INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS Int. J. Numer. Anal. Meth. Geomech., 2006; 30:45–70 Published online 7 September 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/nag.474 Interaction between different internal length scales in strain localization analysis of fully and partially saturated porous media}the 1-D case B. A. Schrefler 1,n,y , H. W. Zhang 2 and L. Sanavia 1 1 Department of Structural and Transportation Engineering, University of Padova, via F. Marzolo 9, 35131 Padova, Italy 2 Research Institute of Engineering Mechanics, Department of Engineering Mechanics, State Key Laboratory of Structural Analysis and Industrial Equipment, Dalian University of Technology, 116024 Dalian, People’s Republic of China SUMMARY The paper analyses the interaction between two internal length scales during dynamic strain localization in multiphase porous materials. The first internal length is introduced in the mathematical model by the gradient-dependent plasticity for the solid skeleton, while the second one is naturally contained in the multiphase model and is due to the seepage process of the water via Darcy’s law, which induces a rate- dependent behaviour of the solid skeleton. Numerical results of a one-dimensional example of water saturated porous medium demonstrate the competing effect between these two length scales. The porous medium is here treated as a multiphase continuum, with the pores filled by water and air, the last one at constant atmospheric pressure. Copyright # 2005 John Wiley & Sons, Ltd. KEY WORDS: strain localization; saturated and partially saturated porous media; gradient-dependent plasticity; internal length scale 1. INTRODUCTION Strain localization or shear bands are strain accumulations in well-defined narrow zones, which have been observed in many engineering materials including water saturated and partially saturated porous materials. As far as such materials are concerned, it is recalled that Rice [1], Rudnicki [2] and Vardoulakis [3] investigated the stability of water saturated inelastic porous Contract/grant sponsor: EC International Scientific Cooperation Programme Contract/grant sponsor: The Program for Changjiang Scholars and Innovative Research Team in University of China (PCSIRT) and the National Key Basic Research Special Foundation of China; contract/grant number: (2005CB321704) Contract/grant sponsor: National Natural Science Foundation of China; contract/grant number: 10225212, 10421002 Contract/grant sponsor: Italian Ministry of Education, University and Research; contract/grant number: MIUR 2003081021 004 Received 6 October 2004 Revised 21 July 2005 Accepted 25 July 2005 Copyright # 2005 John Wiley & Sons, Ltd. y E-mail: bernhard.schrefler@unipd.it n Correspondence to: B. A. Schrefler, Department of Structural and Transportation Engineering, University of Padova, via F. Marzolo 9, 35131 Padova, Italy.