Total Stress Analysis Research Articles

New stability method for embankments on clay foundations

The proposed method makes use of a conventional total stress analysis, corrected to take into consideration the preconsolidation pressure and the undrained shear strength profiles. Good results are obtained for seven well documented cases of embankment failures, the data of which allow a direct application of the proposed method. Many other failure cases, for which the published data are insufficient to allow such a direct application, are analysed on a statistical basis using a relationship between cuσp′ and Ip. Bjerrum's correction appears as a particular case, only statistically applicable, of the proposed method.

A laboratory simulation of reinforced clay walls

To assess the possibility of using clay fill in the construction of reinforced earth walls a series of simulated wall tests has been carried out using Kaolin clay reinforced with polyethylene mesh. Due to the impracticality of bringing a laboratory model to failure by self-weight only, the simulated walls were failed under the application of a vertical surcharge. To minimize the vagaries of subsequent total stress analyses the surcharge was applied using a rigid platen that has the effect of inducing failure along a preselected plane. Results from these tests are interpreted using simple total stress analyses which relate the surcharge intensity at failure to the geometry and strength parameters of the clay and reinforcement. One of the most critical of these parameters is the soil-reinforcement adhesion which was investigated using both unconsolidated-undrained direct shear and pull-out tests. Pour évaluer la possibilité d'utiliser l'argile comme matériau de remblai pour la construction de murs en terre armee, toute une série d'essais de simulation de murs a été effectuée en utilisant du kaolin renforcé par un treillis en polyéthylène. Etant donné qu'il n'était pas pratique d'amener un modèle de laboratoire a la rupture sous son propre poids seulement, la rupture des murs simulés a été provoquée par l'application d'une surcharge verticale. Pour réduire au minimum les aléas des analyses de contraintes totales ultérieures, la surcharge a été appliquée à l'aide d'un plateau rigide qui a pour effet d'en-traîner la rupture suivant un plan pré-sélectionné. Les résultats de ces essais sont interprétés par l'intermédiaire d'analyses simples de contraintes totales qui mettent en rapport l'intensité de surcharge à la rupture et les paramètres de géométrie et résistance de l'argile et de l'armature. Parmi ces paramètres, l'un des plus critiques est celui de l'adhésion sol-armature; ce paramètre a été examiné à l'aide d'essais de cisaillement direct non consolidés—non drainés ainsi que d'essais d'arrachement.

In situ strength and total stress analysis for an embankment of a soft foundation Poopath, V Proc International Symposium of Geotechnical Aspects of Soft Clays, Bangkok, 5–6 July 1977, P547–577. Publ Bangkok: Asian Institute of Technology, 1977

In situ strength and total stress analysis for an embankment of a soft foundation Poopath, V Proc International Symposium of Geotechnical Aspects of Soft Clays, Bangkok, 5–6 July 1977, P547–577. Publ Bangkok: Asian Institute of Technology, 1977

Simplified Procedure for Effective Stress Anaysis of Ground Response

A review is made of several analytical approaches to the determination of pore-pressure generation and dissipation in saturated sands at level sites during earthquakes and the effects of the resulting pore-pressure buildup on the seismic response of these sites. A simplified procedure is described that uncouples the nonlinearity of the stress-strain relationship of the sand under cyclic shear loading and the gradual stiffness degradation caused by the pore-pressure buildup under the type of loading. It was found that stress reduction between a total stress analysis, where the effective confining pressure is considered to remain constant throughout the earthquake, and an effective stress analysis that accounts for both types of soil nonlinearity did not exceed about 85% under the most severe conditions. Consequently, it is believed that the simplified procedure may be used in conjunction with any ground-response analysis program for the purpose of evaluating ground response including pore-pressure effects.

Use of centrifugal model testing to assess slope stability

Centrifugal model tests increase total stress levels in small models made of natural or reconstituted soil, and allow direct observation of repeated failures. A novel use of a basket centrifuge is described in which slopes can be tested without end effects. When a series of models at various overconsolidation ratios was tested to failure the error in the safety factor predicted by a total stress analysis increased with the overconsolidation ratio. Other centrifuges are discussed and the advantage of large machines with balanced arms and swinging carriages are explained. Soil material behaviour discussed includes heat transfer and leaching. The paper concludes with a review of the present availability of and prospects for centrifugal model tests.

Finite Element Analysis of Immediate and Consolidation Deformations Based on Effective Stress Principle

Elastic and elastic-plastic problems in soil mechanics have been frequently analysed numerically by the finite element method. Most of the problems were analysed in terms of total stress and a few in terms of effective stress and pore water pressure. The aim of this study is to obtain the accurate solutions of immediate and consolidation deformations of anisotropic and heterogeneous poro-elastic media introducing the concept of effective stress by the finite element technique.Firstly, this paper describes the principle of the finite element method in the problem of consolidation accompanying with undrained deformation and the procedure of analysis. It is especially emphasized that the interpolation functions for the pore water pressure must be quadratic in space and those for the displacement must be cubic. Secondly, the deformation and the distributions of the pore water pressure or the stresses in the isotropically homogeneous and elastic foundation layer just after the application of a uniform strip load and during the consolidation process are computed as an example. The superiority of the numerical technique is ascertained by comparing the results with those based on total stress analysis.

Stability of embankment on clay : Hamel, JV Discussion of paper by Wu et al, Sept. 1975 1T, 2R J. Geotech. Engng Div. ASCE, V102, GT5, 1976, P574–576

Comments are made on the possible role of residual strength in effective stress analyses of the embankment failure. It is noted that the minimum effective shear strength likely to occur laong structural discontinuities or progressively developed failure surfaces in clay is the residual strength. The residual friction angle (14 deg.) determined by the authors for slickensides in the foundation is used to estimate safety factors for the three cross sections. Details of the estimation procedure are described, and the estimated safety factors are tabulated. Previous work is quoted that pointed out that stability of embankments on slightly overconsolidated highly plastic clay foundations is generally overestimated by both total and effective stress analyses using shear strengths from conventional laboratory tests. It was proposed that a shear strength reduction factor be used to account for progressive failure of foundation behavior which reduce field mobilized shear strengths. Later studies have shown that rsidual or near-residual shear strengths are mobilized as described.

Field and Laboratory Behaviour of a Lightly Overconsolidated Clay

Synopsis An extensive series of flood levées up to 10 ft high is being constructed in Launceston, Australia. The levées rest on a soft clay and a major failure occurred on raising the levées to 7 ft height. An investigation has been made to analyse the failure and recommend on future construction procedure. Laboratory testing showed the soil to be lightly overconsolidated and observations on a full size instrumented test levée indicate that the degree of overconsolidation, although small, influences field behaviour to a marked extent. Total stress analyses gave a factor of safety against general shear failure of 1·60, and an effective stress analysis gave slightly below unity assuming c′ = 0, indicating that critical state conditions may apply. A four stage construction procedure has been recommended on the basis of results from the investigation. Une série étendue de digues de hauteurs allant Jusqu'à 3,04 m (10 pieds) est en train d'être construite à Launceston en Australie. Les digues reposent sur un argile mou et une rupture de première importance s'est produite lorsque l'on éleva les digues à une hauteur de 2,13 m (7 pieds). Une enquête a été menée pour faire une analyse de la rupture et pour faire des recommendations sur les procédés futurs de construction, Des essais en laboratoire ont fait apparaitre que le sol était légèrement trop consolidé et des observations portant sur une digue d'essai de grandeur nature pourvue d'instruments indiquent que le degré de surconsolidation, quoique réduit, a une influence sur le comportement sur place à un degré notable. Des analyses de la contrainte totale donnèrent un facteur de sécurité par rapport à un défaut de cisaillement général s'élevant à 1,60 et une analyse de la contrainte effective a produit un chiffre légèrement inférieur à l'unité en supposant que c′ = 0, indiquant que des conditions d'état critique e peuvent s'appliquer. Un procédé de construction en quatre stages a été recommandé à partir des résultats de l'enquête.

A Study of a Large Compacted Clay Embankment-Fill Failure

The paper describes the failure during construction of a proposed 70 ft. high railway embankment fill. The fill was built of a uniform clay of medium plasticity which was used at an average moisture content of 3 per cent greater than had been provided for in the design. The fill failed under its own weight when it reached a height of 55 feet.In order to assist in the design of stabilizing works three test holes were drilled in the fill and soil samples recovered. Properties of field compacted and laboratory compacted soil samples are compared. The age of both types of samples is shown to have a significant effect on the test results.Both total and effective stress analyses of the embankment at failure have been performed using the laboratory values of soil strength. The total stress analysis gives a safety factor of 1.0 at failure whereas the effective stress analysis gives a safety factor of 1.2. The hazards of choosing the correct value of laboratory shear strength for the total stress analysis are discussed.

Stability Investigations of the North Bank of the Drammen River

Synopsis The Paper describes a stability investigation of the natural soft silty clay slopes of the north bank of the Drammen River. Both a slip which occurred in 1955 and a stable length of the bank are considered. It is shown that the total stress analysis seriously underestimates the stability of these natural slopes, and the method, therefore, cannot be used to estimate long-term stability. The effective stress analysis in the form derived by Bishop (1954)1 gives a reliable estimate of the stability of the slopes, and confirms past experience that the effective stress analysis is the correct approach to long-term stability problems. It is shown that the value of the factor of safety given by the simplified effective stress method expressed as a percentage of that given by Bishop's more rigorous effective stress method drops rapidly as the central angle of arc increases and also decreases with increase in inclination of the line joining the two points where a trial circle emerges from a slope. The results of the comprehensive series of triaxial tests which were carried out show that the same strength envelope is obtained from consolidated drained triaxial tests, independent of the type of test. This envelope is only very slightly higher than that obtained from consolidated undrained triaxial tests expressed in terms of effective stress provided that the failure criterion of maximum principal effective stress ratio is used. If the maximum deviator stress failure criterion is used, then the shear strength envelope for the undrained tests lies below that for the drained tests. The true angle of internal friction for this silty clay is 30° and the true cohesion contributes only a comparatively small amount to the total shear strength. L'article décrit un enquête sur la stabilité des pentes naturelles d'argile limoneuse tendre sur la rive nord du Drammen. On a étudié un glissement qui s'est produit en 1955 et aussi une longueur stable de la rive. Il est démontré que l'analyse de tension totale sous-estime sérieusement la stabilité de ces pentes naturelles, et par conséquent, la méthode ne peut pas être employée pour évaluer la stabilité de longue durée. L'analyse de tension effective sous la forme déduite par Bishop (1954) donne une évaluation sérieuse sur la stabilité des pentes, et confirme l'expérience passée que l'analyse de tension effective est la méthode d'approche correcte pour les problèmes de stabilité de longue durée. Il est démontré que la valeur du facteur de sécurité donné par la méthode simplifiée sur la tension effective exprimée en pourcentage de celle donnée par la méthode plus rigoureuse de Bishop sur la tension effective, baisse rapidement lorsque l'angle au centre de l'arc croît, et décroît aussi avec l'augmentation de pente de la ligne joignant les deux points où un cercle d'essai déborde d'une pente. Les résultats de la série complète des essais triaxiaux qui ont été faits montrent qu'on obtient la même enveloppe de résistance par des essais triaxiaux drainés consolidés, indépendamment du genre d'essai. Cette enveloppe n'est qu'un tout petit peu supérieure à celle obtenue par des essais triaiaux drainés consolidés exprimés selon des termes de tension effective à condition que l'on employe le critère de rupture du rapport de tension effective principale maximum. Si on employe le critère de rupture de contraite variable maximum, l'enveloppe de résistance au cisaillement pour les essais drainés est inférieure à celle des essais drainés. L'angle véritable de friction interne pour cette argile limoneuse est de 30° et la cohésion véritable ne contribue qu'un peu à la résistance totale au cisaillement.