Rational approach for the design of retaining structures using the mechanics of unsaturated soils

N'eem Tavakkoli, Sai K. Vanapalli

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: Behaviour of Unsaturated Soils

ABSTRACT: The earth pressures acting on the soil retaining structures are conventionally estimated using the mechanics of saturated soils using conventional soil properties such as the saturated shear strength parameters and soil density values. However, the backfill material behind the retaining walls is typically in a state of unsaturated condition. Thus, the influence of the capillary stress above the ground water table in the backfill is conventionally not taken into account; due to this reason the resultant earth pressure is likely to be conservative. Rigorous analyses of the design of retaining walls in semi-arid and arid regions where the backfill materials are typically in a state of unsaturated condition should be based on the mechanics of unsaturated soils taking account of the influence of capillary stress (i.e., matric suction). In this paper, several examples are presented to highlight the differences in the earth pressures calculated using conventional soil mechanics and using the mechanics of unsaturated soils.

RÉSUMÉ: La pression des terres agissant sur les structures de rétention des sols est conventionnellement estimées en utilisant la mécanique des sols saturés, faisant usage des paramètres de sol conventionnels tels que la résistance au cisaillement et la densité d'un sol saturé. Cependant, le matériel de remblayage derrière un mur de rétention se trouve généralement dans une condition non-saturée. On ne tient alors généralement pas compte de l'influence des contraintes capillaires au-dessus du niveau phréatique dans le remblais, ce qui se traduit par des résultats conservateurs. Une analyse rigoureuse de la conception de murs de rétention dans des zones semi-arides et arides où les matériaux de remblais sont typiquement dans des conditions non-saturées devrait être basés sur la mécanique des sols non-saturés, tenant compte de l'effet des contraintes capillaires (c.-à-d. la succion matricielle). Dans le présent article, plusieurs exemples sont résolus afin de mettre en évidence les différences dans la pression des terres calculée en utilisant la mécanique des sols conventionnelle et la mécanique des sols non-saturés. 1 INTRODUCTION Retaining walls are structures that are designed and constructed following the safety codes to withstand pressures associated with soil, water and other loading conditions to restrain the backfill soil movement. They are also used in several situations to stabilize slopes. The earth pressures acting on such structures depend mainly on the type and mechanical properties of backfill soils, wall geometry and its material properties. The frictional resistance that arises between the retaining wall and backfill soil can also be a key parameter in the stability analysis of retaining structures in certain scenarios. The lateral earth pressures associated with these structures are commonly estimated using either Rankine or approaches using the mechanics of saturated soils in practice. Such an approach is simple; however, the estimated earth pressures are typically conservative for backfill soils that are in unsaturated conditions. This is especially true for retaining walls in arid and semi-arid regions which typically have unsaturated soils as backfill. In many cases, the natural ground water table is at a greater depth in these regions. Asurface constitutes of arid and semi-arid regions (Dregne 1976). More recently, geocomposites and geotextiles are being used in the construction of retaining structures to retain the capillary stress or matric suction in the backfill soils to take advantage of the engineering behaviour of unsaturated soils (McCartney et al. 2008). Therefore, it is more rational in such scenarios to estimate the resultant earth pressure extending the mechanics of unsaturated soils rather than using the conventional approaches. In this paper, a brief background of effective and total shear strength of unsaturated soils is provided along with the details of how they can be extended in the estimation of resultant earth pressures on the retaining walls using some practical examples. 2 BACKGROUND Limited studies are available in the literature with respect to the estimation of earth pressures or critical height of vertical cuts in unsaturated fine-grained (hereafter referred to as UFG) soils extending the mechanics of unsaturated soils (Pufahl et al. 1983, Vanapalli et al. 2009 and Zhang et al. 2010). Pufahl et al. (1982) estimated the lateral earth pressure on retaining structures taking account of the influence of matric suction. They suggested that the most efficient way to visualize the influence of matric suction (i.e., negative pore-water pressure) on the lateral earth pressure is to include its

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