the soil water potentials evaluated (0.0 to −15,000 cm H₂O) was adversely affected in the f_<0.5 μm dominated treatments; strong soil water retention was observed with the predominance of f_<0.5 μm, as was confirmed by the high water content at plant wilting point. Based on these findings, the use of field capacity water content as the upper limit of plant available soil water was therefore considered inappropriate for compacted soils.     

The biological and physical stability and resilience of a selection of Scottish soils to stresses

The ability of soil to resist and recover from anthropogenic and environmental stresses defines stability and resilience, respectively; an understanding of this ability is critical to sustainable land-use. In this study of 26 soils from across Scotland, we examine the influence of soil properties and antecedent conditions on physical and biological resilience to stress. The sites studied covered a wide range of soil types and land management, including serpentine soil from the Shetland Islands, a catena on the Highland Boundary Fault, and young aeolian sandy soils on the east coast. Biological resilience was measured as CO₂ evolution from soil with added plant residues after either a transient (heat) or a persistent (copper) stress. Measures of physical resilience were: (i) compression and expansion indices following rapid uniaxial compression (to 50 kPa) and relaxation; and (ii) void ratio changes due to an overburden stress and subsequent cycles of wetting and drying. Evolution of CO₂ from soil with added plant residues after heat or copper stress ranged from 23% to >100% that of the unperturbed soil, while the air-filled void ratio after the overburden stress ranged from 70% to >100% that of the unperturbed soil. Soils were grouped into quartiles based on their resistance to and recovery after each of the four prescribed stresses. Soil organic carbon (SOC) content correlated strongly with resilience after biological and physical stresses, particularly resistance to Cu stress ( r = 0.72) and recovery from compression ( r = 0.67), whereas there were no strong correlations between resilience following heat and any of the measured soil characteristics, although both land use and soil class were helpful in separating the responses. Resistance to compression was negatively correlated with SOC ( r =−0.63).     

秸秆添加与土壤基质势对棕壤压缩回弹特性的影响

土壤板结是农田土壤退化问题之一,秸秆还田是提高土壤有机质含量、改良土壤的重要措施。但秸秆还田和土壤基质势对土壤板结的影响尚不明确。试验设0,3,5 g/kg 3个秸秆添加量处理,以及高(1 000 kPa)、中(100 kPa)、和低(10 kPa)3个土壤基质势处理。通过固结试验,测定不同处理的压缩曲线并计算压缩回弹特性指标。结果表明：秸秆添加量、土壤基质势及其二者间的交互作用对压缩曲线最大曲率、预固结压力值、压缩指数和回弹指数的影响都达到显著性水平。预固结压力值、压缩指数和回弹指数均随着秸秆添加量的增加而增大,而最大曲率则随着秸秆增加量的提高而降低。土壤基质势与压缩曲线最大曲率、预固结压力值和压缩指数均呈正相关关系。回弹指数随着土壤基质势的增加呈现先降低后增加的趋势。秸秆添加有助于提高土壤回弹和抗压缩特性。     

Critical state parameters from intact samples of two agricultural topsoils

The critical state parameters of intact samples of a sandy loam (Eutric Cambisol) and a clay loam (Gleysol) were estimated in a constant cell volume triaxial apparatus. Samples were taken under wet and dry conditions. The parameters describing the clay loam were the more variable. This was true of both its initial condition and its response to deformation. Under dry conditions, the sandy loam was less sensitive to increasing stress but compacted more at low stress than the clay loam. Isotropic stress compacted the wet soils until the percentage saturation reached about 85–95% and axial loading caused little further compaction. The difference in strength between soils was greater for the wet samples, whereas the corresponding compactibility differences were greater under dry conditions. The sandy loam was stiffer than the clay loam and the shear modulus decreased exponentially with increasing specific volume before deformation. The rebound slope was about one-twentieth of the compression index for the dry soils and about one-third of the compression index for the wet soils. A simple model of recompression accounted for plastic deformation below the virgin compression line, where the critical state model usually assumes elasticity. The proposed model reproduced the main observed features of repeated isotropic loading.     

Compression of agricultural soils from Quebec

Static uniaxial compression tests were performed on 26 agricultural soils from Quebec. Compression lines (bulk density vs. applied load) were obtained at different water contents for each soil previously sieved to 6 mm. For soils with clay contents less than 35%, the compression index (slope of the compression line) was best correlated with the mineral fraction of the soil (r = 0.75^** with clay and r =−0.78^** with sand). For clay-rich soils, the compression index was best correlated with organic carbon content (r = −0.75^**). The bulk density under standard compression conditions (100 kPa load and 50% water saturation) was related to both clay (r = −0.80^**) and organic carbon (r = −0.77^**). This parameter was also highly correlated with the soil lower plastic limit (r = −0.95^**) which corroborates the observation that the consistency limits can be good predictors of other mechanical properties which are more difficult to determine. Results suggested that both the mineral and the organic fractions have much influence on the compressive behaviour of Quebec agricultural soils.     

初始含水量和容重对黑土压缩特性的影响

东北黑土区农业机械化水平高,农机作业压实导致的土壤结构和物理性状退化问题日益严重,压缩特性是定量分析土壤压实过程的有效手段,但目前黑土压缩特性随初始含水量和初始容重的变化规律尚不明确。为了解初始含水量和初始容重对黑土压缩特性的影响程度及其变化关系,该研究以重塑黑土为对象,设0.15、0.20、0.25、0.30、0.35、0.40 g/g共6个初始含水量水平,设1.00、1.10、1.20、1.30、1.45、1.60 g/cm³共6个初始容重水平,使用固结仪进行单轴压缩试验测定土壤压缩曲线,分析初始含水量和容重对压缩特性影响。结果表明,土壤初始含水量、容重及两者交互作用均极显著影响重塑黑土压缩特性(P<0.001),据此建立了预测压缩特性的土壤传递函数。黑土的预固结压力为10.42～1 106.17 kPa,与初始含水量显著线性正相关、与初始容重显著线性负相关(P<0.05);压缩指数为0.311～0.852,与初始含水量和容重呈二元多项式方程的关系,随初始容重的增大而降低,在中等含水量时最大;回弹指数为0.007～0.321,与初始含水量正相关,与...     

Effect of soil compaction on hydraulic properties of two loess soils in China

Soil compaction affects hydraulic properties, and thus can lead to soil degradation and other adverse effects on environmental quality. This study evaluates the effects of three levels of compaction on the hydraulic properties of two silty loam soils from the Loess Plateau, China. Undisturbed soil cores were collected from the surface (0–5 cm) and subsurface (10–15 cm) layers at sites in Mizhi and Heyang in Shaanxi Province. The three levels of soil compaction were set by increasing soil bulk density by 0% (C0), 10% (C1) and 20% (C2) through compression and hammering in the laboratory. Soil water retention curves were then determined, and both saturated hydraulic conductivity (K_s) and unsaturated hydraulic conductivity were estimated for all of the samples using standard suction apparatus, a constant head method and the hot-air method, respectively. The high level of compaction (C2) significantly changed the water retention curves of both the surface and subsurface layers of the Heyang soil, and both levels of compaction (C1 and C2) changed the curves of the two layers from the Mizhi site. However, the effects of compaction on the two soils were only pronounced below water tensions of 100 kPa. Saturated hydraulic conductivities (K_s) were significantly reduced by the highest compaction level for both sampled layers of the Heyang soil, but no difference was observed in this respect between the C0 and C1 treatments. K_s values decreased with increasing soil compaction for both layers of the Mizhi soil. Unsaturated hydraulic conductivities were not affected by soil compaction levels in the measured water volume ratio range, and the values obtained were two to five orders of magnitude higher for the Mizhi soil than for the Heyang soil. The results indicate that soil compaction could strongly influence, in different ways, the hydraulic properties of the two soils.     

Influence of soil moisture on sulphur oxidation in brown earth soils exposed to atmospheric pollution

Summary Laboratory studies were conducted to determine the influence of soil moisture on S oxidation in atmospheric-polluted brown earth soils. Elemental S was oxidized to sulphate over a wide range of soil moisture treatments (10%–60% w/w), but occurred optimally at around 40%–50% soil moisture content (0.08 MPa). Thiosulphate and tetrathionate were found only in soils incubated at low moisture contents. S-oxidation generally acidified the soils, but an increase in soil pH occurred at high moisture levels, where soils were waterlogged. The S oxidative ability of soil samples collected at monthly intervals and incubated with elemental S in the field-moist state was also strongly influenced by soil moisture content. The rate of sulphate production was greatest in the brown earth soil exposed to heavy atmospheric pollution from a coking works.     

开垦对黑土表层土壤压缩—回弹行为的影响

为探讨开垦对典型黑土表层土壤压缩与回弹行为的影响,以未经开垦天然次生林和开垦年限为17 a、30 a、40 a耕地的表层(0~10 cm)土壤为研究对象,采用快速固结试验方法,研究了土壤压缩与回弹过程中土壤孔隙比(e)、压缩指数(C_c)、压缩系数(a)和回弹指数(C_s)的变化。结果表明:土壤孔隙比(e)、压缩指数(C_c)、压缩系数(a)和回弹指数(C_s)随着开垦年限的增加而降低,C_c、a、C_s变化范围分别为0.252~0.426、0.002 04~0.003 70 k Pa~(-1)、0.041~0.070;未经开垦天然次生林地土壤C_c、C_s显著高于耕地土壤(p0.05);C_c、a、C_s与容重均呈极显著负相关(p0.01),与有机质含量呈极显著正相关(p0.01)。土壤压缩性与回弹能力随着开垦年限的增加逐渐降低,容重、有机质含量对其影响最大。     

A method for predicting soil susceptibility to the compaction of surface layers as a function of water content and bulk density

Identifying the vulnerability of soils to compaction damage is becoming an increasingly important issue when planning and performing farming operations. Soil compaction models are efficient tools for predicting soil compaction due to agricultural field traffic. Most of these models require knowledge of the stress/strain relationship and of mechanical parameters and their variations as a function of different physical properties. Since soil compaction depends on the soil's water content, bulk density and texture, good understanding of the relations between them is essential to define suitable farming strategies according to climatic changes. In this work we propose a new pedotransfer function for 10 representative French soils collected from cultivated fields, a vineyard and forests. We investigate the relationship between soil mechanical properties, easily measurable soil properties, water content and bulk density. Confined compression tests were performed on remoulded soils of a large range of textures at different initial bulk densities and water contents. The use of remolded samples allowed us to examine a wide range of initial conditions with low measurement variability. Good linear regression was obtained between soil precompression stress, the compression index, initial water content, initial bulk density and soil texture. The higher the clay content, the higher the soil's capacity to bear greater stresses at higher initial water contents without severe compaction. Initial water content plays an important role in clayey and loamy soils. In contrast, for sandy soils, mechanical parameters were less dependent on initial water content but more related to initial bulk density. These pedotransfer functions are expected to hold for the soils of tilled surface layers, but further measurements on intact samples are needed to test their validity.     

Mechanisms of soil vulnerability to compaction of homogenized and recompacted Ultisols

Soil compaction is a main cause of soil degradation in the world and the information of soil compaction in subtropical China is limited. Three main Ultisols (quaternary red clay, sandstone and granite) in subtropical China were homogenized to pass through 2 mm sieve and recompacted into soil cores at two bulk densities (1.25 and 1.45 g cm⁻³). The soil cores were equilibrated at different matric potential values (−3, −6 and −30 kPa) before subjected to multi-step compaction tests. Objectives of this study were to determine how different initial soil conditions and loading time intervals influence pre-compression stress and to evaluate an easy measure to determine soil vulnerability to compaction. It became evident that the soil strength indicator, pre-compression stress, was affected by soil texture, initial soil bulk density and matric potential. The coarser the soil texture, the lower the bulk density and the higher the matric potential, the lower was the pre-compression stress. The pre-compression stress decreased exponentially with increasing initial soil water content. Soil water content and air permeability decreased after compaction. The amount of water loss was affected not only by soil texture, bulk density and initial water content but also by loading time interval. These results indicate soil pore structure and hydraulic conductivity changed during compactions. The applied stress corresponding to the highest changes of pore water pressure during compaction had a significant linear relationship with the pre-compression stress (R=0.88, P<0.001). The correlation was ascribed to that the changes in pore water pressure describe the dynamics of the interactive effects of soil pore characters and soil water movement during compaction. The results suggested the evaluation of soil vulnerability to compaction have to consider the initial soil condition and an easy method to measure the changes in pore water pressure can be applied to compare soil strength and soil vulnerability to compaction.     

Subsoil compaction by heavy sugarbeet harvesters in southern Sweden: III. Risk assessment using a soil water model

Due to its persistence, subsoil compaction should be avoided, which can be done by setting stress limits depending on the strength of the soil. Such limits must take into account soil moisture status at the time of traffic. The objective of the work presented here was to measure soil water changes during the growing period, use the data to calibrate a soil water model and simulate the soil susceptibility to compaction using meteorological data for a 25-year period. Measurements of soil water content were made in sugarbeet (Beta vulgaris L.) from sowing until harvest in 1997 on two sites classified as Eutric Cambisols in southern Sweden. Sampling was carried out at 2-week intervals in 0.1 m layers down to 1 m depth, together with measurements of root growth and crop development. Precompression stress of the soil at 0.3, 0.5 and 0.7 m depth was determined from uniaxial compression tests at water tensions of 6, 30, 60 and 150 kPa and adjusted as a logarithmic function of the soil water tension. Soil water content was simulated by the SOIL model for the years 1963–1988. Risk calculations were made for a wheel load of 8 t and a ground pressure of 220 kPa, corresponding to a fully loaded six-row sugarbeet harvester. Subsoil compaction was expected to occur when the major principal stress was higher than the precompression stress. The subsoil water content was very low in late summer, but increased during the autumn. At the end of August, there was practically no plant available water down to 1 m depth. There was in general good agreement between measured and simulated values of soil water content for the subsoil, but not for the topsoil. In the 25-year simulations, the compaction risk at 50 cm depth was estimated to increase from around 25% to nearly 100% between September and late November, which is the period when the sugarbeet are harvested. The types of simulation presented here may be a very useful tool for practical agriculture as well as for society, in giving recommendations as to how subsoil compaction should be avoided.     

Soil physical properties and soybean (Glycine max, Merrill) root abundance in conventionally- and zero-tilled soils in the humid Pampas of Argentina

In the humid Pampas of Argentina soybean is cultivated in different soil types, which were changed from conventional- to zero tillage systems in the last decade. Little is known about the response of soybean roots to these different soil physical environments. Pasture, and conventionally- and zero-tilled field lots cropped to soybean (R1 and R2 ontogenic stages) were sampled in February–March 2001 in a sandy clay loam and two silty clay loam Mollisols, and in a clayey Vertisol. In the 0–0.05 m layer of conventionally- and zero-tilled lots soil organic carbon represented 53–72% of that in pasture lots, and showed an incipient recovery after 4–11 years of continuous zero tillage. Soil aggregate stability was 10.1–46.8% lower in conventionally-tilled than in pasture lots, and recovered completely in zero-tilled lots. Soil relative compaction ranged 60.8–83.6%, which was below the threshold limit for crop yields (>90%). In change, soil porosity >50 μm ranged 0.91–5.09% soil volume, well below the minimum critical limit for root aeration and elongation (>10%, v/v). The threshold of soil resistance (about 2–3 MPa) was only over passed in an induced plough pan in the conventionally-tilled Bragado soil (5.9 MPa), and in the conventionally- and zero-tilled Ramallo soils (3.7–4.2 MPa, respectively). However, neither the low macroporosity nor the high soil resistances impeded soybean roots growth in any site. According to a fitted polynomial function, root abundance was negatively related to clay content in the subsoil (R² = 0.84, P < 0.001). Soybean roots were only abundant in the subsoil of the sandy clay loam Mollisol, which had <350 g kg⁻¹ clay. Results show that subsoil properties, and not tillage systems, were the primary effect of root growth of soybean.     

Mathematical simulation and calculation of the soil compaction under dynamic loads

The deformation and compaction of loamy sandy soddy-podzolic soils under linear dynamic changes in the compressive stresses and in the course of the soil creeping were studied in field experiments. The rheological properties of these soils occurring in the viscoelastic state were described by a first-order differential equation relating the compressive stresses, the rates of their changes, and the velocities of the relative vertical compressive deformation. Regression equations were derived for the viscoelastic properties of the studied soil as functions of its density, moisture, and linear compaction velocity. Methods were proposed for the calculation of indices of the stress-strain state and the compaction of soils under specified conditions of changes in their compressive stresses with time and in the course of the soil creeping after the initial linear increase in load. Corresponding computer programs were developed. The effect of the main factors due to the linear increase in the compressive loads and in the course of the soil creeping on the rheological properties, the stress-strain state, and the density of soils was quantitatively estimated. The calculation showed that the values of the soil deformation and the density under compressive stresses lower than the ultimate strength were stabilized with time, and the properties of the viscoelastic soil approached elastic ones.     

SOIL DEFORMATION AND SHEAR STRENGTH CHARACTERISTICS OF SOME CLAY SOILS AT DIFFERENT MOISTURE CONTENTS

The soil deformation and shear strength characteristics of three clay soils were determined at different moisture contents and spherical pressures, using a quick un-drained triaxial compression test. The soils were found to deform either in a compressive way or by brittle fissuring, depending upon the relative values of moisture content, dry density and spherical pressure. The critical state concept of soil deformation can explain qualitatively the behaviour of these spils at high moisture contents but not at low. The Bridgman concept for fracture in brittle materials describes the soil behaviour at low moisture contents. The shear strength of the clay soils tested was more closely related to the soil moisture suction and to the amount of shrinkage which occurred on drying than to the absolute dry bulk density. The influence of moisture content and spherical pressure on the effectiveness of certain cultivation operations are considered.