A Benchmark Experiment to Assess Factors Affecting Tilt Test Results for Sawcut Rock Surfaces

From the earliest studies on the topic, plane sliding techniques, usually known as tilt tests, have shown contradictory features. On the one hand, they reflect on a small-scale basic principles regarding definition of the friction angle and can reproduce conditions very similar to those of the sliding of blocks on rock slopes; furthermore, tilt angle results agree with friction angles derived from shear and pull tests (Hencher 1977; Muralha 1996). On the other hand, the literature includes several reports of erratic results and it is recognized that even apparently smooth surfaces are actually rough at the microscopic level (Hencher and Richards 2015). Thus, adhesion and textural interlocking may contribute to the variability and the nonreproducibility of tilt test results. A number of authors have carried out simple tilt tests in the past, and several examples illustrate the above-mentioned arguments. Thus, Hencher (2012) observed extreme variability in the results of tilt tests; Nicholson (1994) found that friction angles for sawcut Berea sandstone in direct shear tests varied by 12.5 , despite great attention paid to sample preparation and reproducibility; Coulson (1972) demonstrated that the friction angle of planar surfaces of rock varied with surface finish; Krahn and Morgenstern (1979) reported similar variation for surfaces prepared in different ways and with different surface finishes; and Kveldsvik et al. (2008) found that the basic friction angle for a rock slope, derived from tilt testing of core, varied between 21 and 36.4 . These great differences in measured tilt angles are mainly attributed to different surface finishing and to wear of the rock contacts (Pérez-Rey et al. 2015, 2016), although other reasons, such as testing and ambient conditions, cannot be ruled out. Mehrishall et al. (2016) revealed that the residual friction coefficients of grinded joint surfaces and of rough rock surfaces were almost identical. This manuscript contains interesting findings in rock mechanics practice, derived from large efforts to test the same rocks under different conditions and different laboratories. At the same time, this manuscript presents part of the work which will be used to prepare a Suggested Method on Tilt Testing. It is published in the form of a Technical Note, although its length exceeds what is generally accepted for this type of publication.