ABSTRACT:

Direct shear tests of 50-mm, 60-mm and 300-mm samples were conducted on materials collected from Questa rock piles and their natural analogs to investigate the role of material scalping and water content on the shear strength of these materials. Normal stresses in the range of 50 to 700 kPa were used for the shear tests. The results of the laboratory tests presented in this paper suggest that scalping of the rock pile material causes reduction in friction angle. The effect of water content was studied as well. Air-dried, moist and saturated samples were tested using direct shear tests. It is shown that the friction angle of Questa rock pile material decreases with increasing water content. Therefore, for unsaturated conditions similar to that of the Questa mine region with a semi-arid environment, shear test results on saturated samples can cause underestimation of the frictional strength of the material.

1. INTRODUCTION

The shear strength characterization of mine rock materials placed in rock piles is challenging as these materials typically contain a large range of particle sizes with the largest sizes often multiple feet in cross sectional dimension. Scalping of a soil sample changes the soil gradation curve and results in the presence of greater percentage of fine particles in the modified sample. As a consequence, the shear strength characteristics of the material can change. This issue has been the subject of many studies in the past. Hennes [1] studied the effect of particle size, shape, and grading on the friction angle of dry, rounded gravel, and crushed rock. They concluded that friction angle increases as the percentage of gravel is increased up to a gravel content of 50% and then starts to decrease, which could be because of less well-graded material in the latter situation.

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