Instability and progressive failure of soil
Published:2015-01-02 Hits:1837
Failure and instability are typical problems in the geomechanics, while they are not synonymous. The failure of soil occurs when the stress difference reaches a limiting value. Instability behaviors, which correspond to the spontaneous change of the deformation mode in the next loading increment occurs. For non-associated materials, strain localization or diffuse instability, often occurs before the peak failure criterion is met. Strain localization demonstrates deformation concentrated in narrow zones, diffuse instability usually happens in the case of homogeneous problem and there may be loss of homogeneity randomly distributed in space (but non localized pattern). The onset and progressive evolution of instability behavior are the focus of this research.
(1)To explore micro-mechanism of fabric anisotropy and develop a micromechanics-based model accounting for non-coaxiality of soil.
(2)To develop a suitable predicting criterion (i.e. by using bifurcation theory and second-order work theory) for instability analysis of soils.
(3)To develop rational numerical method (with effective regularization technique) in modeling the evolution of diffuse instability and the strain localization. 
Predicting criterion for instability strongly depends on constitutive model of soil. Both macro and micro approaches are used to develop suitable non-coaxial models for initiation of instability. Diffuse instability of soil is related to behavior of transition from stationary to dynamic state. Due to Hill’s investigation, the criterion of diffuse instability is that sign of the second-work turns to negative. In the finite element program, the second-order work can be calculated to determine whether the material point is stable or not. The localization is induced by loss of elliptic control equations. When the localization occurs, the numerical resolution of finite element method is seriously dependent on mesh size. A non-local plastic model, which assumes mechanical behavior of a material point is related to its neighborhood, can regularize control equations and eliminate the mesh dependence. 
Significant Results and Potential Impact: 
This research provides a effective predicting criteria for the onset of diffuse instability and strain localization based on non-coaxial elasto-plasticity model. Combined with second order work criterion, a effective numerical method is built for analysis of diffuse instability of soil. This research solves strain localization problems by non-local plastic model, and proves its validity for regularization of control systems and elimination of mesh size dependence. It also provide a new stress return algorithm of non-local Mohr-Coulomb model in finite element method, which solving difficulty for numerical simulation of strain localization. This research is a comprehensive investigation of stability behavior of soil, which leads to a more profound understanding of the progressive failure of soil.

Numerical result of generalized plastic strain for softening plasticity with nonlocal model
Principal Investigators:
Maosong Huang, Jiangu Qian, Xilin Lu.
      China National Funds for Distinguished Young Scientists: Progressive failure theory of soil and engineering application. (50825803) 2009.1-2012.12
      National Basic Research Program of China (973 Program): The instability, flow failure and disaster evaluation of landfill. (2012CB719803) 2011.10-2016.10
      National Natural Science Foundation of China: Analysis and experimental study on the strain localization of saturated soil (50179025) 2002.1-2004.12.
      National Natural Science Foundation of China: Study on the noncoaxial plasticity model and non-proportional loading tests based on micro-fabric evolution.(10972159) 2010.1-2012.12. 
      National Natural Science Foundation of China: Bifurcation analysis and experimental study on the instability of soil under true triaxial conditions. (10402029) 2005.1-2007.12
      National Natural Science Foundation of China: Progressive failure analysis of the face stability of shield tunnel in complicated geological and environmental condition. (50908171) 2010.1-2012.12
Key Publication: 
      1.Maosong Huang, Xilin Lu, Jiangu Qian. Non-coaxial elasto-plasticity model and bifurcation prediction of shear banding in sands, International Journal for Numerical and Analytical Methods in Geomechanics. 2010,34(9):906-919.
      2.Qian JG, You ZP,Huang MS, GU XQ. A micromechanics-based model for estimating localized failure with effects of fabric anisotropy. Computers and Geotechnics. 2013,50:90-100
      3.Qian JG, Bardet JP, Huang MS. Spectral Classification of Non-coaxiality for Two-dimensional Incremental Stress-strain Response, Mathematical Problems in Engineering, 2010,Article ID 963043:1-20(doi:10.1155/2010/963043).
      4.Qian JG, Huang M S, Sun HZ. Macro-micromechanical approaches for non-coaxiality of coarse grained soils. Science China-Technological Sciences.2011,54(s1):147-153.
      5.Xilin Lu, Maosong Huang. Static liquefaction of sands under isotropical and K0 consolidated undrained triaxial conditions. Journal of Geotechnical and Geoenvironmental Engineering-ASCE. 2014, DOI: 10.1061/(ASCE)GT.1943-5606. 0001206
      6.Xilin Lu, Maosong Huang, Jiangu Qian. Prediction of plane strain undrained diffuse and localized instability with non-coaxial plasticity. Soils and Foundations. (in press)
      7.Xilin Lu, Jean-Pierre Bardet, Maosong Huang. Spectral analysis of nonlocal regularization in two-dimensional finite element models, International Journal for Numerical and Analytical Methods in Geomechanics. 2012,36(2):219-235.
      8.Xilin Lu, Maosong Huang, Jiangu Qian. The onset of strain localization in cross-anisotropic soils under true triaxial condition. Soils and Foundations, 2011,51(4):693-700.
      9.Xilin Lu, Jean-Pierre Bardet, Maosong Huang. Length scales interaction in nonlocal plastic strain localization of bars of varying section. Journal of Engineering Mechanics, ASCE. 2010, 136(8): 1036-1042.
      10.Xilin Lu, Jean-Pierre Bardet, Maosong Huang. Numerical solutions of strain localization with nonlocal softening plasticity, Computer Methods in Applied Mechanics and Engineering,2009,198(47-48):3702-3711.
      11.Qian JG, Yang J, Huang M S. Three-Dimensional Non-coaxial Plasticity Modelling of Shear Band Formation in Geomaterials. Journal of Engineering Mechanics, ASCE, 2008, 134(4):322-329