Showing papers in "Geotechnique in 1991"

The critical state of sands

Abstract: There is conflicting evidence regarding the critical state and steady state of sands. It seems that in some cases the critical state and steady state are taken to be equal and in others they differ...

Finite cavity expansion in dilatant soils: loading analysis

Abstract: An analysis of the quasi-static expansion of a cylindrical or spherical cavity in an infinite dilatant elastic-plastic soil is presented Closed form solutions for the stress and displacement fields in the soil during the expansion of the cavity are given The soil is modelled as linear elastic-perfectly plastic, using a non-associated Mohr-Coulomb yield criterion An explicit solution for the pressure-expansion relationship is obtained with no restriction on the magnitude of the deformation It is found, in particular, that the radius of the cavity increases indefinitely as the cavity pressure approaches a finite limiting value This limiting pressure can be determined analytically with the help of a single expansion of an infinite series The novelty of the new solution lies in the introduction of dilation to the analysis of large strain expansion Examples of the implications of the new analysis in geotechnical engineering are discussed L'article presente une analyse de l'expansion quasistatique d'une

An analytical study of the cone penetration test in clay

Abstract: This Paper describes an analysis of the quasi-static penetration of a cone penetrometer into clay. The clay is idealized as a homogeneous elastic–perfectly plastic material obeying tbe von Mises yi...

Settlements of shallow foundations on sand

Abstract: A method to evaluate settlements and rotations of rigid shallow foundations on sand under the combined action of inclined and eccentric loads is presented. Experimental results obtained on a model strip foundation are shown first. Next, a mathematical model is formulated which is based on the hypotheses that (a) the foundation and the soil can be considered as a macro-element for which the loadings act as generalized stress variables while the displacements and rotation of the foundation are the corresponding generalized strain variables; and (b) the constitutive law of the macro-element, that is the relationship between generalized stress and strain rates, is rigid-plastic strain-hardening with a non-associated flow rule. Constitutive functions and parameters are determined by means of simple calibration tests. Predictions of the theory are then compared with experimental results in tests where loadings vary in a complex way up to foundation failure. It is shown that experimental evidence is well matched...

A numerical investigation of the structure of persistent shear bands in granular media

Abstract: The structure of persistent shear bands in granular materials is investigated by numerically simulating an idealized assembly of two-dimensional particles. Flexible stress-controlled boundaries are...

Plane-strain compression experiments on water-saturated fine-grained sand

Abstract: This Paper presents the results from an experimental study of the pre-failure and post-failure behaviour of water-saturated fine-grained sand in biaxial compression. Tbe experiments were performed in a plane-strain apparatus which allowed a detailed study of bifurcation and post-bifurcation pbenomena. Emphasis is given to limiting states, deformation instabilities and failure phenomena (liquefaction and localization). A patterned failure mode is illustrated witb X-ray radiographs. Internal fluid flow and rate senstitivity in relation to mathematical stability are also discussed. Cet article presente les resultats d'une etude experimentale du comportement avant et apres rupture d'un ebantillon de sable fin sature soumis a une compression biaxiale. La cellule biaxiale maintient I'ecbantillon en deformation plane, ce qui permet une etude detailee des phenomhnes de bifurcation et de post-bifurcation. Dans cette etude experimentale, on a mis I'accent sur les questions d'etat limite, d'instabilite de deformatio...

Conduction phenomena: from theory to geotechnical practice

Abstract: Virtually all geotechnical problems involve the flow of fluids, chemicals and energy. A current review of the types of direct and coupled flow through soil is given and assessment of their signific...

Behaviour of offshore soils containing gas bubbles

Abstract: An extensive programme of research into the influence of undissolved gas bubbles on the behaviour of fine-grained onshore soils is reviewed. The programme has been based on the development of a laboratory technique for the preparation of reconstituted soil samples containing a uniform distribution of gas bubbles. The structure of these samples is similar to that observed in sediment recovered from the sea bed, and consists of large gas-filled cavities surrounded by a matrix of saturated soil. It is found that surface tension effects limit the difference between gas pressure and pore water pressure, and that the overall void size is effectively a function of the strength of the matrix, so that changes in void volume may be modelled by cavity expansion and contraction in an ideal plastic medium, leading to limits on the difference between gas pressure and mean total stress. A new parameter, operative stress, is shown to influence both the consolidation and the strength of these gassy soils. Thus, during con...

One-dimensional analysis of soil plugs in pipe piles

Abstract: Under static loading in compression, open-ended piles may fail in a plugged mode, with the soil plug moving with the pile, or in an unplugged mode, with shear failure occurring between the soil plug and the pile shaft. It may be shown that, under drained loading conditions, the former mode of failure will generally occur, because arching action within the pipe pile leads to high frictional capacity of the plug. However, under faster rates of loading relevant to the offshore environment, the increase in effective stresses within the soil plug is limited and the plug capacity is significantly lower. The Paper presents a simple one-dimensional analysis of the soil plug under partially drained conditions. The analysis has been implemented numerically, and the resulting program used to derive design charts which give the plug capacity as a function of the soil plug parameters and the rate of loading. These design charts are presented in appropriate non-dimensional form, with example calculations included for t...

Effects of installing 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

The chemical and mineralogical controls upon the residual strength of pure and natural clays

Abstract: Recent work has shown that the residual strength of clays is, to a large extent, determined by the type of clay minerals and the chemistry of the constituent pore water. Changes in either of these factors cause corresponding alterations in residual shear strength. The findings have important implications for the seasonal reactivation of landslides and may also provide an explanation for the long-term weathering and shear strength reduction of clay slopes. Results are presented here to demonstrate the separate effects of clay mineralogy and pore water chemistry upon the residual strength of pure and natural clays. Explanations are sought to identify the controls of these factors upon residual strength based on brief reviews of the shear strength and physico-chemical behaviour of clays. The findings strongly suggest that residual strength should be considered a dynamic property, with increases or decreases in strength occurring in response to environmental change. Des etudes recentes ont montre que la resis...

An assessment of chamber size effects in the calibration of in situ tests in sand

Abstract: Calibration chamber tests are currently used to establish correlations between in situ testing data and the engineering properties of sands, and play an important role in the understanding of in situ tests. Practical and economic considerations require that calibration chambers must be of limited size, and so the chosen boundary conditions have an important effect on the results. An assessment is made of the effects of chamber size on the measured values of the limit pressure obtained from the pressuremeter test and on the cone resistance values from the cone penetrometer test. Experimental and numerical data suggest that the effect of the proximity of the chamber boundaries is significant for dense and medium sand, and is observed even for loose sand. Les essais en chambre de calibration sont a present employes pour etablir les correlations entre les donnees des essais in situ et les proprietes des sables pour le calcul des fondations et jouent un role important dans l'interpretation de tels essais. Des ...

Continuum simulation of flow failures

Abstract: Dynamic aspects of several catastrophic slope failures have been evaluated by unsteady flow modelling, assuming two-dimensional (or quasi-three-dimensional) transient flow of an incompressible bivi...

Subsidence effects on clay barriers

Abstract: Centrifuge model tests have been performed to study the response of clay barriers subjected to differential deformations. The modes of deformation that have been observed are relevant to those that might occur as the result of differential settlements of waste material leading to deformation of cover liners, or from non-uniform soil strength profiles or the propagation of deep-sited subsidence towards the surface, leading to deformations of the base liner. Plane model liners were constructed both from pure kaolin and from a mixture of sand, silica flour and bentonite. The integrity and performance of these model liners were evaluated on a geotechnical centrifuge at 50 gravities. Physical degradation of the model liners was monitored photographically and their performance as effective hydraulic barriers assessed throughout the deformation process. For all the model liners where no overburden was present, tension cracking of the liner surfaces was observed. These tension cracks were very significant in the ...

Cyclic response of calcareous soil from the North-West Shelf of Australia

Abstract: This Paper reports the results from 23 cyclic CIU and CAU triaxial tests on undisturbed samples of calcareous soil. The samples, which include both uncemented and cemented material, have heen subjected to a range of mean and cyclic shear stresses. Results from the tests on the uncemented soil display many of the characteristics of the cyclic response of other cohesionless soils. The more cemented soils sustained higher cyclic stresses and, depending on the load level, showed either elastic behaviour with no pore pressure build-up, eventual stabilization after some pore pressure build-up, or steady state pore pressure build-up and eventual failure. The effect of frequency on the response in the range 0·1 to 10 Hz has heen studied. Since no dramatic differences in performance could he detected, no firm conclusions could be drawn regarding the effect of testing rate. L'article preente les reultats obtenus a partir de 23 essais triaxiaux cycliques de type CIU et CAU effectues sur des echantillons de sol calca...

Stiffness of granular soils from field performance

Abstract: In 1985 Burland and Burbidge carried out analyses on over 200 cases of foundations on sand and gravel and developed a new approach to forecasting settlement. This study re-examines these results in order to find out whether a consistent picture can be obtained to evaluate an operational stiffness for a continuous mechanics approach. The procedure is outlined and comprises the following stages: (a) evaluation of the average SPT (standard penetration test) blow count within the depth of influence; (b) estimation of the modulus number KE (0.1) using the average SPT blow count corrected for overburden pressure; (c) evaluation of the stiffness using Janbu's method and computation of the settlement by means of the elasticity equation, and (d) correction of the modulus number to allow for non-linearity. Back-calculated stiffnesses are higher than usually accepted for these kinds of deposits and it is suggested that other structural phenomena such as early diagenesis may be involved. (TRRL)

Behaviour of clayey sands under undrained cyclic triaxial loading

Abstract: The response of two clayey sands to undrained cyclic triaxial loading is described Both the stress history and the cyclic stress level have been varied The behaviour is compared with that observed for the clayey sands tested under monotonic loading Effective stress paths for the normally consolidated soils loaded monotonically in triaxial compression and extension are shown to form a hounding envelope which determines the pattern of behaviour under cyclic loading A threshold strain is identified, below which cyclic straining results in stable behaviour Above the threshold strain, pore pressures and strains accumulate under cyclic loading at a rate that increases as the effective stress state approaches the bounding envelope On reaching the bounding envelope in triaxial compression, the clayey sands strain-soften On reaching the bounding envelope in triaxial extension, there is liquefaction L'article decrit la reponse de deux sables argileux au chargement triaxial cyclique non-draine On a fait vari

Finite element analyses of an in situ wall propped at formation level

Abstract: The use of formation-level props to support an in situ retaining wall can result in a structural system which is stiff and remote from rotational failure. However, the interaction between the wail, the soil and a continuous prop slab cannot be analysed using simple techniques. Finite element analyses have been carried out to investigate some of the factors affecting the behaviour of an in situ wall, propped at formation level, retaining 9 m of stiff overconsolidated boulder clay. This Paper describes the results of a parametric study in which the effects of soil/wall/prop stiffness and the pre-excavation earth pressure coefficient were investigated. It is found that, because the wall is very stiff, computed deformations are governed by the assumed stiffness of the soil rather than the flexura! rigidity of the wall. Bending moments in the wall are influenced significantly by the assumed pre-excavation lateral earth pressures and, to a lesser extent, by the nature of the structural connection between the wa...

Accelerated physical modelling of hazardous-waste transport

Abstract: This Paper presents the results of a research programme conducted on the Cambridge University balanced-arm centrifuge, to study the long term transport of contaminants in the soil surrounding an engineered landfill site. The phenomena which govern the transport of hazardous waste through porous media are discussed, the principles of geo-technical centrifuge modelling are outlined, and relevant scaling laws that govern the relationship between the centrifuge model and its corresponding prototype, with respect to the problem of hazardous-waste transport, are presented. A centrifuge test, simulating two-dimensional migration of a conservative pollutant species from a landfill site, is described, and results from three such tests are presented. These results are compared with theoretical predictions from an existing one-dimensional transport code. L'article presente les resultats d'un programme de recherches effectuees sur la centrifugeuse a bras equilibre de l'Universite de Cambridge pour etudier le transpor...

Friction and passive resistance in soil reinforced by plane ribbed inclusions

Abstract: A theoretical analysis, supported by laboratory investigations, of stress transfer between soil and ribbed reinforcement has been performed to evaluate the individual contributions of friction and passive resistance to overall pullout resistance. The laboratory experiments consisted of direct shear tests of rigid ribbed inclusions with various rib spacings. Optical monitoring and a carbowax solidification technique assisted in the identification of failure surfaces. A plasticity model, based on Sokolovski's method, was evolved for a fully developed passive component of the pullout resistance. A parametric study revealed the significance of each of the model parameters. The optimum rib spacing is one which allows the maximum number of full passive zones to develop per length of reinforcement. For a rib height and width of 2·5mm, the optimum spacings were found to be 25mm for loose and 33mm for dense Ottawa sands. Une analyse theorique, basee sur des etudes faites en laboratoire, a ete effectuee du transfer...

An alternative framework for unsaturated soil behaviour

Abstract: the author re-analyses experimental data from experiments performed by toll in 1990, which described shear behaviour of unsaturated soil, in order to suggest an alternative framework for determining a critical state relationship using one less independent variable (the degree of saturation) it is claimed that this method is easier to use in predictive calculations theoretical analysis suggests there is no simple critical state relationship linking specific volume to the stress state variables two critical state relationships are derived for unsaturated soil; they can be represented graphically by two three-dimensional plots rather than the series of surfaces suggested by toll critical state values of deviator stress can be predicted for shearing with the water phase undrained but the air phase drained and for shearing with both air and water phases drained, corresponding to problems of immediate stability and long-term stability these two conditions of shearing are considered most likely to be of practical relevance the two simultaneous equations proposed can be solved because the specific water volume remains unchanged in the first case and values for pore air and water pressure are known in the second case This paper was discussed by Lins,AHP in Geotechnique (1992), 42(3), p525-526 The author's reply to the comments made is found on p526-527 of the same issue

Effects of drying on the engineering behaviour of Cochin marine clays

Abstract: Cochin marine clays are soft alluvial deposits with a high natural water content which pose considerable engineering difficulties. A peculiar aspect of these clays is that drying results in considerable changes in plasticity. This Technical Note attempts to link these variations to the clays' compressibility and shear strength behaviour. Tests were carried out on three states of soil - natural, air-dried and oven-dried to provide information on index properties, grain size distribution, compressibility and shear strength. The consequences of rewetting were also explored. The results are tabulated and presented graphically. It is found that drying has a significant effect on the index properties and compressibility and shear strength behaviour of the clays. It is suggested that the application of standard test procedures which recommend prior air-drying to these soils could be misleading. Normalising the soil data with respect to the liquid limit helped to provide a framework for examining consolidation and shear strength behaviour. (TRRL)

Horizontal drainage during consolidation: insights gained from analyses of a simple problem

Abstract: This paper analyses consolidation with radial drainage to concentric inner and outer fixed drainage boundaries. An analysis in which coupling between flow of pore water and effective stress change is ignored gives results that are not physically reasonable. An exact closed form solution can be developed to the equation of radial pore water diffusion in an elastic material and the results of this analysis are compared with a third, finite element analysis, using an elastic-plastic description of the soil. Elastic and elastic-plastic analyses are compared in terms of isochrones of pore pressure, radial displacement and stress change within the sample, and in terms of the strain and stress paths followed by different soil elements in the consolidating clay. Non-uniformities of effective stress state, and hence of soil stiffness, can develop during consolidation; the pattern of deformations that develops is completely dominated by the occurrence of plastic strains. The oedometer should not be regarded as a si...

Earthquake-induced shallow sildes in volcanic debris soils

Abstract: In March 1987, following two earthquakes and prolonged heavy rain, landslides in northern Ecuador caused extensive damage on slopes of 35–60°. The tectonics of the area and the characteristics of the soil are described. Attention is focused on the aspects of behaviour which are peculiar to volcanic debris residual soils which may affect the stability of such materials on slopes. An analysis of the slides shows the effects of components such as shear strength, pore water pressure and seismic loading. The presence of a cohesive component of strength related to bonding is confirmed. Conclusions are drawn as to the mechanisms of the slides which required the presence of both saturated soil and the earthquakes. Gaps in current knowledge on these soils and future research needs are identified. A la suite de deux tremblements de terre et de grosses pluies prolongees, des glissements de terrain ont provoque de grands dommages sur des pentes entre 35° et 60° dans les regions septentrionales de la Republique de l'E...

A reassessment of the causes of the carsington embankment failure

Abstract: New evidence relating to construction events and field measurements at the Carsington embankment reveals an unusual mechanism of articulated slips commencing within the base of the mudstone fill along shear planes built in at the centre of the valley from an early stage of embanking. While a berm delayed failure at the centre, the drag on the valley side where the berm terminated, coupled with built-in shear planes in the core hoot, brought the foundation of yellow clay in the valley side to the condition of critical state. During the next two weeks the foundation soil proceeded in a normal post-failure mechanism part-way to the residual, while the bank stability was maintained temporarily by a reversal in the direction of drag. Collapse at the side finally dragged the centre to failure, reversing the direction of articulation. Causation of the failure commenced with the use of smooth-wheeled rollers on clay fill which was wet of optimum and had PI > 20% without effective scarification, forming a smooth b...