Reconstituted Clays Research Articles

Micro to macro investigation of clays advising their constitutive modelling - part II

This paper reports the second part of the keynote lecture, whose part I has been already published in this journal, presenting extensive experimental research on the investigation of clay microstructure and its evolution upon loading. Whether the first part focused on the micro to macro behaviour of different reconstituted clays, this part instead concerns the microscale features of the corresponding natural clays, their changes under different loading paths and the ensuing constitutive modelling implications. The experimental investigation is carried out according to the methodology outlined in the part I-paper, hence micro-scale analyses are presented on natural clays subjected to macro-scale mechanical testing, with the purpose to provide experimental evidence of the processes at the micro-scale which underlie the clay response at the macroscale. As for the reconstituted clays in the part I-paper, original results on stiff Pappadai and Lucera clay, this time in their natural state, are compared to literature results on clays of different classes, either soft or stiff. The results presented in this paper, together with those discussed in the part I, allow for a conceptual modelling of the microstructure evolution under compression of natural versus reconstituted multi-mineral clays, providing microstructural insights into the macro-behaviour described by constitutive laws and advising their mathematical formalization in the framework of either continuum mechanics or micro-mechanics.

A microstructural insight into the compression behaviour of scaly clays

Scaly clays are intensely fissured clays with lens-shaped elements of millimetre size and they show a complex compression behaviour that poses challenges to the design and construction of geostructures (excavations, retaining diaphragms and tunnels). Scaly clays show a normal compression line (NCL) where plastic deformation accumulates, as typically observed in non-scaly clays. Yet the response observed upon unloading and subsequent reloading is very peculiar, (a) the unloading–reloading cycle is typically a closed loop with relatively large hysteresis; (b) the compressibility recorded at high overconsolidation ratio of the unloading or reloading branches is close to the NCL compressibility. This paper presents a microstructural study on an Italian scaly clay where scanning electron microscope observations are integrated with mercury intrusion porosimetry analyses and X-ray computed tomography images. The mechanism associated with the closing of inter-scale porosity and the generation of new intra-scale porosity was identified as the process responsible for the plastic deformation. Experimental observation of reconstituted clay showed a ‘quasi-reversible’ behaviour upon loading and unloading and a pore size distribution characterised only by interparticle porosity. The observation that unloading and reloading curves are parallel in natural and reconstituted clays, led to the postulation that the interparticle porosity is controlling the elastic response.

The sensitivity of Prazeres clay – some results on reconstituted samples

Soil’s structure is defined as the issue responsible for the differences observed in the mechanical response of natural soil samples compared to the intrinsic behaviour of reconstituted samples. The parameter that incorporates the differences in microstructure of intact and reconstituted clays is designated by stress sensitivity. The influence of microstructure and fabric on the behaviour of soils has been evaluated by comparing strength and compressibility characteristics of intact and reconstituted samples of the same material. Included on a broad characterization study of Lisbon Miocene Prazeres clay held at Porto University, some oedometer and triaxial tests were undertaken on intact and reconstituted samples. In this paper, the process of preparing reconstituted samples and consolidating them in a large oedometer to the in-situ stress is thoroughly described. The results of oedometer and triaxial tests on reconstituted samples are presented and compared with results for intact samples from the same site. A proposal for parameterisation of the intrinsic characteristics of this clay is established on the basis of test results. Concerning compressibility, oedometer test results show good adjustment to the isotropic compression line (ICL). Triaxial test results for intact samples in the light of critical states soil mechanics are similar to those obtained in s’-t space for the reconstituted samples. The intrinsic parameters show good agreement with those described in the literature for soils of the same nature.

Drying-induced volumetric behaviour of clays interpreted via binary pore-scale modelling

The volumetric response upon drying is relevant in the geotechnical and ceramic industry due to the key role played in crack formation. The volumetric response of clays upon drying is relatively complex; the void ratio decreases considerably in the saturated state, levels off almost abruptly at the onset of desaturation, and increases at very low water contents. This paper presents an original deformable pore-network model (DPNM) to elucidate the micromechanisms controlling the complex volumetric response of clays upon drying. The main features of the DPNM are the ‘binary’ approach (pores in the clay are assumed to be either fully saturated with water or dry) and the use of independent tests to calibrate the parameters of the model (compression behaviour of saturated reconstituted clay and dry powder). The DPNM shows that all pores shrink as pore-water tension increases in the saturated state, although the larger pores contribute more to the overall reduction in void ratio. At the onset of desaturation, the larger pores dry out and rebound, whereas the smaller pores remain saturated and shrink. At very low water content/high pore-water tension, the desaturation of the smaller pores causes them to rebound, thus generating a macroscopic increase in the void ratio.

Effects of organic content on sewage sludge compression behaviour

Five sewage sludge specimens with different organic contents were prepared by mixing mass ratios of 0, 5, 10, 20 and 40% dry soil powder into raw sewage sludge. The addition of dry soil powder can reduce the organic content of sewage sludge without significantly changing its particle composition. Oedometer tests were conducted, starting from a small effective vertical stress [Formula: see text]. It is observed that the [Formula: see text] compression curves show an inverse ‘S’ shape due to suction pressure resisting deformation. The suction pressure decreases exponentially with the organic content, a regression equation for which is provided. Burland’s concept of the intrinsic compression line (ICL) is adopted for correlating the compression curves of sludge with various organic contents. It is found that the ICL with a high organic content lies above the low one – that is, the higher the organic content, the greater the void index. In addition, the shape of the ICL for organic sludges is an inverse S rather than slightly concave upwards for inorganic reconstituted clays. An average ICL is provided to normalise the compression curves of sludges with different organic contents. The intrinsic compression parameters [Formula: see text] and [Formula: see text] can be correlated with the organic content, which increase linearly with the organic content, and the regression equations for them are provided.

Double image stress point bounding surface model for monotonic and cyclic loading on anisotropic clays

In this paper, an anisotropic non-associative constitutive model, based on the well-established critical state soil mechanics framework, is developed for the natural and reconstituted clays subjected to monotonic and cyclic loadings. A robust non-elliptical yield surface is implemented in the model which enables it to capture a wide variety of yield stress points. In addition to a realistic soil stiffness simulation, the employed flexible plastic potential surface equips the proposed model to predict a more accurate coefficient of earth pressure at rest. The model uses a combination of conventional volumetric and comprehensive rotational hardening rules to control the evolution of the yield surface caused by plastic strain increments. The adopted rotational hardening rule uses the concept of governing plastic strain increment which not only ensures the existence of the equilibrium state of anisotropy but also takes the effect of plastic strains at different constant stress ratios into account more sensibly. The proposed model is enhanced with a novel double-image stress point bounding surface plasticity type theory to capture nonlinear behaviour inside the yield surface, and also to simulate the cyclic responses. The detailed model formulation is discussed in both triaxial stress space and the e-mathrm{ln}, p plane, and the important features of the model are elaborately explored. The capabilities of the model are also demonstrated by an extensive sensitivity analysis. In the end, the model is used to carry out simulations of monotonic and cyclic element tests on different clays and the results are compared with the available experimental data.

Creep Behavior of Saturated Clay in Triaxial Test and a Hyperbolic Model

As one of the basic mechanical properties of soil, the creep property of a given type soil is related to stress path, and stress level. In this paper, triaxial shear creep tests under different deviatoric stress levels were performed on both intact sample and the reconstituted sample of clay taken from Hangzhou, China. Based on the Boltzmann linear superposition principle, the creep curves of the clay sample under different levels of deviatoric stress were obtained, and the creep characteristics of the intact sample and the reconstituted sample were compared in both total stress creep analysis and effective stress creep analysis. Furthermore, the creep curves were fitted using a hyperbolic creep model. The results show that (1) under the same stress level, the creep of intact sample evolves more than that of reconstituted sample; (2) the hyperbolic creep model is suited to describe the creep characteristics of intact and reconstituted clay, and the model parameters A s and B s can be linearly correlated to the stress level D r ; (3) for the application of the hyperbolic model, the total stress analysis works better, and the model parameters A s and B s can be determined by a linear relationship with Dr.

Deformation modulus of reconstituted and naturally sedimented clays

The undrained secant deformation modulus Eu is an important parameter to quantitatively estimate the undrained settlement of soft clays. A series of isotropically consolidated undrained triaxial compression shear tests were carried out on four reconstituted clays and two naturally sedimented clays to evaluate the effects of the initial water content w0, liquid limit wL, and soil structure on Eu. The test results showed that the variation in Eu with deviator stress was significantly affected by w0 and wL for reconstituted clays. The normalized modulus Eu/(p′)0.9 linearly decreased with increasing w0/wL. This linear relation at a lower wL lies above that at a higher wL. These observations suggest that the soil with lower plasticity and lower initial water content possessed higher values of Eu. As the deviator stress increased, the effect of w0 and wL on Eu gradually vanished. It is also evidenced that the value of Eu for naturally sedimented clays depended on the soil structure. The soil structure clearly enhanced the Eu in the preyield regime in comparison with the corresponding reconstituted clay. Once the soil structure was damaged with increasing consolidation stress in the postyield regime, a significant reduction in Eu was observed with a lower value than that of reconstituted clays. The difference in Eu/Su between naturally sedimented and reconstituted clays gradually decreased with increasing deviator stress. This implies that the effect of the soil structure on Eu diminished at higher deviator stress levels.

Rate-dependent behavior of a saturated reconstituted clay under different over-consolidation ratios and sample variance

Rate-dependent behavior of a saturated reconstituted clay under different over-consolidation ratios and sample variance

SANICLAY-T: Simple thermodynamic-based anisotropic plasticity model for clays

In this work the anisotropic model for clays SANICLAY proposed by Dafalias and Taiebat (2013) is reformulated within the framework of hyper-elastoplasticity. The model, called SANICLAY-T, is fully defined by two scalar potential functions, the free energy and the rate of dissipation. It is first presented in the triaxial space and then generalised in the multiaxial one. The model reproduces exactly the original one for the case of associate flow rule, while leads to a different outcome for non-associated flow. When compared to existing hyperplastic models accounting for rotational hardening, the proposed one proves to be more versatile, as characterised by less restrictive constraints on the hardening and asymptotic behaviour of the soil. The predictive capability of the model is illustrated with reference to experimental data on natural and reconstituted clays, highlighting its merits and limitations.

Water retention behaviour of compacted and reconstituted scaly clays

The paper presents the results of an experimental research devoted to investigate the response to suction variations of a scaly clay in compacted and reconstituted conditions. Different experimental techniques (axis translation, vapour equilibrium, dew point psychrometer suction measurements) were combined in order to explore the water retention properties in a wide suction range (0 ÷ 110 MPa). Experimental results allowed to define the water retention domains for a constant reference void ratio, highlighting the significant role of the microstructure on the response of the investigated clays. In particular, the collected results showed that in the low-medium suction range, the peculiar microstructural features give to the reconstituted clay a better retention capability than the compacted clay. However, the increasing suction induces a significative volumetric shrinkage on the saturated reconstituted clay, especially when the latter is initially normally consolidated. On the other hand, quite similar retention properties were recognized in the high suction range.

Anisotropy in small-strain shear modulus of permafrost at rising temperatures

Perennially frozen soils, or permafrost, exist extensively on the Arctic Coastal Plain of Alaska and can reach up to 600 m deep. This paper describes the cryostructure of permafrost cores obtained from a deep borehole in this area, presents the shear wave velocities measured in different directions by the bender element method, exhibits the small-strain shear moduli of clay, silt and sand permafrost specimens from −10 °C to 20 °C, and discusses the anisotropy in the small-strain shear modulus of permafrost in both frozen and thawed states. It was found that the specimen site is covered by a few meters of syngenetic permafrost with massive wedge ice, below which is a deep epigenetic permafrost with numerous visible ice lenses primarily located in the top 120 m, oriented in horizontal or subhorizontal directions. The shear modulus ratio found for thawed clay permafrost falls in the higher end of those reported for natural or reconstituted clays, while the ratio for thawed sand permafrost is clearly higher than those previously reported. The anisotropy in the small-strain shear modulus was found to be strongly temperature dependent. The temperature dependency of anisotropy can be attributed to volumetric expansion, occurring during the freezing of pore water followed by the thawing of pore ice that, respectively, disrupts then allows the recovery of the preferential alignment of soil particles, or fabric, formed during geological sedimentation processes. In addition, deep permafrost cores with shear stiffness comparable to hard rock exhibit a very low stiffness ratio Ghh/Gvh, likely due to the in situ geostatic stress-induced anisotropy in pore ice crystals.

One-dimensional nonlinear consolidation behavior of structured soft clay under time-dependent loading

This research investigated the nonlinear compressibility, permeability, the yielding due to structural degradation and their effects on consolidation behavior of structured soft soils. Based on oedometer and hydraulic conductivity test results of natural and reconstituted soft clays, linear log (1+e) ~ log sigma\'v and log (1+e) ~ log kv relationships were developed to capture the variations in compressibility and permeability, and the yield stress ratio (YSR) was introduced to characterize the soil structure of natural soft clay. Semi-analytical solutions for one-dimensional consolidation of soft clay under time-dependent loading incorporating the effects of soil nonlinearity and soil structure were proposed. The semi-analytical solutions were verified against field measurements of a well-documented test embankment and they can give better accuracy in prediction of excess pore pressure compared to the predictions using the existing analytical solutions. Additionally, parametric studies were conducted to analyze the effects of YSR, compression index (lambda_r and lambda_c), and permeability index (eta_k) on the consolidation behavior of structured soft clays. The magnitude of the difference between degree of consolidation based on excess pore pressure (Up) and that based on strain (Us) depends on YSR. The parameter lamda_c/eta_k plays a significant role in predicting consolidation behavior.

Experimental evidence about misconception of Terzaghi's 1-D consolidation theory in terms of degree of consolidation

Terzaghi's 1-D consolidation theory is commonly used for prediction of embankment settlement in soft fine-grained soil. Application of this theory is not always effective due to the simplifications of assumed soil behavior under load. This paper examines relationship between the degrees of consolidation determined by pore water pressure measured at the base of the sample and one-dimensional strain. Theoretically, this relationship should be unique, however experimental relationships correlating the degrees of consolidation are found to be non-linear. The article presents the results of multistage consolidation tests with pore water pressure measurements, carried out on intact and reconstituted deep marine clay. Consolidation tests were conducted with uniform stress distribution and single-sided drainage conditions. It was observed that the relationship between degrees of consolidation for intact and reconstituted samples largely divergent from theoretical unique line.

Thixotropic aging and its effect on 1-D compression behavior of soft reconstituted clays

Abstract The present study investigates the moisture content at which the thixotropic effect is maximum, as well as the effect of such aging on compression. For this research, samples were prepared at moisture contents ranging from 0.55–1.25 times their corresponding liquid limits, and fall cone and vane shear tests were administered to determine the shear strength. Oedometer tests were performed on both thixotropically aged and unaged samples to observe the effects of thixotropic aging on 1-D compression characteristics. It was observed that the maximum thixotropic strength was obtained at a moisture content close to 0.75 times the corresponding liquid limit for all samples, and thixotropic strength recovery was negligible at a moisture content of 0.55 times the corresponding liquid limit. Vane shear strength was found to be lower than fall cone strength, and an increase in the rate of strain resulted in lower estimation of thixotropic strength recovery. The thixotropic strength ratio was observed to be dependent on the plasticity index and activity of the clay sample. Of all of the clays examined, the thixotropically aged sample provided the highest yield stress. The study suggests that in situations where the effective vertical stress is less than 100 kPa, the settlement calculation, using the conventional effective stress principle, needs to be rigorously revised. A scheme for considering both strength and compressibility behavior in design practice is also provided.

The compressibility of natural and reconstituted marine clays

There is mounting evidence in favour of replacing the e against [Formula: see text] diagram, conventionally used to represent soil compressibility in an oedometer, with a log(v) against [Formula: see text] diagram, which significantly improves the linearity of virgin loading and unloading paths. This paper presents a simple, new formulation of the ‘intrinsic’ compression and ‘sedimentation’ compression lines for a range of insensitive, sedimentary marine clays presented in Burland’s 1990 Rankine Lecture by using a log(v) against [Formula: see text] interpretation of the data.

A family of improved yield surfaces and their application in modeling of isotropically over-consolidated clays

Recognizing the limitation of the existing yield surfaces for clays, a novel type of the yield surface is derived. The shape parameters allow for the distortion of the yield surface from an ellipse to a teardrop shape such that it provides more flexibility for better modeling the shear response for over-consolidated clays. A new bounding surface model has been proposed within a critical state framework, and can predict the realistic behavior of over-consolidated clays under both undrained and drained conditions. The capacity of the model is demonstrated by five different reconstituted clays with representative testing results in the literature.

One-dimensional compression behaviour of reconstituted clays with and without humic acid

It is often recognized that organic matter contained in natural clays and dredged clays is detriment to the engineering properties by increasing their compressibility. This study takes humic acid as an example to illustrate that organic matter may decrease the compressibility of reconstituted clays with a wide spectrum of predominant clay minerals: kaolinite, illite and smectite, based on experimental results from thirty types of one dimensional incremental load consolidation tests. It is found that the main factors of influencing the compression behaviour of reconstituted clays with humic acid are initial void ratio and void ratio at liquid limit. The difference in compression curves between reconstituted clays with and without humic acid can be attributed to humic acid effects on particle density and liquid limit of clays. The humic acid effect on the compressibility of smectite-dominant reconstituted clays is found to be much more sensitive than that for kaolinite- and illite-dominant reconstituted clays. A quantitative approach is also suggested to assess the compression behaviour of kaolinite- and illite-dominant reconstituted clays with humic acid using intrinsic compression concept.

Characterisation of state bounding surface at low effective stresses in clayey soils having different structures

Shallow stability analysis of natural or man-made soil slopes requires accurate characterisation of soil strength at low effective stresses (5–20 kPa), corresponding to mechanisms expected at 1–2 m depth. This study focused on the intertwining effects of soil structures at different scales and low stresses on the undrained strength of fine-grained soils, by testing two natural plastic clays and a clay–sand mixed soil at different states (intact, reconstituted and compacted) in a series of constant-volume direct shear and hollow cylinder simple shear tests. The non-linearity of the Hvorslev bounding surface was expressed well by a power law function. The normalised strength and the degree of its non-linearity of the clays at intact states were higher than at equivalent reconstituted states, probably owing to true inter-particle cohesion, as confirmed by the hollow cylinder apparatus tests. The compacted states led to significantly lower normalised strength than the reconstituted state in both clays, while they did not show significant difference in the clay–sand mixed soil. This observation was explained by meso-scale discontinuities by X-ray micro-computed tomography images. Compacted clays’ shear strength is associated with a meso-fabric very different from that in more homogeneous intact and reconstituted clays, and the conventional state normalisation based on the equivalent pressure does not capture it.

Experimental study on the performance of compensation grouting in structured soil

Most laboratory test research has focused on grouting efficiency in homogeneous reconstituted soft clay. However, the natural sedimentary soils generally behave differently from reconstituted soils due to the effect of soil structure. A series of laboratory grouting tests were conducted to research the effect of soil structure on the performance of compensation grouting. The effects of grouting volume, overlying load and grouting location on the performance of compensation grouting under different soil structures were also studied. Reconstituted soil was altered with added cement to simulate artificial structured soil. The results showed that the final grouting efficiency was positive and significantly increased with the increase of stress ratio within a certain range when grouting in normally consolidated structured clay. However, in the same low yield stress situation, the artificial structured soil had a lower final grouting efficiency than the overconsolidated reconstituted soil. The larger of normalized grouting volume could increase the final grouting efficiency for both reconstituted and artificial structured soils. Whereas, the effect of the overlying load on final grouting efficiencies was unfavourable, and was independent of the stress ratio. As for the layered soil specimens, grouting in the artificial structured soil layer was the most efficient. In addition, the peak grouting pressure was affected by the stress ratio and the overlying load, and it could be predicted with an empirical equation when the overlying load was less than the yield stress. The end time of primary consolidation and the proportion of secondary consolidation settlement varied with the different soil structures, grouting volumes, overlying loads and grouting locations.