Analysis of size effects on the Hoek-Brown failure criterion of intact granite samples

Abstract

Scale or size effects of the uniaxial strength response in rock samples have been studied in detail in the past, so a good number of studies on different rocks are available. However, analyses on triaxial strength scale effects in rocks are scarce and they seldom address failure criteria (i.e. Hoek-Brown) evolution with specimen size. This obvious lack of data can be attributed to the difficulties of having available Hoek’s cell of different sizes. With the aim of filling this void, the authors have carried out sets of around 25 stress-strain triaxial tests on intact 30, 38, 54 and 84 mm diameter granite specimens, with various confinements (0.2 to 15 MPa), so reliable estimates of Hoek-Brown strength were obtained for every scale. We compare results with previous studies on UCS scale effects, showing a good correlation. Results suggest the studied granite undergoes a reverse size effect in terms of strength at low confinements. Indeed, the UCS increases as sample diameter increases up to around 50 mm, but decreases thereafter. However, results obtained put forward that this strength variation with scale tends to be mitigated for higher confinements where the scale effect may not be clearly recognised. So increased confinement can be associated with a decreased scale dependency component of strength.

Study of size effects on the peak and residual strength of intact and artificially fissured granite samples

“…Some clear trends are reflected by these data. The average range of variability in terms of the coefficient of variation of peak strength (9-14%) are slightly larger than those observed for intact rocks (8-13 %) [5], but generally consistent with these typically found when testing natural rocks.…”

“…Some values do not follow this trend, notably the case of 5 MPa confined samples, something attributed just to rock behaviour variability. 30 mm diameter tests were not available, but a lower strength would be expected as observed for intact specimens [5]. The graph in Figure 2.b present this peak strength results in normalized form, where the mentioned trends are clearer, as it is the irregular response for the 5 MPa confined cases.…”

“…Following previous test program on intact Blanco Mera granite [5], in this study, 3 different sizes of jointed samples of Blanco Mera granite, a granular rock from NW Spain, were obtained. This rock presents 20% quartz, 27% alkali feldspar, 35% plagioclase feldspar, 7% muscovite, 5% biotite, 4% chlorite and 1% sericite.…”

“…The 1+2 jointed pattern is illustrated in Figure 1.b. A standard Hoek's cell was available for 54 mm specimens, while the Hoek's cells and sleeves for the 38 and 84 mm samples (Figure 1.c and 1.d) were engineered for this general study that started testing intact rock samples [5].…”

“…However, not many studies have addressed the scale effect for the rock triaxial strength. Some of the authors of this paper carried out a lab program to assess scale effects on triaxial strength of intact Blanco Mera granite [5]. In that study, it was observed that increased specimen size is associated with an increased confinement dependency component of strength.…”