Ham River Sand Research Articles

Quality assessment of a new in-mould slurry deposition method for triaxial specimen reconstitution of clean and silty sands

An innovative specimen reconstitution technique for sandy and silty soils that simulates underwater deposition is presented and evaluated. The technique is an upgraded version for triaxial testing of the well-established slurry deposition method. This novel setup integrates the reconstitution mould and the mixing tube into a single unit to avoid transferring the sample from the mixing tube to the mould. This subtle, but critical, modification enables reconstitution of very loose specimens as sample transfer disturbance, which can be significant, is eliminated. The quality of specimens prepared by the new reconstitution method was assessed by experiments on a clean sand from the UK (Ham River sand) and a silty sand from Norway (Øysand). The method, as any slurry-based procedure, is capable of producing homogeneous specimens with high initial degree of saturation, even in the absence of back pressure. The procedure is shown to be suitable for sands with or without fines. Moreover, the new method is able to achieve a wide range of initial void ratios, from very loose to very dense, without imposing any particle crushing in the latter case.

Enhancing Constitutive Models for Soils: Adding the Capability to Model Nonlinear Small Strain in Shear

The deviatoric stress‐deviatoric strain relationship in soils is highly nonlinear, especially in the small strain range. However, the constitutive models which aim to replicate the small strain nonlinearity are often complex and rarely used in geotechnical engineering practice. The goal of this study is to offer a simple way for updating the existing constitutive models, widely used in geotechnical practice, to take into account the small strain shear modulus changes. The study uses an existing small strain relationship to derive a yield surface. When the yield surface is introduced to an existing soil model, it enhances the model with the nonlinear deviatoric stress‐deviatoric strain relationship in the small strain range. The paper also gives an example of such a model enhancement by combining the new yield surface with the Modified Cam Clay constitutive model. The validation simulations of the undrained triaxial tests on London Clay and Ham River sand with the upgraded constitutive models replicate the experiments clearly better than the base models, without any changes to existing model parameters and the core source code associated with the base model.

Assessment of standard research sand for laboratory testing

tandard soils are used worldwide as reference materials with which new model or single element experiments may be performed, assessed and calibrated. The testing databases associated with these are valuable resources that are particularly important when developing new procedures. However, the finite extent and variability of all natural deposits creates the possibility that standard soils may vary, or become unavailable, over time. The Ham River Sand (HRS), from the Thames Valley in the UK has been researched continuously and comprehensively in a series of studies since the 1940s, leading to a large database that includes recent advanced hollow cylinder, stress path triaxial and dynamic testing. Fresh samples are now unavailable and the paper describes a study of alternative sampling sources within the Thames Valley. Microscopic visual inspections, index measurements, direct shear, high pressure oedometer, bender element and stress path triaxial test data are presented in the paper, focusing on the natural variability and the ranges seen in material test response. A replacement for the original HRS is identified, so allowing those developing new tests the possibility of conducting experiments on material that is compatible with the existing HRS database. Reference is also made to advances in bender element testing achieved as part of the study.

Local Boundary Surfaces of a Loose Sand Dependent on Consolidation Path

ABSTRACT In the companion paper of Shibuya et al. (2002), a four-dimensional local boundary surface (4-D LBS) applicable for understanding anisotropic shear behaviour of an isotropically consolidated loose Ham River sand (HRS) has been experimentally established by using four stress parameters : the shear stress, t=(σ'1-σ'3)/2 ; mean effective principal stress, p'=(σ'1+σ'2+σ'3)/3 ; the direction of major principal stress relative to the vertical (=deposition direction), α ; and, the relative magnitude of intermediate principal stress, b=(σ'2-σ'3)/(σ'1-σ'3). In this paper, the results of additional sets of hollow cylinder (HC) tests on the anisotropically consolidated samples are newly presented, and are interpreted after the framework of LBS. Undrained initial anisotropy of the loose HRS when subjected to a consolidation path with the effective stress ratio, K, fixed at 0.5, was manifested in a set of HC tests performed using different, but fixed in each test, α values under the conditions of b = 0.3. The shape of a LBS of the anisotropically consolidated sample was demonstrated in a three-dimensional (t, p', α) space in another set of tests, in which the principal stress directions were continuously rotated in various manners. The undrained LBS of the initially loose samples of HRS which have undrained initial anisotropy and limits the effective stress space to be occupied by any undrained paths, was found to be not unique in shape, but pertinent to an anisotropic fabric formed at deposition and modified throughout the consolidation history which followed. The effects of the consolidation path were highlighted by examining the undrained strength at peak conditions.

FOUR-DIMENSIONAL LOCAL BOUNDARY SURFACES OF AN ISOTROPICALLY CONSOLIDATED LOOSE SAND

ABSTRACT Undrained anisotropy, together with the undrained response to monotonic as well as cyclic rotation of principal stress directions, of an isotropically consolidated loose Ham River sand has been investigated by performing three series of hollow cylinder tests. The initial anisotropy of the sand when subjected to a single consolidation pressure was successfully established in the first series of tests, each using different, but fixed in each test, values of α (i.e., the direction of major principal stress relative to the vertical deposition direction) at different b values (i.e., the relative magnitude of intermediate principal stress). The response against monotonic principal stress rotation was also examined in the second series of tests, in which the total stress path defined using the shear stress, t, and α was varied in various manner with b fixed at 0.5. In the third series of tests, the response to cyclic changes of both α and b was examined, and the results were interpreted with the comparative behaviour of the monotonic loading tests with different combinations of α and b. It was found that the sand’s undrained anisotropy and cyclic response could be interpreted by invoking the concept of undrained local boundary surface (LBS). The four-dimensional LBS could be visualized in (t, p′, α) space as a family of superimposed three-dimensional shapes with different b values.

Undrained Behaviour of Natural and Model Clayey Sands

The undrained behaviour of a natural clayey sand from the site of the Gullfaks C oil production platform in the North Sea is described. The material shows significant undrained brittleness in triaxial compression and extension. Variables that are examined include : clay content, granular void ratio and consolidation history. The behaviour of the natural clayey sand is compared to that of a model soil formed from Ham river sand and kaolin. Differences in response are ascribed to differences in grading between the two materials.

The undrained behaviour of clayey sands in triaxial compression and extension

The Paper describes an experimental investigation of the stress-strain behaviour of anisotropically consolidated clayey sands, carried out using instrumented and computer-controlled triaxial cells. The specimens were prepared by sedimenting Ham river sand into a kaolin suspension. For a given granular void ratio, as the clay content of the sedi-mented clayey sands increases from 4-6 to 10%, the undrained brittleness and strains to phase transformation also increase, when the specimens are sheared in compression. When sheared in extension, liquefaction takes place for clay contents up to 7-5%. Increases in granular void ratio give rise to increased brittleness and potential to liquefaction. The effects of overconsolidation on strength and stiffness characteristics have been examined. The effective stress paths for undrained shear of the normally consolidated soil form a boundary for the effective stress paths for the lightly overconsolidated soil. L'article décrit une étude expérimentale, effectuée a l'aide de cellules triaxiales instrumentées et con-trôlées par des ordinateurs, du comportement de sables argileux consolidés de façon anisotrope. Les échantillons ont été préparés par sédimentation du sable fluviatile (Ham) dans une suspension de kaolin. Pour un indice des vides entre les grains d'une valeur donnée, à mesure que le contenu d'argile dans les sables argileux sédimentés s'accroît de 4,6 a 10% la fragilité non-drainée et la transformation des déformations en phases s'augmentent aussi lorsque les échantillons sont soumis au cisaillement sous compression. Dans le cas du cisaillement sous extension la liquéfaction a lieu pour des teneurs en argile jusqu'à 7,5%. Des augmentations de l'indice des vides entre les grains provoquent un accroissement de la fragilité et de la liquéfaction potentielle. Les effets de la sur-consolidation sur les caractérisques de résistance et de raideur ont été examines. Les chemins effectifs de contrainte pour le cisaillement non-drainé du sol normalement consolide constituent une limite pour les chemins effectifs de contrainte dans le sol lé-gèrement surconsolidé.

A Bounding Surface for Granular Materials

A laboratory investigation has been carried out into initial anisotropy and into the effects of continuous principal stress rotation on the undrained behaviour of two granular materials : a natural angular to sub-angular sand and glass ballotini, which comprises spherical grains. Pluviated samples of loose Ham river sand and of ballotini, having the same grading, were isotropically consolidated, then subject to undrained shear in a hollow cylinder apparatus, using a constant b ( = (σ 2 — σ 3 )/(σ 1 —σ 3 )) = 0.5. Despite the marked contrast in their grain shapes, both materials displayed stress-strain, strength and pore pressure responses which were strongly dependent on α, the direction of the major principal stress to the vertical during shear. For both materials a surface may be defined in q ((σ 1 — σ 3 )/2) — p '((σ 1 ' + σ 2 ' + σ 3 ')/3) — α space which portrays their undrained initial anisotropy. The surface is shown to be a bounding surface in that it separates possible from impossible states. The bounding surface is capable of anticipating the response of the materials to stress paths involving principal stress rotation, both monotonically and cyclically applied. It distinguishes different patterns of behaviour on the basis of the direction in which principal stresses are rotated and whether effective stress paths travel on or below the surface. The bounding surface is restricted to the stage of undrained shear during which positive pore pressures are generated and levels of octahedral shear strain are, by observation, less than 0.5% .

The development of a new hollow cylinder apparatus for investigating the effects of principal stress rotation in soils

In the wall of a hollow cylindrical specimen under combined axial load, torque and internal and external radial pressures, the magnitude and direction of the major and minor principal stresses can be controlled together with the magnitude of the intermediate principal stress. This Paper describes both the design and principles of operation of a new hollow cylinder apparatus and its application to the investigation of principal stress rotation effects in sands and clays. A discussion is presented of the stress and strain nonuniformities which occur in a hollow cylinder specimen as a result of its curvature and of restraint at its ends. The manner in which these non-uniformities have been minimized, by selection of a sample geometry of 203 mm id. × 254 mm o.d. × 254 mm height and by the use of internal instrumentation, is described. The results of tests on dense Ham River sand are presented to demonstrate the capabilities of the apparatus. Dans la paroi d'un échantillon creux cylindrique sous une combination de charge axiale, de moment de torsion et de pressions radiales internes et externes la valeur et la direction des contraintes principales majeures et mineures peuvent être controlées en même temps que la valeur de la contrainte principale intermediaire. L'article décrit la construction et les principes de fonctionnement dun nouvel appareil creux cylindrique et son application a l'étude des effets de la rotation de la contrainte principale dans les sables et les argiles. Les non-uniformités de contrainte et de déformation sont discutées qui se produisent dans un échantillon creux cylindrique comme résultat de sa courbure et de la fixation à ses deux extrémités. On décrit méthode par laquelle ces non-uniformitts ont été réduites au minimum par l'adoption d'un échantillon ayant un diamètre interne de 203 mm, un diamtére externe de 254mm et une hauteur de 254 mm, en employant des appareils de mesure placés à l'intérieur. Les resultats de quelques essais faits sur du sable dense type Ham River sont présentés pour démontrer ce dont l'appareil est capable.

A simple piezometer probe for the routine measurement of pore pressure in triaxial tests on saturated soils

A technique has been evolved for installing a piezometer probe on samples down to a diameter of 38 mm. The probe is based on a miniature silicon diaphragm pressure transducer mounted at mid-height to cause minimum interference between the sample and probe and to provide a rapid response for the piezometer-soil system. Considerable savings can be made in testing times by using the probe at mid-height and by using effective lubricated end platens. Measurements can also be made in cyclic loading tests on clays and are viable in quick undrained tests even at shearing rates used in commercial laboratories. Examples are given of measurements made in undrained compression-extension triaxial tests in cromer fill and medium-dense ham river sand. The probe provides a more accurate method of measuring pore pressures in unconsolidated undrained triaxial tests and can be effectively used to maintain an acceptably low gradient of pore pressure in the sample when following a drained stress path. (TRRL)

On estimating Ko for overconsolidated granular soils

It is possible for horizontal stresses to be significantly different at the same depth within two deposits. The stress-path dependency of load-deformation behaviour in tests must be taken into account when assessing their use. Thus the estimation of ko, the in situ ratio of the horizontal effective stress to the vertical effective stress, is of major importance. The author reviews the results of the laboratory evaluation of ko for various materials and gives details of his work on Ham River sand at various void ratios. Samples were set up in a stress path cell with two LVDT transducers to obtain axial deformations relatively free of bedding errors. A lateral strain gauge was installed at the mid-height of the sample to control or obtain the lateral deformation of the sample as required. Typical stress-paths during the loading and unloading are plotted. The rate of increase of ko was always greater in dense than in loose samples. Observations based on Lambe's plot showed that the unloading paths can be approximated to straight lines up to ocr of 7 used. An equation is given to express ko in terms of the slope of the unloading stress-path and ratio of the change in effective horizontal stress to the change in effective vertical stress. (TRRL)

Homogeneous and layered sand in triaxial compression

Samples of Ham River sand were prepared at high and low porosities by pouring through air or a liquid, the techniques being alternated for layered samples. Some samples were reinforced with horizontal sheets of various materials. To avoid edge effects near lubricated platens in triaxial compression, strains in the middle third of each sample were recorded in accurate detail. Changing the relation of the plane of deposition to the major principal stress direction revealed inherent anisotropy effects. Stress-strain and strength data show the influence of porosity, layer geometry and horizontal reinforcing sheets. In homogeneous samples the orientation of lines of zero extension in the maximum-shear plane depended on porosity but not on current strain or inherent anisotropy. Inherent anisotropy did not affect the stress-dilatancy relationship, nor did changing its axes affect the virtual coincidence of stress and strain rate axes. But pre-failure strains were largely influenced by inherent anisotropy at both porosities and the lower porosity gave appreciable strength anisotropy. Layered samples were weaker, depending on layer geometry and the loose layers sustained large strains while at maximum stress ratio. No thin ruptures formed, but at failure the strains were confined within loose layers, when these were oriented so that a line of zero extension was parallel to their plane. This orientation differed appreciably from that of maximum stress obliquity. The stress-dilatancy relationship for homogeneous samples predicted stress ratios in both layered and reinforced samples. The loose sand dilated much more when in a layer than in homogeneous samples. Des échantillons de sable provenant de Ham River ont été préparés à des porosités faibles et élevées en le versant à travers de l'aire ou un liquide, les techniques ont été alternées pour les échantillons sandwiches. Quelques échantillons ont été renfor avec des feuilles horizontales de materiaux divers. Afin d'éviter des effets de bord prés des plateaux lubrifiés dans une compression triaxiale, les déformations au tiers central de chaque échantillon ont été enrégistrées avec précision. En changeant la position du plan de sédimentation par rapport à la contrainte majeure principale, on a pu mettre en évidence des effets anisotropiques. Les mesures de contraintes-déformations et de résistance montrent l'influence de la porosité, de la géomérie des couches et des feuilles de renforcement horizontales. Dans les échantillons homogènes l'orientation des lignes à extension nulle dans le plan de cisaillement maximum dépendait de la porosité mais non pas de la déformation courante ou de l'anisotropie naturelle. L'anisotropie naturelle n'a pas affecté la relation existant entre la contrainte et la dilatance, de même que le changement de ses axes n'a pas affecté la coïncidence virtuelle des axes de vitesse de contrainte et de déformation. Mais les déformations de pré-rupture étaient largement influencées par l'anisotropie naturelle pour les deux porosités et la porosité la plus faible a donné une résistance d'anisotropie appréciable. Les échantillons feuilletés étaient plus faibles, selon de la géométrie des couches et les couches lâches ont supporté de grandes conrai-ntes pour un taux de contrainte maximum. Il n'est apparu aucune rupture mince mais à la rupture, les déformations étaient en totalité dans des couches lâches, quand celles-ci étaient orientées de telle fa¸on qu'une ligne d'extension nulle était parallèle à leur plan. Cette orientation différait notablement à partir de celle d'une obliquité de contrainte maximum. La relation contrainte-dilatance pour les échantillons homogènes a permis de prèvoir des taux de contrainte aussi bien dans les échantillons feuillétes que dans les échantillons renforcés. Le sable lâche se dilatait beaucoup plus dans une couche feuilletée que dans des échantillons homogènes.