A new design method for retaining walls in clay

Abstract

Geotechnical design engineers used to rely on arbitrary rules and definitions of "factor of safety" on peak soil strength in limit analysis calculations. They used elastic stiffness for deformation calculations, but the selection of equivalent linear elastic models was always arbitrary. Therefore, there is a need for a simple unified design method that addresses the real nature of serviceability and collapse limits in soils, which always show a nonlinear and sometimes brittle response. An approach to this method can be based on a new application of the theory of plasticity accompanied by the introduction of the concept of "mobilizable soil strength." This approach can satisfy both safety and serviceability and lead to simple design calculations within which all geotechnical design objectives can be achieved in a single step of calculation. The proposed method treats a stress path in an element, representative of some soil zone, as a curve of plastic soil strength mobilized as strains develop. Designers enter these strains into a plastic deformation mechanism to predict boundary displacements. The particular case of a cantilevered retaining wall supporting an excavation in clay is selected for a spectrum of soil conditions and wall flexibilities. The possible use of the mobilizable strength design (MSD) method in decision-making and design is explored and illustrated.Résumé : Les ingénieurs géotechniciens avaient l'habitude de se fier dans leurs calculs d'analyse limite à des règles et définitions arbitraires du « coefficient de sécurité » basées sur la résistance de pic du sol. Ils utilisaient la rigidité élas-tique pour les calculs des déformations, mais la sélection de modèles équivalents élastiques linéaires était toujours arbitraire. En conséquence, on a besoin d'une méthode simple et unifiée de conception qui traite de la nature réelle de la praticabilité et des limites d'effondrement des sols, qui montre toujours une réponse non linéaire et parfois fragile. Une approche à cette méthode peut être basée sur une nouvelle application de la théorie de plasticité accompagnée de l'introduction du concept de « résistance mobilisable du sol ». Cette approche peut satisfaire tant la sécurité que la praticabilité, et peut conduire à des calculs simples de conception dans lesquels tous les objectifs de conception géotech-nique peuvent être atteints au cours d'une étape unique de calcul. La méthode proposée considère un cheminement de contrainte dans un élément représentatif d'une certaine zone de sol, comme étant une courbe de résistance plastique du sol mobilisée à mesure que les déformations se développent. Les concepteurs entrent ces déformations dans un méca-nisme de déformation plastique pour prédire les déplacements aux frontières. Le cas particulier d'un mur de soutène-ment en porte-à-faux retenant une paroi d'excavation dans l'argile a été choisi comme éventail des conditions des sols et de la flexibilités du mur. On explore et illustre l'utilisation possible de la méthode de calcul de la résistance mobilisable...

Ground movements due to deep excavations in Shanghai: Design charts

“…The possible use of MSD for flexible structures was first considered by Osman & Bolton (2004) in the context of cantilever walls retaining clay. They compared MSD calculations based on rigid wall rotations with Finite Element Analysis (FEA) that fully accounted for typical soil non-linearity and the flexure of walls with typical stiffnesses.…”

Design of Geostructural Systems

“…Later, the same approach was used by Osman and Bolton (2004) to calibrate MSD against the non-linear finite element analaysis (FEA) of retaining walls of various flexibility, subjected to excavation to various proportional depths, in clays that began with various K0 values and that required different strains to reach failure. Wall displacements predicted by rigid-wall MSD, as a ratio of those computed by FEA, were shown to fall in the range 1.0 to 0.5, depending on the sytem parameters mentioned previously.…”

“…The influences of these parameters on the calculated displacement ratio MSD/FEA were, in descending order of significance: relative wall flexibility, embedment ratio, soil mobilization strain, and initial K0 value. Designers who were able able to apply the simple MSD approach could then use the charts in Osman and Bolton (2004) to recalibrate the MSD approximations accordingly.…”

“…If an in situ wall were to be designed with sufficient embedment to restrict wall rotations to 1/100, for example, a limit of 1/200 would be imposed in an equivalent MSD calculation, which offers the least favourable assessment of ΔMSD/ΔFEA across the range of conditions considered by Osman and Bolton (2004). In conducting that calculation, the shear stress-strain curves would then be constructed based on a mean strength profile, a mobilization strain M=2 obtained from the samples taken approximately from middepth of the wall, and by assuming a power index of b = 0.6.…”

“…Full MSD Predictions with RBD Zhang et al (2015) applied the MSD method from Osman and Bolton (2004) to a database of 45 field case histories and 14 centrifuge tests of unpropped cantilever wall displacements.…”