Search

The mechanical behaviour of clayey loess is strongly affected by the soil structure, but although anisotropy has been identified in loess by some, the anisotropy of small-strain behaviour is rarely reported. This paper presents an experimental study on the inherent and stress-induced small-strain shear stiffness anisotropy of a clayey loess from China. Both undisturbed and reconstituted specimens were tested with bender elements under isotropic compression and shearing conditions. Under an isotropic stress state, an inherent anisotropy was found for undisturbed specimens, while the reconstituted specimens prepared by moist tamping behaved isotropically. During shearing, the ratio of horizontal to vertical shear moduli of the undisturbed specimens decreased due to both an increase of stress anisotropy and the destruction of the intact structure. In contrast, the stiffness ratio of the reconstituted specimens only decreased due to stress anisotropy and it became more anisotropic at the critical state. This study reveals the influence of intact structure and inherent anisotropy on the behaviour of loess soils, which cannot be reproduced by compaction, thus highlighting the importance of characterising the undisturbed loess. [ABSTRACT FROM AUTHOR]

The pore size, shape and orientation of an illite-dominant clay were mapped during one-dimensional compression, using mercury intrusion porosimetry (MIP), scanning electron microscopy and gas adsorption. The total porosity was found to spread over the three International Union of Pure and Applied Chemistry classes of pores sizes: micropores (below 2 nm), mesopores (2–50 nm) and macropores (above 50 nm), and all three pore classes were observed during the compression. The clay structure is aggregated, with visible inter-aggregate pores (about 80% of the total porosity), and the remaining intra-aggregate pores of size approximately equal to the thickness of illite platelets (50–100 layers). During compression the largest pores first collapsed, followed by a progressive collapse, in an orderly manner, of smaller and smaller pores. MIP data suggest that the macroscopic deformation mainly translates at the pore scale into changes of inter-aggregate porosity, while intra-aggregate pores spread over the micro- to mesopore size range. Gas adsorption tests show that the volume of intra-aggregate pores decreases with loading, probably due to rearrangement of particles composing the aggregates, while the specific surface area reduces. Examination of the pores' orientation on both vertical and horizontal planes confirms a preferential orientation of pores normal to the loading direction, with a gradual flattening of the pores. [ABSTRACT FROM AUTHOR]

The surfaces of soil grains are not perfectly smooth, especially examined at small scale. In geotechnical engineering, surface roughness has been found to be able to influence the inter-particle friction angle at micro scale and small-strain stiffness at macro scale. However, the quantity and quality of the studies on surface roughness of natural soils are still limited. In this study, the evolution of surface roughness of natural sand grains with increasing normal load was investigated by a single-particle compression apparatus. Thirty Leighton Buzzard sand (LBS) grains coarser than 2·36 mm were tested, and the surface roughness was measured before and after compression by an optical interferometer. The deformations of the asperities and of the bulk of the sand grains in the vicinity of the contact were mapped. Three stages were identified as the normal load increased: (a) plastic deformation of the asperities; (b) asperities and bulk plastic deformation; and (c) bulk only plastic deformation. At very small normal load, only the asperities were found to deform plastically, and the surface roughness of the sand grains decreases due to the flattening of the asperities. Within this regime, the load–displacement relationship of LBS grains under compression could be simulated by the modified Hertz model, which takes surface roughness into consideration. With increasing normal load, the bulk of the sand grains began to yield near the contact. The geometry of the surfaces of LBS grains in contact with the loading platen is the main factor that influences the plastic deformation of the bulk. Differently from the plastic deformation of the asperities, the plastic deformation of the bulk could both smoothen and roughen the surfaces. When plastic deformation of the bulk occurred, both Hertz and modified Hertz theory could not predict the load and displacement relationship of sand grains. Through analysing the cumulative distributions of surface roughness of 30 LBS grains at different normal loads by the Weibull function, the surface roughness was found to decrease dramatically with increasing normal load at first and then tended to be constant. [ABSTRACT FROM AUTHOR]

The particle morphology of granular materials comprises different characteristic scales, including particle shape and surface texture. Different methods have been proposed to characterise the morphology using three-dimensional parameters, among which is the fractal method. These methods, however, are applied either at the scale of particle shape or surface texture. A framework unifying the multi-scale morphology obtained from different measuring instruments could advance the current understanding to this topic, but is still lacking. This paper proposes a novel methodology to characterise the morphology of sand particles across different scales based on results from two previously adopted instruments with different measuring capabilities – an X-ray micro-computed tomography (μCT) and a high-resolution optical microscope equipped with an interferometer. The methodology is applied to sand-sized particles of a crushed granitic rock and a natural quarzitic sand (Fujian sand). By using spectrum analysis on data from both μCT and interferometer measurements, a single fractal dimension is found linking the spectrum of the two measurements for the crushed granitic rock. For Fujian sand, two self-affine patterns are observed, which serves as a separation between particle shape and surface texture, and also indicates that the fractal dimension obtained at larger scale may not be simply extended to small scales. The translation of surface measurements into numerically reconstructed particle morphology at particle shape and surface texture scale is demonstrated by using spherical harmonic expansion and power spectral density functions. [ABSTRACT FROM AUTHOR]

Sands are naturally hydrophilic granular materials, yet, rendering them hydrophobic could lend them to a wide range of geotechnical applications. This study describes a powder-coating procedure performed after chemically modifying the surfaces of coarse, medium and fine sands and examines its effect on their hydrophobicity. The purpose is to render these granular materials more hydrophobic than what is conventionally achieved by chemical methods using a simple technique. The procedure consists of first silanising both the sands and silica powder at a similar concentration by means of an organosilane to modify their surface chemistry, then the silica powder is adhered to the sands at a mass mixing ratio to alter their hydrophobicity. Irrespective of the concentrations and mixing ratios, the powder-coating procedure enhances the hydrophobicity of sands in comparison to the sole use of the chemical method. Changes in the morphology of the sand grains, such as their particle size, particle shape and surface roughness, resulting from the powder-coating procedure are examined by means of dynamic image analysis, profilometry and scanning electron microscopy. The effects of surface chemistry, surface roughness and air on the hydrophobicity of the sands are discussed based on theoretical wetting models to analyse the experimental results. [ABSTRACT FROM AUTHOR]

The existence and failures of silty sand have been reported worldwide, and the behaviour of sand–silt mixtures at small to large strains has been intensively studied. Owing to the relatively low stress level achieved in laboratory testing, no unifying framework has yet been proposed for sand–silt mixtures. In the presented work, a crushable sand, made of pumice, was used as host sand so as to reach a unique normal compression line under pressures easily attainable in the laboratory. The sand was combined with non-plastic fines in proportions varying between 0 and 100%, and tested in compression and shearing with associated bender element testing for small-strain stiffness measurements. The unique relationship between pressure and volume on the normal compression line allowed for the normalisation of the data in a way that highlights the effects of fines irrespective of the density. It was found that once the effects of the fines on the index properties and compressibility have been cancelled, by normalising for volume, the behaviour of the sand–silt mixtures can be characterised as unique. [ABSTRACT FROM AUTHOR]

A series of tests is carried out using a purpose-built set-up to model compaction grouting around a pile shaft. The tests are performed on Hong Kong completely decomposed granite placed in a Rowe cell, with cement grout directly injected at the bottom of the cell from a pressure interface chamber. The soil specimens are tested under different initial stress and density conditions, either normally consolidated or overconsolidated. Different grouting pressures are used. The results indicate that a target volume of grout can be injected if the grout injection pressure is larger than the total stress by a magnitude at least equal to the effective stress. There is a tendency for the grout to propagate between the pile and the soil, with a residual densification of the soil after drainage. In loose soil, grouting has the effect of reducing the effective stress in the soil to zero, resulting in a homogeneous layer of grout between the shaft and soil. In denser specimens, fractures can occur parallel to the direction of minimum principal stress. A noticeable decrease in void ratio is achieved after grouting, due to both grout injection and drainage. [ABSTRACT FROM AUTHOR]

Reconstituting soil is common in construction yet the method used can affect the mechanical behaviour of the reconstituted soil when the soil contains a significant amount of fines. A completely decomposed granite was reconstituted following different methods, namely, dry deposition, moist tamping, slurrying and reconstituting after separating fines by washing; then it was tested in compression and shearing. The test results highlight the sensitivity of the soil behaviour to the method of preparation used for reconstitution. It was found that preparation methods involving large amounts of water, such as slurrying (S) or reconstitution after washing the fines (F), can lead to a release of fines, with effect to change the grading and change the mineralogy. Differences between dry-deposited (D)/moist-tamped (MT) specimens and slurried (S)/washed-reconstituted (F) specimens were observed mainly in the volumetric response. Two different normal compression and critical state lines were obtained for the two main methods of preparation (D/MT and S/F). The distance between the normal compression line and critical state line is larger for the S/F specimens than for the D/MT specimens. For residual soils like Hong Kong completely decomposed granite, which contains a significant amount of fines, it therefore seems necessary to understand the potential differences caused by different preparation methods of reconstitution. [ABSTRACT FROM AUTHOR]

This paper attempts to bridge the gap between the well-known characteristics of strain rate-independent elastic stiffness and of the time-dependent behaviour of clays at large strains. Effects of acceleration, strain rate and ageing are examined in the very small to small strain region, using results from tests on reconstituted London Clay and kaolin. Static measurements of the shear modulus at small strains, using local instrumentation, indicate that the shear modulus is rate-independent when the soil is subjected to elastic deformations only. Most often, in practice, soil specimens are left to rest between the compression and shearing stages, until deformations associated with creep become negligible. The testing strain rate imposed upon shearing induces a temporary acceleration of strains, which results in very high measured stiffnesses. Test data show, however, that when the shearing axial strain rate is of the same order of magnitude as that measured at the end of the compression/creep stage, the remaining plastic creep strains influence the measured stiffness, which is then measured to be comparatively low. Test data also seem to show that temporary strain acceleration may hide effects of recent stress path rotation. Additionally, bender element tests were performed during isotropic compression tests with changes in stress rate and creep stages. The data indicate that the strain rate affects the value of the dynamic shear modulus of the soil. It is suggested that the increase in dynamic shear modulus during short-term creep is caused by mechanisms linked to the gradual decrease in strain rate during creep, but further investigation would be needed to clarify which physical mechanisms. A new method to account for strain rate in evaluating the dynamic shear modulus of normally consolidated reconstituted clays is finally proposed. [ABSTRACT FROM AUTHOR]

Fibres can be an effective means of reinforcing soils. This paper presents data from laboratory triaxial tests on quartzitic sand reinforced with polypropylene fibres. By keeping the studied composite consistent throughout the study (host sand and fibre characteristics kept constant), it has been possible to develop a framework of behaviour for the sand-fibre material, which provides a solid base for future research on fibre-reinforced soils. Data from previous work and from new tests have been analysed within the Critical State framework, that is in terms of normal compression line, critical state line and state boundary surface. [ABSTRACT FROM AUTHOR]

The article presents an experimental investigation of the behaviour of artificially cemented soil cured under stress. It is indicated that the specimens cured at confining pressures of 50 kPa have larger normalised yield and state boundary surfaces. With regards to samples cured on the isotropic intrinsic compression line, a single state boundary surface is determined, independently of the shearing confining pressure. The effect of volume can be normalised when curing on the normal compression line. The extra strength imparted to the soil may attribute to the lower curing void ratio. The data indicate that the unloading events in the samples cured at higher stresses do not affect the subsequent behaviour.

A letter to the editor is presented discussing the influence of structure on the time-dependent behavior of a stiff sedimentary clay.

The behaviour of deep-ocean sediments from the North Atlantic abyssal plains is examined within the context of a recognised framework, the Sensitivity Framework, which was developed by Cotecchia & Chandler for structured terrestrial clays. The deep-ocean sediments have very high sensitivities, of the order of those of quick clays, but in volumetric compression their destructuration occurs at more moderate rates, similar to those usually found in terrestrial clays of medium sensitivities. The deep-sea sediments were retrieved using different types of sampler, which influenced greatly the strength and stiffness of the different samples. In particular, in the lower-quality samples the greater disturbance caused by sampling is responsible for changing their behaviour from brittle to ductile during shearing after compression to stresses post-gross yield, giving them an apparent insensitivity to shear strains. Results from numerical analyses using a constitutive model that was developed for structured terrestrial clays are presented. They show that such a model, which accounts for natural structure, can be used to simulate the behaviour of deep-ocean sediments provided an appropriate destructuration law is used in the model. [ABSTRACT FROM AUTHOR]

A simple constitutive model for structured clays is presented. The model is based on an existing constitutive model for reconstituted clays. It uses a fixed relationship between change in sensitivity and plastic strain to represent destructuration so that all predicted effects of structure are included in the value of sensitivity. Only three added parameters are required, which can each be derived from a single isotropic compression test. All other parameters are the same as those used in the base model, which are derived from test data on reconstituted specimens. A value of ultimate sensitivity greater than unity allows the model to simulate clays with stable and! or metastable structure. Results from a parametric study show that the model predicts responses during compression and shearing typical of natural clays. The model was applied to test data from a natural soft clay using parameters derived where possible from isotropic compression tests. Results from the analyses prove that the simplified formulation of the model is sufficient to significantly improve predictions. [ABSTRACT FROM AUTHOR]