Granular Matter (2014) 16:69–78 DOI 10.1007/s10035-013-0455-3 ORIGINAL PAPER Strong force networks in granular mixtures N. H. Minh · Y. P. Cheng · C. Thornton Received: 21 January 2013 / Published online: 27 October 2013 © Springer-Verlag Berlin Heidelberg 2013 Abstract Using the results of 3D discrete element method simulations we study the force transmission through binary mixtures of sand and silt sized spheres under one-dimensional compression. Three types of contact are categorized depend- ing on the size of the two spheres in contact. The contributions of each contact type to the deviator stress are dependent on the proportion of silt sized spheres. We demonstrate that the magnitude of the deviator stress is solely due to the normal and tangential forces at contacts transmitting normal forces greater than a characteristic normal force, which is generally slightly greater than the average normal force. The maxi- mum packing efficiency was obtained with the mixture of 30 % silt sized spheres and this mixture corresponds to a transition point when there are enough silt sized particles to start to separate the sand sized particles from each other and establish contacts between silt sized spheres that contribute to the deviator stress. Keywords Granular mixtures · Discrete element · Strong force transmission · Characteristic force · Force distribution · One dimensional compression N. H. Minh (B ) School of Engineering, Nazarbayev University, Astana 010000, Republic of Kazakhstan e-mail: minh.nguyen@nu.edu.kz N. H. Minh· Y. P. Cheng Department of Civil, Environmental and Geomatic Engineering, University College London, Gower Street, London WC1E 6BT, UK C. Thornton School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK 1 Introduction The behaviour of granular systems is highly dependent on the size distribution of the constituent particles. Force trans- mission through assemblies of discrete particles of different sizes can be characterized by a high degree of force inhomo- geneity. This happens when a small number of contacts in the system transmit forces of significantly higher magnitude than the other contacts. The force transmission can be represented by two distinct classes called the strong force and the weak force contact networks [1, 2]. The strong forces were defined as contact forces contributing primarily to the macroscopic deviator stress and hence they determine the shear strength of the sample. Column-like structures [3] carrying strong forces were observed inside granular systems, and the buckling of these structures (i.e. the strong force chains) leads to the development of shear bands [4]. The contacts transmitting weak forces, on the other hand, tend to be oriented perpen- dicular to the loading direction and support the stability of the strong force chains. The weak forces only contribute to the isotropic stress component or have a negligible contribu- tion to the deviator stress. For slightly polydisperse systems [1, 2], the characteristic force that divides the strong forces and the weak forces is approximately the mean normal con- tact force of the whole system. The relative proportion of the strong contacts in a three dimensional system [2] is about one third of the total number of contacts. These characteristics of the contact force network are important features of granular media that reflect the high level of redundancy in the system in which only some of the contacts are required to resist the overall applied deformation. In this study, we investigate the effect of particle size dis- tribution on the compression behaviour of imperfect binary mixtures of a polydisperse sand-sized material and a finer uniform silt-sized material; the maximum size ratio in the 123