Virgin Compression Line Research Articles

Compressibility of Repacked Soil as Affected by Wetting and Drying between Uniaxial Compression Tests

According to the classical concept of precompression stress, soil compaction increases precompression stress up to the maximum applied stress, while it leaves the compression index, i.e., the slope of the virgin compression line, unaffected. In field studies performed under unsaturated conditions, however, soil compaction was usually found to increase the precompression stress less than expected and to decrease the compression index. Our hypothesis was that this discrepancy may result from changes in the soil moisture status between compaction in the field and compression tests, including the common practice of preconditioning samples to a standard initial water potential for compression tests. To test this hypothesis, we performed confined uniaxial compression tests in which samples of repacked agricultural loamy topsoil were subjected to five compression cycles with increasing maximum stress. Part of the samples were subjected to a wetting–drying cycle and subsequently conditioned to water potentials of either −1, −6, or −30 kPa before each of Compression Cycles 1, 3, and 5, while the other samples were conditioned to these water potentials only before Cycle 1 and not subjected to any wetting–drying cycles. The concept of precompression stress was found to be fully valid as long as the samples were not subjected to wetting–drying cycles and not reconditioned between compression events. In contrast, the wetting–drying cycles and sample conditioning between compression events reduced both the precompression stress and the compression index. Such effects are not accounted for by the classical concept of precompression stress.

Variance of mechanical precompression stress in graphic estimations using the Casagrande method and derived mathematical models

Mechanical precompression stress is a yardstick for the strength and compressibility of soils. The default method for the estimation of precompression stress is the graphic method according to Casagrande. It involves a subjective perception by the engineer who not only determines the point of the highest curvature visually, but decides also which points are to be used for generating the virgin compression line. In order to avoid such subjective approaches, mathematical models for the determination of precompression stress have been developed emanating from the Casagrande method. These models estimate the smallest radius of the curvature based on the minimum of the second numerical derivative. The paper has the aim to quantify the variance of subjectivity implied by the person executing the graphic method, the variance of different model approaches and the accuracy of the latter in handling the graphic values. Additionally we wanted to investigate the effect of different parameters on the ordinate of the diagram and the effect of the first load step on the precompression stress. To understand these relationships, stress/bulk density functions and stress/void ratio functions measured on 13 sites were analysed by five experienced but independent engineers and by use of three mathematical models. The mean errors of precompression stress estimations by the different testers were 0.01–0.12 and by the models 0.10–0.87 on a logarithmic scale. Expressed in kPa, increasing mean errors were observed with rising precompression stress, due to delogarithmization. For the graphical determination, they reached approx. 10–20 kPa at precompression stress levels of 60–150 kPa in typical subsoils; this means 15% on average. The handling of graphically obtained values by help of mathematical models disclosed considerable deviations between them. In the logarithmic variant, the mean absolute errors varied from 0.09 (9 kPa) to 0.40 (30 kPa) and the determination coefficients from 0.71 to 0.96. Another influence on the level of precompression stress has been observed when different variables were plotted on the ordinate of the graph. The graphically obtained values of precompression stress and those shown in the dry bulk density graph exceed the values calculated on the basis of the void ratio by the factor 1.2–1.5. Furthermore, it can be stated that in soil-compression tests with an initial load of 25 kPa higher precompression stress values were obtained than with lower initial loads (5 kPa), if the precompression values were low.

An in-situ determination of virgin compression line parameters for predicting soil displacement resulting from agricultural tire passes

This paper demonstrates the determination of the virgin compression line parameters from initial soil density, contact pressure and resulting rut depth in uniform soil conditions for which a constant soil density change to a depth of 500 mm was obtained in soil bin experiments (whereby total soil depth was 750 mm). The density change was determined with a “non-invasive” technique determining soil displacement (strain) by placing talcum powder lines into the soil during preparation of the soil bin and measuring the change in their relative position. The soil compaction model COMPSOIL with these parameters predicted wheel rut depth to within ±5%, from which in turn an absolute soil density increase can be determined to within ±3%. The model was successfully validated against data for uniform initial densities of 1.2 g/cm 3 and 1.6 g/cm 3 and a simulated layered field condition. The estimation of the virgin compression line was validated in the field as well. The parameters of the virgin compression line were estimated using soil density change data for the corresponding average contact pressures of different tires with loads of 4.5–10.5 t.

The effect of tyres and a rubber track at high axle loads on soil compaction: part 3: comparison of virgin compression line approaches

Alternative approaches for deriving virgin compression line (VCL) parameters, and their suitability to predict soil displacement from part I and II of this paper series ( Ansorge and Godwin, 2007 , Ansorge and Godwin, 2008 ) using the critical state soil mechanics model COMPSOIL, are presented. In particular, the in situ approach to determine a VCL from readily available tyre and soil data ( Ansorge & Godwin, 2008 ) was compared to triaxial cell test results. The results showed that a VCL was not a unique line with respect to mean normal pressure. The VCL was dependent on the relationship of minor and major principle stresses. Hence a triaxially derived VCL could only be successfully employed to predict the soil displacement if the relationship of minor and major principle stresses in the triaxial cell test represented those occurring in the field from the passage of a tyre. Subsequently the in situ approach was successfully employed using small scale plate sinkage tests further validating the entire approach and demonstrating the potential of a simple laboratory test to derive VCLs. Applying the in situ approach to soil displacement data from a rubber track showed that rubber tracks have different compaction behaviour than tyres. The rut depth of a rubber track was successfully predicted when the track roller body was modelled with tyres representing the pressure profile occurring underneath a track employing the in situ VCL derived from tyre only data. However, soil displacement at depth was significantly overestimated.

Micro- and macro-mechanical behaviour of DEM crushable materials

This paper provides a micro-mechanical commentary on the macroscopic behaviour observed in DEM simulations of the compression of individual crushable grains, and of triaxial tests on assemblies of both crushable and uncrushable grains. A fragmentation ratio is defined to describe the bond breakage processes, and the significance of other micro-mechanical parameters such as sliding contacts ratio, average coordination number, deviator fabric, and internal energies per unit volume is discussed. Three different modes of grain damage were observed: asperity breakage, internal shear cracking, and internal tensile cracking leading to fast fracture. Energy balances both for the compression of a single grain, and for triaxial tests on assemblies of grains, showed that the loss of elastic energy due to bond breakage was a negligible fraction of the significantly enhanced dissipation encountered with crushable materials. This extra dissipation was associated with frictional sliding triggered by the creation of new degrees of freedom among the breaking fragments. Different modes of grain breakage were found to be representative of different regions of soil states of stress defined with respect to the virgin compression line. The secondary role of elastic grain deformability, increasing coordination number but reducing dilatancy, has also been demonstrated.

Determination of precompression stress from uniaxial compression tests

Soil compaction has been studied for many years due to its implications for crop yield. In addition, precompression stress is important tool in evaluating soil load capacity and its implications for soil structure. This study evaluated whether precompression stress fitted by different methods gave rise to different results when a wide range of soils was tested. Soil cores were collected from different depths at 13 sites in Sweden, Denmark and Brazil and subjected to uniaxial compression tests. Soil precompression stress was determined using five different fitting methods: (1) Casagrande by polynomial fit; (2) Casagrande by van Genuchten equation fit; (3) intercept of the virgin compression line (VCL, straight portion of stress–strain curve) and regression with the first three points of the curve; (4) intercept of VCL and regression with the first two points of the curve; (5) intersection of VCL with the x-axis at zero strain. Method 1 produced the highest precompression stress values, while method 5 differed from other methods for all sites. Methods 1 and 3 did not significantly differ for most sites. Mean precompression stress was 50–130 kPa for the different methods. Polynomial and sigmoidal equations showed a good fit for the compaction curve in our dataset ( R 2 = 0.99), but Casagrande method parameters, e.g. point of maximum curvature, its tangent and slope of VCL, were different, resulting in different precompression stress values. Choice of method proved to have a significant influence on precompression stress values. Although methods 1 and 2 are both Casagrande-based, the results frequently differed. For the stress range tested (up to 1000 kPa), the van Genuchten equation was less suitable than polynomial fitting, since the compaction curve was usually not sigmoidal and the inflection point was sometimes outside observed values. Thus, the method used to fit precompression stress must be carefully chosen to avoid overestimates and underestimates. This restricts use of the absolute precompression stress value as a limit for soil stress in order to avoid soil compaction.

Compressive properties of some Swedish and Danish structured agricultural soils measured in uniaxial compression tests

SummarySoil compaction is recognized as a threat to long‐term productivity of agricultural soils and as a cause of environmental problems such as flooding. The use of models to establish strategies for prevention of soil compaction is hampered by lack of model input parameters describing soil mechanical properties. This paper presents the compressive properties N (specific volume at σ = 1 kPa on the virgin compression line), Cc (compression index) and Cr (recompression index) obtained from uniaxial compression testing of 69 individual soil layers and investigates the relationships between these properties and readily quantifiable soil parameters. No correlation was found between compressive properties and soil texture. Instead, N, Cc and Cr were positively correlated to the initial specific volume (v0). This suggests that compressive properties are more strongly affected by soil structure than by soil texture. Dependency of compressive properties on v0 could not be expected from classical soil compressive behaviour theory but suggests modifications to the theory of soil unloading‐reloading behaviour. We suggest that the latter is dependent on time between unloading and reloading.

Compaction of restored soil by heavy agricultural machinery—Soil physical and mechanical aspects

As construction and open-cast mining activities continue to expand on fertile agricultural land, the removal and subsequent restoration of soil to be re-used for plant growth has become an increasingly important issue in soil protection. A key factor for the success of soil restoration is that the soil is allowed to develop sufficient mechanical strength to withstand the stresses involved in the intended type of land use. The objective of this study was to investigate the effects of the first use of heavy agricultural machinery on the physical and mechanical properties of a restored soil after the period of restricted cultivation (as prescribed by current guidelines), when the soil is re-submitted to normal agricultural management. We performed two traffic experiments on a soil which had been restored according to current guidelines 4 years before the beginning of the study. In the first year of the study, a combine harvester passed two times across the wetted experimental area, and in the following year 10 times. Two passes along the same tracks caused only weak compaction effects, mainly reducing coarse porosity. In contrast, after 10 passes, deep ruts had formed, and coarse porosity was drastically reduced down to the subsoil. Confined uniaxial compression tests revealed an increase in precompression stress and a decrease in the slope of the virgin compression line, i.e. the compression index, after 10 passes. However, precompression stress was still much lower than the exerted soil stresses at the corresponding soil depths, indicating that due to the short duration of the wheel loadings equilibrium conditions were not reached in the traffic experiments and that further compaction would have occurred with additional passes. The decrease in compression index found after 10 passes may be due to the practice that samples are pre-conditioned to a specified water tension for the oedometer tests. The results show that loads may exceed precompression stress for short durations even in a restored soil which is still far from having re-gained normal strength without serious damage. Thus, the use of precompression stress as a criterion for traffickability was on the safe side in preventing damage to the ecological quality of the soil by compaction, even if the concept did not fully apply to the field reality of the mechanical stress conditions.

Soil compressibility and penetrability of an Oxisol from southern Brazil, as affected by long-term tillage systems

The precompression stress value defines the transition from the reloading curve to the virgin compression line in the stress–strain curve, which can be used to quantify the highest load or the most intense predrying previously applied to the soil. Thus, in soils with well-defined structured soil horizons, each layer can be characterized by such mechanical strength. Penetration resistance measurements, on the other hand, can be used to determine total soil strength profiles in the field. The effect of long-term tillage systems on physical and mechanical properties was determined in undisturbed and remolded samples collected at 5 and 15 cm depth, 6 months after applying no-till (NT), chisel plow (CP), and conventional tillage (CT) treatments, along with the application of mineral fertilizer and poultry litter. The compressibility tests were performed under confined conditions, with normal loads varying from 10 to 400 kPa after a defined predrying to −6 or −30 kPa. Penetration resistance was determined in the field, after seeding, in three positions: seeding row (SR), untrafficked interrow (UI), and recently trafficked interrow (TI). No-till system showed greater soil resistance to deformation than tilled treatments, as determined by the higher precompression stress and lower coefficient of compressibility. When original soil structure was destroyed (remolded samples), smaller differences were found. The application of extra organic matter (poultry litter) resulted in a reduction of precompression stress in undisturbed samples. Penetration resistance profiles showed greater differences among tillage treatments in the upper layer of the untrafficked interrow, where NT system showed the higher values. Smaller differences were found in the seeding row (with lower values) and in recently trafficked interrow (with higher values), showing that even traffic with a light tractor after soil tillage reduced drastically the effect of previous tillage by loosening up the soil. On the other hand, the tool used to cut the soil and to open the furrow for seeding, incorporated in the direct seeding machine, was sufficient to realleviate surface soil compaction.

Pressão de pré-compressão e comportamento hídrico de um Nitossolo Vermelho distroférrico em função do tamanho de agregados

Soil compaction has been the subject of intensive research studies in the last ten years, but the mechanisms involved in the soil compaction process still remain barely known. The pre-compression pressure has been used as a procedure for estimating the susceptibility of the soil compaction. The contribution of aggregate size of the soil as well as the effect of water content on soil compaction was investigated in a clay soil during this trial. Disturbed soil samples constituted by aggregated inferior to 2.5 mm A1 and 9.3 to 19.4 mm A2 were submitted to a drained uniaxial compression test. The compression index and pre-compression stress and the behavior of the water content were evaluated. The soil compression index decreased with the increase of aggregate size, showing that the virgin compression lines were parallels only with the values of the soil A2. The pre-compression stress values increased with the reduction of aggregate size and the soil water content. The results of the voids ratios in function of the threshold water content reflect with bigger precision, in soil A2, the regions of the soil behavior to the compaction with all the applied pressures.

EXPERIMENTAL VERIFICATION OF AN INVERSE SOLUTION TECHNIQUE FOR DETERMINING PHYSICAL PROPERTIES OF SOIL

Previous researchers have developed an inverse solution technique that utilized a numerical method to createresponse surfaces and an optimization scheme to determine physical properties of soil in situ. The purpose of this article isto provide an experimental verification of this technique. The mechanical behavior of the soil was modeled using a modifiedcritical state soil constitutive relationship that contained five soil parameters, including two elastic soil parameters[logarithmic bulk modulus (.. and Poissons ratio (..] and three soil plasticity parameters [slope of the critical state line (M),slope of the virgin compression line (.., and intercept of the virgin compression line (.)], to create response surfaces for platesinkage tests. These response surfaces were relatively insensitive to the soil elastic parameters (. and .). The responsesurfaces along with experimental results were used in an inverse solution technique to determine soil properties. Thistechnique worked reasonably well when three parameters related to the plasticity behavior of the soil (M, ., and .) wereidentified for all the soil conditions investigated in this study.

Estimativa do efeito do uso e da umidade do solo sobre a compactação adicional de três latossolos

A compactação adicional do solo ocorre quando a pressão aplicada sobre ele é maior do que a pressão de preconsolidação, fazendo com que o solo se deforme ao longo da reta de compressão virgem. A reta de compressão virgem é a região onde ocorre a compactação adicional. Os objetivos deste estudo foram: testar o modelo proposto por Dias Junior (1994), propor um modelo para quantificar a resistência mecânica do solo e avaliar o efeito do uso do solo e da umidade na reta de compressão virgem e no índice de compressão de três Latossolos. Os solos sob as condições de uso com cultivo anual, mata natural e pastagem cultivada estão localizados na região de Lavras (MG). O trabalho foi executado durante os anos de 1996 e 1997. Para a condição de cultivo anual, mata natural e pastagem, foram coletadas cinco amostras indeformadas, com três repetições, nas profundidades de 0-0,03 e 0,27-0,30 m, e utilizadas no ensaio de compressão uniaxial. Coletou-se também uma amostra deformada, com três repetições em cada condição, para determinar o limite de plasticidade. Verificou-se que, à medida que o valor da umidade aumenta, as retas de compressão virgem são deslocadas para a região de menor pressão, o que indica aumento da suscetibilidade do solo à compactação, diminuindo, entretanto, a resistência mecânica a ser vencida pelo sistema radicular. Os índices de compressão não diferiram estatisticamente para o cultivo anual e mata natural na profundidade de 0-0,03 m, o que não ocorreu para a pastagem cultivada em ambas as profundidades e para o cultivo anual e mata natural na profundidade de 0,27-0,30 m. Nessas condições, a densidade do solo final e o índice de compressão do solo seguiram os modelos propostos por Dias Junior (1994). Em geral, os solos sob mata natural na camada de 0-0,03 m foram mais suscetíveis à compactação por apresentarem maiores valores do índice de compressão máximo.

Estimating critical state soil mechanics parameters from shear box tests

SummaryPrediction of compaction, tillage, root growth or other soil deformation events requires a description of the stress–strain properties of the soil such as the critical state model, but estimating the parameters is time consuming and expensive. I have developed a method of estimating critical state properties from a single shear box test, both saving much labour and providing more information than traditional analyses. The method is based on critical state analyses of the constant stress and constant volume shear box tests using the total stresses applied at the boundary. It derives the critical state property parameters from test data by minimizing the difference between test data and the simulated soil deformation (and hence the properties used in that simulation). The minimization is a form of regression analysis. The analyses resulted in good simulations of the history of states in space defined by τxyγxy and e space (constant stress test) or τxyγxy and σy space (constant volume test).For normally consolidated samples, the analysis of a single constant stress test provided estimates of the slope of the critical state line (M), the slope of the virgin compression line (λ), the slope of the rebound line (K), and the elastic modulus (E). The standard deviation of the estimate of kcould not be found. By contrast, the analysis of the constant volume test resulted in poor estimates, particularly of λ. This is because the test yields no information on changes of volume during deformation: hence volume change parameters are not successfully estimated. The constant volume test is therefore not suitable for back analysis.

Estimating critical state soil mechanics parameters from constant cell volume triaxial tests

SummaryCompaction, tillage, stresses around growing roots and other soil deformation events may be predicted by the critical state model of soil mechanics, but estimating the parameters is time consuming and expensive. We develop a back analysis of the constant cell volume triaxial test, in which the critical state parameters are derived from the results of a single test. This both saves much labour and provides more information than traditional analyses, which require several triaxial compression tests and an isotropic compression test to yield the same information. The method finds, using a minimization algorithm and a quasi‐analytical solution to the stress–strain equations, the simulated soil deformation (and hence the properties used in that simulation) that best fits the test data. The minimization is a form of regression analysis.For normally consolidated samples the method provides stable estimates of the slope of the critical state line (M), the slope of the virgin compression line (λ) and elastic modulus (E). The standard errors of the estimates are small in relation to the means of these parameters. The estimates appear to be more reliable than those of more commonly used estimation procedures. The slope of the rebound line (κ) is estimated, but a measure of the accuracy of the estimate cannot be calculated.

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.

Overconsolidation in Agricultural Soils: I. Compression and Consolidation Behavior of Remolded and Structured Soils

The identification of meaningful physical indicators of soil quality is an important part of characterizing the present state of our soil resources. A study was undertaken with the main objective of examining the compressive behavior of remolded (saturated only) and structurally intact (variable degrees of saturation including saturated) solum horizons sampled from an agricultural region in south western Ontario, Canada. The measured compression lines for the majority of soils showed significant convergence as Cc, the slope of the virgin compression line (VCL), increased with initial void ratio (e0) and preconsolidation stress (ρc). The deegree of saturation affected Cc mostly through its influence on ρc. Strength gain with progressive desorption was most apparent in nonplastic soils with more than a minimum contnet of structure stabilizing substances (transient strengthening) and in plastic soils with higher e0 values. The widely adopted compaction paradigm derived from compression of sieved aggregates (i.e., constant Cc and VCL displacement to higher void ratios for a given stress with progressive desorption) was found to hold only for soils with relatively uniform e0 and weaker grades of structure. Samples identified as highly overconsolidated (mostly subsoils) had their measured VCL displaced below the normal compression line (NCL) at ρc, which occurred when the void ratio difference between the NCL at unit stress and e0 exceeded ≈0.36. Factors contributing to subsoil overconsolidation were believed to be mostly natural, including secondary compression and soil dessication. A pedotransfer function based on VCL reconstruction and ρc estimation is proposed for assessing the degree of overconsolidation in agricultural soils located in this part of Ontario.

Overconsolidation in Agricultural Soils: II. Pedotransfer Functions for Estimating Preconsolidation Stress

A series of soil survey interpretive procedure, or pedotransfer functions (PTFs), are presented that follow from the hypothesis corroborated in the companion paper that the satureated compressive behavior of structured and of corresponding remolded agricultural soils in southern Outario were strongly related. The three PTFs were useful in characterizing the degree of overconsolidation as a physical indicator of soil quality on a regional basis. PTF1 identified soils as being highly overconsolidated when the void ratio difference between the normal compression line (NCL) at unit stress and e0 exceeded 0.36. The preconsolidation stress (ρ'c) of a soil was estimated from the dry bulk density measured in situ and from other soil properties needed to estimate the NCL of remolded soils (PTF2) or the virgin compression line of structured soils (PTF3). The PTFs sere tested on a data set comprised of soil horizons characterized in five county-level soil inventories that met the minimum PTF data requiremetns and other defined criteria (n = 210). All PTFs showed that the degree of soil overconsolidation increased significantly (P<0.0001) with depth and with increasing clay conten. For PTF2, the mean estimated ρ'c for the Ap horizons (n = 47) was only 20 kPa due to the effect of repeated tillage and natural turbational processes. the unitlled solum (B) horizons were found to be in a less consolidated condition than the C horizon subsoils (mean ρ'c = 97 and 138 kPa, respectively). This suggested that effective stresses from wheel traffic and tillage operations have generally been no greater than those originating from natural causes int heis region. Many of the clay-rich subsoils were quite overconsolidated (mean ρ'c > 150 kPa), whereas the coarser textured soils appeared to be closer to a normally consolidated condition. PTF3 corroborated most of these findings.

A simple procedure for estimating preconsolidation pressure from soil compression curves

Classical graphics and regression procedures have been used to estimate preconsolidation pressure (σ p) from soil compression curves, but none of these procedures is easy to use and they often involve subjective judgement. This paper presents a simple procedure for estimating σ p from uniaxial compression tests for either saturated or unsaturated soil conditions. We evaluated five methods for estimating σ p from standard soil compression curves for an applied stress sequence of 25, 50, 100, 200, 400, 800, and 1600 kPa. Four methods estimated σ p as the intersection of two lines: (a) the regression line obtained for the first two, three, four or five points of the applied stress sequence in the secondary compression portion of the compression curve and (b) the extension of the virgin compression line determined from the points associated with applied stress of 800 and 1600 kPa. Method 5 consisted of the Schmertmann method. The σ p determined for each method was compared to σ p estimated using the graphical procedure of Casagrande for 288 soil compression curves from three soils in Michigan and from values reported in the literature. Methods 1 and 5 fit our data best at low σ p (high soil water content) while methods 2 and 3 fit the data better at high σ p (low soil water content). Based on a low RMSE (18), a high R 2 (0.92), and closeness of fit to the 1:1 line, a combination of methods 1 and 3 was selected as the best estimation procedure. For data from the literature, methods 1 and 2 provided the best estimate based on lowest RMSE of 5 to 9, R 2 of 0.98 to 0.99, and the closest fit to the 1:1 line. The combined methods were not tested for published data since matric potentials for measured values were unknown. The final procedure, combined methods 1 and 3, was programmed into a computer spreadsheet provided in an Appendix. This procedure provides a fast and reliable estimation of σ p for saturated and unsaturated soil conditions and eliminates subjective judgment associated with classical graphical procedures.

Compression characteristics of a clay soil as influenced by crops and sampling dates

This study was undertaken to determine the influence of previous crop sequences on soil compression behaviour. Soil compression tests were performed on undisturbed soil samples of a Kamouraska clay which had been cropped continuously for 4 years to barley, corn, alfalfa, or timothy. Samples were taken from the surface soil in the non-trafficked zone, five times during the season. Cropping treatments had small but significant effects on soil compression characteristics. Soils on which crops were grown presented lower bulk density under standard compression (300 kPa) and higher pre-compression pressure values than the soil left unplanted (fallow). These effects were attributed to water content at time of sampling which was lower in the cropped than in the bare soils. Seasonal variations of these two virgin compression parameters were important and were related to soil moisture. Conversely, the soil compression index (slope of the virgin compression line) was not affected by crops or water content and seems to be a characteristic dependent on soil constituents.

The variation of soil critical state parameters with water content and its relevance to the compaction of two agricultural soils

SummaryExamination of the previously published results of laboratory compression tests on a loam and a sandy loam has shown that as the water content and degree of saturation of a soil increase, the gradient of the virgin compression line, expressed in terms of specific volume and log of spherical pressure, increases and its intercept decreases. The water contents of the soils ranged from 5% to 30% and the degrees of saturation ranged from 10% to 40%. For both soils the gradient of the recompression line for previously compressed soils was shown to decrease with decreasing initial specific volume (increasing density) and to approach zero at a specific volume of 1.5 (dry bulk density of 1750 kg/m3). It was deduced that the position of the critical state line also varies with soil water content and that the critical state theory can be extended to unsaturated soils and therefore be of use in predicting the mechanical behaviour of agricultural soil during cultivation and compaction.