The tilt-angle equals the material friction of the discontinuity wall plus the roughness i-angle (tilt-angle = φwall material + i) if no real cohesion is present (i.e. no cementing or gluing material between the two blocks), no infill material is present, the asperities do not break, and the walls of the discontinuity are completely fitting at the start of the test, while if the walls of the discontinuity are completely non-fitting, the tilt-angle equals the friction of the material of the discontinuity walls (tilt-angle = φwall material). If cementation or gluing material is present or asperities break, the tilt-angle represents a combination of the (apparent or real) cohesion and the friction along the discontinuity. If infill material is present, the tilt-angle is governed partially or completely by the infill, depending on the thickness of the infill and height of asperities.3
Hoek, E.; Bray, J. (1974). Rock slope engineering. London: Institute of Mining and Metallurgy. pp. 358. ISBN 978-0-419-16010-6. 978-0-419-16010-6 ↩
Xian-Qin, H.; Cruden, D.M. (1992). "A portable tilting table for on-site tests of the friction angles of discontinuities in rock masses". Bulletin of the International Association of Engineering Geology. 46 (1): 59–62. doi:10.1007/BF02595034. /wiki/Doi_(identifier) ↩
Phien-wej, N.; Shrestha, U.B.; Rantucci, G. (1990). "Effect of infill thickness on shear behaviour of rock joints". In Barton, N.R.; Stephansson, O. (eds.). Rock Joints. Balkema (Taylor & Francis), Rotterdam. pp. 289–294. ISBN 978-90-6191-109-8. 978-90-6191-109-8 ↩