The basic friction angle of rock joints is usually obtained from tilt tests, being the most common the laboratory tilt tests. This test has been standardized according to the ISRM. However, most of the times when calculating the shear strength of discontinuities, reference tables are used to obtain the basic friction value for the lithology under study. These tables omit some lithologies complicating the search of adequate references. An alternative, straightforward and economical way to obtain ϕb is through the field tilt test, which is carried out by sliding two blocks aside a joint. It is a well-known test, but there are few references to its implementation. In this test, unlike the laboratory tilt test, the samples are not "polished" and it is necessary to evaluate the roughness of the joint and the normal component to the weight of the upper blocks. The idea is to calculate the term of ϕb from the Barton-Bandis’ equation and include the tilt angle α. Various tilt-test measurements were carried out with field blocks on both sides of the same joint, considering different lithologies (granite, limestone, andesite, dacite, coal and slate) and block sizes, evaluating the ideal ranges of applicability of the test.
The basic friction angle of joints is a fundamental parameter in the kinematic analysis of different types of block failures in slopes and underground excavations. Its adequate evaluation in a daylight and friction circle stereographic diagram can make the difference that something becomes stable or unstable.
One of the most widely used criteria for determining the shear strength of discontinuities is the nonlinear Barton-Bandis [1]. In a previous version of this criterion [2] a term referred to the residual friction (ϕr), is introduced.
Basic friction angle is a key parameter in the determination of residual and peak friction angles of a discontinuity according to Barton–Bandis [1] criteria. The tilt test is widely used for the determination of basic friction of discontinuities. This test is usually performed at laboratory using two slab-like specimens, or three drilling cores, although the test can also be performed in the field. However, in practise, it is quite common to use tables to estimate the value of the basic friction angle for each type of rock [3, 4]. The problem is that some rock types are not included in the above-mentioned tables since their basic friction angles have not yet been determined or published. It is sometimes very complicated and arduous searching for references, especially those referring to rocks like granodiorites, rhyolites, dacites, andesites or carbon.