Mechanical behaviour of an unsaturated compacted residual soil

F.A.M. Marinho, O.M. Oliveira, S. Vanapalli, H. Adem

In the proceedings of: 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: This paper provides a series of experimental test results performed on a residual soil from a site of the University of São Paulo in São Paulo, Brazil. The focus of these tests were to study the shear strength behaviour by conducting modified unconfined compression tests as well as constant water (CW) content triaxial tests on specimens compacted at different initial water content conditions. The matric suction in the soil specimens was precisely measured during the tests by using high capacity tensiometer (HCT). The experimental results suggest that the shear strength behaviour is significantly influenced by the initial compaction water content. An empirical model is proposed using the results of the present study to predict the 3D failure envelope of the tested compacted residual soil for any condition of the initial stress state.

RÉSUMÉ: Cet article présente une série d'essais expérimentaux menés sur un sol résiduel d'un site de l'université de São Paulo, au Brésil. L'emphase de ces essais a été mise sur l'étude du comportement de résistance au cisaillement lors d'essais de compression non-confinés et d'essais triaxiaux à teneur en eau constante (EC) sur des échantillons compactés sous différentes conditions de teneur en eau initiale. La succion matricielle dans les spécimens a été mesurée avec précision durant les essais en utilisant un tensiomètre à haute capacité (THC). Les résultats expérimentaux suggèrent que la résistance au cisaillement est influencée de manière marquée par la teneur en eau initia le lors du compactage. Un modèle empirique est proposé faisant usage des résultats de la présente étude afin de prédire itions de contrainte initiale. 1 INTRODUCTION Residual soils formation is attributed to the mechanical and chemical weathering or disintegration of rocks. The behaviour of these soils is dependent on their composition and also on the environmental conditions under which they were formed. The upper layers of the residual soil are typically fine-grained in nature and below which are partially disintegrated parent rocks. In other words, residual soils commonly remain in place over the rocks from which they are originally formed. Such soils are found in many regions of the world and more widely distributed in Brazil. These types of soils are used commonly in the construction of both geotechnical and geoenvironmental structures such as embankments, pavements and soil barriers. The residual soils show significantly different characteristics in comparison to other natural soils and are difficult to be classified using conventional procedures. The conventional soil mechanics principles also do not offer rational interpretation of residual soils mechanical behaviour because they are typically in a state of unsaturated condition. Therefore, geotechnical engineers are interested in understanding the mechanical behaviour of both natural and compacted residual soils which are sometimes referred in the literature as problematic soils (Rahardjo, et al, 1995, Pereira, et al., 2000; Toll and Ong, 2003; Rahardjo, et al, 2004a, Rahardjo, et al, 2004b; Futai and Almeida, 2005; Matsushi and Matsukura, 2006; Kayadelen, et al., 2007; Jotisankasa and Mairaing, 2010). The residual soils mechanical behaviour is also sensitive to wetting and drying cycles (Toll and Ong, 2003; Rahardjo, et al, 2004a). For this reason, the influence of initial compaction water content as it relates to the soil shear strength behaviour is important and hence investigated in this paper. The soil used in the present study is a residual soil obtained from the experimental site of the University of São Paulo in São Paulo, Brazil. The unsaturated shear strength of compacted residu-al soil is studied by conducting unconfined compression tests and constant water content (CW) tests. The CW tests were conducted in the triaxial shear test equipment introducing some modifications to accommodate the high capacity tensiometer (HCT). Along similar lines, HCT was used to measure the matric suction changes during the shearing stage of conducting the unconfined compression tests. The HCT used for the present investigation was built at the University of São Paulo to measure matric suction in the range of 0 to 500 kPa (Oliveira and Marinho, 2008). The shear strength behaviour of the soil under saturated conditions was performed by Oliveira (2004). The experimental results of the present study and that of Oliveira (2004) have been used to develop a 3-D empirical model to predict the failure envelope of compacted residual soils of São Paulo, Brazil for any condition of the initial stress state. The results of the study suggest that the initial moulding water content of the compacted residual soils has considerable influence on the shear strength.

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