Extensive data of the strength and dilatancy of 17 sands in axisymmetric or plane strain at different densities and confining pressures are collated. The critical state angle of shearing resistance of soil which is shearing at constant volume is principally a function of mineralogy and can readily be determined experimentally within a margin of about 1°, being roughly 33° for quartz and 40° for feldspar. The extra angle of shearing of ‘dense’ soil is correlated to its rate of dilation and thence to its relative density and mean effective stress, combined in a new relative dilatancy index. The data of o′max – o′crit in triaxial or plane strain are separately fitted within a typical margin of about 2°, though the streneth of certain sands is underpredicted in the 1000–10000 kN/m2 range owing to the continued dilation of their crush-resistant grains. The practical consequences of these new correlations are assessed, with regard to both laboratory and field testing procedures. L'auteur analyse de nombreuses d...

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The strength and dilatancy of sands

Water Retention: Laboratory Methods

Soils can rarely be described as ideally elastic or perfectly plastic and yet simple elastic and plastic models form the basis for the most traditional geotechnical engineering calculations. With the advent of cheap powerful computers the possibility of performing analyses based on more realistic models has become widely available. One of the aims of this book is to describe the basic ingredients of a family of simple elastic-plastic models of soil behaviour and to demonstrate how such models can be used in numerical analyses. Such numerical analyses are often regarded as mysterious black boxes but a proper appreciation of their worth requires an understanding of the numerical models on which they are based. Though the models on which this book concentrates are simple, understanding of these will indicate the ways in which more sophisticated models will perform.

Soil Behaviour and Critical State Soil Mechanics

Synopsis The equation governing the one-dimensional consolidation of a fully saturated clay layer are derived here on the basis of assumptions more general than those usually adopted. The limitation of small strains has not been imposed and the variation of soil compressibility and permeability during consolidation has been taken into account. Furthermore, although Darcy's law is assumed to be valid, it is recast in a form in which it is the relative velocity of the soil skeleton and the pore fluid that is related to the excess pore fluid pressure gradient. The consolidation of a thin clay layer, the self-weight stresses of which are negligible compared with those applied, is examined in detail. Non-homogeneity, time-effects intrinsic to the soil skeleton and compressibility of the pore fluid are incorporated in the theory, but a detailed consideration of their importance is deferred to later papers in which the case of the thick clay layer will also be presented. Les equation regissant la consolidation a...

The Theory of One-Dimensional Consolidation of Saturated Clays

Within the framework of an extended elastoplastic constitutive model for unsaturated soils (loading-collapse (LC) model), an experimental programme was performed in an osmotically controlled suction triaxial apparatus. The laboratory behaviour of a statically compacted silt was studied, and particular attention was given to the volume changes monitored during shear. Isotropic loading tests confirmed the main features of the LC model related to the effect of suction on volume changes, and allowed a direct determination of the LC curve. Constant σ3 and a few constant q shear tests were performed in order to study yielding and plastic flow at various increasing suctions, starting from the as-compacted condition. Several yield criteria were considered, depending on the type of test performed. Some similarities between compacted unsaturated soils and natural soft soils were shown, such as the inclined elliptical form of the yield curve, which results from the anisotropic state of stress prevailing during k0 co...

Yielding and plastic behaviour of an unsaturated compacted silt

One of the main criticisms of the triaxial test is the nonuniformity of stress and deformation at all but small strains. The main cause of this nonuniformity is friction at the end platens, which causes both the barrelling effect and the concentration of dilation in local zones which in turn results in the premature development of a failure surface. These disadvantages are largely overcome by the use of lubricated end platens which, by removing the dead zones, allow the use of shorter test samples. Short samples with lubricated end platens have proved an advantage in triaxial research on particulate matter for a number of years. Use of this technique has the consistency of the test results. Preliminary tests on clays indicate that this is a promising method of reducing the testing time necessary for accurate pore pressure measurement. An additional benefit from their use is the reduction of piston friction. The use of electrical pressure transducers for measuring pore pressures is also described; this system is particularly suitable if automatic recording is required.

Importance of Free Ends in Triaxial Testing

Synopsis A consolidation cell is described in which the total stress is applied by means of hydraulic pressure acting across a convoluted rubber jack. This arrangement improves control of the drainage conditions, allowing the separation of the instant of loading from the commencement of drainage and permitting the measurement of initial pore pressures. Back pressures are readily applied. A range of cell sizes meet the need to test larger samples of non-uniform deposits. The cell allows both vertical and radial flom tests for consolidation or direct permeability measurements. On decrit une cellule de consolidation dans laquelle la contrainte totale est appliqudee au moyen de pression hydraulique agissant par un verin a caoutchouc convolute. Cet arrangement ameliore le controle des conditions de drainage, permcttant la separation de l'instant de charge du commenccmcnt du drainage ct par consequent la mesure des pressions interstitielles initiales. Les centre-pressions sont appliquees rapidement. Une gamme d...

A New Consolidation Cell

Synopsis The cross-anisotropic elastic half-space, with isotropy as a special case, is suggested as an improved mathematical model of natural soil deposits. A solution by Michell is modified to produce the important stresses and displacements in terms of the five elastic constants required to define cross-anisotropy. Where the exact expression for stress is strongly dependent on the values of Poisson's ratios, and hence of doubtful practical value in soil mechanics, an approximate expression is suggested. It is shown that expressions for surface settlement can be easily modified to include anisotropy. Consideration of the type of anisotropy existing various undisturbed soils indicates that anisotropy contributes to the over-estimation of settlements overconsolidated clays, and is partly responsible for the observed stress distribution in sand. Le semi espace elastique anisotropique simultane, en traitant l'isotropie comme un cas special, est propose comme modele mathematique ameliore des depots de sol nat...

Stresses and Displacements in a Cross-Anisotropic Soil

Synopsis A review is made of published evidence on the physical properties of unsaturated and compacted clays with the object of recognising the more important factors affecting the consolidation process in such soils. An attempt is then made to formulate and solve a set of general equations to describe the consolidation process. Because of the complexity of these equations it is found desirable to sub-divide the process into a number of less general treatments, each being relevant to a particular range of saturation values. The treatments include the effects of decreasing permeability and a compressible pore fluid, and the resulting non-linear partial differential equations are solved by finite difference approximations on a digital computer. The computed results exhibit the two important characteristics found in consolidation tests on unsaturated clay, viz. a continuously curved settlement/root-time plot and a mid-plane pore pressure dissipation plot which is flatter than the theoretical Terzaghi plot. ...

Consolidation of Compacted and Unsaturated Clays

Compression tests under controlled effective stress paths show the stress path independence of volumetric strain and degree of saturation. Stress path dependency appears to be caused by a reversal in the direction of saturation. The swelling process does not in itself appear to introduce an important path dependency. The collapse mechanism is controlled by a potentially unstable structure, a high applied stress and a high suction pressure. The absence of one of these three factors removes the possibility of significant collapse. The effect of molding water content and compaction method on the structure of compacted clay is illustrated by the different compression behavior of clays compacted dry and wet of optimum. The compression behavior is treated best in terms of the separate components of applied stress and suction pressure, and for one-dimensional compression, the stress-strain relationship for a clay can be stated in a simple form.

Laing Barden

Papers

Importance of Free Ends in Triaxial Testing

A New Consolidation Cell

Stresses and Displacements in a Cross-Anisotropic Soil

Consolidation of Compacted and Unsaturated Clays

Volume change characteristics of unsaturated clay