The relevance of the Yield Shear Strength of Plastic Clays as the boundary between elastic and progressive plastic settlement of Foundations

Jaime Graterol M.

Dans les comptes rendus d’articles de la conférence: GEO2011: 64th Canadian Geotechnical Conference, 14th Pan-American Conference on Soil Mechanics and Geotechnical Engineering, 5th Pan-American Conference on Teaching and Learning of Geotechnical Engineering

Session: Shallow Foundations

ABSTRACT: The bearing capacity and settlements of foundation on saturated normally and over consolidated plastic clays are analyze under the concept of the yield shear strength, following the criteria that cohesive clay is a plastic solid and could expected to exhibit the basic properties of such material. A comparative study is made of the elastic and progressive plastic settle-ments of foundations in this kind of soils following the yield strength concept with the consolidation settlements from the theory of consolidation. Finally the inaccuracy in settlement prediction following the theory of consolidation in this kind of soil is explained and several foundation failures are analyze.

RESUMEN: La capacidad admisible de carga y de asentamientos de fundaciones apoyadas en arcillas plásticas saturadas normalmen-te consolidadas y preconsolidadas son analizados bajo el concepto de la resistencia al esfuerzo cortante del límite cedente y siguiendo el criterio de que una arcilla cohesiva es un sólido plástico, pudiéndose esperar que desarrolle las mismas propiedades de dicho material. Se hace un estudio comparativo entre los asentamientos elásticos inmediatos y aquellos plásticos progresivos de fundaciones en este tipo de suelos, siguiendo el concepto de la resistencia al límite cedente y los asentamientos por consolidación obtenidos de la Teoría de Consolidación. Finalmente se explica la inexactitud en la pre-dicción de los asentamientos, siguiendo la Teoría de Consolidación en este tipo de suelo y se analizan algunos casos de falla de fundaciones. 1. INTRODUCTION 40 years ago when I began my first design of earth works and foundations in plastic clays, among all other investi-gation, I had the opportunity to read the extensive work on shear Resistance of Plastic Clays, it™s application in foun-dation engineering and field observations developed by W.S. Housel, University of Michigan. 2. BASIC CONCEPTS IN HOUSEL´S WORK 2.1 Shearing Resistance Due to Cohesion: Shearing resistance due to cohesion or cohesion is that property of soil which provides finite static resistance to tangential displacement through mutual attraction be-tween particles of the mass, characteristic of microscopic and sub-microscopic matter. Shearing resistance due to cohesion is independent of applied normal pressure, a re-lationship inherent in any material capable of sustaining a permanent constant difference in principal stresses. 2.2 The Ring Shear Test: Accepting the definition of cohesion as being independent of normal pressure, the ring shear test procedure was set up to measure the transverse shearing resistance at zero normal pressure. Setting up the test procedure with defini-tive control of the other factors to be measured, that co-hesive clay is a plastic solid and could be expected to ex-hibit the basic properties of such a material, in Fig. 1 is il-illustrated the relationship between shearing stress and rate of shearing deformation, in accordance with the long accepted definition of a plastic solid. With normal pressure eliminated as a variable in test procedure, there remain three variables to be measured: time, shearing stress, and rate of shearing displacement. It follows that a valid relation between the two variables, shearing stress and rate of shearing displacement, can only be obtained by holding the third variable, time, con-stant. Figure 1. Properties of plastic solids.˘ˇˆ˙˝˛˚ˇˆ˜˝˛

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