Virgin Compression Research Articles

A new framework for evaluating the virgin compression curves of reconstituted clays at different initial water contents

The influence of initial water content on the virgin compression of reconstituted clays is a topic of great importance. This study introduces a novel perspective based on the concept of intrinsic compression. It is demonstrated that the virgin compression curves of the reconstituted clays are similar in shape to the intrinsic compression line and can be obtained through vertical shifting and stretching. Building upon this concept, the intrinsic compression line is further extended to the compression curve cluster, enabling a direct evaluation of the influence of initial water content. The validity of the compression curve cluster is verified using independent data from the literature as well as data collected by our research team. In addition, a comparison with other relevant theories further underscores the superiority of the proposed framework.

One-dimensional compression behavior of granular soils around virgin compression line (VCL)

One-dimensional compression behavior of granular soils around virgin compression line (VCL)

Analysis of the micro to macro response of clays to compression

An investigation of clay microstructure and its evolution under one-dimensional (1D) and isotropic compression is presented for different clays. Data from the literature are compared to original results on two Italian clays, obtained using scanning electron microscopy, image processing, mercury intrusion porosimetry and on-purpose swelling tests. The effects of composition and loading history on clay microstructure, as well as its changes along the compression path (pre- and post-gross yielding) are analysed and a conceptual model of microstructure evolution is proposed for the clays under study. Normally consolidated clays at early virgin compression, either natural or reconstituted, are found to possess an open fabric of random to low orientation, complying with a prevailing inter-aggregate and a smaller intra-aggregate porosity, whose size and distribution depend on composition. Under 1D compression, either in the field or in the laboratory, the inter-aggregate porosity is lost, at a rate dependent on composition and loading history, and the dominant intra-aggregate micropore is progressively reduced. Accordingly, perfectly oriented stacks of domains are recognised which, however, embed preserved random particle arrangements even at large pressures, resulting in an increase of average orientation up to the reach of a steady orientation degree. Isotropic compression causes faster microstructure evolution, although large pressures are required to change 1D-induced fabric orientation.

Volume change response and fabric evolution of granular MX80 bentonite along different hydro-mechanical stress paths

Despite the increasing understanding of bentonite behaviour, there is still missing evidence on how different hydro-mechanical loadings, including sequences of hydration and compression, affect the fabric and the volume change behaviour of the material. It is generally assumed that the interplay between the behaviour of clay assemblages and the overall fabric of the material is the reason of having final states that are dependent on the stress path followed. Here the results of an experimental campaign aiming to study these factors are reported and discussed. Free swelling and swelling pressure tests were performed, both followed by compression to a relatively high stress. The experimental program involved various samples that were dismantled at intermediate states in order to perform microstructural observations by means of mercury intrusion porosimetry and electronic scanning microscopy. It was observed that while the void ratio at a given stress level depends on the stress path, subsequent compression led to a unique virgin compression line. The data obtained at the microscale gave further insight for an interpretation of the volume change behaviour observed at the macroscale, showing that at high stress the material tends to recover the same fabric regardless of the path to saturation.

Evaluation of a new grain breakage factor based on the single grain crushing strength

In granular soils subjected to high-level shear stress, a micro-damage occurs in the soil's internal structure, referred to as grain breakage. The main factors controlling grain breakage are the applied stress level and the single grain crushing strength of the individual grains (σf). This paper presents a laboratory study on ballast grains' single grain crushing behavior. Double-plate grain crushing tests were carried out on dolomite, calcerous, and basalt origin grains, having sizes ranging from 2.00 to 37.50 mm. Factors affecting the single grain crushing strength, such as grain shape, loading rate, and grain size, were investigated. Individual grains' single grain crushing strength was observed in three phases related to granular soils' 3-staged one-dimensional compression behavior. An equation that considers grain shape in terms of grain sphericity (form) has been proposed to determine the single grain crushing strength. One-dimensional compression tests were performed on uniformly graded soil samples consisting of calcerous and basalt origin grains. The single grain crushing strength of the calcerous origin soil sample’s mean grain-sized grains was determined to be 12.7 MPa, and this value was determined to be 25.6 MPa for the sample consisting of basalt origin grains. One-third/fourth of this single grain crushing strength value was obtained as the virgin compression line (VCL) initial stress of the soils’ one-dimensional compression curve (4.3 and 6.0 MPa for calcerous and basalt origin soil samples, respectively). The final stress of the VCL was determined to be close to 1–1.25 times of this specified single grain crushing strength (16 and 26 MPa for calcerous and basalt origin soil samples, respectively). The change in the mean grain size of the soil samples (d50) during one-dimensional compression was defined as an exponential function. A new grain breakage factor depending on the single grain crushing strength of the soils’ mean grain-sized grains has been proposed to determine the grain breakage that develops under one-dimensional compressional conditions.

Determination method and strain-attribute interact in the calculation of precompression stress from soil compression curves

Determination of soil precompression stress (σp) from compression curves has markedly changed in recent decades, with several available methods of varying complexity. The graphical procedure proposed more than eight decades ago remains a standard, though it is now emulated by statistical and numerical procedures. We tested nine determination methods based on linear, polynomial, nonlinear and spline regressions. Four attributes were used to represent soil deformation during compression (strain-attributes): bulk density (ρb), void ratio (e), total porosity (ϕ) and strain (ε). Undisturbed soil samples equilibrated either at 10 or 100 kPa water tension were subjected to confined, drained uniaxial compression tests. The experimental factors (determination method, strain-attribute, and water tension) and their interaction affected the determination of σp. Sigmoid equations converged for 84%–95% of the compression curves, failing for samples at 100 kPa with with steep virgin compression lines. Methods based on fourth-degree polynomials resulted in 10-94% of the values outside the range 10–1550 kPa, depending on the strain-attribute. Although ε and e are linearly related, the former suits better the application of methods based on polynomial and spline regression, also fitting well for methods based on sigmoid equations with samples at 10 kPa. The bi-linear intersection methods presented relaistic σp values for bi-linear curves and should be solely applied to ρb. The spline method resulted in σp values clustered along the loads applied during the compression tests.

Hydromechanical behavior of unsaturated soils: Interpretation of compression curves in terms of effective stress

This state-of-the-art paper on the hydromechanical behavior of unsaturated soils focuses on the interpretation of the compression curves of unsaturated soils in terms of effective stress, with the goal of understanding the relative impacts of suction on the effective stress, net yield stress, effective yield stress and slope of the virgin compression line (VCL) during a monotonic increase in net stress. A database of compression curves was compiled for both high and low plasticity fine-grained soils under a wide range of suctions, isotropic or oedometric stress states, drainage conditions (constant suction or constant water content) and preparation techniques (impact compaction, static compaction, consolidation from slurry). Most of the compression curves plotted in terms of effective stress revealed a consistent hardening response with increasing suction and a slight suction dependency on the slope VCL. Interpretation of the compression curves in terms of effective stress led to load-collapse curves with a similar shape for a wide range of soils. Most soils evaluated had a greater rate of increase in effective yield stress with suction than the rate of increase in suction stress with suction, implying that these compacted soils may be susceptible to collapse upon wetting. Inconsistent trends were observed in some studies, which were attributed partially to natural variability but also experimental issues and limitations on the range of conditions investigated. Accordingly, recommendations are provided for future studies on the compression curves of unsaturated soils to ensure that results can be clearly interpreted in terms of effective stress.

Empirical Compressibility Index Equations for Artificial Remolded Clay Mixtures

A series of one-dimensional consolidation tests was conducted on reconstituted soil mixtures with different mineral compositions. Test groups were prepared by mixing various proportions of kaolinite, quartz, and montmorillonite in which the initial water contents were equal to the liquid limits of the soils. The focus was on the compression behavior of reconstituted clay mixtures, including the rarely reported triple mixtures, as well as establishing a link between the liquid limit and the clay mineral ratio of the specimens. The intrinsic compression indexes of the soils were highly dependent on the composition of the soils and the initial water contents. New equations were proposed to estimate the compression index based on the clay mineral content, liquid limit, plasticity index, and initial void ratio. A triangular chart was proposed to estimate the compression indexes of reconstituted soils. A simple way to determine the virgin compression lines of reconstituted specimens is described.

Suitability of swelling and collapse theory proposed based on virgin compression surface

This study investigates the suitability of the swelling and collapse theory proposed based on a constitutive virgin compression surface (VCS) developed within the modified Monash Peradeniya Kodikara (MPK) framework. The modified MPK approach incorporates net stress, void ratio, and moisture ratio as state variables to interpret the swelling-collapse behavior of unsaturated soils. The soil selected for this research was an expansive Quaternary age basaltic residual clay located in Victoria, Australia. The experimental program included testing on unsaturated compacted clay specimens and clay specimens stabilized with lime at the optimum lime content (OLC). The OLC was obtained based on the swelling potential. Static compaction tests were conducted on untreated and lime-treated samples to establish the VCS and to propose the relationship between moisture ratio and net stress. Next, 1-D compression and consolidation laboratory tests were carried out to investigate and compare the mechanism of collapse and swelling based on well-established suction-based theories and the moisture content-based approach of this research. The swelling and collapse response obtained through both theories were very close which verified the suitability of the moisture content-based approach, being the Modified MPK framework. This study also investigated the application of suction-controlled experimental data extracted from the relevant literature within the modified MPK framework. The novelty of this study is proposing and validating a framework for conversion of the results between two different partial theories to interpret the volumetric behavior of expansive clay. Finally, a new method is proposed to estimate the swelling-collapse state of a sample through wetting without the need to establish the VCS.

Експериментальні дослідження деформаційних характеристик закладальних масивів

Purpose of work is to determine the deformation characteristics of the backfill massifs used to support the side rocks behind the working face in the cut area of a coal mine. To achieve this goal, laboratory studies were carried out on samples of crushed rock of different particle-size composition, which were subjected to uniaxial compression in a steel cylinder (virgin compression) and in a special bag (with the possibility of lateral expansion of the filling material). As a result of the studies performed, it was found that the relative deformation of a compacting body from crushed rock during virgin compression is less than the relative deformation of the same body when lateral expansion of the filling material is possible. It has been experimentally proven that with the possibility of lateral expansion of the filling material, the values of the relative deformation change according to a linear relationship, when with an increase in the bulk density of crushed rock after uniaxial compression, a decrease in the vertical size of the compacting body is recorded. For crushed rock of heterogeneous particle-size composition, at maximum values of the compaction coefficient and virgin compression, the vertical size of the compacting body is 36 % less than in the case of uniaxial compression of the filling material with the possibility of lateral expansion of the crushed rock. Moreover, the smaller the fraction of crushed rock, the greater the recorded difference. The change in the cubic capacity of the compacting body occurs as a result of repackaging of particles of different sizes in the total volume of the filling material and the spread of crushed rock along the plane under the action of a compressive load. The value of Poisson’s ratio under conditions of uniaxial compression of crushed rock with the possibility of lateral expansion of the compacting body changes according to a parabolic dependence and reaches minimum values in the presence of inhomogeneous (in particle size) crushed rock in the filling material. The maximum values of the investigated coefficient are provided after compression of the initial material of coarse or fine fractions, which initially compose the filling mass. Ensuring the stability of side rocks, the operational state of mine workings in the excavation areas of a coal mine and preventing subsidence of the earth's surface are achieved after the construction of a filling mass in the worked-out space, consisting of inhomogeneous (in size) pieces of crushed rock, which has compression characteristics. Therefore, the requirements for the properties of filling arrays are determined in accordance with their purpose.

Estimation of the compression behavior of sandy clay considering sand fraction effect based on equivalent void ratio concept

Sandy soils produced from offshore engineering and dredging activities usually possess a fine fraction beyond the “transitional fine content”. The soils have a wide range initial water content and sand fraction. Previous models based on mixture theory is not straightforward, and the effect of initial water content is not explicitly incorporated. To this end, a novel compression model is formulated by incorporating the equivalent void ratio into the tangent stiffness of clay matrix. The virgin compression behavior of clay matrix is captured by incorporating two physical variables, termed as the reference void ratio and the remolded yield stress, which are correlated with the initial water content of the clay matrix. The equivalent void ratio concept is then introduced by incorporating a structure variable which relies uniquely on the volume fraction of sand. The structure variable denotes the sand fraction effect linking to partial contacts and densified clay bridges between aggregates. It relies on basic features of sand, e.g., the particle shape and particle size distribution. Morphological analysis reveals that the model is versatile to capture the two basic features of the sandy clays, e.g., the increasing compressibility and decreasing remolded yield stress with rising initial void ratio of clay matrix, the sand fraction effect arising from the partial contacts and clay bridges between aggregates. The proposed model has five material constants, and only three oedometer tests are required for the calibrating these parameters. Validation of the model reveals that the model is effective to reproduce the compression behavior of sandy soils with a wide range of initial void ratio and sand fraction.

Laboratory Investigation of the Mechanical Properties of a Rubber–Calcareous Sand Mixture: The Effect of Rubber Content

This paper introduces a rubber–calcareous sand mixture as a lightweight building material in offshore engineering. The mechanical properties of mixtures of varying rubber contents were investigated by performing a one-dimensional (1-D) compression test in a modified oedometer cell, as well as a resonant column test. A discussion on the test results, along with detailed interpretations regarding the role of rubber chips in the mixtures, are provided. It was found that the virgin compression curves of the rubber–calcareous sand mixtures tended to converge at a certain stress level, whilst the stress level depended on the rubber content. Moreover, the relative breakage was examined by comparing the particle size distribution curves of the calcareous sand before and after the compression test. It was shown that the grain crushing of calcareous sand was less remarkable with the inclusion of rubber chips. Furthermore, the small strain shear modulus (G0) of the mixtures decreased with the rubber content, yet the modulus reduction and damping curves exhibited little difference for the specimens of varying rubber contents.

Анализ методов аналитических расчетов осадок фундаментов во времени

Currently, there exist three main methods for calculating foundation settlement: the method of linearly compressible layer (introduced by Prof. K. E. Egorov), the method of layer-by-layer summation (described in SP 22.13330.2016), and the method of one-dimension virgin compression (a special case of the equivalent layer method introduced by Prof. N. A. Tsytovich). All the above-mentioned methods for calculating foundation settlement are based on the linear dependence of deformations on stresses. The principal differences between these methods are in taking the account of natural soil stress from its own weight, the technique of assessing the compressible soil strata and evaluating the distribution of stress in depth from external pressure. In the practice of bases and foundations design, the layer-by-layer summation method is considered to be the most highly reputable and widely used. It should be noted that each method was initially developed to solve some specific problem. However, not every engineer at present knows the limits of a particular method application for calculating foundation deformation. The article presents calculations of the non-linear settlement development in the foundation performed by various methods. Comparison with actual deformations is made based on the results of long-term monitoring of foundation settlement of a well-known building in Saint Petersburg, and recommendations are offered regarding the applicability of the discussed methods.

United void index for normalizing virgin compression of reconstituted clays

The intrinsic compression framework that uses the void index for normalizing the virgin compression of reconstituted clays has been widely applied for academic and practical purposes. Past studies have shown that the data of void index are scattered when the stress is out of the range from 100 to 1000 kPa. In this study, the key cause responsible for the scatter problem in the existing intrinsic compression framework is identified. A united void index is introduced for normalizing the compression curves of reconstituted clays over a wide stress range starting from the remoulded yield stress to 1000 kPa. The normalized unique line is termed the unified normalized compression line (UNCL). Its constitutive equation is established in terms of the united void index versus the effective vertical stress. The uniqueness of the UNCL is validated based on independent data from the literature and the data from the research team. It is suggested that the UNCL should be directly measured from the virgin compression. In the case without conducting consolidation tests, the correlations between the intrinsic parameters in the UNCL’s equation and two physical parameters are proposed for indirectly determining the UNCL. The accuracy of the empirical correlations is investigated via the comparisons between the calculated intrinsic parameters and the measurements.

Scale-dependent soil strengthening processes – What do we need to know and where to head for a sustainable environment?

Due to the need of securing food production for a growing world population, modern agriculture enhances the risk of land degradation. Thus, knowledge of biological, chemical and physical soil properties and processes under in situ conditions at all scales are needed to give a precise quantification of relevant soil functions. Scenarios for political discussions must be included. Parameters and their boundary conditions need to be defined. These parameters include the interparticle forces on the micro-scale, which are determined by the zeta potential and rheological properties. They are affected by matric potential and organic carbon. In addition, fertilizer application (amount and composition) and micro (aggregate) stability, which includes the processes of repulsion and dispersion, affect these forces. The latter are strongly related to the ionic composition and concentration. Strengthening processes can be also documented on the meso- and macroscale. On all scales, we need to regard tensorial properties and functions, as they also serve as a feedback mechanism for anthropogenic effects including changes in fluxes, accessibility of particle or pore surfaces for filter and buffer processes, or plant growth. Land use affects the physical soil functions, which may result in yield decline, enhanced gas emission, enhanced soil erosion, or more frequent flooding, if the rigidity of the pore system is not maintained. Soil degradation due to inadequate management mainly occurs when the internal soil strength (i.e., the precompression stress range, as its transitions from the recompression to the virgin compression range) is exceeded. The degradation can occur with any stress whether it is static or dynamic or mechanical or hydraulic. In the field, it can be verified by visible deformation (as a result of too much traffic) and, by means of critical physical values, the consequences on soil functions can be estimated using Compaction Verification Tools (CVT) that document harmful (subsoil) compaction.Thus, to avoid irreversible soil degradation because of subsoil compaction, precompression stress values (or ranges), in combination with hydraulic conductivity and air capacity values, are suggested to be the basis for values used in the German soil-protection law.

An extended modified cam clay model for improved accuracy at low and high-end stress levels

The Modified Cam Clay (MCC) model is extended based on S-shaped compression so that the quantitative inaccuracies and the qualitative errors of the model associated with both low and high stress levels can be removed. The following modifications are made: (i) a material constant r, the spacing ratio, is introduced; (ii) the yield surface is modified for r ≠ 2; and (iii) the ratio of the elastic compression index to the virgin compression index is assumed to be constant. The compression and shearing behavior of reconstituted clays for p′ < ∝ can be successfully described. Following the same method, the proposed complete S-shaped compression curve can be implemented to many existing models, removing errors of the models at the extremities of stress level and improving the performance of the models for different stress levels with one set of values of the model parameters.

Characteristics of volume and suction of undisturbed unsaturated loess under triaxial compression

The compression of unsaturated loess was explored with triaxial instrument for unsaturated soils. The test covered 3 different water contents of unsaturated loess and the saturated loess at initial confining pressures of 100 kPa, 200kPa and 400 kPa. The matric suction and volume change were obtained during virgin compression. Test results showed unsaturated loess have similar compression properties on the compression curve of saturated soil on the semi-logarithmic plane of average net stress, with yield points, which can be expressed by straight lines before and after yielding. With an increase of degree of saturation, compressibility gradually increases. The matric suction of unsaturated loess gradually decreases, and the variation is similar to that of volume. The parameters for the Modified Cambridge model were obtained to describe the compression of unsaturated soils.

Modelling virgin compression line of compacted unsaturated soils

In this paper, the volumetric collapse of an unsaturated soil, upon soaking to saturation under a certain stress level, is referred to as soaking collapse. The soaking collapse for a soil under virgin condition is assumed equal to the difference between the soil’s virgin compression line (VCL) and its normal consolidation line (NCL) at saturated condition based on results of oedometric constant water content compression and soaking tests performed on a compacted Nanyang expansive clay. A one-parameter model is proposed to describe the variation of the soaking collapse with the pre-soaking degree of saturation under virgin condition once (1) a soil’s yielding point can be clearly defined and (2) the degree of saturation during virgin compression is known. This model can be used to predict the VCL from the NCL. Data of the Nanyang expansive clay, along with published data of nine soils that were derived from constant water content or constant suction compression tests (including oedometric and triaxial compression), were used to calibrate and validate the model. It is shown that the model, when combined with a degree of saturation-volume model and the information of yielding and NCL, can predict reasonable VCLs for all examined soils and suitably capture several characteristics of the VCL including the nonlinearity and pressurised saturation. A constant model parameter of 1.5 is found suitable for all examined soils. The model is also used to predict the VCLs of two compacted soils from the constant degree of saturation compression, which is a recently developed testing technique to evaluate the compression behaviour of unsaturated soils, with reasonable agreement achieved.

Hydromechanical behavior of a natural swelling soil of Boumagueur region (east of Algeria)

This work presents an experimental study of the hydromechanical behavior of a natural swelling soil taken from Boumagueur region east of Algeria. Several pathological cases due to the soil shrinkage / swelling phenomenon were detected in this area. In a first part, the hydric behavior on drying-wetting paths was made, using the osmotic technics and saturated salts solutions to control suction. In The second part, using a new osmotic oedometer, the coupled behavior as a function of applied stresses and suction was investigated. It was shown that soil compressibility parameters was influenced by suction variations that an increase in suction is followed by a decrease in the virgin compression slope. On the other hand, the unloading slope of the oedometric curves was not obviously affected by the imposed suction. The decrease in suction strongly influences the apparent preconsolidation pressure, ie during swelling of the samples after wetting.

Interpretation of piezocone penetration and dissipation tests in sensitive Leda clay at Gloucester test site

A modified piezocone penetration test (CPTu) analytical solution based on spherical cavity expansion and critical state soil mechanics (SCE–CSSM) is employed for assessing yield stress, undrained shear strength, and flow parameters in sensitive Leda clay at the Gloucester test site. For sensitive and structured clays, the formulation relies on the mobilized effective stress friction angle ([Formula: see text]) defined at two parts of the stress–strain curve: (i) peak stress ([Formula: see text]) and (ii) maximum obliquity ([Formula: see text]). Input parameters for assessing the overconsolidation ratio ([Formula: see text], where [Formula: see text] is preconsolidation stress and [Formula: see text] is current effective vertical stress) from CPTu results include: undrained rigidity index (IR= G/su, where G is shear modulus and suis undrained shear strength), plastic volumetric strain potential (Λ = 1 – (Cs/Cc), where Csis swelling index and Ccis virgin compression index), and effective friction angles ([Formula: see text] and [Formula: see text]). A direct CPTu means of assessing the undrained rigidity index in a reliable manner is also developed that gives the Nktcone factor and matches profiles of undrained shear strength from triaxial compression tests (suTC). The modified solution is also implemented on two additional sites: a sensitive-quick clay in Norway and structured varved clay from New England. Interpretations of the coefficient of consolidation and permeability from pore-water pressure dissipation tests at Gloucester are evaluated using the SCE–CSSM formulation and shown to be comparable with independent laboratory and field tests.