The Hoek-Brown with softening model (HBS model) has been recently added to the PLAXIS material library. This was a good opportunity to experiment the PLAXIS software capabilities for underground excavation in rock where usually FLAC is often being used. An example related to a 2D excavation analysis of a tunnel in a rock mass is presented. Extensive elements of comparison are proposed with an identical analysis run in both PLAXIS 2D and FLAC 2D to analyse the possible differences due to the implementation specificities. Results obtained will be presented in terms of tunnel convergence, structural forces in shotcrete and axial forces in rock bolts for which very good agreement between both solutions is observed.
The elasto-plastic characteristics of the Hoek & Brown with softening (HBS) implemented in PLAXIS (2019) have been defined according to the yield surface proposed by Jiang and Zhao (2015) which represents a generalization of the Hoek & Brown criterion through the invariants associated with the stress tensor:
(equation)
where p, q and θ represent the mean stress, the stress deviator and the Lode angle respectively. σci represents the uniaxial compression strength and the function A(θ) is defined as follows:
(equation)
with -1 ≤ κ ≤ 0. The parameters mb, s and a are dimensionless parameters which are determined through the empirical correlations proposed by Marinos et al., (2015) and Brown (2008) (i.e., the GSI system):
(equation)
(equation)
(equation)
where GSI represents the Geological Strength Index which is aimed to determine the quality of the rock mass from geological observations of joints, fractures, and discontinuities. Although the GSI classification enables to differentiate the initial yielding according to the spatial distribution of discontinuities, there is no specific reference to their rock-quality (i.e., the opening and the roughness of joints and fractures). For this purpose, a disturbance factor D has been introduced by Hoek et al. (2002) to calculate the material properties mb, s and a.
