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Title: Evolving anisotropy in unsaturated soils : experimental investigation and constitutive modelling
Author: Al-Sharrad, Muayad A.
ISNI:       0000 0004 2728 3071
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 2013
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This work explores the influence of evolving anisotropy on the stress-strain behaviour of unsaturated soils and proposes a new constitutive elasto-plastic model for unsaturated soils accounting for evolving anisotropy. An extensive campaign of laboratory tests on both isotropically and anisotropically compacted soil samples under a wide range of stress paths was performed. These experimental data were then employed in developing the new model and investigating its performance. A programme of controlled suction triaxial testing was performed on unsaturated and saturated samples of Speswhite kaolin prepared by two different methods of compaction: isotropic and anisotropic. Tests involved probing stress paths, to investigate the initial forms of the yield surface for isotropically compacted and anisotropically compacted samples at different suction values, and how the yield surface was altered by plastic straining caused by loading stages or by wetting stages with significant collapse-compression. Tests also included shearing to failure, to investigate critical state conditions. Experimental results were interpreted in terms of mean net stress p ̅, deviator stress q and suction s as stress state variables and, alternatively, interpreted in terms of mean Bishop’s stress (defined as p^*=p ̅+ S_r s), deviator stress q and modified suction (defined as s^*=ns, where n is the porosity). The experimental results showed that fabric anisotropy can evolve during plastic straining even for a soil that starts isotropic but is then loaded to anisotropic stress states. Also, the results showed that fabric anisotropy can evolve during wetting stages that involve collapse-compression. Furthermore, the results showed no apparent influence of initial or evolving anisotropy on the critical state, where both the initially isotropic and initially anisotropic samples, loaded at various stress path slopes, showed nearly the same critical states. Critical states can be represented in the q:p ̅ plane by a series of parallel lines for different values of suction and the constant suction cross-sections of the yield surface can be represented by distorted ellipses in the q:p ̅ plane, intersecting the negative axis at the point of intersection of the corresponding critical state line. Alternatively, critical states can be represented in the q:p^* plane by a single straight line (for all values of suction) passing through the origin, and constant suction cross-sections of the yield surface can be represented in the q:p^* plane by distorted ellipses passing through the origin (suggesting that the yield surface expression is simpler when expressed in terms of q,p^* and s^* rather than in terms of q,p ̅ and s). A new constitutive model was formulated in terms of Bishop’s stresses and modified suction based on the above observations and other considerations such as that representing the coupling between mechanical and water retention behaviour is easier with Bishop’s stress than with net stress. The new anisotropic model combines features from the isotropic model for unsaturated soils of Wheeler et al. (2003a) with features for modelling of anisotropy taken from the anisotropic model for saturated soils S-CLAY1. The new anisotropic constitutive model was developed solely as a mechanical model, unlike the constitutive model of Wheeler et al. (2003a), which is a combined mechanical and water retention model. Model simulations of mechanical behaviour with the new anisotropic model were performed by using experimental values of S_r (with no attempt to predict values of S_r), because it was then possible to check whether mechanical aspects of the model were performing well. Model simulations showed that significant improvement in the accuracy of the predicted soil behaviour was achieved by incorporating the role of evolving fabric anisotropy. However, model performance appears more satisfactory in simulating soil behaviour under unsaturated conditions than under saturated conditions. Also, the model is not entirely successful in predicting some aspects of anisotropic soil behaviour such as differences in initial specific volume between isotropically and anisotropically compacted samples.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TA Engineering (General). Civil engineering (General) ; TH Building construction