Simulating a direct shear box test by DEM S.H. Liu Abstract: Distinct element simulation was performed for direct shear box (DSB) tests on a dense and a loose two- dimensional (2D) sample of 3259 cylinders. Special attention was devoted to the effect that the frictional force between the inside surface of the upper shear box and the sample had on the measured shear strength in the DSB test. Some ways of minimizing this interface frictional force were introduced in the paper. Given that the deformation approxi- mates simple shear within the deforming zone across the sample centre (referred to as the shear zone), a method was proposed to evaluate the overall strains in the DSB test. The numerically simulated data were used to interpret, on a microscopic scale, the angle of internal friction and a 2D stress–dilatancy equation for the mobilized plane in granular material. It was found that the angle of internal friction in granular material is not directly related to the interparticle friction angle ( φ μ ). Instead, it relates to the average interparticle contact angle ( θ) on the mobilized plane and the ratio k/f 0 , representing the degree of the probability distribution of the interparticle contact forces that is biased toward the positive zone of the contact angle θ (along the shear direction), where k is the slope of the linear distribution of the av- erage interparticle contact forces against the interparticle contact angle; and f 0 is the average interparticle contact force. Key words: angle of internal friction, direct shear box test, distinct element method, friction, granular material, stress– dilatancy. Résumé : On a réalisé une simulation en éléments distincts pour les essais à la boîte de cisaillement direct (à laquelle l’abbréviation DSB est attribuée dans cet article) sur un échantillon 2D dense et lâche de 3 259 cylindres. On a ac- cordé une attention particulière à l’effet de la force de frottement entre l’échantillon et la surface intérieure de la partie supérieure de la boîte de cisaillement sur la résistance au cisaillement mesurée dans l’essai DSB. On a introduit dans cet article des façons de minimiser la force de frottement à l’interface. Considérant que la déformation est proche du cisaillement simple à l’intérieur de la zone en déformation au centre de l’échantillon (soit la zone de cisaillement), on a proposé une méthode pour évaluer les déformations globales dans l’essai DSB. Au moyen des données numériques simulées, l’angle de frottement interne et l’équation de contrainte de dilatance bidimensionelle sur le plan mobilisé pour un matériau granulaire ont été interprétés à l’échelle microscopique. On a trouvé que l’angle de frottement interne du matériau granulaire n’est pas en relation directe avec l’angle de frottement interparticule φ μ . Il est plutôt en relation avec l’angle moyen de contact interparticule θ sur le plan mobilisé et le rapport k/f 0 représentant la distribution de la probabilité des forces de contact interparticule tendant vers la zone positive de l’angle de contact θ (le long de la di- rection du cisaillement), où k est la pente de la distribution linéaire de la moyenne des forces de contact interparticule par rapport à l’angle de contact interparticule, et f 0 est la force moyenne de contact interparticule. Mots clés : angle de frottement interne, essai de cisaillement direct, méthode d’éléments distincts, frottement, matériau pulvérulent, contrainte–dilatance. [Traduit par la Rédaction] Liu 168 Introduction The testing of soils by applying a shear load (or displace- ment) has resulted in a worldwide revival of interest over the last few decades. Several types of laboratory device have been developed for directly determining the shear strength envelope for soils. Among them, the direct shear box (DSB) test, with both an upper shear box and a lower one, has most commonly been used, because the testing procedures are simple, and it is capable of approximately simulating the de- formation conditions of plane strain as occurs in many field problems. In the conventional DSB test, shearing of the sample is often achieved by pushing the lower shear box horizontally while the upper shear box is restrained verti- cally and horizontally (Taylor 1948; Skempton and Bishop 1950), as shown in Fig. 1. The shear force is measured with a bearing ring or a load cell that is attached to the upper shear box. In this DSB device, a frictional force is generated at the attachment point when the upper shear box moves up or down as a result of the volume change in the sheared sample (dilation or contraction). Sometimes, to prevent tilt- ing of the upper shear box during the shearing process, a clasp is set opposite the attachment point. In turn, the fric- tional force at the attachment point and the clasp restrain the upward or downward movement of the upper shear box. Consequently, a frictional force between the inside surface of the upper shear box and the sample is generated when the volume of the sheared sample changes (dilation or contrac- tion). Owing to this frictional force at the shear box – sam- Can. Geotech. J. 43: 155–168 (2006) doi:10.1139/T05-097 © 2006 NRC Canada 155 Received 4 February 2004. Accepted 19 October 2005. Published on the NRC Research Press Web site at http://cgj.nrc.ca on 26 January 2006. S.H. Liu. College of Water Conservancy and Hydropower Engineering, Hohai University, Xikang Road 1, Nanjing 210098, PR China (e-mail: sihong_hhu@yahoo.com.cn). Can. Geotech. J. Downloaded from www.nrcresearchpress.com by UNIVERSITY OF VIRGINIA on 10/07/13 For personal use only.