Showing papers in "Geotechnique in 1995"

An elasto-plastic critical state framework for unsaturated soil

Abstract: Data from a series of controlled suction triaxial tests on samples of compacted speswhite kaolin were used in the development of an elasto–plastic critical state framework for unsaturated soil. The framework is defined in terms of four state variables: mean net stress, deviator stress, suction and specific volume. Included within the proposed framework are an isotropic normal compression hyperline, a critical state hyperline and a state boundary hypersurface. For states that lie inside the state boundary hypersurface the soil behaviour is assumed to be elastic, with movement over the state boundary hypersurface corresponding to expansion of a yield surface in stress space. The pattern of swelling and collapse observed during wetting, the elastic–plastic compression behaviour during isotropic loading and the increase of shear strength with suction were all related to the shape of the yield surface and the hardening law defined by the form of the state boundary. By assuming that constant–suction cross–secti...

Interpretation of bender element tests

Abstract: J. C. Santamarina, Georgia Institute of Technology, and M. A. Fam, University of Waterloo Bender elements have surfaced as versatile transducers for low-strain testing of soils in a variety of cells and load conditions. However, lack of guidelines leads to different implementations among laboratories. The authors present an interesting evaluation of effective length and travel time determination. The purpose of this discussion is to contribute complementary information, summarizing our experience with bender elements in various short-term and long-term tests (a typical installation and examples of application are described in Fam & Santamarina (1995)).

Stiffness of fine-grained soil at very small strains

Abstract: The stiffness of soil at very small strain Go is a useful parameter for characterizing the non-linear stress–strain behaviour of soil for monotonic loading and it is required for analyses of the dynamic and small strain cyclic loading of soils. Tests were carried out on fine-grained soils in a hydraulic triaxial cell fitted with bender elements and with local axial gauges. From the results of these tests simple expressions were obtained which describe the variation of G>o with current state in terms of the current stress and overconsolidation ratio. The parameters in these expressions were found to depend on plasticity index. The simple expressions for Go were found to apply generally at larger strains, with the values for the parameters also depending on the current strain. Values of Go measured in laboratory tests on reconstituted London clay agree well with values measured in tests on undisturbed samples and in field tests which make allowance for the different states in the various tests. La rigidite ...

Compression model for cohesionless soils

Abstract: A simple four-parameter elasto-plastic model describes the non-linear volumetric behaviour of freshly deposited cohesionless soils in hydrostatic and one-dimensional compression. It expresses the tangent bulk modulus as a separable function of the current void ratio and mean effective stress using natural strains. Specimens compressed from different initial formation densities approach a unique response at high stress levels—the limiting compression curve (LCC)—which is linear in a double logarithmic void ratio-effective stress space. The model describes irrecoverable, plastic strains which develop throughout first loading and represent mechanisms ranging from particle sliding and rolling at low stresses to crushing—the principal component of deformation for LCC states. The three input parameters describing plastic deformation can be readily estimated from a hydrostatic or one-dimensional compression test loaded to high stress levels; the elastic bulk modulus requires accurate small strain measurements in...

An experimental and theoretical study of the behaviour of a calcarenite in triaxial compression

Abstract: An experimental and theoretical investigation on the behaviour of a calcarenite subjected to various axisymmetric loading programmes is reported. The essential feature of the observed behaviour is the occurrence of a destructuration phase, which marks the transition from rock-like to soil-like behaviour. During this phase the state of stress remains constant, while strains increase steadily. Three phases can be distinguished: an initial elastic, a destructuration phase and a hardening or softening phase which ends on an ultimate state locus which is linear in the p'–q plane. The observed behaviour is more and more ductile for increasing confining pressures. In the softening regime the specimen is unstable. It is shown that by means of a mathematical model based on the theory of strain-hardening plasticity it is possible to describe mathematically the overall behaviour of the calcarenite in various types of triaxial compression test. Qualitative and quantitative accuracy is generally satisfactory, especial...

Slope stability analysis: a kinematical approach

Abstract: A stability analysis of slopes based on a translational mechanism of failure is presented. The collapse mechanism is assumed to be in the form of rigid blocks analogous to slices in traditional slice methods. The proposed analysis, although based on the kinematical approach of limit analysis, always satisfies the equilibrium of forces acting on all blocks in the selected mechanism. All slope stability analyses based on the limit equilibrium of slices can be interpreted in the context of their implicitly assumed collapse mechanisms. The static assumptions made are equivalent to assuming an arbitrary strength of the soil on interfaces between slices. Solutions to stability factor &ammaH/cfrom all analyses based on the limit equilibrium of slices fall into a relatively narrow range bounded by the solutions using the proposed analysis for two extreme assumptions of soil strength between the blocks. Solutions beyond this range obtained by any method of slices indicate unreasonable consequences when interpreted...

Analysis of coupled heat, moisture and air transfer in a deformable unsaturated soil

Abstract: A new theoretical formulation for the analysis of coupled heat, moisture and air transfer is presented, which is applicable to a deformable unsaturated soil. The approach proposed extends previous analyses of the coupled transport of heat, pore water and pore air to take account of the deformation behaviour of partially saturated soil. A numerical solution of the governing differential equations is achieved by the use of the finite element method as a spatial discretization technique coupled with a finite difference recurrence relationship to describe transient behaviour. Three isothermal test problems are then considered to model the behaviour of unsaturated soil under varying stress and suction conditions. Both heave and collapse due to wetting are simulated. The model is applied to experimental work performed by others for the case of heating of an unsaturated montmorillonite clay and is shown to be capable of producing quantitatively physically correct results. These indicate that strong interactions ...

Pile end-bearing capacity in crushable sands

Abstract: The occurrence and formation of crushable soils is a widespread phenomenon. Warm tropical seas give rise to deep deposits of biogenic skeletal sediments in coastal shelf areas, or exposed granitic ...

Yielding and critical state of a collapsible unsaturated silty soil

Abstract: Isotropic and anisotropic compression tests, and drained triaxial shear tests were performed on a collapsible unsaturated silty soil under various suctions. The results show that the concepts of yielding and critical state originally developed for saturated clays can be applied to this unsaturated material when the matrix suction is considered as an independent variable. Pre-yield and post-yield behaviour is clearly defined, the yield curve increases in size with suction, and the effect of suction on the critical state strength decreases with increasing net mean stress. Des essais de compression isotrope et anisotrope et des essais triaxiaux de cisaillement draine ont ete realises, pour differentes valeurs de succion, sur des cols silteux non satures effondrables. II apparait que les concepts d'etat limite et d'etat critique, initialement developpes pour les argiles saturees, sont applicables a ce type de materiau non sature si la succion matricielle est consideree comme variable independante. Les comport...

Theoretical investigation of the undrained stability of shallow submerged slopes

Abstract: The problem of the criteria of stability of the equilibrium state of an infinitely long slope, with constant inclination, is tackled and the conditions of spontaneous collapse of the slope are proposed. An experimental programme is described to define the stability locus for loose Hostun RF sand. The validity of the instability line concept introduced by Lade for isotropically consolidated virgin specimens is corroborated. It is further shown that this instability line can still be defined when specimens are anisotropically consolidated, although then the slope of the line is greater. It is shown that an instability line cannot be defined for specimens pre-loaded in drained conditions and subsequently reloaded undrained, as the level at which peak occurs in undrained conditions is dependent on the pre-loading history. The results are examined in the light of elastoplastic strain-hardening theory. By using a model to allow for mixed isotropic–kinematic hardening, all the experimental phenomena observed can...

Shear band formation in sand according to non-coaxial plasticity model

Abstract: The inadequacy of the ordinary flow and deformation theories of plasticity in explaining the experimental results on shear band formation in sand has led to the development of a consistent non-coax...

Block theory and its application

Abstract: Block theory is a geometrically based set of analyses that determine where potentially dangerous blocks can exist in a geological material intersected by variously oriented discontinuities in three...

Depositional and post-depositional geochemistry, and its correlation with the geotechnical properties of marine clays in Ariake Bay, Japan

Abstract: The mineralogy, geochemistry and geotechnical properties of clay samples in Ariake Bay, Japan are presented. The mineralogy and geochemistry are discussed in terms of depositional and post-depositional processes, and the correlations of the clay mineralogy and geochemistry with the geotechnical properties were examined by multiple regression analysis. The most predominant clay mineral in the deposits was found to be smectite. The vertical distribution of trace elements in the deposits was indicative of the periodical changes in the parent materials involved in the depositional processes. The chloride concentration in the pore water varied, according to a change in the sea level, from brackish to marine, and then brackish towards the surface. The higher iron oxide contents near the surface and in the deepest zones were the results of pyrite oxidation in the deposits due to weathering under subaerial conditions. Smectite content was the most predominant governing factor for the consistency limits and activi...

A method for the study of crack propagation in rock structures

Abstract: A method is presented for analysing the stability of rock structures based on the propagation of geological discontinuities. Discontinuities are modelled as flat cracks and their propagation is analysed using fracture mechanics methods. As rock structures are brittle, these phenomena can be studied using linear elastic fracture mechanics (LEFM). Problems in the application of LEFM methods are analysed, with regard to the states of compressive stress present in rock. A numerical procedure is described which simulates the propagation of natural discontinuities in rock; the crack propagation trajectories determined from this analysis are compared with experimental results obtained on mortar specimens. The applicability of the method to the study of rock mass stability is shown with reference to the behaviour of rock slopes. L'article presente une methode permettant d'analyser la stabilite de structures rocheuses. Cette methode est fondeve sur Petude de la propagation de discontinuites geologiques. Ces discon...

The intrinsic behaviour of a decomposed granite soil

Abstract: The intrinsic mechanics of a decomposed granite soil were investigated by triaxial tests at standard pressures and at high pressures up to 35 MPa to compare its behaviour in its saturated, partially saturated and dry states. In each case it was found that the behaviour could be described within a critical state framework with particle breakage as the principal means of plastic volumetric compression. The behaviour of partially saturated samples compacted at around the optimum water content was found not to differ appreciably from that of saturated samples. In contrast, tests on dry samples revealed a state boundary surface for the dry soil which exists outside that of the saturated soil. Sieve analyses showed this to result from the fact that the soil in its dry state was subject to a smaller amount of particle breakage. Although the critical state friction angle is not affected by the presence of water, the location of the state boundary surface at higher volumes/stresses for the dry soils results in the...

Quantifying soil microstructure using fractals

Abstract: This Paper develops the theoretical basis and presents the results of experimental studies to determine a fractal dimension associated with soil microstructure. The topics addressed include the development of techniques for obtaining images of real soils in a format suitable for analysis, the development of imageprocessing techniques for extracting the fractal characteristics of the image, the development of a rational analytical basis for determining the fractal dimension associated with soil microstructure, the analyses required to quantify the fractal dimension of the soil samples tested, and parameter studies to determine the effect of particle angularity, grain size distribution and sample density on the fractal dimensions of selected sands. L'article presente les bases th´oriques et les r´sultats exp´rimentaux permettant de d´terminer la dimension fractale associ´e a la microstructure d'un sol. Des techniques permettant d'obtenir des images d'un sol r´el sous un format utilisable et de traiter ces i...

Evaluation of in situ anisotropy from crosshole and downhole shear wave velocity measurements

Abstract: Downhole and cross hole shear wave velocity determinations are becoming a routine part of dynamic site evaluation studies. The in situ measurements are frequently performed in boreholes using a variety of techniques. A seismic cone system has been developed for determining both downhole and crosshole shear wave velocity. The system consists of a source and two receivers, all embodied in penetrometers. At any depth three different polarized sets of shear wave signals can be generated and are recorded by both receivers, located at different distances from the source. It has been suggested from laboratory tests in large chambers that the measured shear wave velocities can be used to evaluate the in situ stress conditions. Tests have been performed at research sites in Vancouver to evaluate the in situ stress dependence of the measured shear wave velocities. The results of this study suggest that variations in structural anisotropy are the predominant factors controlling the directional shear wave velocities,...

Bearing capacity of a foundation resting on a soil reinforced by a group of columns

Abstract: A new design method for a foundation on a soil reinforced by columns is described. A lower bound of the bearing capacity is determined within the framework of the yield design theory. It takes into account the three-dimensional nature of the problem and is applicable to a wide range of geometries. A parametric study on the improvement of the bearing capacity as a function of the proportion of reinforcement, and on the strength characteristics, is presented. A complete analytical solution is given for the strength of a composite cell subjected to a triaxial loading, which provides an insight into the reinforcement mechanism.

Geotechnical behaviour of saline sabkha soils

Abstract: The behaviour of salt-cemented sabkha soil was investigated by considering the effect of brine and distilled water on its properties. Laboratory and field testing programmes were conducted to examine the effect of soaking and leaching on the structure, compressibility, collapse, permeability, load-carrying capacity and settlement of sabkha soil. Results indicate that the percolation of distilled water through the sabkha causes destruction of the natural cementation, leading to collapse, increase in permeability, reduction in strength and increase in settlement. Dissolution and leaching of halite, gypsiferous and calcarenite cements occur, leaving the quartz particles covered by a thin loose mat of illitic clay with large voids. The collapse deformation of sabkha is inclusively linked to the ambient salt complex, which determines the structural properties of the soil and promotes the development of collapse on wetting. The soaking of sabkha produces practically negligible collapse; leaching causes a signif...

Some aspects of fabric anisotropy of soil

Abstract: An important feature controlling the elastic and plastic constitutive behaviour of a soil is the orientation of the particles. The overall behaviour of a soil on application of a load depends not o...

An approximate analysis of the three-dimensional effects of diaphragm wall installation

Abstract: The Paper investigates the three-dimensional effects of diaphragm wall installation in stiff clays by an approximate numerical analysis using two simplified plane section analyses. Horizontal displ...

Vertical loading experiments on rigid pad foundations at Bothkennar

Abstract: The Paper describes tests on instrumented rigid square pads performed at the EPSRC soft clay test site at Bothkennar in Scotland. The aims were to investigate bearing capacity and load-displacement behaviour under short-term and long-term conditions. The instrumentation and loading schemes also allowed the soil's in situ stress–strain, consolidation and creep properties to be compared with those established in high quality laboratory tests. There was generally good agreement. The results emphasize that non-linearity has an important effect on the behaviour of spread foundations, even when they are subject to relatively light loads. KEYWORDS: bearing capacity; case history; clays; compressibility; footings/foundations; full-scale tests; stiffness. L'article presente les essais realises sur des surfaces carrees rigides et instrumentees installees sur le site experimental `argiles molles' du ESERC Bothkennar, Ecosse. Le but est d'etudier la charge admissible et le comportement chargementdeplacement a court e...

Shaft capacity of displacement piles in a high OCR clay

Abstract: The Paper describes a series of field experiments using extensively instrumented closed–ended steel piles installed in heavily overconsolidated London clay. Each pile was equipped to measure the ef...

Analysis of downdrag on pile groups

Abstract: This Paper describes a new iterative procedure for the analysis of the downdrag forces in end-bearing pile groups. Special emphasis is placed on the rate of increase of downdrag forces with free field soil settlement. Pile–soil–pile interaction is modelled using Mindlin's elastic solution for a point load in an elastic half-space. Pile–soil slip is taken into account by limiting the shaft friction at the interface. Parametric studies were conducted to elucidate the parameters that govern the relationship between free field soil settlement and downdrag forces induced in a pile group. Results indicated that interior piles of a large pile group experience relatively small downdrag forces, especially when the pile spacing is small. The validity of the proposed method is demonstrated by back-analysing two model studies of negative skin friction on pile groups. The proposed method is also validated against the field test results of a 38 pile group. The computed results show an encouraging degree of agreement wi...

Pressure measurements in a rectangular silo

Abstract: The pressure exerted by the material stored in rectangular flexible-wall silos is investigated during filling. Tests on a large-scale steel model silo with a pyramidal hopper showed that the stress state of the material stored is influenced significantly by the wall flexibility and boundary conditions. The measured wall pressures were very different from pressures predicted by traditional theories and existing design codes. This is because most traditional theories were developed for rigid-wall circular silos and ignored the effects of interaction between stored material and structure. The causes of the observed pressure redistribution are explained. L'article etudie la pression exercee sur les parois flexibles d'un silo rectangulaire, tors du remplissage, par le materiau stocke. Les essais realises sur une maquette grande echelle d'un silo muni d'une tremie pyramidale montrent que l'etat de contrainte dans le materiau stocke est fortement dependante de la flexibilite des parois et des conditions aux limi...

Geotechnical materials and the critical state

Abstract: Comparison is made between the triaxial compres-sion behaviour of soil, soft rock and hard rock. On the basis of the evidence, it is argued that the brittle and ductile fields of rock behaviour can be interpreted with reference to the critical state framework originally developed for soils. Within this framework, the brittle—ductile transition in rock is the equivalent to the change in overconsolidated to normally consolidated behaviour found in soils. The critical states for soils and soft rocks represent the frictional plasticity of porous materials, whereas the equivalent critical state for hard rocks is heavily influenced by the cohesive plasticity present in mineral solids. A view of geotechnical materials emerges to emphasize the simiarity of soil and rock behaviour. This suggests that the critical state concept has application to all geotechnical materials. L'article etablit une comparaison entre le comportement en compression triaxiale d'un sol, d'une roche tendre et d'une roche duve. Au vu des re...