Use of the equivalent continuum approach to model the behavior of a rock mass containing an interlayer shear weakness zone in an underground cavern excavation

• A concept of a virtual rock mass composed of ISWZ and host rock was proposed.
• An equivalent continuum approach including a rock–soil composite material (RSCM) model was elaborated.
• The input parameters expressions for the RSCM model was presented.

An interlayer shear weakness zone (ISWZ) is a weak zonal geotechnical system of variable thickness that occurs between different rock strata (e.g., tuff and basalt). At the site of the future Baihetan hydropower station, Sichuan Province, China, because of the relatively poor ISWZ mechanical properties, the overall stability of the underground powerhouse is potentially at risk. In this study, to evaluate the effects of ISWZs on the stability of the future underground powerhouse by means of three-dimensional continuum modeling (3-D continuum modeling), the concept of a virtual rock mass composed of ISWZ and host rock is proposed. An equivalent continuum approach, including a rock–soil composite material (RSCM) model, is elaborated, with corresponding expressions for the input parameters. Comparisons were made between the predictions from the RSCM model, the results obtained by an analytic method, and existing data from physical model tests. The comparison showed that all three types of information showed good consistency in terms of failure mode and strength. This indicates the suitability of the RSCM model for describing the behavior of a rock mass containing discontinuities. Furthermore, comparison between the predictions of the proposed equivalent continuum approach, the joint element approach, and the solid element approach for a deformation of a test tunnel section containing an ISWZ show that the results produced by the first two approaches are similar, but much smaller than that using the third approach. Further comparison of the actual state of the ISWZ-containing rock mass in the test tunnel section confirmed the applicability of the proposed equivalent continuum approach to prediction of deformation of the rock masses containing ISWZs at the future Baihetan underground powerhouse site.