Use of a tilting table to determine the basic friction angle of hard rock samples

Abstract

A tilting table can be used to determine the basic friction angle of a rock. Dolostone and quartzite sliders with natural and sanded surfaces move at similar angles on sanded plates of dolostone and quartzite. Friction angles measured from conventional direct shear tests on the same specimen are similar.When the natural or sanded sliders are tilted on polished plates, lower friction angles result: about 12° for quartzite and 16° for dolostone. The dolostone sliders leave a loose powder on the polished plates. If the powder is accumulated over successive slides, tilt angles for sliding increase to 30°. Friction angles on polished plates approach a minimum, the mineral friction angle, [Formula: see text], which represents the friction between the minerals making up the rock surfaces. The basic friction angle, [Formula: see text], is the sum of [Formula: see text] and the surface roughness produced by sanding.Tests of a new tilting table design suggest that basic friction angles should be determined with naturally surfaced sliders on plates wet lapped with No. 80 grit. Vibrations from the electric motor driving the table reduce measured friction angles by about 1°. Sliding angles increase up to 2° in very humid conditions. Key words: friction, rocks, tilting table, roughness, humidity, dolostone, quartzite, limestone.