Effect of seepage forces on circular openings excavated in Hoek-Brown rock mass based on a generalised effective stress principle

When an opening is excavated below groundwater table, the groundwater flows into the excavated zone and seepage forces apply to the rock mass surrounding the opening. Such seepage forces significantly affect the elasto-plastic responses of the rock mass based on effective stress law. In this regard, it is recognised that the failure criterion of a rock mass is a function of Terzaghi's effective stress, while its elastic deformations are a function of Biot's effective stress. Based on this generalised effective stress law, seepage forces arising from the groundwater flow into a cylindrical opening are estimated quantitatively, in the present study. An elastic-brittle-plastic analysis of deep circular openings in an infinite Hoek-Brown medium with consideration of the seepage forces under steady-state flow and the generalised effective stress law is derived. The proposed solution can be effectively used for the actual design of underwater openings below deep groundwater table. It is shown that, in openings below deep groundwater tables, the effect of seepage and pore pressure, and consequently, the effective stress law on mechanical analysis may be significant.