Three-dimensional ground settlements and stress-transfer mechanisms due to open-face tunnelling

Abstract

A series of three-dimensional, numerical, elastoplastic, coupled-consolidation analyses was conducted to investigate the effects of the stiffness ratio (n = E ′h/E ′v) and the initial coefficient of earth pressure at rest (K0) on ground deformations and stress-transfer mechanisms due to idealized open-face tunnelling (i.e., New Austrian Tunnelling Method or NATM) at a given unsupported length. As n increases, smaller plastic extension strain zones are mobilized at the crown and invert for a given K0, leading to a deeper and narrower transverse surface settlement trough. On the other hand, for a given n, the lower the initial K0 is, the deeper and narrower the computed settlement trough will be. Three-dimensional stress-transfer mechanisms at a tunnel heading are demonstrated through the changes in normal stresses and induced shear stresses around the tunnel. As the tunnel excavation advances, the maximum total normal stress reduction in the lateral direction (Δσxx) and in the vertical direction (Δσzz) occurs at the tunnel heading, whereas the largest total normal stress reduction in the longitudinal direction (Δσyy) takes place at about 0.5D (D = diameter) ahead of the tunnel face. A total stress reduction zone can be identified within 2.0D ahead of the tunnel face, and the stress influence zone diminishes at a distance of about 1.0D beyond the tunnel face. A significant reduction in pore-water pressure, of up to about 75% of its initial value, can be identified around the tunnel heading. Around a tunnel open face, the effective stress coefficient Kxz (= σ′xx/σ′zz) and Kyz (= σ′yy/σ′zz) rise to approximately 1.8 and more than 2.0 at the crown and invert in the initial K0 = 1.0 ground conditions, respectively. On the other hand, Kxz falls to about 50% of the initial K0 value at the springline, and the coefficient at the shoulder and heel drops to between 10% and 20%.Key words: open face, tunnelling, NATM, anisotropy, three-dimensional, stress transfer, ground settlements.