Shear test results for cohesion and friction coefficients for different granular materials: scaling implications for their usage in analogue modelling

Laboratory tests have been carried out on dry granular materials such as quartz sand, glass microspheres and sugar with different grain size, rounding and sphericity. The measurements have been made with a simple sheer test machine for different values of normal stress (similar to 50-900 Pa). Shear stress has been plotted against normal stress in order to determine the cohesion and coefficient of internal friction for the investigated materials. Resulting values of cohesion and coefficient of internal friction are mainly dependent on rounding and sphericity, while grain size has a less significant influence. Further, the behaviour of the materials for very small normal stresses (similar to 0-400 Pa) is more complex than previously assumed. The fracture envelopes for all materials investigated are convex-outward for this small range and converge towards a straight failure envelope with increasing normal stress. Finally, in extensional faulting experiments, there is no significant change in fault dip with increasing depth. Therefore, the non-linear behaviour for small normal stresses is best described as a dependence of the cohesion on the normal stress and not as a dependence of the coefficient of internal friction on the normal stress. Values for cohesion increase from similar to 0 Pa (+/-15 Pa) at zero normal stress to 137-247 Pa (+/-15 Pa) for normal stresses greater than similar to 250-400 Pa. The results show that well-rounded, spherical material is better suited to model brittle behaviour of rocks in crustal and lithospheric scale analogue models than less well-rounded material, since it has a smaller cohesion and a coefficient of internal friction, which is closer to values of natural rocks. (C) 2000 Elsevier Science B.V. All rights reserved.