GSI Conversion Equations and Indirect Estimates of JRC and JCS Values – Applicability for the Conditions of the ONKALO Facility

Salminen, P., Lindfors, U., Haapalehto, S.

English

66

This report provides results from a small-scale mapping campaign, which was conducted in ONKALO. The purpose of this study was to investigate the applicability of indirect ways to determine the Geological Strength Index (GSI) values for the brittle deformation zones and the Joint Roughness Coefficient (JRC) and Joint wall Compressive Strength (JCS) values for the fractures in the ONKALO conditions. Previously, GSI and JRC values have not been mapped at all from ONKALO tunnels and JCS values have not either been systematically mapped from ONKALO. Instead, they have been estimated indirectly from other mapped parameters.

Obtaining GSI values is important for the quantitative parametrization of the brittle deformation zones. GSI value is used as an input parameter for Hoek-Brown failure criterion (Hoek et al. 2002). For the parametrization of the brittle deformation zones intersecting ONKALO, GSI values corresponding to the core zones of the brittle deformation zones have been derived from mapped Q´ parameters from the drill core, tunnel and shaft intersections (e.g. Salminen 2018, Salminen et al. 2018).

JRC and JCS values are used for the quantitative characterization of discontinuities within the rock mass (Barton-Bandis joint criterion; Barton & Bandis 1982, Barton & Choubey 1977). Salminen et al. (2018) provided indirect estimates of JRC and JCS values for fractures in the area of entire ONKALO facility (outside of the brittle deformation zone intersections).

In this study, GSI values were mapped from several pilot hole and tunnel sections within brittle deformation zone intersections in ONKALO. A range of GSI values was obtained for each mapped section – sometimes there were even two possible ranges, depending on the observation scale. From these same rock sections, the Q´ parameters and the RMR₈₉´ parameters were also mapped. GSI values were calculated from the mapped Q´ and RMR₈₉´ parameters using three different conversion equations: (1) the ”conventional” equation connecting the Q´ value and the GSI value (Hoek et al. 1995), (2) a newer equation connecting the RQD, J_r and J_a values (three of the Q´ parameters) to the GSI value (Hoek et al. 2013), and (3) another new equation connecting the JCond₈₉ and RQD values (two of the RMR₈₉^´ parameters) to the GSI value (Hoek et al. 2013). The calculated GSI values were compared to the mapped GSI values. In the case of the studied tunnel sections, the new equation (2) most often provided GSI values closest to the mapped GSI values. In the case of pilot holes, the conventional equation (1) most often provided the best compatibility, but the new equation (2) also gave GSI values close to the mapped ones. For the conditions of ONKALO in general, based on this study, Equation (2) is recommended to be used when deriving GSI values from other mapped parameters. 

JRC and JCS values were determined from six test fractures in ONKALO. These test fractures were also mapped for other parameters, which were needed in the indirect schemes to estimate JRC and JCS values presented in the report of Salminen et al. (2018) and its synthesis report by Salminen (2018). JRC value can be estimated based on the J_r value and the associated fracture profile. JCS values can be calculated based on the ratio of the rock type specific uniaxial compressive strength (UCS) of intact rock and an ”alteration” value of the fracture. In this study, this alteration value of a fracture was estimated based on the thickness of the fracture fill and the J_r profile (fracture profile). JRC and JCS values estimated with the indirect schemes were compared to the values determined directly from the fractures.

JRC values were measured with a ”comb” (a roughness profile tool). These measurements resulted to ranges (sometimes quite wide) of JRC values. The indirectly estimated JRC values fit well within these ranges, except for one fracture. It seems that the indirectly estimated JRC values are applicable – especially when taking in account that the measured JRC values are not either accurate, single values. Note, however, that only six fractures were investigated in this study.

Schmidt hammer measurements were made for the determination of the JCS values. A rebound value (R-value) was calculated from the measurement results. The JCS value itself was determined based on the rebound value and the average density of the rock. These measured JCS values are not far away from the indirectly derived JCS values, if the inaccuracies in the determination of both types of JCS values are taken in account. So, it seems that the methodology for indirect estimation of JCS values is also applicable. However, the amount of studied fractures was small (6).

GSI, JRC, JCS, brittle deformation zone, fracture

WR 2017-04_web (pdf) (7 MB)