Pre- and post-cyclic behavior of mixed clayey soils Abbas Soroush and Hossein Soltani-Jigheh Abstract: Soil structures are often comprised or supported by soils that are mixtures of cohesive and granular geomateri- als. These soils are termed as intermediate or mixed soils and are widely found in natural deposits and man-made soil structures. The objective of this paper is to study the behavior of mixed clayey soils under monotonic, cyclic, and post- cyclic monotonic loading, with the main focus on the last of these three. For this purpose, a number of strain-controlled monotonic, cyclic, and post-cyclic triaxial compression tests were carried out on ‘‘clay–sand’’ and ‘‘clay–gravel’’ mixtures in undrained conditions. The effects of sand (or gravel) contents, number of cycles, cyclic strain amplitude, consolidation pressure, and grain size on the mechanical behavior of the mixtures are evaluated. The major findings from the laboratory study are: (i) the addition of granular materials to the clayey soils increases excess pore-water pressure during strain- controlled monotonic undrained shearing and cyclic loading and (ii) during post-cyclic monotonic shearing, mixed clayey soils behave similarly to overconsolidated soils; the higher the granular material is, the higher the overconsolidation ratio would be. Based on the research results, an algorithm for estimating post-cyclic mechanical parameters of mixed clayey soils, without conducting cyclic and post-cyclic tests, is suggested. Key words: mixed clayey soils, post-cyclic loading, undrained triaxial testing. Re ´sume ´: Les structures de sol sont souvent compose ´es ou supporte ´es par des sols qui sont des me ´langes de ge ´omate ´riaux cohe ´sifs et granulaires. Ces sols sont nomme ´es interme ´diaires ou sols me ´lange ´s et sont retrouve ´s fre ´quemment dans les de ´- po ˆts naturels et dans les structures de sol fabrique ´es. L’objectif de cet article est d’e ´tudier le comportement de sols argi- leux me ´lange ´s soumis a ` des chargements monotoniques, cycliques et post-cycliques monotoniques, avec l’emphase mise sur ce dernier type de chargement. Pour ce faire, des essais en compression triaxiale monotoniques, cycliques et post- cycliques avec de ´formation contro ˆle ´e ont e ´te ´ effectue ´s sur des me ´langes « argile–sable » et « argile–gravier » dans des conditions non-draine ´es. Les effets sur le comportement me ´canique du contenu sable (ou gravier), du nombre de cycles, de l’amplitude de la de ´formation cyclique, de la pression de consolidation et de la granulome ´trie ont e ´te ´e ´value ´s. Les prin- cipaux re ´sultats de l’e ´tude en laboratoire sont : (i) l’ajout de mate ´riaux granulaires aux sols argileux augmente la pression interstitielle pendant le chargement en cisaillement et cyclique monotonique non-draine ´ avec de ´formation contro ˆle ´e et (ii) durant le cisaillement monotonique post-cyclique, les sols argileux me ´lange ´s se comportent de fac ¸on similaire aux sols sur-consolide ´s; plus il y a de mate ´riaux granulaires, plus e ´leve ´ sera le ratio de sur-consolidation. Selon les re ´sultats de la recherche, un algorithme qui estime les parame `tres me ´caniques post-cycliques de sols argileux me ´lange ´s est sugge ´re ´, sans avoir a ` effectuer les essais cycliques et post-cycliques. Mots-cle ´s : sols argileux me ´lange ´s, chargement post-cyclique, essai triaxial non-draine ´. [Traduit par la Re ´daction] Introduction Soil structures are often comprised or supported by soils that are mixtures of cohesive and noncohesive geomaterials, so-called intermediate or mixed soils. These soils contain broad grading, ranging from clay and silt particles to gravel, cobble, and even boulder. ‘‘Alluvial deposits and tills’’ and ‘‘mixed clayey soils used as the sealing element in earth dams’’ are, respectively, examples of natural and artificially- blended mixed geomaterials. Knowledge of strength and de- formation characteristics of such materials is necessary for stress–strain and stability analyses of soil structures. It is common practice to design soil structures, such as earth dams, to withstand static and dynamic (e.g., earthquake) loads. However, a review of the performance of earth dams during earthquakes shows that most of the failures of earth dams have occurred either a few hours or up to 24 h after an earth- quake. This phenomenon, called delayed failure or delayed re- sponse, means that the critical period for an earth dam is not only the period of shaking, but also a period of hours following the earthquake. Therefore, this necessitates investigation of the post-earthquake behavior of such structures (Seed 1979; Ishihara 1984; Gu et al. 1993; Soroush et al. 1996). According to published literature, some researches have been carried out to understand the behavior of clay–granular material mixtures under monotonic and cyclic loading. How- ever, these researches are not sufficient; more importantly, the post-cyclic behavior of such soils has never been studied. Received 7 January 2008. Accepted 30 September 2008. Published on the NRC Research Press Web site at cgj.nrc.ca on 2 February 2009. A. Soroush 1 and H. Soltani-Jigheh. 2 Department of Civil and Environmental Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran. 1 Corresponding author (e-mail: soroush@aut.ac.ir). 2 Present address: Azarbaijan University of Tarbiyat Moallem, Tabriz, Iran. 115 Can. Geotech. J. 46: 115–128 (2009) doi:10.1139/T08-109 Published by NRC Research Press