Showing papers in "Geotechnique in 1978"

Wave-induced pore pressures and effective stresses in a porous bed

Abstract: A general theory for the pore pressures and effective stresses induced in a porous bed by ocean waves is developed. The pore fluid as well as the soil skeleton are considered compressible and the f...

The behaviour of axially and laterally loaded single piles embedded in nonhomogeneous soils

Abstract: An approximate elastic analysis is made of the working load responses of single piles embedded in a soil of linearly increasing modulus with depth. The formulation employs, within a boundary element algorithm, the fundamental solution for point loads acting at the interface of a two-layer elastic half-space. This analysis is of comparable generality and computational efficiency to earlier elastic solutions of the corresponding homogeneous problem and has distinct advantages in practical applications over the simpler homogeneous soil model, namely, improved predictions of the response due to lateral loads and the rationalization of the selection of soil moduli. These advantages are highlighted in the comparisons made between reported observations on the behaviour of single piles from model and full scale field tests and those predicted by the theory. Comprehensive plots of the results of a parametric study are presented in non-dimensional form. Une analyse elastique approximative est faite a partir des rep...

A double hardening model for sand

Abstract: An elastoplastic model is established for the initial loading, unloading and reloading of sand. The plastic strains are described as the sum of two independent parts. The most important part of the plastic strains is described by means of a shear yield surface and a non-associated flow rule in which Rowe’s stress-dilatancy equation is incorporated. During continued deformation the shear yield surface changes not only in size but also in shape. Close agreement is obtained with experimental data on yield surfaces. The other part of the plastic strains is purely volumetric. This part accounts for the plastic volume strain in isotropic compression. It cannot be disregarded, particularly in the case of loose sands. Un modele elastoplastique est etabli pour le chargement initial, le dechargement et le rechargement de sable. Les deformations plastiques sont decrites comme la somme de deux parties independantes. La partie la plus importante des deformations plastiques est derite a l'aide d'une surface de cisaille...

Theoretical analysis of the stability of slopes

Abstract: The problem of limiting equilibrium of a slope in a state of plane strain is formulated in terms of the variational calculus. Formulated that way, the analysis is carried out without any a priori assumptions with respect to the shape of the slip surface, or the normal stress distribution along it. Thus, on the basis of only a formal definition of the concepts of limiting equilibrium, and factor of safety with respect to strength, it is proven that the minimal factor of safety must occur on slip surfaces with a special geometrical property. This geometrical property ensures that the resultant of the infinitesimal normal and frictional forces either pass through a common point or are parallel to a common direction. It is shown that as a result of this geometrical property the minimal factor of safety is independent of the normal stress distribution along the critical slip surface. In the homogeneous and isotropic case the analysis shows that the critical slip surface may be either a log-spiral or a straight...

Gypsum and anhydrite in foundations of hydraulic structures

Abstract: There have been many recorded cases of distressed dams caused by the solution of anhydrite or gypsum from their foundations. The solution processes have now been examined quantitatively and predictive methods put forward on the basis of laboratory measurements. Solution mechanisms have been established and specific solution rates determined which lead to the conclusion that gypsum tends to be less dangerous than anhydrite. Gypsum, whether in massive or particulate form, dissolves producing caverns and/or progressive settlements; accelerating seepage flows and accompanying deterioration of foundations are unlikely if provision is made to keep the initial seepage flow rates to low values. However, conglomerates cemented by gypsum are particularly hazardous. Anhydrite can dissolve dangerously so that rapidly accelerating seepage flows occur within a few years unless there are rigorous control procedures. A rational form of site investigation is proposed for suspect foundations. Il y a eu beaucoup de cas de r...

Effect of inclusion properties on the behaviour of sand

Abstract: The Paper describes the findings of plane strain unit cell tests on Leighton Buzzard sand with and without inclusions of aluminium foil, aluminium mesh and a non-woven melt bonded hetrofilament fabric. The results are analysed in terms of boundary stresses and strains and also internal deformation patterns. These data indicate that reinforced earth is a limiting condition where ideally inextensible materials are included in soils to strengthen them and inhibit both internal and boundary deformations. Another limiting condition is where ideally highly extensible materials are included. The composite system is in this case termed ‘plysoil’ as it exhibits some strengthening but more importantly has greater extensibility and smaller losses of post peak strength compared to sand alone or reinforced earth. This alteration to the stress-strain behaviour of the sand is shown to be a consequence of alterations to the internal deformations induced by the presence of the extensible inclusions. From this understandin...

The analysis of flexible raft-pile systems

Abstract: An analysis has been developed to predict the behaviour of a raft-pile foundation system. The analysis considers the system as a flexible elastic plate supported on a group of compressible friction piles, and the supporting soil is represented as an elastic homoeneous or non-homogeneous material. The ultimate load capacity of the piles is taken into account by a ‘load cut-off’ procedure. The analysis is used to establish the effectiveness of the pile group in reducing settlement of the raft and the effects of raft flexibility and size, and pile group parameters are considered. Two raft-pile systems, namely, the La Azteca building, Mexico City, and the Hyde Park Cavalry Barracks, London, are reanalysed. There is encouraging agreement between measured and predicted settlements and pile loads. Une methode a ete developpee afin de prevoir le comportement d'un systeme de fondation par radier general surpieux. L'analyse considere le systeme comme une plaque elastique flexible supportee par un groupe de pieux fl...

A model for the behaviour of overconsolidated soil

Abstract: A mathematical model is formulated to represent the stress-strain behaviour of overconsolidated soil. The full range of behaviour from lightly to heavily over-consolidated states is covered. The basis of the model is the utilization of a constitutive equation for a work-hardening plastic material. The model fits into the general framework provided by the critical state view of soil behaviour. As with the other critical state models four parameters characterize a particular soil. These are determined from routine laboratory tests. The object of the model is to enable good qualitative predictions of a wide range of soil behaviour to be made using only four easily measureable parameters. The Paper is confined to modelling behaviour under the stress conditions imposed in the conventional triaxial apparatus and oedometer, but extension to more general stress systems is possible. In Part I, the model is developed for material with a spherical past stress history; in Part II, it is extended so that loading from ...

Coeffcient of consolidation by the velocity method

Abstract: The velocity method described is based on reducing the curve representing the Terzaghi consolidation rate and an exponential settlement-time curve from oedometer tests to a form in which they are geometrically identical. The classical solution of the consolidation equation is given as a velocity plot and all experimental behaviour must be geometrically similar except for secondary compression. This curve may thus be used as an overlay for any experimental curve. Both curves are plotted to a double logarithmic scale and relationships can then be established by direct super-position of the two curves. The velocity method described thus overcomes the disadvantages of the traditional log t and t 90 methods that require graphical constructions that are not amenable to automatic processing. (TRRL)

Kinematics of the mass flow of granular material through a plane hopper

Abstract: A kinematically admissible velocity field for the mass flow of a granular material in a plane converging hopper is presented in this Paper. The proposed solution is based on assumptions suggested by the flow pattern observed during experimental work. In particular, the rupture surfaces observed during flow are incorporated into the solution. The material filling the hopper is treated as plastic, though different flow rules are specified for difference areas of the material. Stresses are not considered because the static counterpart of the kinematic solution does not contribute to the approach adopted. The main aim of the work is to provide a realistic mathematical model for understanding the velocity fields observed in experiments, rather than a complete solution based on simplifying assumptions. A criterion is suggested which may govern the transition from mass to core flow as the head of material in the hopper is reduced. Un champ de vitesse cinematiquement admissible in-situ pour l'ecoulement d'une mas...

The rate of swelling of overconsolidated clays subjected to unloading

Abstract: Time-rate of swelling under one-dimensional drainage and either one-dimensional or isotropic deformation conditions was investigated using reconstituted specimens of Duck Creek. Crab Orchard, Cucaracha aid Bearpaw shales. Cylindrical specimens were used, drainage was allowed from the top, and pore water pressure was measured at the base of all specimens. Load-deformation-history variables included maximum pressure (80 psi, 500 psi and 1500 psi), overconsolidation ratio (1.07–250), decrement ratio (0·07–15·7), and sustained secondary swelling (up to 3 log time cycles). The swelling index, Cs, is a function of the overconsolidation ratio, OCR. Cs, changes with effective stress and the rate of excess negative pore pressure dissipation is very sensitive to the changes. Secondary swelling can be very sig nificant. The ratio of secondary swelling index, Cas, to Cs, can be as high as 0·4. The maximum value of 0·4 for Cas/Cc, contrasts with the maximum observed value of 0·1 for Cα/cs, for consolidation. However, ...

The displacement field due to a point load at the interface of a two layer elastic half-space

Abstract: Displacements in both layers of a two-layer elastic half-space, due to horizontal and vertical point loads at the layer interface, are obtained by integration of infinite series solutions derived by the Integral Transform technique. Rapid convergence assures accurate and efficient evaluation of displacements in all cases. The solution can be used as a fundamental unit solution to some problems of soil and rock mechanics. Les deplacements dans l'une et l'autre des couches d'un espace semi infini elastique constitue par deux couches, sous l'effet de charges ponctuelles, horizontale et verticale, appliquees sur l'interface, sont obtenus par integration de series de solutions infinies derivees de la technique de Transformation Integrale. Une convergence rapide assure une evaluation precise et efficace des deplacements dans tous les cas. La solution peut-ere utilisee comme une solution specifique d'unite fondamentale pour certains problemes de mecaniques des sols et des roches.

The behaviour of Drammen Clay under cyclic loading

Abstract: A model for the behavour of clay under static and cyclic loading is developed, based on existing test results for Drammen Clay. The static part of the model is an adapted version of ‘Modfied Cam Clay’. The effects of cyclc loading are described in terms of a single fatgue parameter, namely the pore-water pressure generated by cyclic loadng. The cyclic part of the model comprises three basic formula, and contains only four parameters. It has been formulated throughout in terms of effective-stress and strain invariants, and can be applied to arbtrary loading histories, for clay elements with arbitrary initial states or consolidation histories. Un modele pour le comportement de I'argile sous chargement statique et cyclique est developpe, base sur des resultats d'essais existants, pour I'argile de Drammen. La partie statique du modele est une version adaptee de ‘L'argile de Cam’. Les effets de chargement cyclique sont decrits sous forme d'un seul parametre de fatique, a savoir pression de I'eau interstitielle...

Ground supports for tunnels in weak rocks

Abstract: The various factors and how they influence the loading and deformation of supports needed in deep tunnels in weak rocks are presented. Broadly, these are the rock and water conditions and their response to excavation, the excavation and support timings and procedures, and the support details and their workmanship. Particular attention is drawn to the need to consider the truly three-dimensional influence of rock discontinuities on excavation collapse mechanisms with the aid of models and field observation as a guide to support and safety requirements; to consider during construction the positioning and timing of support placement in relation to the rate of advance of the face (another three dimensional effect); to allow a degree of rock yielding by means of yielding supports on the basis of concepts developed from simple yielding rock/support interaction theory combined with construction monitoring; and to consider quantitatively the real load-deformation properties of the structural supports and their in...

The development of centrifugal models to study the influence of uplift pressures on the stability of a flood bank

Abstract: A series of small-scale two-dimensional models has been tested on the Cambridge geotechnical centrifuge. The models represent a clay flood embankment built on a clay foundation layer that overlies a pervious granular layer. High water pressures in the pervious layer represent the uplift pressures that are known to act on Thames flood embankments. The models have shown that the uplift pressures may modify the nature of the most critical failure mechanism, but do not appear to affect greatly the factor of safety against collapse. The mechanism of failure observed is thought to have contributed to the breach at Dartford Lock in 1953. A simple analysis is proposed, derived from consideration of the failure mechanism observed in the models, which promises to afford a rational design approach for landside loading berms where uplift-interaction failure is thought to be a danger. Une serie de modeles bidimensionnels d'echelle reduite a ete sur la centrifugeuse geotechnique de Cambridge. Les modeles representent u...

The filtration behaviour of construction fabrics under conditions of dynamic loading

Abstract: The use of a construction fabric at the subgrade/sub-base interface in road construction, for the prevention of upwards migration of clay particles into the sub-base is considered. This article describes tests using a cohesive subgrade soil, a number of fabric types, and a single sized 20 mm aggregate as sub-base material. The soil was compacted in a steel box, covered with the fabric to contain the aggregate which was subsequently saturated. The stress normal to the place of the fabric was varied sinusoidally for 24 hours. The aggregate contamination due to upward movement of the clay particles was estimated. Test results for various fabrics, and a no-fabric control test are tabulated and discussed. (TRRL)

A study of the variation and effects of water pressures in the pervious strata underlying Crayford Marshes

Abstract: Changes in piezometric pressure in an array of piezometers installed in a layer of sandy gravel under the alluvium in Crayford marshes, on the south side of the River Thames, have been measured over a number of tide cycles. An analytical model has been developed which reproduces these variations with reasonable accuracy. Using the model the aquifer parameters have been determined from the measurements made over normal tides. These parameters have been used to predict the increase in pore water pressure due to a storm surge and the maximum piezometric pressures which are likely to be developed during a design tide having a return period of 1 in 1000 years in the year 2030. These estimates indicate that the uplift pressures which develop under the storm conditions could reduce the stability of both the flood defences and buildings constructed on the marsh. It is recommended that the flood defences should be reinforced on the landward side with large loading berms and that an average depth of 0·5m of heavy f...

Estimating foundation settlements in sand from plate bearing tests

Abstract: The use of a single extrapolation formula is shown to be inadequate in calculating foundation settlements in sand from test plate results. Any such extrapolation must take account of soil conditions. Four distinct soil conditions have been assumed and settlement ratios calculated as a function of foundation width by elastic methods and by a method based on assumed penetration test values. The range of settlement ratios calculated by these methods are shown to agree with published field values. The influences of water table and foundation excavation on settlement ratios are also examined. L'utilisation d'une seule formule d'extrapolation se revele insuffisante dans le calcul des tassements de fondations porees sur du sable, a partir des resultats d'essais a la plaque. N'importe quelle extrapolation doit tenir compte des conditions du sol. Quatre conditions de sol distinctes et ete envisagees, et des taux de tassement calcules en fonction de la largeur de la fondation par des methodes elastiques et par une ...

A one point cone penetrometer liquid limit test

Abstract: In 1975 the cone penetrometer test was introduced by British Standards Institution as the preferred method for determining the liquid limit of soils. This paper presents the results of examining 600 liquid limit (ll) cone penetrometer test results and the calculation of correction factors similar to those for the one point cassagrande test method. It is shown that a reliable estimate of the liquid limit can be obtained by determining the cone penetration at one moisture content, instead of at several as presently required by B S procedure, and adjusting this moisture content by means of the correction factor to that required to give a soil penetration of 20 mm, ie, the liquid limit. The conversion factors vary with soil plasticity, hence soils are grouped into those with low, intermediate, and high plasticity (ll 20-35, ll 35-50 and ll >50), thereby ensuring that 95 per cent of the one point results will be within 3.5 per cent of the actual 11 value. (TRRL)

A note on Skempton's A parameter with rotation of principal stresses

Abstract: This Paper proposes a system of defining the major and minor principal stress increments in the Skempton's (1954) pore pressure equation. Using this system, a unified trend emerges for use in analysing published data that are seemingly conflicting. The system is also shown to give a consistent interpretation, both theoretically and experimentally, to triaxial stress states involving principal stress rotation. The practical implications resulting from the study of this system are examined. Cette etude propose un systeme pour la determination des augmentations de contraintes principales majeure et mineure, dans l'equation de pression interstitielle de Skempton (1954). En utilisant ce systeme, une tentative d'unification en decoule pour etre utilisee dans l'analyse des donnees publiees qui sont apparement contradictoires. Le systeme parait aussi donner une interpretation satisfaisante theoriquement aussi bien qu'experimentalement, aux etats de contraintes triaxiales impliquant la rotation de la contrainte pr...

Soil compressibility as measured in the oedometer

Abstract: The evaluation of oedometer tests and the calculation of settlement can be based on the linear plot of the constrined modulaus D with consolidation pressure p. This plot has a U shape and presents the following advantages: it reveals the minumum value of D and the pressure where it occurs; it shows the range of pressures where the soil is strain softening; it illustrates one of the causes of the variation of cv with p; it is applicable to a wide range of soil types. The D against p model is compared with other current models of the variation of compressibility with pressure. It is in agreement with the widely accepted e against log p model but inconsitent with the exponential and linear models. The value of D which is needed for a settlement calculation can be estimated roughly from a settlement calculation can be estimated roughly from the natural dry density or, in the case of saturated soil, from the natural water content. The value of D which enters this calculation is the specific constrained modulus...

Drawdown in embankments

Abstract: The recession of the phreatic line in an embankment following sudden drawdown of a reservoir is studied. The non-equilibrium equation for flow in porous media is simplified and solved to produce general charts indicating the variation of the water surface with time. Charts for the cases of a battered cut and a triangular embankment are presented. The point of emergence of the phreatic line is considered and the relationship between the storage coefficient and coefficient of consolidation is explained. On etudie l'abaissement de la nappe phreatique dans un remblai a la suite d'un affaissement brutal d'un reservoir. L'equation de non-equilibre pour l'ecoulement dans un milieu poreux est simplifiee et resolue pour produire des graphiques generaux indiquant la variation de la surface de l'eau avec le temps. Des graphiques pour les cas d'une coupe inclinee et d'un remblai triangulaire sont presentes. Le point d'emergence de la nappe phreatique est considere et la relation entre le coefficient d'accumulation et...

The calculus of variations applied to stability of slopes

Abstract: These notes report a discussion on a paper by J. Revilla and E. Castillo. In that paper the authors presented a method, based on the theory of the calculus of variations, for the determination of the actual safety factor of a slope, and hence a more complete knowledge of the safety of the slope. The contributors to the discussion comment on the technique employed by the authors, and relate the results obtained by the use of the method to those put forward by earlier research workers in their considerations of the same problem. In particular, reference is made to the use by the authors of Janbu's simplified method for the stability presentation for frictionless soil, leading to the introduction of errors in subsequent calculations, and differences between the authors' results and those obtained application of this technique. The authors' reply to the discussion is appended, indicating that further research into the variation technique was in progress, and that the purpose of the paper was to demonstrate the importance of the calculus of variations technique. Janbu's method had been selected as an example of possible application of this technique. /TRRL/