The Research Of Hypoplastic Constitutive Model Of Cohesionless Soil

Based on hypoplasticity, mechanism of cohesionless soil is researched. The latestresearch and development of hypoplasticity is summarized. Fundamental of hypoplasticity isintroduced. The two hypoplastic models—Wu-Bauer hypoplastic model and Gudehus-Bauerhypoplastic model are thoroughly investigated. The simulation and ameliorated work ofGudehus-Bauer hypoplastic model under complex stress state are at emphasis discussed. Thespecific research work can be summarized as follows:(1) The basic concept of hypoplasticity is briefly introduced. Differences betweenhypoplasticty and traditional elastoplastic model are discussed. Classification ofhypoplasticity is studied. Evolvement from cohesionless soil to cohesive soil in hypoplasticityis analyzed. Application of hypoplasticity to shear band, liquefaction of saturated sand andprediction of the bearing capacity of footings is summarized, suggestions are made for futurestudies for hypoplastic model.(2) Determination of constitutive parameters in Wu-Bauer hypoplastic model andGudehus-Bauer hypoplastic model is introduced. The defect of method of determiningparameters in Wu-Bauer hypoplastic model is discussed. According to simplex designs inoptimization, amelioration of parameter determination is made. The method is proved byvirtue of triaxial consolidated drained test, oedometric test and load-unload test. It is shownthat this method can ascertain the parameters. The merit and demerit of Wu-Bauer hypoplasticmodel are analyzed.(3) The stress-strain relation of cohesionless soil can be modeled with Gudehus-Bauerhypoplastic constitutive model, in which the magnitude and direction of stress incrementdepend on not only the previous stress state, but also the bulk and direction of current strainincrement. For analyzing differences of this model and elastoplastic theory, the linear termand nonlinear term of Gudehus-Bauer model can be investigated. The stress-strain relation ofGudehus-Bauer hypoplastic model in differently original void ratio is discussed. The resultsshow that Gudehus-Bauer hypoplastic model can consider irreversible deformation withoutrecourse to decomposition of strain into elastic and plastic parts. This model can also embodydilatancy, strain-softening of dense sand and contraction, strain-hardening of loose sand.Gudehus-Bauer hypoplastic model also consider some stress path and strain path ofcohesionless soil. (4) The reason that Gudehus-Bauer hypoplastic model can not consider the effect ofmedium principal stress is analyzed by virtue of Gudehus's response envelope theory. Aimingat the defect without considering medium principal stress, the three ameliorations are broughtforward. The formerly two ameliorations are based on revision of Duncan-Zhang modelconsidering medium principal stress. The last ameliorations is the nonlinear term of the modelmultiplying by one factor that considers medium principal stress parameter. Those threeameliorations enhance stiffness of soil and change deformation of soil. The revised modelsare compared with true triaxial test of constant medium principal stress parameter. It is shownthat the revised models can simulate that medium principal stress affects on strength anddeformation of sand at degree and predict approximately volumetric strain caused by mediumprincipal stress.(5) Based on basic principle of nonlinear continuum mechanics, the amelioratedGudehus-Bauer hypoplastic model considering principal stress axes rotation is presented. Theameliorated model insists that the magnitude and direction of stress increment depend on thebulk of current principal stress state and the direction of current increment of principal stressaxes rotation. The computing results of the ameliorated model compare with the relevantexperimental data by the cases. The results show that the revised Gudehus-Bauer hypoplasticmodel can consider strength and deformation of sand under principal stress axes rotation.(6) As Gudehus-Bauer hypoplastic model can not consider cross-anisotropy of sand, themodel introduced by matrix of cross-anisotropy is improved. The cross-anisotropy of sand isdistinguished with the angle of deposited orientation in the revised Gudehus-Bauerhypoplastic model. The change of density is dominated by the current void ratio. Thecross-anisotropy of sand in consolidated drained triaxial test is computed by the improvedGudehus-Bauer hypoplastic model and computing results compare with the relevantexperiment. It is shown that the improved Gudehus-Bauer hypoplastic model can simulatecross-anisotropy of sand and distinguish quantitatively that initial void ratio affects onstrength and deformation of sand with cross-anisotropy.(7) As the forged ratcheting considering cycle loading with hypoplastic model isproduced, intergranular strain tensor considering contact deformation of grains is introducedin hypoplastic model. Compared with original hypoplastic model, the revised hypoplasticmodel can reflect mechanics of sand in cycle loading. That includes accumulation ofdeformation and accumulation of pore water pressure and decrease of effective stress.