Behavior Of Clay Research Articles

Multi-scale description of hydro-mechanical coupling in swelling clays. Part I: Nonlinear poroelasticity

This paper presents a micromechanical description of the swelling behavior of partially saturated clays following a two-stage homogenization procedure on a three-dimensional Representative Elementary Volume (REV) of clay. At the nano-scale, the REV of clay particles includes idealized parallel clay platelets and oblate spheroidal pores in between them that are saturated with an electrolyte solution. Swelling forces at all spacing ranges, including the crystalline and osmotic regimes, are considered. At the micro-scale, the REV of clay is comprised of dispersed clay aggregates and partially saturated (variably-sized) spherical pores in between them. At issue, is reconstructing the couplings of mechanical, hydraulic and electrochemical forces at the macroscopic level and their effect on the overall behavior of clay. In Part I, we focus on reversible deformation mechanisms in clay. A generalized nonlinear form of the classical Biot’s poroelasticity relation is obtained with additional terms accounting for capillary and swelling stresses. Capillary stress emerges from interaction of fluid phases at different scales, and takes into account the surface tension effects. The swelling part on the other hand, correlates with the net disjoining hydration and electrochemical forces at the nano-scale. As a result, the stress description of clays is embedded with microstructural information. In addition, a robust localization procedure is utilized to track the microstructural changes associated to micro-porosity and clay platelet spacing. The nonlinearity of the stress–strain description is shown to originate from the dependency of the particles’ elasticity on spacing with electrochemical origins. Finally, base-line features of the model are investigated through material point simulations of a clay swelling test. The discrepancies between the model predictions and experimental measurements are resolved in Part II of this paper by considering also the plastic deformations within the framework developed in Part I.

Multi-scale description of hydro-mechanical coupling in swelling clays. Part II: Poroplasticity

This paper presents a micromechanical description of the stress-deformation in partially saturated swelling clays, which accounts for the plastic dissipation induced due to the coupled action of mechanical, swelling and capillary forces that operate at the underlying small scales. This is achieved via a two-stage homogenization procedure based on the average-field theory on a three-dimensional Representative Elementary Volume (REV) of clay which includes clay aggregates and partially saturated inter-aggregate micro-pore space. For the aggregates, which are assumed saturated with an electrolyte solution, the multiscale nonlinear poroelasticity model developed in Part I of this two-part paper is used here, which accounts for swelling forces that develop within the aggregates and span the crystalline and osmotic regimes. The interplay between mechanical, capillary and swelling forces across the material scales leads to the clay aggregates gliding upon each other. The behavior of the interface between adjacent clay aggregates, which describes their sliding, plays a prominent role in the macroscopic behavior of clays. This is manifested in the form of induced plasticity, described here using a simple plasticity law, which is upscaled using principles of continuum micromechanics. This work reconstructs all underlying complex couplings at play to finally arrive at a poroplasticity model at the macroscopic scale, where the macroscopic plastic strain is directly related to sliding action between clay aggregates at the micro-scale. Such a micromechanical description of plasticity in clays is made possible by utilizing the enhanced localization procedure presented in Part I. Finally, the model is shown to be successful in predicting the response of clays in a typical swelling test, when comparing the model predictions against experimental measurements.

Geomechanical Behaviour of Clay Stabilised with Fly-Ash-Based Geopolymer for Deep Mixing

Geopolymer has recently become an attractive alternative to traditional binders (e.g., cement and lime) used for chemical soil improvement, with several environmental benefits including lower toxic emissions and energy consumption. This paper presents an evaluation of the geomechanical behaviour of soft clay treated with fly-ash-based geopolymer incorporating slag for deep soil mixing (DSM) applications. The geomechanical properties of stabilised clay were evaluated using unconfined compressive strength (UCS) tests and durability against wetting–drying. Thermal conductivity and pH tests along with microstructural analysis using scanning electron microscopy (SEM) were also performed to provide insights into the effect of geopolymer on treated clay. The results indicate that the inclusion of geopolymer with the increase in curing time and activator content considerably improves the geomechanical performance of geopolymer-treated soft clay in terms of stress–strain response and attainable peak compressive strength. Although it was found that a small percentage of geopolymer can enhance the initial compressive response, a larger dosage of geopolymer up to 30% was necessary to maintain stable durability performance over successive wetting–drying cycles. Such improved durability performance is related to the enhanced soil structure due to the cementation development and overall reduction in thermal conductivity. The reduction in thermal conductivity of treated clay was found to be activator-dependent and was suppressed steadily with the increase in activator concentration. Overall, geopolymer-treated clay showed promising potential for DSM applications due to its enhanced strength and durability responses.

Perubahan Nilai CBR Terhadap Penambahan Fly Ash dan Bottom Ash pada Tanah Lempung

Clay soil has a characteristic that is wet, it will be soft and plastic and cohesive. To overcome this problem, clay soil with high plasticity needs stabilization. The stabilization method using additives such as chalk, fly ash and bottom ash can be used as a stabilization material to increase the carrying capacity of high plasticity clay. This study is commended to know the behavior of high plasticity clay which is stabilized with lime, fly ash and bottom ash. Increased crease of the stabilized clay soil is seen from the CBR test. CBR testing is done under curing conditions for 0, 14, and 28 days and soaking for 0 and 4 days. The results showed that the CBR value increased with duration of curing and decreased throughout the duration of immersion. Then the CBR value increases with the addition of lime percentage and each mixture in a mixture variation. The highest CBR value occurred in soil L 5% + MS5 95% with a CBR value of 75.37% under conditions for 28 days of curing and 4 days of immersion. The lowest CBR value occurs in soil L 5% + MS2 95% with a CBR value of 12.85% in conditions without curing and without immersion.

Effects of Freeze–Thaw Cycles on Volume Change Behavior and Mechanical Properties of Expansive Clay with Different Degrees of Compaction

Freeze–thaw (F-T) cycles are one of the main deterioration factors for the performance of expansive clay in seasonally frozen soil regions. This study designed a unidirectional environmental boundary loading device for F-T cycle tests. Laboratory tests were conducted to quantitatively evaluate the effects of the F-T cycle on volume change behavior and mechanical properties of expansive clay with different degrees of compaction below the canal in Xinjiang. The SEM image processing was performed with the Image-Pro Plus image processing software to get pore parameters. The results showed that during the F-T cycle, the volume change of the sample gradually changes from contraction to expansion as the degree of compaction increases. As the degree of compaction increases, the hardening characteristics of the expansive clay are weakened, and the F-T cycle promotes the softening of the stress–strain relationship. The first F-T cycle resulted in a significant decrease of failure strength, and the failure strength of different compaction degrees decreased by 56.07%–67.54% after seven cycles. The failure strain and resilient modulus of the samples are attenuated to varying degrees after F-T cycles. The correlation rate between the porosity and the attenuation rate of the failure strength is 0.719–0.882, the corresponding weight reaches 0.419–0.445, and there is an excellent linear relationship between them. The research offers some important insights into the construction and maintenance of expansive clay engineering such as the Xinjiang Canal.

Mechanical Behavior of Clay Soil Stabilized with Fly Ash and Bottom Ash

Soil is one of the most important aspects of an infrastructure given its functions of receiving and holding structural loads. However, not all soils have good physical and mechanical properties. To overcome those conditions, stabilization of the soil is practiced to meet the technical requirements. This study aims to determine the behavior of high plasticity clay stabilized with lime, fly ash, and bottom ash. These additives can be used as a stabilizing agent to increase soil strength. The percentage of additives varied from 5%–25% then mixed with clay and were tested through the (Unconfined Compression Strength (UCS) test. The UCS test was carried out with 6 treatments, including consecutively non-curing and unsoaked, non-curing and soaked, curing for 7 days and unsoaked, cured for 7 days and soaked, cured for 28 days and unsoaked, and cured for 28 days and soaked. Results show that the UCS value increased along with the span of curing time. Meanwhile, the soaking treatment of the sample decreased the UCS value.

Compressive behavior of kaolinitic clay under chemo-mechanical loadings

Compressive behavior of kaolinitic clay under chemo-mechanical loadings

A Coarse-Grained Model for the Mechanical Behavior of Na-Montmorillonite Clay.

Sodium montmorillonite (Na-MMT) is one of the most commonly found swelling clay minerals with diverse engineering and technological applications. The nanomechanical properties of this mineral have been extensively investigated computationally utilizing molecular dynamics (MD) simulations to portray the molecular-level changes at different environmental conditions. As the environmentally found Na-MMT clays are generally sized within hundreds of nanometers, all-atomistic (AA) MD simulations of clays within such size range are particularly challenging due to computational inefficiency. Informed from atomistic modeling, a coarse-grained (CG) modeling technique can be employed to overcome the spatiotemporal limitation. The current study presents a modeling strategy to develop a computationally efficient model of Na-MMT clay with a typical size over ≃100 nm by shrinking the atomistic platelet thickness and reducing the number of center-layer atoms. Using the "strain-energy conservation" approach, the force field parameters for the CG model are obtained and the developed CG model can well preserve in-plane tension, shear, and bending behaviors of atomistic counterparts. Remarkably, the CG tactoid model of Na-MMT, a hierarchical multilayer structure, can reproduce the interlayer shear and adhesion as well as d-spacing among the clay sheets as of atomistic one to a good approximation while gaining significantly improved computational speed. Our study demonstrates the efficacy of the CG modeling framework, paving the way for the bottom-up multiscale prediction of mechanical behaviors of clay and related minerals.

Role of free iron oxides in the physicochemical and mechanical properties of natural clay

While it is widely acknowledged that interparticle bonding due to free iron oxide (FIO) dramatically affects the behavior of natural clay, the exact role of FIO in determining soil physicochemical and mechanical properties and the underlying mechanism remain unknown. In response, this study removes FIO from natural Zhanjiang clay, a structured marine clay with cementation, to investigate how FIO affects the physicochemical and mechanical behaviors of soil. The immersion method and leaching method (with dithionite-citrate-bicarbonate solution) are used for FIO removal. The test results indicate that the removal of FIO significantly changes the soil physical indices and degrades the mechanical properties. The effects of FIO removal are explained from the perspective of microstructural alterations as revealed by scanning electron microscopy (SEM) observations, mercury intrusion porosimetry (MIP) tests, energy-dispersive X-ray spectroscopy (EDS), and zeta potential measurements. FIO is distributed at the contacts between soil particles and exists in the bridging form. Due to the attraction between the positively-charged FIO and the negatively-charged clay mineral surfaces, soil particles are bounden to form larger soil aggregates with enhanced stability. The removal of FIO leads to the breakdown of soil aggregates and the generation of large pores, thereby increasing soil compressibility and decreasing the shear strength. This study provides some new micro-level understandings of how FIO affects the behavior of natural clay.

Effect of vertical drains and preloading on the creep behavior of soft clay

Effect of vertical drains and preloading on the creep behavior of soft clay

Fire behaviour of hemp, clay and gypsum-based light biobased concretes and renders

Greenhouse gas emissions from cement manufacturing account for about 8 to 10 percent of total CO2 emissions worldwide. To reduce these emissions, researchers are developing new concrete manufacturing techniques and processes to reduce high energy consumption and environmental impacts. Biobased concretes are eco-friendly insulating materials for building industry that can respond to this problem. However, there is a lack of knowledge regarding how these plant-based fillers ignite and contribute to heat release in case of fire. In this work, the fire behaviour of a series of hemp-based earth and/or gypsum concretes covering a large range of densities (180–1500 kg/m3) is investigated using the cone calorimeter at an incident heat flux of 50 kW/m2. The fire performances are mainly monitored by the thermal inertia of the materials. Only the lightest concretes ignite with a density threshold for ignition occurrence around 500 kg/m3. For a density of 261 kg/m3, the critical heat flux of an earth-hemp concrete was found to be close to 27 kW/m2. The flaming period remains very short in all cases.

Unified bounding surface model for monotonic and cyclic behaviour of clay and sand

Unified bounding surface model for monotonic and cyclic behaviour of clay and sand

Effects of frequencies on the cyclic behavior of marine soft clay under different consolidated stress paths

To experimentally investigate the effect of frequency on the cyclic behavior of marine soft clay under different consolidated stress paths, a series of cyclic tests on remolded Wenzhou marine clay were conducted under undrained conditions covering various frequencies and consolidated stress paths using a triaxial apparatus. The development of pore water pressure, stress–strain relationship, and resilient modulus was analyzed. Several useful conclusions were reached from this study. First, the trends of pore water pressure development can be divided into three stages: linearly increasing, slowly increasing, and stable. Second, the frequency has a more pronounced influence on the number of cycles of the steady value of pore water pressure than the counterparts of values of the ratio of the radial to vertical consolidation stresses, K. Then, a unified linear equation was established based on the consideration of different K values on a logarithmic scale. Finally, the shapes of the stress–resilient strain hysteretic loops and the range encompassed by the stress–resilient strain hysteretic loops are slight as the value of K decreases, which indicates that the K value has a slight effect on the shift in the type of elastic behavior of clay.

Evaluation of stress history and undrained shear strength of three marine clays using semi-empirical methods based on Piezocone Test

The paper presents a comparative study between semi-empirical methods for the estimation of pre-consolidation pressure and undrained shear strength from Piezocone (CPTu) data. The first method, proposed by Massad, was developed from observing the variation of these parameters with depth; the second method, proposed by Mayne, was developed from simplifications and relationships between the Spherical Cavity Expansion Theory (SCET) and the Critical State Theory; the third method was proposed by Mayne, which considers the variations due to soil type from the CPT Index to estimate the pre-consolidation pressure. The methods were validated based on their applications to the marine clay from Santos Coastal Plain, Brazil, Bothkennar clay from Scotland, and Torp Clay from Sweden. It is intended to verify if the results are consistent with each other, with the stress history of these soils and with the available test results. The application of the Massad’s method led to results close to the available reference values. The results of the Mayne’s method based on SCET showed great variability in behavior comparing to the test data depending on the case study. By the Mayne’s method based on CPT Index values, the calculated pre-consolidation pressures were slightly higher than the values of the available test data. The variations in the results highlighted the importance of validating estimates based on semi-empirical methods through specific tests and the knowledge of geological history contributes to predicting the behavior of clays, since they showed good agreement with the available data from oedometer tests.

The Strength Behavior and Desiccation Crack Development of Silty Clay Subjected to Wetting–Drying Cycles

It is commonly accepted that wetting–drying cycles have an effect on the soil strength behavior. Crack development in soil is observed by many engineers during wetting–drying cycles, which may give a good explanation for the change in soil strength. A series of laboratory tests were conducted in this study to investigate the desiccation crack development and the strength change law for silty clay subjected to different numbers of wetting–drying cycles. The results show that the desiccation cracks at the end of drying process developed in two stages: the stage of rapid growth and the stage of steady state. The change law of soil strength is similar to the cracking that decreases quickly in the former stage and slowly in the latter stage, which indicates that the cracking in the soil is the main reason for strength reduction. Based on the assumption of an isotropic and linear elastic soil mass at rest earth pressure conditions, an equation for the depth of desiccation cracking after different numbers of wetting–drying cycles was obtained with soil mechanics for unsaturated soils. Finally, the applicability of the equation was verified compared with the experiment results.

ELASTO-PLASTIC BEHAVIOR OF NATURALLY DEPOSITED CLAY DURING/AFTER SAMPLING

Changes in soil structure and overconsolidation in highly structured natural deposited clay as a result of sampling and setting in laboratory tests, together with their effects on the subsequent mechanical behavior, are numerically simulated in terms of unconventional elasto-plastic mechanics. To describe the mechanical behavior, a Super/subloading Yield Surface Cam-clay model is employed. This model allows a description of the decay of soil structure, loss/ increase of overconsolidation and evolution of anisotropy with ongoing plastic deformation from the process of the soil sampling through the unloading with swelling up to the subsequent tests. The sampled clay subjected to isotropic unloading and/or one-dimensional unloading with swelling down to the first stage of vertical loading level, displays a response to some degree, different from that of the soil at the original sampling site; for example, an overconsolidated sample subjected to isotropic unloading with swelling will display rewinding behavior in a subsequent undrained triaxial compression test, and secondary consolidation/delayed compression in a one-dimensional consolidation test. However, one-dimensional unloading with swelling, which is followed by one-dimensional compression test, will give better prediction of the in-situ consolidation yield stress than isotropic unloading. Artificially disturbed clay subjected to repeated undrained shearing after sampling suffers a significant loss of structure. This results in a drop in peak strength under subsequent monotonic undrained shearing tests, and in the compression index in one-dimensional compression tests, as described by Schmertmann.

SIMULATION OF SHEAR AND ONE-DIMENSIONAL COMPRESSION BEHAVIOR OF NATURALLY DEPOSITED CLAYS BY SUPER/SUBLOADING YIELD SURFACE CAM-CLAY MODEL

Naturally deposited clays exhibit complicated mechanical behavior that differs from that of remolded clays. For example, clay in a normally consolidated state commonly exhibits softening in undrained shear tests or rewinding in a heavily overconsolidated state. The Super/subloading Yield Surface Cam-clay model (Asaoka et al., 1998, 2000, 2002) was proposed in an attempt to clarify the complicated mechanical behavior in naturally deposited clays. In this constitutive model, the concepts of structure, overconsolidation, anisotropy, and their evolution laws, are introduced into the modified Cam-clay model. In the present study, undrained triaxial compression tests and oedometer tests were carried out on two types of naturally deposited undisturbed clay, Pleistocene clay and Holocene clay, and the behavior was then simulated using the Super/subloading Yield Surface Cam-clay model. The findings of the present study are as follows: 1) For the two types of undisturbed clay, the Super/subloading Yield Surface Cam-clay model can simulate undrained triaxial compression behavior ranging from the normally consolidated state to the overconsolidated state, corresponding to various isotropic pressures using a single set of material constants. 2) In addition, the model can simulate one-dimensional compression behavior using the same material constants as those used for the simulation of the undrained triaxial compression behavior. 3) Through the simulation, the mechanical behavior of Pleistocene clay and Holocene clay, which have different loading histories and have undergone different aging effects, can be described by the different evolution parameters, as well as the elasto-plastic parameters.

Assessment of alkali-induced heave in soil and its stabilisation using slag

Soil–alkali-interaction results in unexpected heaving and other negative effects of soil properties, which can lead to complete failure of structures. An investigation was carried out on the impact of highly concentrated sodium hydroxide alkali on the heaving behaviour of low-plasticity clay. The effect of ground-granulated blast-furnace slag as a stabiliser for alkali-interacted soil was also investigated. The interaction of alkali with the soil showed heaving up to 5.25% after 30 days, while a slag content of 20% showed maximum suppression of heave. The alkali-interacted soil stabilised with slag was also compared with un-interacted soil mixed with slag. There was a marginal increase in friction angle with an increase in curing period, and a significant increase in cohesion for both. A substantial improvement in unconfined compressive strength was also evident with increased slag content and curing period. The results were supported by microstructural examinations. The application of slag as a liquid-based stabiliser for alkali-interacted soil was also conducted using electrokinetics. Statistical analysis indicated that the curing period was a dominant parameter in affecting the compressive strength of un-interacted soil while, for alkali-interacted soil, slag content was more dominant.

Study on the frost heave behavior of the freezing unsaturated silty clay

The frost heave behavior of unsaturated soils is related to the water-vapor redistribution, whereas the water-vapor migration processes cannot be directly observed due to the limitations of measurement techniques. To reveal the frost heave behavior of the unsaturated silty clay, five specimens for unsaturated silty clay were prepared. One-side freezing tests for the five specimens were carried out under uniform conditions. The test durations are 12 h, 24 h, 48 h, 96 h, and 192 h, respectively, and the water content profiles in the freezing process were studied by testing final water content profile of each specimen as the profile at that time. The heat-water-vapor migration and frost heave behavior of the freezing unsaturated silty clay were interpreted. The results showed that the temperature variations can be divided into three stages (a sharply cooling stage, a slowly cooling stage, and a steady stage) according to the change rate of temperature, and the duration times of the first and the second stage are short. A mix migration of liquid water and water vapor in unsaturated soils can be experimentally determined during the freezing, and a thin ice forms at the top of the specimens due to the vapor migration and condensation. Meanwhile, the liquid water in unfrozen zone mainly migrates to frozen fringe, and segregated ice forms in the freezing unsaturated soil once enough water accumulates at the frozen fringe. In addition, the frost heave of the specimens is discrete under the uniform test conditions. We selected the relative error and absolute error of the frost heave as evaluation indexes, and further evaluated the repeatability of the test results. Evaluation results showed that the one-side freezing test results are repeatable, and it is an useful testing method to study the heat-water-vapor migration through one-side freezing tests with different test durations under uniform testing conditions. Moreover, the confusion that the frost heave mainly occurred in the top layer of high-speed railway subgrade was explicit solved by the behavior revealed in this paper.

Hydro-thermal characteristics and deformation behaviors of silty clay subjected to freeze–thaw cycles

Hydro-thermal characteristics and deformation behaviors of silty clay subjected to freeze–thaw cycles