A new slurry-based method of preparation of specimens of sand containing fines

A new method of specimen reconstitution is presented that is appropriate for element testing of sands containing either plastic or nonplastic fines. The method allows reconstitution of homogeneous, saturated specimens of sands containing fines whose stress-strain response closely resembles the stress-strain response of natural soil deposits formed underwater (e.g., alluvial and offshore submarine deposits, hydraulic fills, and tailings dams). A procedure is described to evaluate the maximum void ratio (emax) of sands containing fines under conditions that more appropriately represent soil deposition at its loosest state in aquatic environments. For soils deposited in water, the data obtained with the procedure proposed in this paper suggest that ASTM D 4254 overestimates the emax of sands containing plastic fines and underestimates the emax of sands containing nonplastic fines. Copyright © 2008 by ASTM International

Thermally induced pore water pressure of reconstituted London clay

Different forms of thermo-active structures have been proposed as a way of making use of the ground temperature to achieve renewable low-carbon heating and cooling in civil engineering construction. Such structures comprise piles, retaining walls or tunnel linings, and are used both as structural components and as conduits for utilising geothermal energy. In the scenario of the underground space in London, it is the thermo-active piles that have received most attention. However, little experimental evidence exists on the thermal behaviour of London clay to aid the design of thermo-active structures. This paper presents advanced laboratory testing on the reconstituted London clay to characterise the effect of temperature on its mechanical behaviour. Particular emphasis is given to thermally induced pore water pressures, as their evolution is not well understood. Tests are conducted in a temperature-controlled isotropic cell developed at Imperial College London. The emphasis of the current paper is on the temperature-based calibrations of different transducers. Soil specimens are isotropically consolidated and then subjected to undrained heating-cooling in the temperature range of 21 to 37 °C. Results obtained are compared with an existing laboratory study on another type of clay

Linking carbonate sand fabric and mechanical anisotropy from hollow cylinder tests: motivation and application

In addition to density and stress, fabric is also a key state variable strongly affecting soil behavior. While fabric influence on mechanical behavior of soils has been investigated experimentally, the available database is limited in terms of boundary conditions and soil types tested. Offshore carbonate sediments are of special interest for offshore geotechnical analyses due to their prevalence in tropical waters and unique mechanical behavior that stems from their mostly biogenic origin. A key gap in the availability of experimental data on soil fabric relates to the anisotropy of offshore carbonate sediments. In practice, anisotropy studies (whether rigorously correlated to fabric or not) are typically carried out experimentally for simple boundary conditions such as idealized plane strain and axisymmetric states. In real geotechnical applications, stress paths subjected to soil elements in the field are far more complex, often involving the combined variations of both the orientation and magnitude of all three principal stresses. This paper presents a new multi- scale approach to assess soil fabric at the micro-scale level and relate it to the macro- mechanical response observed for generalized loading conditions. A new sampling method is illustrated that enables preservation and evaluation of the fabric of offshore sediments specimens following generalized stress disturbances imparted by a hollow cylinder apparatus. The link between fabric evolution and the observed stress-strain behavior of sand is discussed along with preliminary results. The approach is part of a broad framework that will be used to systematically study the evolution of soil fabric and anisotropy and their relationship to multi-directional loading scenarios

A semi-empirical re-evaluation of the influence of state on elastic stiffness in granular materials

This study uses data acquired from three-dimensional discrete element method simulations to reconsider what measure of state can be used to predict stiffness in granular materials. A range of specimens with linear and gap-graded particle size distributions are considered and stiffness is measured using small amplitude strain probes. Analysis of the data firstly confirms that the void ratio, which is typically used as a measure of state in experimental soil mechanics, does not correlate well with shear stiffness. However, the empirical expressions developed by Hardin and his colleagues can capture variations in stiffness, provided an appropriate state variable is used. The study then highlights that the contribution of individual contacts to the overall stiffness is highly variable, depending on both the contact force transmitted and the particle size. Analyses explore how the stress transmission both within and between the different size fractions affects the overall stiffness. This heterogeneity in stiffness relates to the heterogeneity in the stress transmission amongst the different fractions. By considering the heterogeneity of stress distribution amongst different particle size fractions, a new semi-empirical stress-based state variable is proposed that provides insight into the factors that influence stiffness

Analysis of simple shear tests with cell pressure confinement

The high cost of offshore infrastructure provides continuous encouragement for optimisation of design practices. Development of a more rational method to interpret results from simple shear tests with cell pressure confinement can reduce costs and improve reliability of offshore infrastructure. This paper addresses a commonly overlooked issue affecting design parameter selection: specimen shape varies from right cylinder to oblique cylinder after loading along a single shearing direction. Thus, horizontal stresses are not always equal to the cell pressure and their magnitude varies throughout the specimen’s lateral surface. An analysis is proposed that accounts for changing specimen geometry and lateral surface area during shearing and for the actual effect of cell pressure during testing. The analysis also describes how the intermediate principal stress can be assessed. Test results for medium dense silica sand are interpreted following de Josselin de Jong’s alternative shearing mechanism hypothesis. Conventional interpretation methods yield conservative design parameters for this soil. Failure states develop when the intermediate principal effective stress is halfway between major and minor principal effective stresses. Typical results for the soil tested show equipment performance meets standard direct simple shear requirements for shear strain rate, vertical stress and specimen height control

Stress inhomogeneity in gap-graded cohesionless soils - A contact based perspective

Gap-graded cohesionless soils, comprising mixtures of fine and coarse grains, pose a particular challenge in soil mechanics. Reasoning and experimental data indicate that some of the finer grains may exist in the void space without transmitting any stress. A number of authors have proposed considering at least some of the volume of these particles along with the void space when calculating the void ratio in the case of low fines contents. The concept of a transitional fines content has been proposed, i.e., a fines content delineating materials whose behavior is dominated by the coarser grains and materials whose behavior is determined by the finer grains. This contribution uses discrete element method (DEM) simulations to explore the nature of stress transmission in gap-graded materials comprised of spherical particles. Partitioning the stress tensor by considering the contributions of the contacts between coarse particles, the contacts between coarse and fine particles, and the contacts between fine particles is shown to provide useful insight into the contribution of each type of particle to the overall stress transmission. In general, for the mixtures considered here, the coarse-coarse contacts transmit a greater range of forces and a greater average force. For the mixture with size ratio of 3.7, the range of contact force magnitudes transmitted by each contact type reduces with increasing fines content and increasing sample density. This sensitivity is more evident for the lower fines contents studied

Shear modulus of sand-tyre chip mixtures

This paper presents experimental results on the shear modulus of sand–tyre chip (STCh) mixtures. A series of bender element tests was carried out on specimens of sand mixed with varying proportions of tyre chips (TCh). Tests were carried out on STCh mixtures at a constant initial relative density of 50% for different initial effective confining pressures. The bender element test results indicate that the maximum shear modulus of the STCh mixtures increases with effective confining pressure and decreases with the gravimetric proportion of TCh, which ranged from 23 to 138kPa and 0 to 40%, respectively. However, the strain-controlled cyclic triaxial test results show that the shear modulus at large shear strains decreases with increasing single-amplitude shear strain as a function of the proportion of TCh in the mixture. The lower the proportion of TCh, the larger the degradation observed. The modified Hardin and Drnevich mathematical formulation adequately captures the variation in shear modulus of the STCh mixtures for a wide range of shear strain amplitudes

Effects of sampling disturbance in geotechnical design

This paper describes an experimental study of the effects of sampling disturbance in an Australian natural soft clay and the consequences of different sample quality on the representativeness of soil parameters used in geotechnical designs. The paper is divided into three sections. Laboratory test results obtained from specimens retrieved using three different tube samplers as well as the Sherbrooke (block) sampler are first described. Then, the sample quality assessment, using available indices proposed for soft soils, is presented. It is shown that sample quality varies with the stress paths and boundary conditions applied in laboratory tests. Finally, mechanical soil properties derived from specimens retrieved using the different samplers are used in the prediction of two classical problems in soil mechanics: the settlement and excess pore pressure response underneath an embankment as well as the settlement and bearing capacity of a shallow footing. These two examples are used here to highlight the consequences of poor sampling in practice

The influence of particle size distribution on the stress distribution in granular materials

This study systematically explores the effect of the shape of the particle size distribution on stress transmission in granular materials using three-dimensional discrete element method simulations. Extending prior studies that have focussed on bi-modal mixtures of coarser and finer grains, a broad range of isotropically compressed specimens with spherical particles and linear, fractal and gap-graded particle size distributions are considered. Considering isotropic stress conditions the nature of stress distribution was analysed by determining the mean effective particle stresses and considering the proportion of this stress transmitted by particles with different sizes. For gap-graded materials a contact-based perspective was adopted to consider the stress transmission both within and between the different size fractions. A clear correlation emerged between the cumulative distribution of particle sizes by volume and the cumulative distribution of particle sizes by mean effective stress for specimens with continuous PSDs. This correlation does not hold universally for gap-graded materials. In gap-graded materials the distribution of effective stress between the different size fractions depends upon the size ratio and the percentage of finer grains in the specimen. In contrast to specimens with continuous gradings, in the gap-graded specimens the distribution of effective stress amongst the different size fractions exhibited a marked sensitivity to density. Basic network analysis is shown to provide useful insight into effective stress transmission in the bimodal gap-graded materials