Modeling shear behavior and strain localization in cemented sands by two-dimensional distinct element method analyses M.J. Jiang a,⇑ , H.B. Yan a , H.H. Zhu a , S. Utili b,1 a Dept. of Geotechnical Engineering, Tongji University, Shanghai 200092, China b Dept. of Engineering Science, Oxford University, Oxford OX1 3PJ, UK article info Article history: Received 25 November 2009 Received in revised form 7 September 2010 Accepted 8 September 2010 Available online 12 October 2010 Keywords: Cemented sand Bond breakage Strain localization Numerical analyses Distinct element method abstract This paper presents a numerical investigation of shear behavior and strain localization in cemented sands using the distinct element method (DEM), employing two different failure criteria for grain bonding. The first criterion is characterized by a Mohr–Coulomb failure line with two distinctive contributions, cohe- sive and frictional, which sum to give the total bond resistance; the second features a constant, pressure- independent strength at low compressive forces and purely frictional resistance at high forces, which is the standard bond model implemented in the Particle Flow Code (PFC2D). Dilatancy, material friction angle and cohesion, strain and stress fields, the distribution of bond breakages, the void ratio and the averaged pure rotation rate (APR) were examined to elucidate the relations between micromechanical variables and macromechanical responses in DEM specimens subjected to biaxial compression tests. A good agreement was found between the predictions of the numerical analyses and the available experimental results in terms of macromechanical responses. In addition, with the onset of shear band- ing, inhomogeneous fields of void ratio, bond breakage and APR emerged in the numerical specimens. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Most natural soils are characterized by a bonded structure aris- ing from various processes, for example, the solution and deposi- tion of silica at particle contacts [1]. Soils are also sometimes artificially cemented by chemical agents in ground-treatment pro- cesses. Experimental results in the literature have shown that ce- mented soils have peculiar behaviors different from uncemented ones, such as stiffening at low pressure followed by yielding in a manner similar to overconsolidated soils [2,3], enhanced strength [4–8], a relatively brittle stress–strain response and a more dilative volumetric response [6,9–11]. These findings have encouraged extensive research into constitutive models of cemented soils. Several continuum constitutive models have been suggested to describe some important features of structured soils by previous researchers [12–21]. Although the models differ in mathematical details, they are all based on the principle that the size of the state-boundary surface increases with interparticle bonding (for instance, see [20]). However, concerning cemented soils, only the macroscopic response can be observed in the laboratory, while the mechanisms taking place at the micromechanical level remain mostly unknown. One of the motivations for the present study was to bridge the gap between macro- and micromechanics for these types of soils. In the past decades, a number of theoretical, experimental and numerical works have been carried out on strain localization, par- ticularly in granulates [22–26]. Regarding experimental works, the 1c2e apparatus developed in Grenoble in the 1990s was the first intended to relate micromechanics to macromechanics by running biaxial tests on Schneebeli wooden rods [27,28]. However, only a few reports can be found on strain-localization analysis in clays due to the theoretical, numerical and technical difficulties related to this research field [29–33]. For example, it is very difficult to gather sufficient microscopic data from specimens in the labora- tory even with advanced technologies such as X-ray techniques [34–36], stereophotogrammetric techniques [37], or particle-im- age velocimetry [38]. Such an unsatisfactory situation extends to the analysis of strain localization in cemented sands, which consti- tuted another strong motivation for this study. It is authors’ opinion that the distinct element method (DEM) presents an effective method to investigate the global mechanical behavior, strain localization, and associated micromechanisms occurring in cemented sands. The DEM was first proposed by Cundall and Strack [39], in which a detailed description of the method can be found. The main objective of this study was to provide insight into the shear behavior and strain localization tak- ing place in cemented sands by DEM analyses. For this purpose, a series of biaxial compression tests was run on assemblies of 2D 0266-352X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.compgeo.2010.09.001 ⇑ Corresponding author. Tel./fax: +86 21 65980238. E-mail address: mingjing.jiang@tongji.edu.cn (M.J. Jiang). 1 Formerly at Strathclyde University, Glasgow, UK. Computers and Geotechnics 38 (2011) 14–29 Contents lists available at ScienceDirect Computers and Geotechnics journal homepage: www.elsevier.com/locate/compgeo