Study On Large Strain Consolidation Of Soft Soil Considering Rheological Effect

Consolidation theory is one of the basic subjects of soil mechanics, since one dimensional consolidation theory was proposed by Terzaghi in 1920s, there were many researchers had made further researches and continuously improvement on the basis of Terzaghi's theory. There is widely distributed of deep soft soil along the coastal areas and rivers all over the world, people found that the consolidation displacement usually was very large, then the hypothsises of Terzaghi's theory were not suitable any more, and the hypothesis of large strain was needed. In the same way, with the increasing strict requirement of foundation post-construction settment, the requirement of reasonably and accurately prediction of foundation settment process was higher and higher, the rheological property of soft soil was paid more and more attention. Based on this, the thesis studied the large strain consolidation of soft soil considering rheological effect through theory analysis, laboratory experiment and engineering example. The main work of this thesis is as follows:(1) On the basis of continuum mechanics principle, the balance equation and the continuity equation of large strain consolidation were derived by updated Lagrange description considering the geometric nonlinearity and material nonlinearity simultaneously, further more:the FEA control equation of large strain consolidation was obtained.(2) A series of one dimensional consolidation rheological tests were taken on Ningbo soft soil, and triaxial consolidated drained rheological tests were taken through self-refitted stress-controlled triaxial apparatus. The results of one dimensional consolidation rheological tests and triaxial rheological tests were compared, based on which the rheological property of Ningbo soft soil was analyzed systematically.(3) The Nishihara model was extended to three dimensions, and the plastic yield of soil was described by modified Cambridge model combined with over-stress theory. The constitutive relationship of soil was derived, and a three dimensional rheological model which can describe the rheological property of soil comprehensively was set up. The model was fitted by triaxial rheological tests, and the model parameters of different confining pressure and deviatoric pressure were obtained.(4) On the basis of general FEA software ABAQUS, the user material subroutine (UMAT) of the rheological model proposed by this thesis was coded. The correctness of the UMAT was verified by a uniaxial compression numerical simulation. Based on the model of this thesis, large strain consolidation of soft soil was studied considering rheological effect, self load and permeability nonlinearity simultaneously, and the rules of the dissipation of the excess pore pressure and the displacement of soil was analyzed. The results of large strain were compared with that of small strain, and the sensitivity of the model parameters was analyzed.(5) Using the proposed rheological model and the fitted model parameters, three dimensional numerical simulation was performed on a surcharge preloading test of soft soil. The measured data of the displacement of soil and the dissipation of excess pore pressure was compared with the computed results, through which the property of the proposed model and the UMAT was verified.Considering the large strain effect and rheological effect simultaneously, the results of this thesis have important reference significance in the foundation treatment of soft soil. The results also will be an enrichment of the consolidation theory of soft soil and an establishment of the foundation of further study of complex nonlinearity consolidation problem.