Behaviour of clay treated with cement & fibre while capturing cementation degradation and fibre failure - C3F Model

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Journal Article
International Journal of Plasticity, 2016, 81 pp. 168 - 195
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Crown Copyright © 2016 Published by Elsevier Ltd. All rights reserved. Soil treated with cement becomes brittle because its shear strength decreases rapidly in a post-peak state, which is why in recent years the inclusion of fibre into soil treated with cement has become an increasingly popular research area. This paper presents a constitutive model to simulate the behaviour of the fibre reinforced cement treated soil, referred to as the improved soil composite. In this model, a non-linear failure envelope was formulated to merge with the Critical State Line (CSL) of the reconstituted soil mixture at high levels of stress in order to capture the broken cementation bonds and ruptured fibre. A non-associated plastic potential function and a general stress strain relationship that includes the softening of the composite soil were also proposed to simulate the pre-and-post peak state. Moreover, many researchers focus on the addition of fibre into sand, soft clay, and sand treated with cement, whereas the behaviour of soft clay treated with fibre and cement requires further investigation. Hence, in this study a series of undrained triaxial tests were carried out on natural Ballina clay treated with cement and 0.3%-0.5% of fibre to determine how the amount of fibre and cement affects the behaviour of soft clay. SEM images were also analysed to study the structure of the improved Ballina composite at the micro-structural level. The laboratory results indicated that the combined effects of cementation and fibre reinforcement increased the shear strength and ductility of treated soft clay. Under triaxial conditions the peak shear strength of soft clay treated with cement and fibre increases dramatically due to the formation of cementation bonds and the bridging effect provided by the fibres, and the brittleness caused by the cementation bonds breaking also improves significantly due to the inclusion of fibre. However, when shearing at a high mean effective stress the cementation bonds break and the fibre ruptures due to the mean effective stresses and plastic deviatoric strain which caused major cracks to appear within the sample. The performance of the model was evaluated by comparing its predictions with the results of the undrained triaxial tests conducted on the improved Ballina clay composite. By capturing the main features of the composite soil the model provided reliable predictions that agreed with the experimental results.
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