Large rock fragments were encountered during the in situ shear testing on the Questa mine rock pile materials, New Mexico, USA. The in situ shear boxes used in the field were 30 cm × 30 cm and 60 cm × 60 cm in size. In order to evaluate the effect of these oversize particles on the friction angle and cohesion of the rock pile material, several laboratory shear tests with a box 6 cm × 6 cm in size were conducted where the oversize particle was simulated using stainless steel spheres that were placed along the shear plane. The experimental findings suggest that the presence of the oversize particle causes an increase in friction angle while the cohesion can either decrease or increase depending on the size of the oversize particle with respect to the size of shear box and the location of the oversize particle along the shear plane.
1. INTRODUCTION
The effect of oversize particles on shear strength of soil has been studied by some researchers with emphasis on the role of scalping on the mechanical behavior of rock fill dam material. Holtz and Gibbs [1] conducted several triaxial tests on river sand and gravel and quarry material to investigate the effect of the density, maximum particle size, particle shape, and amount of gravel on the shear strength of the material. All specimens were saturated before testing and the loading rate was slow enough to allow free drainage of the material without generating any excess pore water pressure. To study the effect of specimen size on the shear strength, very small (34.9 mm × 76.2 mm), small (82.5 mm × 206.4 mm), medium (152.4 mm × 381 mm), and large (228.6 mm × 571.5 mm) specimens were tested. Test results on sand specimens (passing No. 4 sieve size) compacted at 70% relative density showed similar friction coefficients irrespective of the specimen size except for the very small specimen that showed marked increase in shear strength. The authors attributed this increase in shear strength to the appreciable coarse sand of 4.763 mm (3/16-inch) maximum size that resulted in a maximum particle size to cell diameter ratio of 0.14. Hennes [2] investigated the effect of particle size, shape, and grading on the friction angle of dry, rounded gravel and crushed rock. The shear test apparatus was 152.4 mm × 304.8 mm (6-inch × 12-inch) in size. He concluded that as the maximum particle size increases, the friction angle increases. The same conclusion was made as some complementary triaxial tests were conducted by this author. Note that in this research work, the specimens were compacted following identical vibration and tapping of the materials without verifying the resulting porosity of the specimens. Vallerga et al. [3] did not observe any change in friction angle of granular materials using traixial tests for the situation that the specimen diameter to maximum particle size ratio was larger than about 15. In the series of tests conduced, the compaction of specimens were controlled to achieve a fixed void ratio irrespective of the maximum particle size.
