Laboratory Tests - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 449

Figure 14.27 Tip of the steel pin before and after CAI test (West 1989)

Table 14.2 Classifi cation of abrasiveness of intact rock based on CAI values obtained with steel pins
of hardness HRC 54-56 (Thuro & Käsling 2009) and potential uncertainties when using steel pins with
hardness HRC 40-43

classifi cation CAI 54-56 CAI 40-43

extremely abrasive 4.0 < CAI ≤ 6.0 4.7 < CAI ≤ 16.0

very abrasive 2.0 CAI 4.0 2.3 CAI 10.7

< ≤ < ≤

medium abrasive 1.0 CAI 2.0 1.2 CAI 5.3

< ≤ < ≤

slightly abrasive 0.5 < CAI ≤ 1.0 0.6 < CAI ≤ 2.7

not very abrasive 0.3 < CAI ≤ 0.5 0.35 < CAI ≤ 1.3

not abrasive 0 ≤ CAI ≤ 0.3 0 ≤ CAI ≤ 0.8

The steel pin is recommended to have a tensile strength of 20 N/mm2 and a Rockwell
hardness of HRC 54-56 according to CERCHAR (1986) and a Rockwell hardness of
HRC 40 according to West (1989). The infl uence of steel pin hardness on the CAI was
investigated by Rostami et al. (2005) and Michalakopoulos et al. (2006) by evaluating
the results of CAI tests performed in several laboratories on different rocks with steel
pins of different hardness. In Fig. 14.28, the CAI values obtained with steel pins of
HRC 54-56 are plotted versus CAI values determined with steel pins of HRC 40-43
(Käsling et al. 2007). The CAI values, except for a few outliers, lie within the yellow
shaded area between the two red lines in Fig. 14.28. When assuming a linear relation-
ship between the CAI values of hardness HRC 54-56 and hardness HRC 40-43

(14.55)

the red lines can be described by A = 1.17 and A = 2.67, respectively. As a result, Table
14.2 (third column) indicates potential uncertainty with regard to the classifi cation of
abrasiveness of intact rock when determining the CAI value using steel pins of hardness
HRC 40-43 instead of HRC 54-56. Because of this uncertainty it is recommended to
use a steel pin hardness of HRC 54-56 as a standard (Käsling et al. 2007).
Also, the type of the testing device, Cerchar or West apparatus, can have an infl uence on
the results of the CAI tests. For coarse-grained crystalline rocks such as granite the CAI