Compression Curves Research Articles

Effect of cell size and wall thickness on the compression performance of triply periodic minimal surface based AlSi10Mg lattice structures

Triply periodic minimal surface (TPMS) are mathematical surfaces with zero mean curvature and are spatially invariant. Recently, these surfaces are investigated for potential application in the manufacturing of cellular materials or lattice structures for a variety of purposes. However, designing such structures is a difficult task as the dedicated design platforms impose higher costs. Therefore, in this study, lattice structures have been designed using an in-house developed opensource tool for different cell sizes and cell wall thicknesses. The lattice structures have been additively manufactured through laser powder bed fusion (LPBF) and their design compliance and manufacturing defects have been investigated through optical micrography and SEM. The lattice structures have been subjected to quasistatic uniaxial compression test in as-manufactured and heat-treated conditions and the resulting compressive stress-strain curves have been analyzed in detail to draw a relationship between the two varying quantities and the mechanical properties of the lattice structure. The results show that there has been small but visible differences between the as-designed and as-manufactured lattice structures. The mechanical properties of lattice structures deteriorated with increasing cell size at constant wall thickness and improved with increasing cell wall thickness at constant unit cell size. The heat treatment procedures homogenized and increased plastic deformation while decreasing the strength and modulus values. Overall the study outlined a significant effect of cell geometry, size, and cell wall thickness on the mechanical properties of the TPMS lattice structures.

An analytic and complete equation of state for condensed phase materials

Analytic equations of state (EOS) are intended to reproduce theoretical and experimental data in a single phase portion of the thermodynamic space. We devise a complete and thermodynamically consistent model with four distinct features: (1) a reference isotherm that remains thermodynamically stable, (2) a flexible specific heat model based on a fourth-order rational polynomial, (3) a Grüneisen parameter that depends on specific volume and temperature, and (4) pressure and internal energy functions that can be inverted analytically in temperature. The model aims to improve the accuracy of existing equations of state while remaining computationally efficient. To demonstrate its features, we include calibrations for single-crystal pentaerythritol tetranitrate (PETN), liquid nitromethane (NM), and hexagonal close-packed beryllium (Be) metal. The parameter optimization uses the specific heat capacity, Grüneisen parameter, and static compression curves obtained from density functional theory for the crystalline solids and molecular dynamics simulations for liquid NM. We also present a velocity autocorrelation function that yields accurate phonon densities of states for the EOS calibration from the molecular dynamics trajectories. Each of the three calibrations is constrained to enforce the ambient state from experimental measurements and validated against experimental Hugoniot data from multiple sources. We also include one-dimensional hydrodynamic simulations of the isentropic compression experiments for beryllium conducted at the Z facility.

Modeling compression curves of reconstituted clays based on logistic function

The logistic function is introduced for modeling the compression curves of reconstituted clays over a wide stress range beginning from the initial void ratio. Three parameters in the model can be determined by the least square method based on the measurements. It is found that the compression curves of reconstituted clays can be interpreted very well by the proposed model. The error in the void ratio between the calculations and the measurements varies within a narrow range of ±3%, based on the databases available from our research team and the independent researchers. The proposed model possesses the merit of continuity and derivability without singularity, compared with the conventional piecewise linear method of interpreting the compression behavior. In this study, a simply empirical way is also suggested for determining the compression curves of reconstituted clays in the case without performing consolidation tests. The three parameters in the model are correlated with two physical indices: the void ratio and the void ratio at the liquid limit.

Rapid inverse design of metamaterials based on prescribed mechanical behavior through machine learning

Designing and printing metamaterials with customizable architectures enables the realization of unprecedented mechanical behaviors that transcend those of their constituent materials. These behaviors are recorded in the form of response curves, with stress-strain curves describing their quasi-static footprint. However, existing inverse design approaches are yet matured to capture the full desired behaviors due to challenges stemmed from multiple design objectives, nonlinear behavior, and process-dependent manufacturing errors. Here, we report a rapid inverse design methodology, leveraging generative machine learning and desktop additive manufacturing, which enables the creation of nearly all possible uniaxial compressive stress‒strain curve cases while accounting for process-dependent errors from printing. Results show that mechanical behavior with full tailorability can be achieved with nearly 90% fidelity between target and experimentally measured results. Our approach represents a starting point to inverse design materials that meet prescribed yet complex behaviors and potentially bypasses iterative design-manufacturing cycles.

An Investigation into the Influence of Sample Height on the Consolidation Behaviour of Dredged Silt

This study delves into the effects of sample height on consolidation behaviour, utilising the automatic air pressure consolidometer. Extensive tests were conducted on three varieties of dredged silt samples of varying heights from Qianwan, Shenzhen, China. The salient findings can be summarised as follows: (1) Compression curves for samples of different dimensions transitioned through three distinct phases: minimal load disturbance, elastic deformation, and plastic deformation. Notably, the void ratio during the latter two phases diminished as sample height increased. (2) A rising sample height corresponded to a reduced stable strain and compression index. Furthermore, the consolidation coefficient notably diminished with an escalation in the sample height, whereas the structural yield stress remained largely unaffected. (3) Given the disparate formation processes, stress histories, and material compositions between dredged and marine silts, the permeability coefficient of dredged silt was found to be superior to that of marine silt. Within the typical preloading pressure scope (50~300 kPa), the consolidation coefficient of dredged silt was lower compared to marine silt. However, as the consolidation pressure significantly surpassed this threshold, the coefficient disparity between the two silts narrowed.

Study on dynamic mechanical properties and meso-damage mechanism of carbon fibers recycled aggregate concrete under freeze-thaw environment

The uniaxial compression mechanical properties tests of carbon fibers recycled aggregate concrete (CFRAC) were carried out in order to investigate the effect law of coupling freeze-thaw cycles (FTCs) and dynamic strain rates on the mechanical properties of CFRAC. Two cases of carbon fibers (CFs) doping of 0% and 0.3% were considered. The full CFRAC uniaxial compressive stress-strain curves at different strain rates (10−5/s, 10−4/s, 10−3/s, 10−2/s) after different FTCs (0, 50, 100) were obtained. The microscopic characteristics and microcrack propagation process of the specimens were analyzed by means of nuclear magnetic resonance (NMR), scanning electron microscope (SEM) and acoustic emission (AE). The results showed that the specimens still had significant strain rate sensitivity after FTCs. With the same amount of CFs doping and strain rate, the strength and elastic mode (E) of the specimens kept decreasing as the number of freeze-thaw cycles (N) increased. With the same N, the strength and E of the specimens increased and the deformation capacity decreased as strain rate increased. With the addition of 0.3% CFs, the frost resistance and crack resistance of the specimens were significantly enhanced. Combined with the statistical damage constitutive model, the influence of FTCs and dynamic strain rates coupling on the meso-damage mechanism of CFRAC was analyzed. The meso damage evolution process was characterized by five characteristic parameters E0, εa, εh, εb and H. The results showed that there was an obvious regularity between the change of characteristic parameters and CFs content, dynamic strain rate and the N. An effective link between the fine-scale damage mechanism and the macroscopic nonlinear intrinsic behavior was established. The phenomenon of AE signal lag in uniaxial compression was explained from the perspective of effective stress.

Appropriateness of hydrostatic pressure-based modeling for strength differential effect in advanced high strength steel

Although numerous models have been proposed to describe the strength differential (SD) effect, their appropriateness and efficiency in the case of advanced high strength steel (AHSSs) have not been investigated. To this end, this study investigates the SD effect in AHSS using a hydrostatic-pressure-dependent plasticity theory. Although pressure model can effectively predict the SD effect in AHSSs with only one coefficient, the original formulation is experimentally inconsistent in the first quadrant of a yield surface. Hence, this study developed a new formulation to remove the inconsistency in a simple manner and validated the pressure model using new experimental data reported for two recently developed AHSSs: DP980 and MART1500Y. The SD effect is characterized based on monotonic compression and tension tests in three different directions with respect to the rolling direction to obtain the stress–strain curves and r-values. Next, the pressure model is used to predict the plastic behavior of both steel sheets. The parallel transition of flow stress and similarity in r-values between the tension and compression states supports the appropriateness of the proposed pressure-based modeling. Furthermore, these insights substantiate the fact that deformation mechanisms cannot explain the SD effect in both steel sheets. Therefore, complex yield functions that describe the SD effect in general materials are not necessary in the case of AHSS. Additionally, the pressure model is incorporated into a distortional plasticity model for applying it to springback in the bending process. The distortional plasticity model with and without the pressure model is calibrated using tension–compression curves, thereby revealing that the SD significantly affects the calibration. Finally, a U-draw bending experiment and the corresponding finite element simulation are conducted to compare the springback; the results show that considering the SD effect improves the springback simulation accuracy.

In Situ Alteration of the Hydro-Mechanical Behaviour of a Compacted Stabilised Expansive Soil

This paper assesses the performance of an embankment constructed in 2010 with a stabilised expansive soil. Two types of treatment were employed at construction time: 4% lime and a mix of 2% lime and 3% cement. A sampling campaign was carried out in 2021 to evaluate the long-term performance of the stabilised soil properties. To assess the compressibility of the soil, oedometer tests were carried out on samples from different parts of the embankment. The results were compared to the compression curve of the untreated soil, also sampled in the same embankment. Complementary shrinkage tests were performed to investigate the effect of the treatment on swelling and shrinkage. The obtained results show that the yield stress of the material from the outer part was inferior to 100 kPa, similarly to the yield stress of the untreated soil, demonstrating a strong alteration in the effect of both treatments over time. This alteration was noticeable to a distance of approximately 2 m from the external surface. Beyond this distance, the performance of the soil was comparable to the behaviour of recently treated soil, with yield stresses close to 1000 kPa. These observations, similar for each treatment dosage, raise questions as to the durability of the treatment on the outer part of the backfill.

Study on the impact of HTPP fibers on the mechanical properties of ceramsite concrete

In this paper, the effect of High-Toughness Polypropylene (HTPP) fibers admixture (0, 0.3 %, 0.6 %, 0.9 %) on the mechanical properties of ceramsite concrete was investigated, and four kinds of ceramsite concrete specimens were prepared and tested for various kinds of ceramsite concrete compressive strength, flexural strength, and stress-strain curves, and the performance characteristics obtained include the strength-to-weight ratio, flexural-to-compressive ratio, tensile-to-compressive ratio, compressive toughness index, and failure morphology. Furthermore, combined with Scanning Electron Microscope (SEM) images, we analyzed the enhancement mechanism of basic mechanical properties of ceramic concrete by HTPP fibers. The results show that HTPP fibers can effectively improve the strength and toughness performance of ceramsite concrete, and the effect on improving toughness performance is more significant; After adding HTPP fibers, the compressive strength of ceramsite concrete increased by 26.3 %, the flexural strength increased by 29.7 %, the toughness index increased by 28.9 %, and the strength to weight ratio increased by 24.1 %; After adding HTPP fibers, the brittle failure characteristics of concrete are significantly weakened, and when it fails under load, it can still maintain good integrity, without scattering or continuous cracking. Based on the SEM test results, the mechanism of HTPP fibers reinforced ceramsite concrete was explained, mainly attributed to three aspects: fibers preventing ceramsite from floating, fiber surface maintaining good bonding with cement matrix, and effective synergistic bearing between fibers and ceramsite concrete. The research findings of this study can provide a theoretical foundation for the structural analysis and design of this type of concrete. It will be beneficial for the use of ceramsite concrete as a structural material and holds significant implications for the advancement of prefabricated lightweight wall panels.

Mechanical Properties of Dahurian Larch Wood under Cyclic Loading: Experiments and Constitutive Model

This article presents a study on the mechanical properties and constitutive model of Dahurian larch wood under parallel-to-grain (ParG) and perpendicular-to-grain (PerG) cyclic loading. A total of twenty-four dog-bone specimens were designed and prepared. Failure modes, stress–strain curves, elastic moduli under monotonic loading, and unloading/reloading moduli under cyclic loading were analyzed. Results indicated that the tensile and compressive envelope curves of wood under cyclic loading are similar to the monotonic stress–strain curves. The unloading and reverse reloading stiffness of wood are significantly degraded in both material directions. Furthermore, a constitutive model that is capable of considering the degradation of cyclic compression unloading stiffness and the change in reverse compression (tension) stiffness after tension (compression) unloading was proposed. A comprehensive comparison with test results was conducted, and they are in good agreement. Thus, the correctness of the proposed constitutive model is verified.

Study on the compressive mechanical properties of mixed recycled aggregate concrete with silane modified

This study used a 10% silane solution to strengthen recycled brick aggregates (RBA). Subsequently, mixed recycled aggregate concrete (MRAC) was prepared by using recycled concrete aggregate (RCA) and RBA modified with silane (Si-RBA). The properties of RBA and RCA were compared. The effects of the Si-RBA/RCA ratio on the failure modes, compressive strengths, and characteristics of the compressive stress strain curves under monotonic loading and cyclic loading were tested. The microstructures of the RBA, Si-RBA, and MRAC were analyzed by scanning electron microscopy (SEM). The test results showed that RBA modified with silane reduced water absorption by 51.61% and improved the crushing index by 14% compared to RBA. When the Si-RBA/RCA ratio changed from 0:10 to 10:0, the failure mode of MRAC transformed from interfacial transition zone (ITZ) failure to aggregated failure, while the compressive strength decreased. The MRAC specimens exhibited similar failure phenomena, regardless of monotonic or cyclic loading. The peak stress and elastic modulus decreased as the Si-RBA/RCA ratio increased, whereas the peak strain showed a fluctuating growth trend, and the plastic strain increased. In addition, SEM images showed that the reaction products of silane filled the pores of the RBA, and the silane solution made the ITZs of the RBA much denser. Based on the test data, a constitutive model for MRAC under monotonic and cyclic loadings was proposed, which showed good accuracy in predicting the response of MRAC under a compressive load.

Microstructure and activation volume of a Cu-6 wt %Zn brass processed by equal channel angular pressing

The generation of a high density of twins few tens nanometers in size can enhance both strength and ductility. The present work aims to produce highly twinned microstructure in Cu-6 wt % Zn alloy of a moderate stacking fault energy (SFE) by equal channel angular pressing (ECAP) using a die with angles ϕ= 110° and Ψ = 0. X ray diffraction (XRD) shows that a high density of defects is stored up to 2 passes then a recovery takes place. EBSD imaging reveals a high density of twins and a fibrous microstructure. Grains close to 100 nm in size are formed. Strain rate sensitivity (SRS) was investigated in compression and nanoindentation (NI). Compression curves show that deformation occurs by slipping with a contribution of twins to hardening. A high contribution of shear bands was revealed during ECAP and compression tests. The activation volume V* in compression and NI is in the range 70b3 - 100b3, consistent with the emission of dislocations from grain boundaries and twin boundaries The experimental values of V* were compared to those expected from defect densities.

Study of Constitutive Models of Reconstituted Clay with High Initial Water Content

The compression and shear behavior of reconstituted clay are closely related to the initial water content of the reconstituted soil. It is difficult to obtain the compression and shear test data of clay with a high initial water content. This study aims to propose a model to predict the compression deformation and strength characteristics of reconstituted clay prepared with any initial water content using less data. Based on the concept of the disturbed state, this paper establishes a mathematical model that can simulate the compression and triaxial shear characteristics of reconstituted clay with different initial water contents. This model uses three compression curves of reconstituted clay with different initial water contents to calibrate the model parameters, and can predict the compression deformation characteristics of reconstituted clay prepared with any initial water content. The model can simulate the consolidated undrained shear behavior of clay reconstituted with different initial water contents. Comparing the measured data shows that the model is in good agreement with the measured compression curve and the triaxial stress–strain curve. The error of the predicted pore ratio and test pore ratio before yield is within 5%, and the error of the predicted pore ratio and test pore ratio after yield is within 10%. The stress–strain relationship of clay hardening with different water contents can be captured. The model can provide a preliminary prediction for the mechanical properties of clay with a high initial water content.

Compressive and flexural toughness indices of lightweight aggregate concrete reinforced with micro-steel fibers

There remains a lack of comprehensive investigations examining ductility of the fiber-reinforced lightweight aggregate concrete (LWAC) elements, necessitating a deeper exploration. The purpose of this study is to assess the effect of micro-steel fibers on the compressive and flexural toughness capacities of LWAC. A total of twenty-one LWAC mixtures were prepared, varying the test parameters of the compressive strength of concrete and fiber volume fraction. The toughness capacities of fiber-reinforced LWAC were calculated using the stress–strain curves in compression and flexural load–deflection curves measured for each specimen, in accordance with ASTM C1018. These were compared with those of the previous normal-weight concrete and LWAC specimens reinforced by single macro-steel fibers or a combination of macro-steel fibers and micro synthetic fibers. The effect of numerous fiber parameters on the toughness of concrete was considered as a function of the fiber reinforcing index. Simple closed-form equations from the literature were modified to reliably determine the compressive and flexural toughness indices of micro-steel fiber-reinforced LWAC. Additionally, the relationship between equivalent flexural strength ratio and flexural toughness indices was formulated from a regression analysis using test datasets. Test results showed that the addition of only 0.25% micro-steel fibers significantly enhanced strength and toughness in compression and flexure. Moreover, better dispersion of steel microfibers enabled greater LWAC strength and toughness increases through a denser restriction of crack propagation.

Effects of Salinity and Curing Time on Compression Behavior of Fly Ash Stabilized Marine Clay

This study involves an experimental examination of compression behaviors of fly ash stabilized marine clay cured at different times concerning the influences of pore water salinity. A set of laboratory one-dimensional compression tests was conducted on soft Lianyungang marine clay specimens with various pore water salinities and stabilized with fly ash. Test results indicated that both salinity and curing time influenced the compression characteristic of fly ash stabilized clay. The presence of fly ash obviously reduced the compressibility of the untreated marine clay sample. The semi-logarithmic compression curve of untreated pure soil was a straight line, whereas fly ash stabilized clay samples exhibited the pattern of two straight lines. All the fly ash stabilized soil specimens were well represented by two straight lines in the bilogarithmic coordinate, making it convenient to determine the compression yield stress. The compression index at the pre-yield stage was not susceptible to the variation in the salinity and curing period, whereas at the post-yield stage it exhibited a downward tendency as salinity and curing time increased. The value of yield stress for stabilized marine clay specimen exhibited a downward tendency with increasing salinity, whereas it increased significantly within 7 days and tended to level off with the elapsed curing time. The adverse effect of salinity and the positive influence of curing time on yield stress should be considered in engineering applications.

Effect of the Load Application Angle on the Compressive Behavior of Al Honeycomb under Combined Normal-Shear Stress.

A comparison of the compressive behavior of Al honeycomb under pure normal stress and combined normal-shear stress was analyzed in this work. The typical working stress of honeycomb is a compressive load along the direction parallel to the axis of the cells. However, the component can also undergo shear stresses during operation, which can cause premature failure. This work analyzes the mechanical behavior in compression by normal stress (0°) and in conditions of combined normal-shear stress (at 15° and 25°) using a special pair of wedges. The samples were obtained from a 3000 series Al alloy sandwich panel and tested according to the ASTM C365/C365M-22 standard. The different deformation modes of the cells in the combined compression were examined for three angles (0, 15°, and 25°). A theoretical model of combined compression was used to derive the normal and tangential components starting from the total stress-strain curves. A compression curve analysis was conducted at different angles θ, allowing for considerations regarding changes in strength, absorbed energy, and deformations. Overall, as the load application angle increased, both the shear resistance of the honeycomb and its tangential displacement up to densification increased, which is the opposite of what occurs in normal behavior. The cell rotation angle was calculated as the load angle varied. The rotation angle of the cell increased with the displacement of the crosshead and the application angle of the force.

Comparison of gyratory compaction effect on coarse and fine-grained soils

Filling materials should have high strength and required gradation. It is important to determine properties of the filling material in the laboratory to satisfy required codes. The optimum water content and maximum dry unit weight can be designated by performing Proctor tests, which have been used for nearly a century. The impact loads are used to compress the soil in an inflexible mold in Proctor tests, but in reality, field compaction methods rely on a combination of kneading, vibration, and augmented normal pressures to achieve required densities in field. In this study, firstly standard proctor tests, modified proctor tests and Supervave Gyratory Compaction tests were conducted on different soil types. Then the compression curves were compared and the statistical analysis (ANOVA) were performed to find out the best methodology. Dry unit weight obtained using the SGC were compared with the maximum dry unit weight obtained using laboratory impact methods. The results showed that soil samples compacted with a gyratory compactor give the same performance over impact compaction methods.

Elastoplastic Modeling of Sandy Clays Based on Equivalent Void Ratio Concept

Naturally sedimentary soils, such as marine clays, weathered residual soils, and glacial deposits, usually possess a certain amount of sand particles. These types of soils could be classified as sandy clay, because the coarse fraction is usually under the transitional content. Their mechanical properties (e.g., compressibility and shear strength) are dominated by the clay matrix and significantly affected by the evolving interparticle structure. Previous research into sandy clay is experimental or numerical, and theoretical investigations that considered the reinforcing effect of coarse inclusions have seldom been reported. Therefore, the effect of sand content was incorporated based on the equivalent void ratio (e*) and equivalent compression curve (ECC) of sandy clays. A unified structure parameter (χ) was introduced, which could capture the reinforcing effect of sand particles on the stiffness and shear strength of sandy clays well. The constitutive relationship was formulated by incorporating the equivalent void ratio concept into an elastoplastic framework. Validation of the proposed model was achieved by comparing the predicted results with experimental data that were compiled from the literature. This revealed a satisfactory prediction for the mechanical response of sandy clays and other gap-graded soils (i.e., gravel–clay mixtures) with a broad spectrum of coarse fractions.

Long-term mechanical performance of high fluidity fiber reinforced concrete modified by metakaolin

To clarify the long-term strength and toughness of metakaolin (MK) and steel fiber (SF) modified concrete with higher fluidity and water/binder ratio, a series of tests including slump tests, compression tests, splitting tests, digital image processing and Scanning Electron Microscope (SEM) tests were performed on MK-SF concrete cured for 7–360 days. Results reveal that the slump of fresh concrete decreased with an increase in the MK and SF replacement rates. Moreover, the impact of MK on the slump of steel fiber reinforced concrete (SFRC) was more pronounced when combined with a lower water/binder ratio, resulting in increased viscosity. At the pre-peak stress region of the strain-stress curve, the compressive strength fc, tensile strength ft, Young’s modulus Ec, elastic modulus E0, and tensile strain at peak stress εt-max of high fluidity MK-SF concrete increased with increasing MK and SF admixing ratio, regardless of curing age. Notably, the coupling effects of MK and SF became more prominent after long-term curing. Without MK incorporation, the effects of SF and curing time on the above indices were relatively implicit. At the post-peak stress region of strain-stress curves, there existed a residual stage. The inclusion of MK significantly improved the long-term residual strength and strain of SFRC. Additionally, the toughness index Mc, which represents the total area of the compressive strain-stress curve containing both the pre-peak and post-peak regions, also exhibited substantial development with curing time, primarily attributed to the incorporation of MK and SF. The coupling of MK and SF led to a transformation of the concrete failure mode from brittle to ductile. Regression analysis reveals that a linear equation adequately described the long-term relationships of fc-ft, fc-Ec, fc-E0, fc-Mc, and fc-εt-max in MK-modified SFRC. Based on the testing data, a relative strength or toughness index λ and a new generalized hyperbola model were proposed to predict the long-term mechanical behavior mentioned above. Through crack morphology and microstructure analysis, the distinct roles of MK and SF in the composite material were examined.

Hydro-mechanical behavior of unsaturated intact paleosol and intact loess

Many geo-hazards occurred in regions incorporating loess-paleosol sequences. Understanding the hydro-mechanical behavior of unsaturated intact paleosol and intact loess is a prerequisite to address these geo-hazards. So far, the loess has been extensively investigated, while the study of paleosol is rare. This study investigates the unsaturated hydro-mechanical behavior of intact paleosol and intact loess, including water retention, compression, and collapse. The study of intact loess serves as a benchmark for that of intact paleosol. Results show that the intact paleosol and intact loess have similar air entry value (AEV) and average degree of hysteresis in WRC, although the former soil has a 20.2% smaller void ratio and a 38.8% larger clay fraction. This could be attributed to the larger volume of mega-pores (diameter > 360 μm) owned by the intact paleosol, which offset density and clay fraction effects on the AEV and hysteresis in WRC. On the other hand, the intact paleosol has a smaller structure permitted space (i.e. the area bounded by the compression curves of reconstituted and intact specimens) than the intact loess at a given suction. This is because that point-to-point and point-to-face contacts are widely observed between particles in the intact loess, while face-to-face particle contacts dominate in the intact paleosol. The former particle arrangement results in a metastable skeleton, while a stable skeleton is expected for the latter particle arrangement. In addition, the maximum collapse potential of the intact paleosol, which is calculated by the compression curves under suctions of 200 kPa and zero, is 42.8% smaller than that of the intact loess, mainly because of the metastable skeleton of the latter soil.