A Constitutive Model For Bonded Soil And Unsaturated Soil Based On The Micro-Mechanism Of Bonding-Breakage

The natural soil has many complex mechanical properties that are different from those of the remolded soil made in laboratory and the relevant study is a hot topic in geo-mechanics. It is said that "The establishment of mathematical model of natural soil is the key issue of geo-mechanics in 21-th century." On the other hand, most of the soil in engineering is unsaturated. So it is necessary to construct unsaturated soil constitutive models which can take account of the soil structural property. The state of the art in the constitutive relations for structural soil and unsaturated soil was summarized, including modified elastic-plastic models, damaged models, bounding surface models, Alonso model, Gallipoli D model, Wheeler model and so on. It also presents a constitutive model for construct unsaturated soil guidance in the binary-media model:Based on the micro observation with SEM and some assumption, a new method to partition the bonding element and frictional element in artificially bonded soil has been proposed according to the contact property between particles. A new breakage parameter has been defined clearly with bonding element volume in micro level. The basic equation of the breakage mechanics have been deduced by the basic integrating equation of the mechanics from micro to macro level.On the basis of analysis the experimental results of the unsaturated soil which subjected to isotropic conditions and the elastic-plastic theory, the relationship between plastic strain and stress of unsaturated soil has been deduced from the Gallipoli D model, which is formulated in terms of two constitutive variables directly related to these suction mechanisms: the average skeleton stress and an additional scalar constitutive variable,ξ, related to the magnitude of the bonding effect exerted by meniscus water at the inter-particle contacts. After differential processing , the control variable dp' and dξwere written with related to dp and ds . The comparison between the results of calculation with this method and the experiments which subjected to isotropic conditions and wetting-drying cycle of a sample subjected to a constant isotropic net stress in this thesis show ideal result.In the frame of the breakage mechanism, the new binary-media model has been established with the research results of the artificially bonded soil. This model considers the bonding element as elastic-brittle material and the frictional element as elastic-plastic material which can be described by the unsaturated soil elastic-plastic model. The number of parameters in this model is moderate and all of them have the defining mechanical meanings. The value of these parameters can be calibrated by the triaxial test with simple stress path in laboratory. The comparison between the results of calculation with this method and the experiments which subjected to isotropic conditions in this thesis show that this model can predict the main stress-strain relationship of the unsaturated artificially bonded soil in isotropic conditions.