Virgin Compression Research Articles

Effect of soil microstructure on the compressibility of natural Singapore marine clay

The effect of soil microstructure on the compressibility of natural Singapore marine clay is studied in this paper. It was found that natural Singapore marine clay can sustain higher void ratio than reconstituted Singapore marine clay at the same effective stress in one-dimensional compression. This difference is most noticeable between yield stress and two times the yield stress. This void ratio difference is commonly attributed to soil microstructure, and the decrease in this difference at higher stresses is attributed to progressive destructuration. As a consequence of progressive destructuration, the virgin compression curve is nonlinear and the compression index along the virgin compression range is variable, which is only noticeable during the constant rate of strain consolidation test on high-quality samples. In the absence of high-quality samples and constant rate of strain consolidation tests, an empirical method is proposed to obtain a first-order estimation of a nonlinear virgin compression curve using index properties and yield stress. This paper demonstrates that the proposed method works reasonably well with natural Singapore marine clay. The error bound for the predicted compression indices is approximately ±20% of the compression indices measured with constant rate of strain test on high-quality samples.

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.

Secondary Compression Features of Jiangsu Soft Marine Clay

The results of one-dimensional compression tests conducted on undisturbed specimens of Jiangsu soft marine clay is presented. Because of its high in situ void ratios and natural water content, Jiangsu soft marine clay displays high values of both the virgin compression index, Cc, and the secondary compression coefficient, Cα. The laboratory data indicates that the value of the ratio Cα/Cc for Jiangsu soft marine clay is constant. However, neither Cα nor Cc are constant: they both depend upon the natural water content (or void ratio) and thus are also dependent on the deformation (or compression) of Jiangsu soft marine clay. Settlement analyses show that the secondary settlement of Jiangsu soft marine clay is a significant component of the field settlement. The concept of a constant value for Cα/Cc is used to predict the secondary settlement of a surcharged embankment founded upon Jiangsu soft marine clay. The predictions are in agreement with the limited post-construction field measurements of the embankment settlement.

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.

An analytical study of the effect of penetration rate on piezocone tests in clay

Finite element cavity expansion analysis investigating the effect of penetration rate on piezocone tests in clay is presented. A coupled analysis was performed, in which the rate of cavity expansion was linked to the penetration rate of the cone and the cone angle, using the assumption that the deformation was wholly radial, and took place only between the cone tip and the cone shoulder. The soil was modelled using modified cam clay with two sets of parameters and varying values of overconsolidation ratio (OCR). The influence of penetration rate on the stress and pore pressure distributions was examined. For slower penetration rates, the excess pore pressure at the cone shoulder is lower since consolidation is permitted coincident with penetration. The radial profiles of post-penetration voids ratio demonstrate that partially drained penetration is permitted by volume change in the near field, in addition to radial movement in the far field. The radial distribution of excess pore pressure after slow penetration differs from the undrained case, with a relatively low radial gradient existing at the cone face. As a result, the dissipation curves after slow penetration lag behind those following fast penetration. The cone velocity is made dimensionless by normalizing with the coefficient of consolidation and the cone diameter. ‘Backbone’ curves of normalized velocity against normalized tip resistance and excess pore pressure capturing the transition from undrained to drained penetration are derived. The normalized pore pressure backbone curve is unique, whilst the normalized tip resistance shows a small dependency on OCR. These backbone penetration curves are compared with centrifuge model piezocone tests conducted at varying rates, and subsequent dissipation tests. The numerical and experimental results suggest that the value of consolidation coefficient operative during the dissipation phase is 2–4 times higher than the virgin compression value due to changes in the operative soil stiffness, as demonstrated from the stress paths of individual soil elements. The use of multi-rate penetration tests to deduce values of consolidation coefficient is discussed, in light of these differences. Copyright © 2005 John Wiley & Sons, Ltd.

Calculation of the compression index and precompression stress from soil compression test data

Abstract When compressing soil, there is a characteristic relationship between compressive stress and volume change that can be used to define important soil mechanical properties. Two defining features can be determined – the compression index (Cc – the modulus of the slope of the linear virgin compression curve) and the precompression stress ( σ ′ p – the transition point between the elastic rebound curve and the virgin compression curve). These are indicators of compressibility and stress history respectively. The purpose of this paper is to evaluate different ways of estimating these indicators based on laboratory test data. Repacked soils with a range of textures were subjected to sequential compressions of 50, 100 and 200 kPa, which provided two compression characteristics with “known” σ ′ p of 50 and 100 kPa. Three functions were fitted to the measured test data (fourth-order polynomial, symmetrical logistic sigmoidal and asymmetrical Gompertz sigmoidal). Values of Cc were estimated by linear regression (for the data later fitted with a polynomial function) or by the tangent at the inflection point derived from model parameters (logistic and Gompertz functions). Three estimates of σ ′ p were calculated for each of the three functions: the standard Casagrande method (C), the intercept of the virgin compression curve and the initial (no stress) horizontal line (V–I), and the point of maximum curvature (MC) derived from the curvature function (κ). The accuracy of estimating σ ′ p and the magnitude of Cc generally increased with clay content. Estimates of Cc based on sigmoidal curves did not differ greatly from the linear regression estimate. Sigmoidal curves yielded σ ′ p estimates with lower absolute deviations from known values than polynomial-based estimates. The MC calculation based on the Gompertz function gave the most accurate estimate of σ ′ p . The lower asymptote of sigmoidal curves may also correspond to the water-filled pore space. Thus, despite the fact that all three functions fitted the measured data equally well, characteristics based on the sigmoidal curves were deemed to be most appropriate. The greater accuracy of the prediction of σ ′ p favoured the Gompertz function. Wider applicability of this was further checked with data selected from an independent database on subsoil compaction. We recommend fitting the Gompertz function to measured soil compression characteristic test data, and to define Cc objectively as the modulus of the slope of the tangent at the inflection point, providing this lies within the measured data range, and σ ′ p as the point of maximum curvature as defined by κ.

Fractal model for virgin compression of pure clays

The well-known linear relation between the void ratio and the logarithm of pressure for the one-dimensional consolidation has widely incorporated into elastoplastic constitute equation of soils, however, it is not at all straight line for more compressible clays and has several physically unaccepted properties. The compression of clays is explained by fractal theory, and is seen as a balance between the vertical pressure and repulsive force between clay plates. The distance between clay plates depends on the structure formation of the clay flocculation and clay surface. The structure formation of the clay flocculation and clay surface can be modeled by fractal approach. A new formulation of one-dimensional virgin compression of pure clays, expressed by the linear function of ln ν–ln p or ln e–ln p, is proposed based on the fractal model for clay structure. The linear function of ln ν–ln p is suitable to express the virgin compression of the extremely high plasticity clays up to the high pressure of 5000 kPa. For low plasticity clays, the virgin compression satisfies with the linear relationship of ln e–ln p at the pressure higher than 100 kPa. These too are show to be in satisfactory agreement with experimental data. The gradient of the plots of ln ν–ln p and ln e–ln p is related to the fractal dimensions of clay structure, the fundamental material parameters for the first time.

Mechanical Resilience of Degraded Soil Amended with Organic Matter

Organic matter may help prevent the degradation of the soil pore structure by mechanical stresses, and consequently the impact of compaction and surface erosion. To obtain a mechanistic understanding of some of the processes involved, we amended an Ultisol with peat particles, at amendment rates of 0, 10, and 50 g kg−1, and subjected the peat‐soil mixtures to different wet/dry cycles (1, 4, or 10). Mechanical stresses were imposed to the soil as direct shear, surface shear, and virgin compression tests. The objective was to determine the effect of increasing organic matter on the resistance and resilience of the soil pore structure to mechanical stresses, after wet/dry cycles. Peat amendment increased soil porosity, mainly in the fine pores (<6 μm), whereas an increased number of wet/dry cycles affected the coarse pore fraction (>50 μm). Soil shear strength decreased with a greater peat‐amendment rate. For soil amended with 50 g kg−1 of peat, increasing the number of wet/dry cycles increased the compressibility index from 1.14 to 1.43 and the friction angle under shear by 6°. Although peat may decrease the resistance to compression, the initial soil pore structure and the resilience to this mechanical stress was greatly improved. Wetting and drying cycles improved the impact of peat on the mechanical resistance of the soil, but reduced resilience slightly. The results suggest that to amend with organic matter will improve recovery from vehicle traffic damage and improve water retention during dry periods, providing better conditions for plants and microbes.

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.

Stress–strain effects in structured unsaturated soils on coupled mechanical and hydraulic processes

The stress–strain processes in structured unsaturated arable and forest soils depend very much on the internal soil strength and on the existing hydraulic and mechanical boundary conditions. These variables affect soil deformation by compaction and shearing to a great extent. As soon as the internal soil strength, defined as the precompression stress value, is exceeded by external forces, an intense virgin compression process occurs in combination with shearing forces at high pore water pressure values. These processes result in a complete homogenisation of the soil profile down to a certain depth. Consequently, both the hydraulic and the gas fluxes are affected, which results in an intense alteration of ecological and mechanical properties of the site. The consequences of such compaction and shearing on soil strength and on sustainable land use must be considered with respect to the sustainability of the system.

Ko-Volume Change Characteristics of an Unsaturated Soil with Respect to Various Loading Paths

Abstract The volume change behavior of an unsaturated soil is a function of the stress state variables; namely, net normal stress and matric suction. The total and water volume changes during drained and constant water content loadings are investigated, and their behavior is described with respect to changes in the stress state variables. Volume changes are also presented in terms of void ratio and water content changes. The differing characteristics of volume change during drained and constant water content loadings are reflected clearly through the void ratio and water content changes. The significance of the highest, past matric suction is shown to be similar to the preconsolidation pressure influence on a saturated soil. The laboratory test results show that the water coefficient of volume change changes significantly from the reloading curve to the virgin compression curve. The effect of preconsolidating the soil was also shown to influence significantly the measured soil-water characteristic curve.

Prognose der mechanischen Belastbarkeit und der auflastabhängigen Änderung des Lufthaushaltes in Ackerböden anhand von Bodenkarten

Die Ermittlung/Berechnung der mechanischen Belastbarkeit von Böden als Pflanzenstandort durch den Wert der Vorbelastung kann als tiefenabhängige Kenngröße anhand von Regressionsanalysen vorgenommen und für einzelne Bodentypen der Bundesrepublik Deutschland kartenmäßig dargestellt werden. Außerdem können auflastabhängige Änderungen von physikalischen Kenngrößen im Erstverdichtungsbereich prognostiziert und dokumentiert werden. Auf dieser Grundlage können Empfehlungen zur standortspezifischen Bewirtschaftung bei gleichzeitiger Verhinderung von zusätzlichen Komprimierungen (z.B. Konzepte für bodenschonendes Befahren, Folgen der Umstellung von Bewirtschaftungen: konventionell/konservierend, Ableitung von Maschinenmassen und Reifendrücken) gegeben werden. Darüber hinaus können standortabhän-gige Gefährdungspotenziale in Abhängigkeit von der hydrologischen/klimatischen Situation definiert werden. Eine kontinuierliche Weiterentwicklung der Datengrundlage führt zu weiter verbesserter Karteninterpretation oder Weiterentwicklung von entsprechenden Beratungsschemata. Prediction of soil strength of arable soils and stress dependent changes in ecological properties based on soil maps Based on a database of at present 160 mechanical soil profile datasets, the site and horizon dependent mechanical soil strength expressed as precompression stress can be predicted by multiple regression analysis and used for documentation in maps at different scales. Stress dependent changes in air permeability or air capacity can be derived for the virgin compression stress range as well as the effect of stress propagation in soils or stress attenuation capacity and depth dependent changes of ecological properties. Thus, areas with defined mechanical sensitivity as a function of depth can be derived and recommendations for site adjusted farming techniques can be given. In addition it allows the agricultural machine industry to develop site adjusted machines to support the ideas of good farming practice, defined by the soil protection law of Germany.

Modelling the destructuring of soils during virgin compression

INTRODUCTION The important in uence of soil structure on the mechanical properties of soil has long been recognized (e.g. Casagrande, 1932; Mitchell, 1976). The term `soil structure' is used here to mean the arrangement and bonding of the soil constituents, and for simplicity it encompasses all features of a soil that cause its mechanical behaviour to be different from that of the corresponding reconstituted soil. The removal of soil structure is referred to as `destructuring', and this is usually a progressive process. In recent years there have been numerous studies in which a theoretical framework for describing the behaviour of structured soils has been formulated (e.g. Burland, 1990; Leroueil & Vaughan, 1990; Gens & Nova, 1993; Cotecchia & Chandler, 1997). The authors have also made a study of the virgin compression behaviour of structured soils (Liu & Carter, 1999), and in particular have proposed the hypothesis that, during virgin compression, the additional voids ratio sustained by soil structure is inversely proportional to the current mean effective stress. It was found that this hypothesis gives reasonable predictions of the virgin compression behaviour of many soft structured soils, although discrepancies were observed for some soils. The in uence of destructuring, as well as the development of soil structure on the compression properties of soft clays, have been successfully quanti®ed, and it was demonstrated that the hypothesis may be employed as a basis for formulating a complete stress±strain model for structured soils. In this article, the previous hypothesis is generalized so that the virgin compression behaviour of stiff clay, clay shale and ®ssured clay can also be described accurately. A general discussion on the modelling of destructuring of soils is also given.

Some measurements of permeability and effective stress on a heterogeneous soil mixture: implications for recovery of inelastic strains

The results of permeability variations in response to both changes in isotropic mean effective stress and internal fluid pressure gradients for an artificial silty clay are presented. While changing confining stress induces a typical elasto-plastic response, with a small increase in permeability during unloading, raising the fluid pressure results in a more pronounced increase in permeability, the magnitude of which is similar to that seen during virgin compression. The behaviour is interpreted as reflecting compressibility contrasts between rigid quartz grains and more compressible clays. Fluctuating the confining pressure has a more direct influence on the less compressible quartz particles, with a subsequent smaller change in permeability upon unloading. Conversely, raised pore fluid pressure affects the clay matrix more directly and can therefore prompt a different permeability response. The results have implications for predicting the permeability characteristics of overconsolidated soils that are heterogeneous in nature.

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.

Virgin compression of structured soils

This paper describes a study of the virgin compression behaviour of both naturally and artificially structured soils under one-dimensional compression or isotropic compression. It is proposed that during virgin compression, the additional voids ratio sustained by soil structure is inversely proportional to the current mean effective stress. The proposed formula has been verified for 20 different naturally and artificially structured soils, and it is seen that the proposal describes well the behaviour of all these soils. Finally, a general discussion on soil structure and its features is presented, and the proposed compression equation is extended to describe the compression of structured soil along a general stress path. Cet expos&acaute; est une étude du comportement d'une consolidation vierge sur des sols structur&acuate;s de mani&gravere naturelle et artificielle, avec une consolidation unidimensionnelle on isotrope. Nous avançons que pendant la consolidation vierge, le taux de pore additionnel subi par la structure du sol est inversement proportionnel &grave la contrainte effective moyenne actuelle. La for-mule propos&acuate;e a &acuteté v&acuute;rifi&acuate;e sur vingt sols structurés de manière artificielle et naturelle et l'on volt que la proposition décrit bien le comportement de tons ces sols. Enfin, nous présentons une &acuate;tude générale de la structure du sol et de ses caractéristiques et nous d&acuate;veloppons l'équadon de consolidation proposée pour d&acaute;crire la consolidation du sol structuré le long d'une trajectoire de contrainte g&acutge;nérale.

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.