Over-consolidated Clay Research Articles

Soil parameters used for design of gravity platforms in the North Sea

The foundation soils for most of the North Sea gravity platforms installed so far have been very stiff over-consolidated clays and dense sand. Two actual platform sites are described that are representative of these soil conditions. It is shown how the design soil parameters required for a geotechnical analysis of the platform foundation have been selected for these two sites. The results of in situ testing, laboratory tests and experience from existing platforms are used, with the main emphasis placed on procedures used at the Norwegian Geotechnical Institute.

Discussion on ‘Geotechnical, mineralogical and chemical interrelationships in weathering profiles of an over-consolidated clay’ by D. J. Russell and A. Parker

D. Robinson writes: The authors have clearly demonstrated in their Fig. 3 a the change in shape of the illite basal 10 Å reflection with depth in a weathering profile of the Oxford Clay. They attributed this change to a decrease in crystallinity caused by weathering which results in a progressive loss of interlayer potassium and leads to an increased shoulder on the low 2θ side of the 10 Å peak. Quantification of the shape factor, as the authors suggest, has been widely attempted in clay mineral studies. In the examples quoted (Gill et al. 1977; Weaver 1960) the ratio of the peak-minus-background intensity at 10 Å to that at 10.5 Å is utilized in the examination of crystallinity changes through diagenesis into low-grade metamorphism. This ratio therefore is a measure of the similarity to an ideal 10 Å peak rather than reflecting the importance of the 11 Å d -spacing as in the shape-factor chosen by the authors. The changes demonstrated in peak-shape, however, could also be caused by the presence of varying proportions of random mixed-layer components in the illite (Thorez 1975). The authors will be aware that there are various pretreatment methods that allow the recognition and semi-quantification of interlayer components (Hower & Mowatt 1966; Weaver 1956). It may be that the various geotechnical parameters could be better understood if there is a varying proportion of interlayer component in the illite, especially if it is of a swelling character. It is obvious that the authors have many more data

Foundation performance of Gardiner Dam

Gardiner Dam is founded on a highly overconsolidated clay shale that had a profound influence on embankment design and performance. The shale contains preconstruction shear zones whose specific location, extent, and strength characteristics were virtually impossible to determine from test samples and soil logs. Analyses of slumped valley slopes and of shallow excavation slopes that moved during early stages of construction overestimated the strength of dormant shear zones that were activated during subsequent embankment construction. Major design modifications consisting of extensive slope flattening and toe-berm loading were undertaken to compensate for the weakness of the shear zones.About 2 m of foundation settlement and over 2 m of lateral displacement occurred under the central section of the 64 m high embankment during construction, but virtually none happened at the toe. Annual post construction increments of lateral displacement are related mainly to the yearly cyclic reservoir load. Although these deformations have decreased to the order of 2 cm/year, comprehensive monitoring and critical evaluation are maintained in order to ensure long-term safety and functional adequacy of the project.The design and construction of Gardiner Dam proceeded at the forefront of knowledge and understanding of the residual shear strength and behavior of highly overconsolidated clay shales. The construction and postconstruction performance of the foundation, which is described in this paper, indicates that much more information and research are needed to fully understand the long-term behavior of such materials.

The growth of slip surfaces in the progressive failure of over-consolidated clay

In heavily over-consolidated clays there is a marked peak in the observed relation between shear stress and shear strain. As the strain increases, the stress falls from a peak to a much smaller residual stress. Slopes made from such a clay often fail progressively many years after construction. Sliding occurs on a concentrated slip surface, and it is found that the mean resolved shear stress on that surface is markedly less than the peak shear strength. Concepts from fracture mechanics, and in particular the J -integral, are used to derive conditions for the propagation of a concentrated shear band of this kind. The results indicate the presence of a strong size effect, which has important implications for the use of models in soil mechanics. An elastic analysis makes it possible to determine the size of the end zone in which the shear stress on the shear band falls to its residual value. An attempt is made to assess the possible sources of the time-dependence governing propagation speed of the shear band. They include pore-water diffusion to the dilating tip of the band (which governs the rate at which local strength reductions can occur), viscoelastic deformation of the clay (which allows a gradual build-up of strain concentration at the tip of the band), and the weathering break-down of diagenetic bonds.

First-Time Slides in Over-Consolidated Clays

(a) After a slide has taken place the strength on the slip surface is equal to the residual value. The residual strength is associated with strong reorientation of the clay particles and is represented by an angle of shearing resistance $= which in most clays is considerably smaller than the value of 4’ at peak strength (Fig. 1). (b) First-time slides in slopes in non-fissured clays correspond to strengths only slightly less than the peak. (c) First-time slides in fissured clays correspond to strengths well below the peak. (d) Some form of progressive failure must be operative to take the clay past the peak. This could be simply the result of a non-uniform ratio of stress to strength along the potential slip surface; but probably the fissures play an important role as stress concentrators and in leading to softening of the clay mass. (e) Granted a progressive failure mechanism the limiting strength would be residual, and it is therefore reasonable to express the actual strength at the time of failure as a function of the upper (peak) and lower (residual) limits, i.e. by the residual factor. (f) In natural slopes of London CIay the strength has fallen approximateIy to the residual value. (g) The residual strength obtains on pre-existing shear surfaces, whether these are the result of tectonic shearing or old landslides.

Observational Data on Ground Movements Related to Slope Instability

Data from field measurements verify the progressive nature of landslides resulting from excavations at the toe of slopes in over-consolidated highly plastic clays and clay-shales. The data are in agreement with generally accepted hypotheses concerning the mechanism of such progressive failures. The distribution of such movements changes with time, and both total movements and velocities must be reviewed to understand the failure process. Horizontal movements resulting from the construction of embankments on soft clays are shown also to be progressive and to be similar to those resulting from cuts in over-consolidated clays. These movements often extend well beyond the toe of the embankment except when the fill is placed on a stiff layer overlying soft materials. Several of the progressive slides were stabilized by corrective treatment which included cylinder pile retaining walls, underground concrete buttresses and slope flattening. Analysis of field observational data leads to a better understanding of the mechanism of failure and ultimately may lead to improved methods of stability analysis.

Stability of Slopes Undergoing Creep Deformation

An analysis of a long slope undergoing creep deformation is presented. The analysis is based on assumptions that the soil flows viscously with no volume change, and that the creep deformation ensues when the shear stress due to gravity exceeds the soil residual shear strength. Expressions to describe the velocity field in the creep zone of the slope are derived. The results are discussed for the cases of with or without surcharge loads and results are presented in dimensionless charts. Because of the assumptions used, the theoretical results appear to be applicable to slopes of over-consolidated clay or clay shales. In order to determine the average coefficient of viscosity of the soil, field displacement versus time records of the slope undergoing creep is necessary. Two published soil creep case histories are utilized to verify the theoretical analysis and the results are reasonable.

Orientation of Fissures in Stiff Overconsolidated Clay of the Siwalik System

Synopsis Measurements of the attitude of fissures in stiff-fissured overconsolidated clay bedrock are described together with the technique of stereographic projection, point and contour diagrams to portray their attitude. This technique was used on Siwalik clay to investigate directions of orientation of fissures which could act as paths of weakness for failure surfaces in dam foundations and deep excavations. Both random and preferred orientations of fissures were found, the latter appearing to be related to the local fault, shear and fold pattern of the bedrock. The working hypotheses of fissure formation, used during the investigation, are given. On décrit des mesures sur la disposition des fissures dans des assises rocheuses d'argile surconsolidée fissurée raide ainsi que la technique de projection stéréographique, des diagrammes de points et de contours pour décrire leur disposition. Cette technique a été utilisée sur des argiles Siwalik pour faire des recherches sur les directions d'orientation des fissures qui pourraient agir comme lignes de faiblesse pour les surfaces de rupture dans les fondations de barrage et les excavations profondes. On a découvert des orientations aléatoires comme des orientations préférées, ces dernières semblant être apparentées à la faille locale, au type de cisaillement et de pli de l'assise rocheuse. On présente les hypothèses admissibles sur la formation des fissures qui ont été utilisées au cours de l'enquête.

Foundation of Structures on Over-Consolidated clay Layers in São Paulo

Synopsis The problem of determining the settlement and bearing capacity of building and bridge foundations on partially-saturated, over-consolidated clay layers is approached in this Paper from an experimental point of view. The data for this study have been provided by the fast development of the city of São Paulo, Brazil, which is built on an extremely heterogeneous clayey formation. Emphasis is given to the relation between geological formations and foundation problems, each stratum having its peculiarities, a consideration of great importance in the solution of specific problems. As would be expected, the φ= 0 method for determination of the bearing capacity of foundation on these clays is not always quite valid. Although settlements of foundations on over-consolidated clay layers are not important they can be predicted with fair accuracy if certain assumptions are made. Le problème qui consiste à déterminer l'effet detassement et la capacité de portage des fondations de bâtiments et de ponts sur des couches argileuses en partie saturées et surtassées est traité dans cet Article d'un point de vue expérimental. Les donées pour cette étude ont été fournies par le développement rapide de la ville de São Paulo, au Brésil, cette ville étant bâtie sur une formation argileuse très hétérogène. L'auteur y souligne en particulier le rapport entre les formations géologiques et les problèmes de fondations, chaque couche géologique ayant ses particularités dont l'étude est d'une grande importance pour la solution de problèmes déterminés. Comme on peut le prévoir, la méthode φ = 0, pour le calcul de la capacité de portage des fondations sur une telle argile, n'est pas toujours tout à fait valable. Bien que le tassement des fondations sur des couches d'argile surtassée ne soit pas de grande importance, on peut cependant en prédire l'ampleur avec quelque précision, si certaines suppositions sont faites.

A Landslide at Jackfield, Shropshire, in a Heavily Over-Consolidated Clay

A Landslide at Jackfield, Shropshire, in a Heavily Over-Consolidated Clay

The Sensitivity of Clays

Synopsis The sensitivity of clays is defined as the ratio of their undisturbed and remoulded strengths, and varies from about 1·O for heavily over-consolidated clays to values of over 100 for the so-called extrasensitive or “quick” clays. As a result of experimental work it is shown that thixotropy can account for low or medium sensitivity but not for high sensitivity. Both laboratory and field evidence, however, show that a reduction in the salt concentration in the pore water, caused by leaching clays of marine or estuarine origin, can result in high sensitivities. Moreover, of the small number of clays so far investigated, those which have not been leached are found to exhibit only low or medium sensitivities of the order of magnitude explicable in terms of thixotropy. But it is, nevertheless, not suggested that leaching is the sole cause of high sensitivity and other possibilities are briefly discussed. Reasons are given, supported by experimental work, for the insensitivity of heavily overconsolidated clays with water contents approximating to the plastic limit. A result of particular interest from the scientific point of view is that the water content and undisturbed strength of a clay remain unaltered by learhing, although the liquid limit and remoulded strength are appreciably reduced. It is suggested that this implies effectively “solid” contact between the particles of an undisturbed clay' Finally some evidence is given which indicates that the fissured structure present in many clays may be due to syneresis. La sensitivité des argiles est définie comme étant le rapport entre leur résistance à l'état brut et leur résistance après remoulage, et varie de 1,0, environ, pour les argiles trop fortement tassées, jusqu'à des valeurs de plus de 100 pour des argiles dites extrasensitives ou “sables mouvants”. A la suite de travaux d'expérience, on montre que la thixotropie peut entre en ligne de compte pour les basses ou moyennes de sensitivités, mais non pas pour les fortes sensitivités. Cependant, les travaux de laboratoire et sur les lieux montrent qu'une réduction de la concentration en se1 dans l'eau de porosité, causée par le lavage des argiles d'origine maritime ou alluvionaire, peut provoquer de fortes sensitivités. Qui plus est, du petit nombre d'argiles qui ont fait l'objet de recherches jusqu'à maintenant, celles qui n'ont pas été lavées n'ont présenté seulement que des sensitivités faibles ou moyennes de l'ordre de grandeur explicable en fonction de la thixotropie. Cependant, il n'est pas suggéré que le lavage soit la seule cause de haute sensitivité et d'autres possibilités font l'objet d'une discution rapide. On donne des raisons, s'appuyant sur des travaux expénmentaux, pour le manque de sensitivité des argiles trop fortement tassées dont la teneur en eau approche la limite plastique. Un résultat d'un intérêt tout particulier du point de vue scientifique est que la teneur en eau et la résistance à l'état brut restent inchangées par le lavage, alors que la limite liquide et la résistance après remoulage soient sensiblement réduites. II est suggéré que ceci signifie qu'il y ait un contact “solide” effectif entre les particules d'une argile à l'état brut et s'oppose à la théorie communément acceptée que les particules sont séparées par des couches relativement épaisses d'eau adsorbée. Finalement il est donné des preuves qui indiquent que la structure fissurée qui se rencontre dans beaucoup d'argiles puisse être due à une synérèse.