Saturated Fine-grained Soils Research Articles

Influence of stress history on thermal conductivity of saturated fine-grained soils

This research investigated variations in thermal conductivity <i>k</i> for normally consolidated (NC) and overconsolidated (OC) saturated fine-grained soils at different levels of vertical effective stress. A series of undrained heating tests were performed using a specially designed consolidometer with central radial heating arrangement and temperature measurements within a soil specimen. For different soils investigated in this research, <i>k</i> correlated well with soil consolidation characteristics (compression index <i>λ</i> and swelling index <i>κ</i>) and vertical effective stress <i>σ</i><sub>v</sub>ʹ at both NC and OC states. However, the rates of increment of steady- and transient-state values of <i>k</i> (<i>k</i><sub>st</sub> and <i>k</i><sub>tr</sub>, respectively) with <i>σ</i><sub>v</sub>ʹ differed from one soil to another. For OC clays, both <i>k</i><sub>tr</sub> and <i>k</i><sub>st</sub> increased with preconsolidation stress <i>σ</i><sub>c</sub>ʹ. A set of equations were proposed to estimate both <i>k</i><sub>st</sub> and <i>k</i><sub>tr</sub> for saturated fine-grained soils as functions of <i>σ</i><sub>v</sub>ʹ, <i>λ</i>, <i>κ</i>, and stress history. At a particular value of <i>σ</i><sub>v</sub>ʹ, the dependence of <i>k</i> on overconsolidation ratio (OCR) and <i>σ</i><sub>c</sub>ʹ marked the importance of considering stress history in the determination of <i>k</i>. The influence of soil mineral composition on <i>k</i> was also explored. Quantification of soil mineralogical composition and available data on mineral thermal conductivity enabled prediction of <i>k</i><sub>tr</sub> with reasonable accuracy.

Three-dimensional finite element analysis of the effect of soil liquefaction on the seismic response of a single pile

Soil liquefaction is considered as one of the most significant issues that leads to failure of shallow and deep foundations. However, the effect of liquefaction on the seismic response of piles still poorly understood. Therefore, this research examines the seismic response of a pile embedded in soil stratum of saturated fine-grained soils. Midas GTS/NX is used to carry out the number assessment. In addition, the modified UBCSAND soil constitutive model is used to depict the nonlinear features of saturated sand during earthquake waves. The developed three-dimensional model is first validated using the results of a shaking table test of a pile embedded in coarse-grained soil, where good agreement is obtained between the finite element model and the experimental results for the displacement, acceleration, and liquefaction ratio demonstrated good agreement. Furthermore, the orientations of the vectors produced by the numerical study, that matched a global circular flow characteristic, reflected the movement of the liquefied soil all around pile. The findings showed a considerable decrease in the pile frictional resistance during the seismic events as a consequence of increasing the pore water pressure and subsequent liquefaction. Regarding this, before the soil was entirely softened, resistance due to friction was observed near the ground, in correspondence with the loose sand layer. In addition, the pile showed excessive settling, which is due to the decrease of the soil stiffness caused by the increase of the pore water pressure. The results of this research provide an insight into the mechanism of the behavior of pile in saturated coarse-grained soils and thus, it helps to improve future research on the topic and also achieve better design of piles embedded in saturated coarse-grained soils.

Modelling the volume change of saturated fine-grained soils under cyclic temperatures

New thermomechanical models for saturated soils recently developed in the literature very well capture the thermal cyclic deformations of normally consolidated (NC) soils, but this is not the case for slightly overconsolidated (OC) soils. To improve this, a framework based on the reference line (RL) and thermal stabilisation line (TSL) is proposed in the e-ln(p) plane. Whatever the applied thermal solicitations, monotonic or cyclic, the thermoplastic volumetric strains of slightly OC soils are easily predicted from their NC states using the RL and TSL lines concept. It should be noted that the TSL already exists in the literature but requires at least three tests at different over consolidation ratios (OCRs) for its calibration. For this purpose, a simple method is also proposed for determining the TSL line explicitly from the total deformation recorded at the end of the thermal cyclic test (on NC sample). In this way, the number of tests is reduced from three to one. Besides, to better predicting the thermomechanical behaviour of saturated soils, the hardening law is calculated independently for both thermal and mechanical loading cases. Finally, the proposed model is compared with existing recent models as well as experimental data from the literature.

State parameter–based thermomechanical constitutive model for saturated fine-grained soils

This paper presents a two-surface constitutive model for describing thermomechanical behaviour of saturated fine-grained soils at both normally consolidated and overconsolidated states. A thermal-dependent stress ratio-state parameter relation is adopted to account for the effects of temperature on the shape of the state boundary surface (SBS) of soils. In the model, both the size and the shape of the SBS are allowed to vary with temperature, which is evidenced by thermal variation of the mechanical yield loci and the shifts of the normal consolidation line (NCL) and the critical state line (CSL) upon heating and (or) cooling. A thermal yield surface is added for modelling the isotropic thermal deformation of soils more accurately, in particular at overconsolidated states. The mechanical and thermal yield mechanisms are coupled through the temperature-dependent preconsolidation pressure that is controlled by a volumetric hardening law. Based on experimental observations, a nonlinear relationship between the spacing ratio and temperature changes is defined and a simple thermal dependent non-associated flow rule is proposed. The model is validated against some selected experimental results of several soils tested under various mechanical and thermal paths such as drained isotropic heating and cooling, drained and undrained triaxial compression at non-isothermal conditions.

Studying Performance of PVDs on Consolidation Behavior of soft Clayey Soils Using EFM, Mahshahr Oil Storages

The main problem with saturated fine-grained soils is the slowness of consolidation procedure and the occurrence of large soil settlements. Generally, to expedite consolidation procedure, preloading method would be used along with prefabricated vertical drains. Soil improvement process under this condition would be subjected to design method and vertical drains’ modeling. In the current paper and to understand the consolidation behavior of clayey soils improved with vertical drains, a parametric study has been performed by PLAXIS 2D finite element software. The results are indicative of an increase in average consolidation degree from 74 to 84% after a period of 6 months through the reduction of vertical drains’ distances from 4.5 to 1.5m. Moreover, it became clear that an increase would be made in the rate of settlement, consolidation, and dissipation of excess pore water pressure through increase made in length and diameter of vertical drains and increase of their discharge capacity. Also, it became specified that increase of diameter is less effective on the expedition of consolidation procedure compared to that of distance reduction among drains.

A new relationship between the mobile and the adsorbed water in fine-grained soils using an effective void-ratio estimation

Firmly adsorbed water on the surfaces of clay minerals and the water that fills the unconnected and dead-end pores in saturated fine-grained soils is immobile, which means that it cannot impact on the hydraulic conductivity of these soils. Therefore, only the volume of the voids occupied by mobile water is important. This volume can be expressed as the effective porosity or the effective void ratio. In the past, many methods were proposed to determine these values: however, the obtained values varied, leading to a difference in the proposed correlation with the saturated hydraulic conductivity. This article describes a process for determining the effective void ratio and has a completely different basis to so-far suggested procedures. Determining the volume of the mobile and immobile water in the saturated fine-grained soils assumes that the quantity of the mobile water and the thickness of the adsorbed water film on the external surfaces of the clay grains are equal at the same effective stress. The total quantity of adsorbed (immobile) water on the external surfaces of clay minerals in this case depends on their size and quantity in the soil’s composition. On the basis of the determined quantities of immobile and mobile water, the ineffective and effective void ratios are calculated. The results show that the interdependence between the total void ratio and the ineffective void ratio is linear, which allows for a simple calculation of the effective void ratio. However, it should be taken into consideration that this expression can only be used for soils that do not contain swelling clay minerals. The evaluation of the hydraulic conductivity of the soils using effective void ratios, determined with the proposed method, shows good agreement with the measured values.

A critical assessment on seismic liquefaction potential of fine-grained soils

Devastating ground failures at seismically active sites underlain by saturated fine-grained soils have led to a need for revealing liquefaction potential of fines and effects of the characteristics governing this potential. A cyclic pore pressure model is introduced to examine the effects of plasticity index, ratio of water content to liquid limit and cyclic stress ratio on liquefaction susceptibility of such soils. The model is developed using data of 68 cyclic triaxial tests compiled through a comprehensive literature review. By means of pore pressure behavior estimated by the model and strain tendencies observed during the tests, a critical assessment on liquefaction susceptibility criteria for fine-grained soils is provided. Beyond evaluation of the susceptible and non-susceptible fines, this assessment partially provides distinction between liquefaction-related phenomena one of which is induced by complete loss of strength, whereas the other is induced by high strain levels without complete loss of strength.

A Study on Correlation Between Liquid Limit, Plastic Limit and Consolidation Properties of Soils

Adequate and reliable data on soil compressibility are often not available either due to time constraint or heterogeneity of soil deposits or non availability of undisturbed soil samples. This paper suggests a correlation between compressibility characteristic and liquid limit and plastic limit of soil that may be adopted under such circumstances. The proposed method is based on a pressure—void ratio equation derived in case of reconstituted saturated fine-grained soils making use of the experimentally verified fact that the major principal (vertical) effective consolidation stresses at liquid limit and plastic limit are around 6.3 and 630 kN/m2 respectively. The experimental verification has been made in three different ways using three different soil state parameters. In the process it has also been shown that the compression index is an exclusive function of plasticity index and specific gravity of soil.

Numerical 3-D FEM and Experimental Analysis of the Open-Ended Coaxial Line Technique for Microwave Dielectric Spectroscopy on Soil

Open-ended coaxial line probes (OCs) are systematically analyzed by means of numerical 3-D finite element calculations in combination with experimental investigations for microwave dielectric spectroscopy on fine grained soils. The probes, based on conventional coaxial lines and connectors (N, SMA), are broadband characterized in the frequency range from 1 MHz to 10 GHz. The sensitive region for dielectric measurements is ±7-mm lateral and 7-mm perpendicular to the midpoint of the sensor aperture. The spatial spreading of the sensitive zone is stable for the investigated low-loss and high-loss strongly dispersive standard liquids, as well as the saturated and unsaturated soils. Dielectric spectra are determined based on a bilinear relationship between effective permittivity and complex reflection coefficient of the probe after probe-calibration with known standards. The mean relative error of the real part of the complex permittivity from 100 MHz to 10 GHz is smaller than 3.5% and is less than 10% for the imaginary part. A lower limit of the measurement range of 50 MHz with the used procedure and materials is suggested. Complex effective permittivity of saturated fine-grained soils is determined with the developed probes and procedure. The soil dielectric spectra were analyzed with a broadband relaxation model, as well as a novel, coupled hydraulic-dielectric mixture approach. The results demonstrate the suitability of the investigated OCs for the determination of high resolution soil dielectric spectra.

Desiccation cracks in saturated fine-grained soils: particle-level phenomena and effective-stress analysis

The formation of desiccation cracks in soils is often interpreted in terms of tensile strength. However, this mechanistic model disregards the cohesionless, effective-stress-dependent frictional behaviour of fine-grained soils. An alternative theory is explored using analyses, numerical simulations based on an effective-stress formulation, and experiments monitored using high-resolution time-lapsed photography. Results show that desiccation cracks in fine-grained sediments initiate as the air–water interface invades the saturated medium, driven by the increase in suction. Thereafter, the interfacial membrane causes an increase in the local void ratio at the tip, the air-entry value decreases, the air–water interface advances into the tip and the crack grows. The effective stress remains in compression everywhere in the soil mass, including at the tip of the desiccation crack. This crack-growing mechanism can explain various observations related to desiccation crack formation in fine-grained soils, including the effects of pore fluid salt concentration, slower crack propagation velocity and right angle realignment while approaching a pre-existing crack, and the apparent strength and failure mode observed in fine-grained soils subjected to tension. Additional research is required to develop a complementary phenomenological model for desiccation crack formation in coarse-grained sediments.

Predicting the normalized, undrained shear strength of saturated fine-grained soils using plasticity-value correlations

In soil mechanics, the plasticity of fine-grained soils is expressed using the plasticity index. This represents the difference between the water content at the liquid and plastic limits, which are often collectively referred to as the Atterberg (or consistency) limits. They are of key importance in soil mechanics because they determine, in a simple way, the interaction between the solid and liquid phases in soils, and thus provide the possibility to classify soils into groups with similar mechanical properties. As the Atterberg limits are the most distinctive and the easiest property of fine-grained soils to measure, several researchers have tried to use them to predict various mechanical properties, such as the normalized, undrained shear strength of fine-grained soils. The most widely used expression for predicting this value from the plasticity index was suggested by Skempton. Since then, there have been several studies to examine the validity of his relation. A lot of the data agree with this relation; however, there is an equally large volume of data that contradicts it. The present study shows that the correlation between the normalized undrained shear strength of cohesive soils and the plasticity index really exists, but it is not as simple as was suggested by previous studies. By considering the mineralogical properties of clay minerals and their impact on the quantity and state of water in soils, it was found that there is no uniform criterion to determine the normalized undrained shear strength from the plasticity index for all fine-grained soils.

Thermal Conductivity Evolution of Saturated Clay under Consolidation Process

This paper presents the results of a study on the thermal conductivity of a soft saturated clay (Bangkok clay) carried out in relation to an investigation into thermal ground improvement using prefabricated vertical drains. The thermal conductivity of clay specimens was measured, at different porosities and temperature levels, using a simple nondestructive steady-state test method. In addition, a theoretical mixture model to simulate the evolution of thermal conductivity of saturated fine-grained soils has been introduced. It is formulated in terms of thermal conductivity and volume fraction of each soil phase (solid and water), and a morphological parameter controlled by the soil fabric condition. The proposed model has been validated against thermal conductivity results reported in the literature and results obtained from the present investigation. Reasonable agreement has been obtained between the predicted and measured thermal conductivity values.

Volume change behaviour of saturated clays under drained heating conditions: experimental results and constitutive modeling

One of the essential features of thermomechanical behaviour of saturated fine-grained soils is the thermally induced volume change occurring under a drained heating condition. In this study, a modified oedometer apparatus, able to handle temperatures up to 100 °C, was used to investigate the thermally induced volume change behaviour of soft Bangkok clay. The results of this experimental study provide additional experimental evidence and insights that may enhance the understanding of volume change behaviour of saturated clays at elevated temperatures. This paper also introduces an isotropic thermoelastoplastic constitutive model that closely predicts the thermally induced volume change behaviour of saturated clay specimens under normally consolidated conditions as well as overconsolidated conditions obtained by unloading process in the temperature range of 20–95 °C. Two yield limits, namely: loading yield limit and thermal yield limit, each of which can be measured from simple experimental results, are proposed to capture the volumetric plastic strain behaviour induced either by mechanical or thermal loading (or any combination thereof). The capability of the proposed model has been verified against experimental results obtained on soft Bangkok clay as well as on two other clays (Boom clay and MC clay) as reported in the literature.

Analysis and Estimation of the Coefficient of Consolidation

Abstract Consolidation is one of the most important behaviors of saturated fine-grained soils that needs to be understood for settlement analysis of these soils. The two most important aspects of laboratory consolidation tests are: (1) estimation of the compression index (Cc), used to predict total settlement of normally consolidated soils provided the void ratio versus log (effective stress) is linear; and (2) the coefficient of consolidation (Cv), used to predict the rate of settlement in the range of primary consolidation. Several researchers have proposed various graphical methods for obtaining estimates of Cv from laboratory oedometer tests. Olson (1985) concluded that since different graphical methods estimate different values for Cv for the same data, the only rational way to estimate the Cv value is to base it on the coefficient of volume compressibility (mv) and the coefficient of permeability (k). Guided by these considerations, an attempt has been made to establish the feasibility of estimating Cv using stress state-permeability relationships (Nagaraj et al. 1993). The input parameters are the liquid-limit void ratio (eL) and the overburden pressure (p).

Hydraulic conductivity in surface active soils

Kozeny's equation based on a capillary tube model with constant viscosity does not give satisfactory results for fine-grained soils. When surface forces dominate over gravity forces, the pore water behaves abnormally and the physical properties of this pore water are found to be quite different from free water. Flow through saturated fine-grained soils is known to be affected by the properties and thickness of loosely and strongly bound water, whose viscosity is observed to be higher because of the modified water structure induced by clay-water interaction. An analytical solution based on a capillary tube model taking into account the changed viscosity of bound water and variation of viscosity within the bound water in relation to its thickness is attempted. The derived equation appears to be general in nature and is applicable for both surface active and inactive soils. It is shown that Kozeny's equation turns out to be the particular case of the derived equation when the thickness of the bound water is zero or when the variation of viscosity is not taken into account.

Influence of viscosity and inertia on the penetration resistance of saturated fine-grained soils: by G. W. Turnage

Influence of viscosity and inertia on the penetration resistance of saturated fine-grained soils: by G. W. Turnage