Creep behavior of unsaturated cohesive soils subjected to various stress levels
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ORIGINAL PAPER
Creep behavior of unsaturated cohesive soils subjected to various stress levels Jamel Kamoun 1 & Mounir Bouassida 1 Received: 17 January 2017 / Accepted: 8 January 2018 # Saudi Society for Geosciences 2018
Abstract This paper presents the results of an experimental investigation carried out on disturbed clay samples to analyze their creep behavior. The experimental program included compression triaxial tests with varied degree of saturation of the samples and stress level. Triaxial tests were performed at imposed strain rate varied from 0.005 to 0.10%/min. Various stress paths have also been tested including isotropic consolidation and deviatoric stress. The obtained results showed that shear deformation increases with the initial degree of saturation and stress level of clay samples. At fixed stress level and for both isotropic and deviatoric stress paths, the initial degree of saturation and the second invariant of the strain tensor varied with a similar trend. The variation of axial creep strain is correlated with time in a semi-logarithmic function. Keywords Creep . Degree of saturation . Clay . Experimental behavior . Strain rate . Stress level
Introduction A creep test is performed when a specimen of any material is subjected to a constant stress level for which the variation of strain is observed during time. The study of long-term behavior of soils is essentially conducted under constant loading. Creep strain is generally observed in partially or totally saturated cohesive soils subjected to constant stress level after the end of primary consolidation, e.g., when excess of pore pressure is completely dissipated. For saturated cohesive soils, after the primary consolidation which takes a long period, the creep phase is usually observed. During the creep phase, the void ratio gradually decreases under constant effective stress (Gudehus 1990). The creep behavior of cohesive soils happening after the first loading is clearly more important than that observed during the reloading phase. Further, for the same clay, the rate of creep of normally consolidated samples is higher than that of over-consolidated samples (Schiffman et al. 1964). The creep behavior is also affected among other factors by the stress * Mounir Bouassida [email protected] 1
Université de Tunis El Manar, Ecole Nationale d’Ingénieurs de Tunis, Ingénierie Géotechnique, LR14ES03. BP 37 Le Belvédère, 1002 Tunis, Tunisia
conditions as well as by the degree of saturation (Kamoun 2003). The creep behavior of cohesive soils under constant effective stress is characterized by the factor CB, which is the slope of the semi-logarithm time-strain line; this factor was introduced by Gudehus (1990): CB ¼
ε2 −ε1 ln ðt 2 =t 1 Þ
ð1Þ
t1 and ε1 are the time and the axial deformation at the end of the consolidation phase. t2 and ε2 denote the same data in the creep phase. CB can be also expressed in terms of the void ratio e: CB ¼ −
e2 −e1 1 þ e0
1 ln ðt 2 =t 1 Þ
ð2Þ
e0 = initial void ratio of the tested soil. e1 and e2
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