Application	PLAXIS 2D
Version	PLAXIS 2D 2024.1
Date created	21 June 2024
Date modified	21 June 2024
Original author	Richard Witasse - Principal Application Engineer
Keywords	PLAXIS, Rock, Tunnel closure, Plastic radius, Hoek-Brown, Mohr-Coulomb, Field stress

Description

This example demonstrates the practical application of PLAXIS for the calculation of the plastic zone extent and tunnel wall closure. It has been inspired by an article from Hoek (1998).

The problem involves a 10 m span modified horseshoe-shaped tunnel driven through very poor-quality rock at a depth of 50 m. The rock mass is a graphitic phyllite with a uniaxial compressive strength of σ_ci = 15 MPa. The rock mass is described as 'disintegrated', consisting of a poorly interlocked mixture of angular and rounded pieces, and 'poor' with slickensides and highly weathered surfaces corresponding to an approximate Geological Strength Index GSI = 25.

The uniaxial compressive strength of the rock mass is estimated as σ_cm = 1 MPa (deduced from the equivalent cohesive strength c_eq = 330 kPa and equivalent friction angle φ_eq = 24.9°). It is assumed that the rock mass properties and the ratio of the in-situ stresses remain constant. The horizontal stress is assumed to be 1.3 times the vertical stress.