Showing papers in "Geotechnique in 1985"

A state parameter for sands

Abstract: The requirement for a rational engineering approach to constructing structures using undensified hydraulic sand fill has identified several deficiencies in current technology. In particular, there is no single parameter measure of sand behaviour. This Paper presents an appropriate physical parameter, termed the state parameter, that combines the influence of void ratio and stress level with reference to an ultimate (steady) state to describe sand behaviour. Data from a triaxial testing programme on Kogyuk sand with four fines contents is presented and the significant engineering design parameters are demonstrated to be dependent on the state parameter. The concept of state is a fundamental physical concept and has wide applicability both as an empirical normalizing parameter and for constitutive modelling of soil behaviour. Plusieurs insuffisances de la technologie habituelle ont ete revelees lorsqu'on a tente de trouver une methode de construction rationnelle pouvant etre utilise dans le cas de structure...

Residual strength of clays in landslides, folded strata and the laboratory

Abstract: The post-peak drop in drained shear strength of an overconsolidated clay may be considered as taking place in two stages. First, at relatively small displacements, the strength decreases to the ‘fu...

Permeability tensor for discontinuous rock masses

Abstract: Rock masses, which commonly contain a large number of discontinuities, are treated as homogeneous, anisotropic porous media to formulate the corresponding permeability tensor. This has been successfully achieved by introducing a symmetric tensor (crack tensor) which depends only on the geometry of the related cracks (aperture, size and orientation). The principal directions associated with the symmetric crack tensor are coaxial with those of the permeability tensor. The first invariant of the crack tensor is proportional to the mean permeability, while the deviatoric part is related to the anisotropic permeability. These results are well supported by numerical experiments on the permeability of cracked media by Long, Remer, Wilson and Witherspoon. An actual rock mass (moderately jointed granite) was studied to see whether the crack tensor can be determined in situ. Stereology, based on geometrical statistics, provides a sound basis for determining the crack tensor in situ. The crack tensor is obtained by ...

Stress–strain–strain rate relation for the compressibility of sensitive natural clays

Abstract: Four types of oedometer tests (constant rate of strain tests, controlled gradient tests, multiple-stage loading tests and creep tests) were carried out on a variety of Champlain sea clays. Results obtained on five sites are used to demonstrate that the rheological behaviour of these clays is controlled by a unique stress-strain-strain rate relationship. This relationship can be simply described by two curves (σ-p′-ɛv and (σ′/σp′)-ɛv). The significance, the practical implications and the limitations of the model are also discussed. Quatre types d'essais oedometriques (a vitesse constante de deformation, a gradient controle, a chargement en plusieurs etapes et avec fluage) ont ete effectues sur diverses argiles marines Champlain. Les resultats obtenus sur cinq sites sont utilises pour demontrer que le comportement rheologique de ces argiles est regi par une relation unique contrainte-deformation/vitesse de deformation. Cette relation peut etre decrite de facon tres simple par deux courbes (σp-ɛv et (σv′/σp′...

Wave-induced stresses in unsaturated submarine sediments

Abstract: A simple method to solve the problem of pore pres-sure and effective stresses induced by harmonic waves in a poroelastic sediment is presented by using Biot's equation and a compatibility condition for elasticity. The sediment is assumed to obey the effective stress principle in the normal definition for saturated sedi-ments, although it may contain gas bubbles occluded in the porewater. The governing equation is a fourth-order partial differential equation in terms of pore pressure or effective mean normal stress. The solutions contain Skempton's pore pressure coefficient B. For unsaturated sediments, with B < 1, the pore pressure and effective stresses induced by wave action consist of two parts. One part depends only on the wavelength and the other part depends on both the sediment and the wave characteristics. The second part is out of phase with the wave. When the submarine sediment is unsaturated, the wave-induced negative effective nor- mal stresses become greater under the trough of waves. A zone ...

Calibration of constitutive models for granular materials using data from biaxial experiments

Abstract: The predictive capability of a class of rate-type constitutive equations for describing the behaviour of frictional dilatant materials is tested in plane strain compression experiments. The examination is restricted on shear band bifurcation states, where small deviations of the deformations from rectilinear extensions can be assumed. For predicting correctly both the state of bifurcation and the shear band inclination, the incipient shear modulus parallel to the geometric axis has to be estimated as the modified secant modulus. In addition a second hardening modulus, related to incipient contraction during the process of shear band formation, must be considered. On teste la possibilite de prevoir le comportement de materiaux possedant frottement et dilatance a partir de quelques lois rheologiques du type rate. On se limite au cas d'un etat de bifurcation avec bandes de cisaillement, ou l'on peut supposer que les deviations par rapport a l'extension rectilineaire sont petites. Pour prevoir correctement l'...

A constitutive model for the stress–strain–time behaviour of ‘wet’ clays

Abstract: A constitutive model is developed to describe the stress–strain–time behaviour of ‘wet’ clays subjected to three-dimensional states of stress and strain. The model is based on Bjerrum's concept of total strain decomposition into an immediate (time-independent) part and a delayed (time-dependent) part generalized to three-dimensional situations. The classical theory of plasticity is employed to characterize the time-independent stress–strain behaviour of cohesive soils using the ellipsoidal yield surface of the modified Cam Clay model presented by Roscoe and Burland. The time-independent strain is divided into an elastic part and a plastic part. The plastic part is evaluated using the normality condition and the consistency requirement on the yield surface. The time-dependent (creep) component of the total strain is evaluated by employing the normality rule on the same yield surface as in the time-independent model and the consistency requirement which requires that the creep strain rate reduces to phenome...

Induced and inherent anisotropy in sand

Abstract: Data from an extensive series of drained tests on dense sand in a directional shear cell have been used to explore the interaction of inherent and induced anisotropy. The essential link between previous strain and induced anisotropy has been demonstrated. This link is shown to hold regardless of any inherent anisotropy or previous cyclic rotation of principal stress directions. In dense sand a small permanent shear strain of the order of 0·5% leads to significant induced anisotropy. Induced anisotropy is shown as causing very large directional variations in stiffness of sand and associated capability for porewater pressure generation. The underlying causes of inherent and induced anisotropy are discussed and related to the widely recognized persistence of inherent anisotropy which frequently controls peak shear resistance. Possible methods for predicting the effects of anisotropy are briefly discussed. Des donnees obtenues a partir d'une serie grande d'essais draines effectues sur du sable dense dans une ...

Geotechnical model tests of bearing capacity problems in a centrifuge

Abstract: The Paper introduces the centrifuge research activities undertaken by the soil mechanics group at the Tokyo Institute of Technology during the past ten years and focuses on the bearing capacity of shallow foundations in dense sand. A series of experiments was conducted to explain de Beer's scale effect and to investigate the effect of the roughness of the footing base, the anisotropy in dense sand, the embedment of footings and the effect of slopes near footings. Measurements of deformations were attempted during loading tests using an automatic tracking device developed by the group and radiographs were used to detect slip lines. Progressive failure was found to be more marked for wider footings, which is considered to be the main cause of the scale effect; the smaller bearing capacity factors occur for wider footings. It is shown that strong anisotropy in dense sand is also responsible for the scale effect. A smooth footing, made by lubricating the base with a greased membrane, yielded two symmetrical w...

Some fall-cone tests

Abstract: Fall cone tests have been used to determine the liquid limit of cohesive soils, and estimations of strength and sensitivity have been possible by making correlations with other strength measurements. Recent theoretical analyses allow a direct estimation of strength from fall-cone tests. Results of some fall-cone tests performed with cones of different angles are presented together with results of some vane strength tests. The relation between cone penetration and strength is compared with theoretical predictions and with earlier experimental correlations. (TRRL)

Soil models in offshore engineering

Abstract: The offshore activities in the North Sea have been a very demanding but fruitful challenge to geotechnical engineering for more than a decade now. It soon became evident that there was an urgent ne...

Low Gradient Permeability Measurements in a Triaxial System

Abstract: Permeability measurements were conducted with the flow-pump method on sand, sandy silt and silty clay specimens in a conventional triaxial system by introducing and withdrawing water at known constant flow rates into the base of a specimen with a flow-pump, and by monitoring the head difference induced across the length of the specimen with a sensitive differential pressure transducer. The results show that the previously reported advantages of the flow-pump method, compared with conventional constant head and falling head methods, were realized for permeability measurements in conventional triaxial equipment. These advantages are that direct flow rate measurements are avoided together with the associated errors that arise from the effects of contaminants on capillary menisci and the long periods of time involved in flow rate measurements; permeability measurements can be obtained much more rapidly and at substantially smaller gradients; errors from the small intercept in the otherwise linear flow rate ve...

Comparison of frictional material models with respect to shear band initiation

Abstract: The existing elasto-plastic models of geomaterials have been developed on the basis of experiments in which uniform straining was attempted. The shear bands observed in experiments have been described directly without direct relation to the elasto-plastic models. However, shear band initiation can be considered as an instability of plastic flow; consequently the material behaviour under uniform deformation and the occurrence of a shear band can be considered as coupled phenomena. In this Paper the coupling relations are elaborated and applied in the calculation of shear band initiation for a range of popular constitutive models of materials like drained clay, sand, concrete and rock. It is found that amongst the popular elasto-plastic models large variations in the predicted slope of the shear band and the predicted instant of initiation occur. Les modeles elasto-plastiques des sols existants ont ete developpes sur la base d'essais dans lesquels une deformation uniforme est imposee. La formation des surfa...

Yield of a soft, high porosity rock

Abstract: A series of triaxial compression tests was carried out on air-dried samples of Bath Stone, a soft, high porosity, oolitic limestone. Tests were carried out to investigate the brittle, transitional ...

A note on sedimentation and consolidation

Abstract: An extended equation is derived which governs the sedimentation and simultaneous consolidation of pelagic sediments. An example of the implementation of this theory is also given in this note. (TRRL)

Simulation for risk analysis with correlated variables

Abstract: A Monte-Carlo simulation technique for geotechnical risk analysis with correlated random variables is presented. The proposed method is illustrated by an example of time required for water-table draw-down near a box cut of an open strip coal mine. The permeability and storage coefficient are considered as correlated stochastic variables. The results are compared with those achieved by using Rosenblueth's point estimate method for finding statistical moments as well as the well-known Taylor series approach. Excellent agreement is achieved for the first two moments. For higher moments the Rosenblueth method provides a better approximation to the simulation solution than does the Taylor series method. The role of a valid risk analysis technique for correlated variables is discussed and the application of the Paper to stability analysis of slopes is highlighted. L'article presente une technique de simulation type Monte-Carlo pour l'analyse geotechnique des risques avec des variables aleatoires correlatives. L...

Centrifuge scaling considerations for fluid-particle systems

Abstract: Two simulations are involved when a centrifuge is used to test models. First, the behaviour of the model in a uniform ng field is assumed to be similar to that of the prototype. Then the centrifuge is assumed to produce an equivalent ng gravitational field. For most static problems, the centrifuge does produce an equivalent ng gravitational field, but for some dynamic problems involving saturated soil these assumptions can break down. When the soil particles and fluid are moving relative to one another, the behaviour in the ng field is not similar to that in the 1g field unless the Reynolds number in both conditions is less than unity. Since this is a special circumstance, the centrifugal behaviour is not similar to that of the prototype in most cases. To illustrate this, the similarity requirements are examined for a single particle moving in a fluid. If different fluids are used in the model and prototype, then the difference in densities must also be accounted for.

A Pore pressure and stability model for a silty clay under repeated loading

Abstract: The behaviour of saturated clays subjected to low frequency cyclic loading is of considerable importance in the design of offshore structures. Monotonie strain-controlled and low frequency (0·1 Hz) repeated load triaxial tests have been carried out on reconstituted samples of Keuper Marl, a silty clay, under undrained conditions. The strain-controlled tests were carried out at overconsolidation ratios of 1, 4, 10 and 20 and the data from these tests were used to establish the stable state boundary surface. Results from repeated load tests on samples with overconsolidation ratios of 1 and 4 were compared with this boundary surface. Previous investigations have shown that the dry side of the state boundary surface for clays, sometimes known as the Hvorslev surface, forms an empirically determined stability criterion for cyclic effective stress paths of samples of all stress histories. Normally consolidated and lightly overconsolidated soils subjected to undrained cyclic loading become heavily overconsolidat...

Non-uniformity of triaxial samples due to consolidation with radial drainage

Abstract: This note describes a limited series of tests in which 38 mm diameter triaxial samples of kaolin clay were consolidated isotropically with radial drainage and the variation in water content across the diameter of the sample was measured. The results showed that significant non-uniformities can occur during consolidation of soil samples in routine triaxial tests in which side drains are used. A consolidation increment of 200 kpa gave rise to non-uniform water contents which imply substantial variations in effective stress. The problem can be avoided by removing radial drainage or it can be minimized by slow drained loading if radial drainage is necessary. (TRRL)

The effect of wall stiffness on the behaviour of a propped retaining wall

Abstract: In this note numerical analysis has been used to investigate the effect of wall flexibility on the behaviour of a propped embedded retaining wall. It has been shown that for flexible walls of the sheet pile type the predicted bending moments are much lower than that given by the simple limit equilibrium type of design calculation. As the stiffness of the wall increases then so do the bending moments. For walls installed in low k0 (=0.5) soils the bending moments approach the limit equilibrium values as the wall stiffness increases. These results are in full agreement with the model tests of Rowe (1952). However, for stiff walls in high k0 (>1) soils the bending moments greatly exceed the limit equilibrium values. A similar trend is observed for the prop forces. The results imply that for a diaphragm or secant pile wall in stiff clay with a high k0 the bending moments and prop forces could exceed design values based on the limit equilibrium type of design calculations. Curves showing the variation in the ratios of: (a) maximum bending moment to that predicted by limit equilibrium; (b) the prop force to that predicted by limit equilibrium, with wall stiffness for various in situ k0 values are given. These may be useful for preliminary design. (TRRL)

Elastic settlement of arbitrarily shaped foundations embedded in half-space

Abstract: The settlement of foundations under working loads which induce relatively small strains in the soil is frequently estimated in practice on the basis of results from the theory of elasticity. Despite its shortcomings in modelling actual soil behaviour, elastic theory may be quite useful especially in predicting immediate settlements on saturated clayey deposits, provided that an appropriate value of undrained secant Young’s modulus has been experimentally determined. Most of the available elastic solutions refer to loads acting directly on the ground surface. To estimate the settlement of an embedded foundation without resorting to expensive numerical (mainly finite element and boundary element) techniques, engineers apply reduction factors to the settlement of the corresponding surface foundation. While most available reduction factors are purely empirical in nature, an approximate solution by Fox (1948) has been particularly popular for embedded flexible rectangular foundations. Fox’s solution has been reproduced in the form of a simple chart by Janbu, Bjerrum & Kjaernsli (1956). This chart is still widely used in geotechnical engineering, being recommended in several textbooks of soil mechanics and foundation engineering. Unfortunately, as Burland (1970) and Christian & Carrier (1978) have shown, Fox’s factors may grossly exaggerate the effect of embedment, when compared with the results of finite element analyses. This is hardly surprising: Fox’s results, based on Mindlin’s solution for a point load within a half-space, refer to a uniformly loaded area surrounded by (and bonded to) an elastic continuum, rather than to an embedded foundation. That is, Fox’s results implicitly assume that part of the carried load is transmitted to the ground through tension between the upper side of the foundation mat and the overlying soil. However, no (net) tension can usually develop between soil and foundation and, in most cases, the ‘overlying’ soil has been removed by excavation. The results of the studies by Burland and

Centrifuge tests of embankments on strengthened and unstrengthened clay foundations

Abstract: Centrifuge tests were conducted to study the behaviour of embankments on soft clay foundations prepared with an overconsolidated crust over normally consolidated kaolin clay. Tests were performed with embankments constructed during flight in stages with measurements of pore pressures at selected locations and displacements using a marker grid. This Paper concentrates on two tests in the series. One test was performed with the clay foundation strengthened using granular columns; the other test had an untreated foundation. In the test using an untreated foundation the embankment failed at a height of 11·6 m. The strengthened foundation supported an embankment of 13 m without failure. Large lateral displacements were measured in unstrengthened clay during consolidation periods and it was found that large displacement increases are a good indicator of a factor of safetybelow about 1·3. At corresponding loadings the use of strengthened foundations decreased lateral and vertical movements by about 50%. Total st...

Salt heaving in the sahara

Abstract: Salt damage to pavements has been studied on the roads of the Algerian Sahara and particularly on the runway of the old airport of Adrar. Salt damage results from the heaving of the priming coat an...

The efficiency of partially penetrating vertical drains

Abstract: The Paper deals with the problem of consolidation of a clay layer in which vertical partially penetrating drains have been installed. The influence of well resistance is considered in the analysis, which is performed by means of the finite element method. The drainage pattern for each drain is assumed to be axisymmetric. Diagrams of the average degree of consolidation versus time for some selected non- dimensional parameters are presented. These dia-grams indicate, for partially penetrating drains, that the delay in the consolidation process within the zone penetrated by drains (due to higher excess porewater pressure in the underlying undrained zone) is noticeable only to a very limited height above the drain tips. In the case studies presented the delay is negligible to a depth corresponding to at least 80% of the partially penetrating drain length. L'article concerne le probleme de la consolidation d'une couche d'argile dans laquelle on a installe des drains verticaux qui penetrent partiellement. L'inf...

Dynamic penetration resistance and the prediction of the compressibility of a fine-grained sand - a laboratory study

Abstract: Dynamic penetration tests, such as the standard pene-tration test, are widely used in conjunction with em-pirical or ad hoc relationships to determine the com-pressibility of granular soil under foundations. This Paper reviews those factors which control the com- pressibility of sand, and those which are thought to influence dynamic penetration resistance. The results of a laboratory study of the compressibility and dynamic penetration resistance of an uniform fine sand at a single density are presented. It is shown that while the compressibility of the sand is strongly dependent on its previous stress path, and is relatively unaffected by its current stress level, dynamic penetration resis-tance is strongly influenced by current stress level and is unaffected by stress path. Des essais dynamiques de penetration, tel que le ‘stan-dard penetration test’, s'emploient tres frequemment en combinaison avec des rapports empiriques ou specialement elabores dans le cas particulier pour determiner la compressibili...