Showing papers in "Geotechnique in 1988"

Simple method for dynamic stiffness and damping of floating pile groups

Abstract: A simple analytical solution is developed for computing the dynamic impedances of floating rigidly-capped pile groups with due consideration to pile-soil-pile interaction. The method introduces some sound physical approximations and considers the interference of cylindrical wave fields originating along each pile shaft and spreading radially outward. Axial, lateral, and rocking oscillations of rigidly-capped pile groups are studied parametri-cally. Results are presented for the dynamic stiffness and damping of the whole group, and for the distribution of dynamic loads amongst the individual piles. The predictions of the simple method for vertical and rocking oscillations compare extremely well with rigorous numerical solutions, thereby offering a valuable insight into the nature of pile-soil-pile interaction. It is demonstrated and explained how the dynamic efficiency may far exceed unity at certain resonant frequencies due to destructive wave interference. The proposed method can be readily applied by en...

Calibration chamber tests of a cone penetrometer in sand

Abstract: The results of a series of tests using a cone penetrometer in sand in a large calibration chamber are reported. Sand at three different densities and a range of stress states was studied. The principal finding from the tests is that cone resistance in sand depends primarily on the horizontal stress and the angle of friction but is relatively unaffected by the vertical stress. A relationship between qc, σh′ and φtc′ is proposed and compared with previous work in calibration chambers. L'article presente les resultats d'une serie d'essais effectues avec un penetrometre a cone dans du sable dans une chambre d'etalonnage. Le sable a ete etudie avec trois densites differentes dans un intervalle d'etats de contrainte. Les essais ont demontre surtout que la resistance au cone dans le sable depend d'abord de la contrainte horizontale et de l'angle de friction mais reste relativement peu influencee par la contrainte verticale. On propose une relation entre qc, σh′ et φtc′ et on la compare avec des recherches anteri...

Liquid limit of kaolinitic soils

Abstract: This paper elucidates the relative importance of soil structure and various compositional factors in influencing the liquid limit of natural kaolinitic soils. Earlier studies dealt with purified systems and anticipated that the liquid limit of the soils would increase with percentage clay size fraction and surface area, and that soils with a greater degree of paricle flocculation would possess a higher liquid limit than soils with a more parallel particle arrangement. The results revealed that the inter-particle attraction and repulsion forces have a prominent role in determining the liquid limit of kaolinitic soils. These forces determine the particle arrangement (clay fabric) which in turn regulates the liquid limit values. The influence of clay fabric was ascertained from the relationships of liquid limit with shrinkage limit and sediment volume (in water) values. It was anticipated that kaolinitic soils with a greater degree of particle flocculatin and a higher liquid limit would shrink less and occupy a higher sedimentation volume. As expected an increase in liquid limit was accompanied by an increase in shrinkage limit and sediment volume in water.

Three-dimensional analysis of slides in cohesive soils

Abstract: The Paper presents the three-dimensional stability analysis of homogeneous, isotropic and purely cohesive slopes. Rotational slides are assumed with a cylindrical central part terminated with either plane or curved ends. Analytical expressions for the three-dimensional factor of safety are obtained assuming plane ends. Critical three-dimensional factors of safety are computed using a family of power curves to generate the end surfaces. Relevant case records are reviewed and compared with the analyses. The errors involved in neglecting end effects are found to be significant in many cases (up to 30% in the cases reviewed) and in back analyses will generally be on the unsafe side. L'article decrit une analyse de stabilite tridimensionnelle de pentes homogenes, isotropes et purement coherentes. On admet des glissements circulaires avec une partie centrale cylindrique qui se termine par des extremites planes ou courbees. Des expressions analytiques pour le facteur de securite tridimensionnel sont obtenues ave...

A conceptual model for soils containing large gas bubbles

Abstract: Offshore site investigations have indicated the presence of undissolved gas bubbles in the soils forming the sea-bed at various locations throughout the world. The gas bubbles are typically much larger than the normal void spaces, so they cannot be considered as occluded bubbles within the pore water which simply change the compressibility of the pore fluid. The effect of these large bubbles on the engineering properties of the soil must be understood if offshore construction is to take place in areas of gas-bearing sediments. This Paper describes a conceptual model for soils containing large gas bubbles. The model consists of a matrix of saturated soil surrounding isolated gas-filled cavities. The cavities forming the bubble sites are fixed within the soil, so that gas drainage occurs only by the movement of dissolved gas. The behaviour of the model is complicated by the processes of localized consolidation and bubble flooding. Des recherches conduites en mer ont indique qu'il y a partout dans le monde d...

The undrained shear strength of soils containing large gas bubbles

Abstract: Measurements of undrained shear strength are reported from a series of triaxial tests on reconstituted soil samples containing large bubbles of methane gas. The experimental results indicate that the undrained strength can be either increased or reduced by the presence of large gas bubbles, depending upon the values of the consolidation pressure and the initial pore water pressure. A conceptual model for soils containing large gas bubbles, developed in the companion Paper, is analysed to provide theoretical predictions for the undrained shear strength. The analysis leads to theoretical upper and lower bounds for the undrained strength, which show good agreement with the experimental data. The experimental and theoretical developments have led to a better understanding of the strength of soils containing large gas bubbles. This could be of considerable benefit to the offshore construction industry, if platforms, pipelines and other structures are to be built in areas of gas-bearing sediments. L'article pre...

In situ and laboratory stress–strain relationships

Abstract: The question of whether or not the end of primary consolidation curve from laboratory oedometer tests is representative of in situ behaviour has been the subject of much discussion. The behaviour of the Berthierville test embankment is described and in situ effective stress-strain curves are defined for two well-defined clay sublayers. These compressibility curves are then compared with laboratory end of primary consolidation curves. Similar comparisons are also made for sublayers under three other well-instrumented embankments. All the results show that the laboratory end of primary consolidation curve strongly underestimates in situ settlements and thus that theory A is not valid. On s'est souvent demande si la courbe de la fin de consolidation primaire obtenue a partir d'essais oedometriques en laboratoire represente effec-tivement le comportement en place. L'article decrit le comportement du remblai experimental de Berthierville et definit les courbes contrainte-deformation effectives en place pour de...

Analysis of the cone pressuremeter test in clay

Abstract: The cone pressuremeter test is a new in situ test which combines a standard cone penetrometer with a pressuremeter. An analysis of the cone pressuremeter test in clay is presented. The analysis shows that important information about soil properties can be gained from the unloading section of the pressuremeter curve, as well as the loading section. Seven tests using a prototype cone pressuremeter are analysed using the new method, and the derived soil parameters are compared with other tests at the same site. The analysis is shown to fit closely the behaviour observed in the tests, and the derived values of shear modulus and undrained strength agree well with the results of other tests. Le nouvel essai au pressiometre a cone combine un penetrometre a cone de type standard avec un pressiometre. L'article presente une analyse de l'essai au pressiometre a cone dans l'argile. L'analyse demontre que des indications importantes au sujet des proprietes du sol peuvent s'acquerir a partir des zones de dechargement ...

Centrifuge liquefaction tests in a laminar box

Abstract: The difficulties associated with instrumenting earthquake sites in order to record pore pressure changes in a future event led to the use of scaled model tests performed in a centrifuge. Both dry and saturated sands were employed, contained in a box constructed of aluminium laminae designed to move freely on each other. This would result in shearing distortions developing in the soil unimpeded by the container. Accelerometers, displacement transducers and pore pressure sensors were attached to the box and embedded in the soil at various elevations so as to record the response of the soil to an earthquake-like excitation supplied to the base of the container. A special apparatus was constructed to imitate earthquake motion. In some tests on saturated sand, the soil profile was liquefied. Test results of accelerations, lateral and vertical displacements and pore pressures against time for typical earthquake inputs are given. The data, obtained under controlled conditions, can be compared with the various calculation methods for dynamically generated pore pressures.

Drained principal stress rotation in saturated sand

Abstract: An investigation into the effects of drained principal stress rotation on the behaviour of a saturated medium-loose sand is described Using a hollow cylinder apparatus the principal stress directions were rotated both at a constant shear stress and under an increasing shear stress during drained monotonic loading to failure The experimental results are presented The findings are used to support the notion that a bounding surface, defined by tests with fixed principal stress orientations, can be used to make a qualitative prediction of drained and undrained behaviour during monotonic loading to failure which includes rotation of principal stress directions The bounding surface conveys the importance of both anisotropy in determining strength and the direction of principal stress rotation in determining the magnitude of volumetric strains or pore pressures which develop The findings concerning the bounding surface apply to a medium-loose sand which has a well-defined con-tractant region Strains which

Behaviour of diaphragm walls in clay prior to collapse

Abstract: Centrifuge model tests have been used in an attempt to gain a coherent view of the soil-structure interaction behaviour following the excavation of soil in front of a pre-constructed wall. Excavation was simulated by the removal of a suitably heavy fluid from a preformed cavity. The broad replication of stress magnitudes and stress paths permitted the full representation of wall deformation, soil strain and swelling, completing a 50 year full-scale lifespan in under 24 hours of continuous centrifuging. Measurements were made of soil displacement vectors, pore water pressures, wall displacements and bending moments together with forces in props when they were present. These have made possible the validation of simplified ‘geostructural mechanisms’ which offer the same degree of advantage to the designer as does the idealization of heam behaviour encapsulated in engineer's beam theory. A serviceability criterion for soil or wall displacements can be entered into simplified admissible strain fields appropria...

Two- and three-dimensional dynamic finite element analyses of the Long Valley Dam

Abstract: Earthquake response records from the well-instrumented Long Valley Dam in the Mammoth Lake area of California are compared with numerical prediction made using finite element models The soil response to cyclic loading is accounted for by the use of a multi-surface plasticity model The input and output to the finite element analyses take the form of accelerations at the base and at various crest locations The computed and measured crest acceleration are compared in both the time and frequency domains Natural frequencies have also been obtained for the finite element models and for the real structure The time-domain results give good agreement and high correlation in the up/down-stream direction but poor agreement in the vertical direction The failure of the finite element models to capture the high-frequencies present in the vertical and transverse directions is thought to be partly due to the crude finite element discretization used L'article compare les donnees de reponse aux tremblements de terre

Using the BS cone penetrometer for the determination of the plastic limit of soils

Abstract: The use of the BS cone penetrometer test for the determination of the plastic limit of soils based on the relationship between water content and undrained shear strength is described. The experimental results indicate that the assumptions used which are based on the results of Skempton and & Northey (1953) and the suggestions by Wroth and Wood (1978) agree closely with the experimental results. By performing cone penetration tests with a range of penetration values from about 5 mm to 25 mm, the w-d (water content/penetration) plot can be constructed. The plot on a semi-logarithmic graph will produce two straight lines intersecting at a point with d about 14 mm. From the lower line of the w-d plot, the water content corresponding to a penetration value of 2 mm (i.e. the plastic limit) can be ascertained.

Uplift resistance of soils

Abstract: The uplift resistance of soils is normally associated with the design of foundations for structures such as transmission line towers where an adequate margin of safety is required against failure. This represents the design for safety case. The Author describes a scheme for the water coolant system at Sizewell A nuclear power station, where eight shafts were jacked up from tunnels to the sea-bed through 6 m of cohesionless soil. This represents the design for failure case. Existing theories based on the design for safety case were inadequate and the required jacking pressures, to overcome the uplift resistance of the soil, were successfully determined from laboratory model tests where dimensional similarity was satisfied between model and prototype. The Sizewell and field test results were compared with predictions from centrifuge tests and finite element predictions which were carried out later for the design for safety case. For cohesionless soils the centrifuge tests gave similar results to the model a...

Uplift resistance soils

Abstract: A scheme is described for the water coolant system at Sizewell A nuclear power station, where eight shafts were jacked up from tunnels to the sea-bed through 6 m of cohesionless soil. This represents the design for failure case. Existing theories based on the design for safety case were inadequate and the required jacking pressures, to overcome the uplift resistance of the soil, were successfully determined from laboratory model tests where dimensional similarity was satisfied between model and prototype. The Sizewell and field test results were compared with predictions from centrifuge tests and finite element predictions which were carried out later for the design for safety case. For cohesionless soils the centrifuge tests gave similar results to the model and field tests, whereas the finite element predictions grossly underestimated the ultimate uplift resistance for other than loose sands. A comparison is also made between predictions of the uplift resistance of cohesive soils from model tests and from theoretical studies, including finite element methods. Recent field tests on belled-out piers in cohesionless soils are compared with the results from the original Sizewell model tests. Attention is drawn to the increasing number of specializations within geotechnical engineering, the fragmentation that could develop between geotechnical engineers because of these specializations, and the necessity of keeping an open mind on, and a broad approach to, the solution of problems in geotechnical engineering.

Dynamic shear modulus and fabric: part I, depositional and induced anisotropy

Abstract: A torsional simple shear-resonant column device was used to perform experiments on hollow cylindrical samples consisting of glass spheres with two different diameters. The purpose of the investigat...

Analysis of pile group by infinite layer method

Abstract: This Paper presents an infinite layer method for the analysis of the settlement interaction between piles embedded in layered soil. In this method, the soil, which can have properties varying with depth, is divided into a number of infinite layers whereas the pile is divided into the same number of solid bar elements, which deform as a rigid section in the horizontal plane. Based on the single pile model, the increase in settlement of each pile due to interaction with the adjacent pile was calculated by an iteration procedure and is expressed in terms of an interaction factor. The influence of the layered soil, pile spacing and pile stiffness on the interaction effect is also examined. Assuming that the principle of superposition is applicable, the interaction between piles in a pile group can then be easily evaluated. The effect of the rigidity of the pile cap on the load distribution among the piles and the settlement of the foundation system has also been investigated. L'article presente une methode po...

Implications of weathering on the engineering properties of the Fuller's Earth formation

Abstract: The Fuller's Earth formation is an overcon-solidated, jointed, randomly Assured, calcareous mudstone with some thin to medium, jointed argillaceous limestone bands. South of Bath, the formation contains the commercial (montmorillonite rich) Fuller's Earth bed. This Paper reviews the effect of the proportion of calcite present on the moisture content, Atterberg limits, particle size, and residual shear strength. Attention is drawn to the importance of appreciating that the calcite percentage and the clay mineralogy are both likely to change with time as a result of weathering processes. This will affect the stability of natural slopes and man-made cuttings, excavations, etc. La formation de terre a foulon est un limon compact calcaire surconsolide jointe et fissure au hasard qui contient des bandes de chaux argillacee jointees d'epaisseur faible ou moderee. Au sud de Bath la formation comprend la couche de terre a foulon commerciale qui est riche en montmorillonite. L'article examine l'influence de la prop...

Continuum model for natural slopes in slow movement

Abstract: A two-component mixture model is proposed to describe the slow motion of natural slopes, which may creep and slide along their basal surface under the effect of gravity. The field equations comprise the balance laws of mass and momentum and constitutive relations for the partial stresses of the fluid and the solid as well as the seepage force. The fluid is modelled as an incompressible perfect liquid and the soil as a viscous isotropic body in which the stress may also depend on the porosity. The seepage force is described by Darcy's law. Explicit constitutive formulas are presented. The kinematic and dynamic boundary conditions for the fixed basal surface and the movable free and phreatic surfaces are stated. The shallowness of the moving mass permits reduction and simplication of the governing equations describing the initial boundary value problem. This makes it possible to separate the determination of the stress and velocity distributions from the evolution of the movable surfaces. Examples are given...

Hutton tension leg platform foundations: prediction of driven pile behaviour

Abstract: This Paper describes an approach to the prediction of axial load-displacement relationships for large piled foundations. The method is based on effective stress principles and attempts to take account of the effects of pile installation and equilibration, the possible formation of an interface zone (with reduced frictional strength) near the pile shaft and strongly non-linear soil constitutive behaviour. The theoretical and experimental background of the topic is reviewed and the approach then applied in a finite element analysis of the piled foundations of the Hutton tension leg platform. A superposition procedure is used to estimate group behaviour from predictions made for a single pile. The results are found to give a close match with the four independent foundation compliance parameters measured on-site during platform installation. L'article decit une methode pour prevoir les relations charge/tassement axiales pour des pieux de grandes dimensions. Cette method est basee sur les principes de contrain...

Use of a rotary shear box for testing the shear strength of rock joints

Abstract: This note describes a rotary shear machine similar to those designed by Bishop, Green, Garga, Andersen & Brown (1971) and Bromhead (1979), but suitable for testing rock surfaces, especially the shear strength of rock surfaces separated by an infilling of weaker material. The machine was constructed in 1986 and has recently been used to complete a series of trial tests on clean, flat rock surfaces and similar surfaces separated by clean rock gouge. It was specifically designed to accurately measure shear parameters up to peak failure and to follow these parameters beyond failure to their residual or basic angle of friction. This permits an accurate study of the mechanisms governing shear resistance in rock joints which are subjeced to large displacements.

Interpretation of flat plate dilatometer tests in sands in terms of the state parameter

Abstract: The use of in situ tests to determine design parameters is particularly important in sands where undisturbed sampling is difficult. Unfortunately, most in situ tests do not measure actual soil properties but instead yield some index parameters which are converted to design values by means of empirical correlations. Thus the quality of the empirical correlation ensures tbe quality of the design. This Paper assesses the key parameters which govern the expansion stage of a flat plate dilatometer (DMT) test in cohesionless material in order to develop relevant correlations with respect to the compressibility of sands. Laboratory tests in a small calibration chamber were conducted to obtain a response from the DMT during expansion in undisturbed Ottawa sand under a given stress field. The deformation characteristics inferred from the DMT data compared well with those obtained from triaxial tests. The results from the DMT are best interpreted in terms of the state parameter which combines the influence of void ...

Centrifuge modelling of sand embankments and islands in earthquakes

Abstract: The bumpy road shaking table system at the Cambridge geotechnical centrifuge was used to conduct an experimental investigation into the effects of earthquakes on sand embankments and islands. The results of the study show that, when a loose or medium dense embankment or island is subjected to an earthquake, positive pore pressures are generated, particularly at the crest. This pore pressure generation softens the embankment and lowers its resonance frequency, resulting in liquefaction and decoupling of surcharge motions from crest motions in the extreme case. When a dense embankment or island is subjected to a strong earthquake, spiky acceleration records are observed at the shoulders of the embankment or island. These records may be attributed to the propagation of dilation fronts through the sand which is in a state of cyclic mobility. If a rigid surcharge is present on the crest, the interaction of the embankment or island with the rigid surcharge gives rise to surcharge accelerations which are charact...

Quantitative definition of highly weathered granite using the slake durability test

Abstract: The results are presented that were obtained from slake durability tests conducted on weathered Korean granites. It is demonstrated that the test can identify granite in the HW grade, but can also provide both an accurate and a sensitive measure of weathering intensity within the HW grade of granite. Sophisticated sampling is not required. Also, the slake durability test provides a quantitative measure of the soil to rock ratio within such materials.

Dynamic shear modulus and fabric: part ii, stress reversal

Abstract: The dynamic shear modulus was measured during stress reversal in a torsional simple shear-resonant column device. The shear modulus measurements were then used to investigate the change in the fabric during shearing. It was assumed that the shear modulus was sensitive to the net loss of contacts associated with the development of fabric in the material. During a complete cycle of loading, unloading and reverse loading, the fabric changed more slowly than the stress and more rapidly than the strain. The decrease in the shear modulus during loading was recovered either totally or partially on unloading, depending on the largest stress in loading. Le module de cisaillement dynamique fut mesure au cours du renversement des contraintes dans un appareil a colonne resonante pour mesurer le cisaillement simple par torsion. Puis les mesures de module de cisaillement furent utilisees pour etudier le changement dans la texture au cours du cisaillement. L'hypothese etait que le module de cisaillement etait sensible a...

Influence of stress-release disturbance, storage, and reconsolidation procedures on the shear behaviour of reconstituted underwater clay

Abstract: Tests have studied the changes in undrained shear strength, porewater pressure parameters and compression modulus produced in clay specimens during sampling, offloading and reconsolidation. Specimens of normally consolidated and over consolidated illite (OCR = 2) were prepared by anisotropic consolidation. Some of these were then tested immediately to provide control information about the undisturbed insitu strength. The remainder were offloaded, stored under conditions of either totally free drainage or totally restricted drainage (undrained) for three fixed periods of time up to one week, reconsolidated using three different procedures, and sheared undrained. The test results varied quite considerably with all of the controlled variables in the program. They provide quantitative data that confirm the established practice that best results are obtained from samples which are stored without drainage for as short a times as possible and are then reconsolidated anisotropically to their in situ stresses befo...