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 approximates 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 ([[phi].sub.[mu]]). Instead, it relates to the average interparticle contact angle ([bar.[theta]]) on the mobilized plane and the ratio [k/[f.sub.0], representing the degree of the probability distribution of the interparticle contact forces that is biased toward the positive zone of the contact angle [theta] (along the shear direction), where k is the slope of the linear distribution of the average interparticle contact forces against the interparticle contact angle; and [f.sub.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.
Resume : On a realise une simulation en elements distincts pour les essais a la boite de cisaillement direct (a laquelle l'abbreviation DSB est attribuee dans cet article) sur un echantillon 2D dense et lache de 3 259 cylindres. On a accorde une attention particuliere a l'effet de la force de frottement entre l'echantillon et la surface interieure de la partie superieure de la boite de cisaillement sur la resistance au cisaillement mesuree dans l'essai DSB. On a introduit dans cet article des facons de minimiser la force de frottement a l'interface. Considerant que la deformation est proche du cisaillement simple a l'interieur de la zone en deformation au centre de l'echantillon (soit la zone de cisaillement), on a propose une methode pour evaluer les deformations globales dans l'essai DSB. Au moyen des donnees numeriques simulees, l'angle de frottement interne et l'equation de contrainte de dilatance bidimensionelle sur le plan mobilise pour un materiau granulaire ont ete interpretes a l'echelle microscopique. On a trouve que l'angle de frottement interne du materiau granulaire n'est pas en relation directe avec l'angle de frottement interparticule [[phi].sub.[mu]]. Il est plutot en relation avec l'angle moyen de contact interparticule [bar.[theta]] sur le plan mobilise et le rapport k/[f.sub.0] representant la distribution de la probabilite des forces de contact interparticule tendant vers la zone positive de l'angle de contact [theta] (le long de la direction du cisaillement), ou k est la pente de la distribution lineaire de la moyenne des forces de contact interparticule par rapport a l'angle de contact interparticule, et [f.sub.0] est la force moyenne de contact interparticule.
Mots cles : angle de frottement interne, essai de cisaillement direct, methode d'elements distincts, frottement, materiau pulverulent, contrainte-dilatance.
[Traduit par la Redaction] Liu 168
Introduction
The testing of soils by applying a shear load (or displacement) 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 deformation 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 vertically 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 tilting of the upper shear box during the shearing process, a clasp is set opposite the attachment point. In turn, the frictional 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 contraction). Owing to this frictional force at the shear box--sample interface, the shear strength is generally overestimated for dilatant specimens (like coarse granular soils) but underestimated for contractive specimens in the DSB tests, as reported by Takada et al. (1996) and Sumi et al. (1997). Moreover, the DSB test is inevitably subject to such criticisms as the full stress and strain states are not defined; and only the horizontal shear stress ([[tau].sub.zx]) and the vertical stress ([[sigma].sub.z]) are available. The strains cannot be obtained from the measured horizontal (shear) displacement (D) and the vertical displacement (h) because of the nonuniformity of the deformation throughout the sample in the DSB test. For this reason, difficulties arise in understanding the relationship between parameters derived from the DSB test and those derived from other laboratory and field testing devices.
[FIGURE 1 OMITTED]
In this paper, distinct element simulation is performed for the DSB test for better understanding of the intrinsic drawbacks in this test, since it can provide microscopic information that is difficult to obtain experimentally, such as particle displacements and the particle-particle contact force network. The possible ways to remove or minimize the frictional force at the internal surface of the upper shear box are then introduced. On the basis of the numerically simulated results, a method to evaluate strains in the DSB test is proposed. Furthermore, the angle of internal friction and a stress-dilatancy relationship for granular material are interpreted on the microscopic scale.
Discrete modeling of the direct shear box test
Distinct element method
The discrete method used is the distinct element method (DEM) pioneered by Cundall (1971) and Cundall and Strack (1979). The DEM is a numerical technique that keeps track of the motion of individual particles and updates any contact with neighboring elements by using a constitutive contact law. In two dimensions (2D) each particle has three degrees of freedom (two translations and one …
No comments:
Post a Comment