Analysis Of Submacroscopic Deformation Mechanism Of Frozen Silty Clay And A Study On Its Anisotropic Damage Models

The rapid development of engineering construction in cold regions urges us to make further studies on the mechanical properties and deformation mechanism in microscopic and submacroscopic scale, and set up more effective constitutive models to describe the stress-strain behavior. It has been shown by the well-documented data that under loads there exist at the same time, the phenomena of emerging and growth of a large number of micro-cracks and reorientation of soil particles, with the former resulting in the reduction of effective bearing area, and consequently the degradation of bearing capacity of frozen soil; the latter leading to anisotropy of mechanical properties. Therefore, it is appropriate and necessary to describe the deformation process of frozen soil by anisotropic damage theories. Based on the experimental results, analysis of submacroscopic deformation mechanism of frozen silty clay has been made, and an anisotropic damage model has been set up for frozen soil. The main results of this paper is in detail listed in the following:1, On the basis of a lot of testing results, the stress-strain properties and variational laws of volume strain versus axial strain are analyzed at different confining pressures and different temperatures. It is found that all the stress-strain curves behave in the same form of "S", and exhibit the similarity in the form at the same confining pressure. Increase in confining pressures will evidently improve plasticity of frozen soil. Because of the dominance of viscoplastic deformation in the creep process, creep curves under triaxial compression behave without the third phase of atypical creep curve, viz. gradual flow phase.2, Triaxial shear strength of frozen silty clay increases with the decrease in temperatures and with the increase in confining pressures and strain rates, and changes in linearity with the absolute value of temperatures and confining pressures, and in exponent with strain rates. Its elastic modulus increases linearly with the decrease in temperatures and exponentially with strain rates, and behaves independent of confining pressures.3, The CT-images and variation of CT-numbers during the triaxial compression reveal that there have already been existing various forms of micro cavities, microcracks and low-density regions in the cross section of frozen soil before loads applied, and structural cells, mineral particles and cemented ice particles distribute randomly. Under loads, the rearrangement and reorientation of mineral particles and ice crystals restrict deformation and breakdown of frozen soil and consequently result in anisotropy in mechanical response. Similar to those of creep process, there also exist strengthening and softening phenomena at the same time in triaxial compression process. Plastic damages, induced by reciprocal sliding between soil particles and/or cemented ice particles in pores and their dislocations, can not be determined by CT-numbers.4, According to the basic principles and theories of irreversible thermodynamics, an elastoplastic anisotropic constitutive equation is formulated by introduction of plastic positive function considering the effect of hydrostatic pressure. The anisotropic damage evolution equation is derived from the presented damage accumulated positive function based on the equivalent strain energy release rate in the space of strain energy release rates. All the parameters in the presented model can be determined by use of data of uniaxial and triaxial tests, and influencing factors on them and laws are also remarked in this paper. Comparisons between the experimental results and calculated ones by the presented model initially prove it correct.Basic FEM equations are derived for elastic-plastic anisotropic damage analysis.5^ By introduction of Perzyna-type equation to the viscoplastic deformation analysis of frozen soil, an elastoviscoplastic anisotropic constitutive equation is obtained, with its special form— creep constitutive equation. An extension of inviscid damage to viscous damage, an viscoplastic anisotropic damage evolution equation is derived. According to existence of the structural strengthening and softening, an anisotropic damage evolution equation form is supposed for creep process, which can reflect changes of submacroscopic structure of frozen soil. The parameters in this model can be determined by triaxial compression and creep tests, and influencing factors on the parameters and their influencing laws are also remarked. Comparisons between the experimental results and calculated ones by the presented model initially prove it correct. Basic FEM equations are derived for elastoviscoplastic anisotropic damage analysis.6^ Calculated results by viscoplastic anisotropic damage model reveal that under the same conditions, damage at a higher strain rate is greater than that at a lower one, which accounts for the fact of greater elastic modulus and shear strength at higher strain rates from damage mechanics point of view, and also for the existence of the retarding of germination and growth of microcracks in frozen soil at higher strain rates.