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

摘要

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 (_xx) and in the vertical direction (_zz) occurs at the tunnel heading, whereas the largest total normal stress reduction in the longitudinal direction (_yy) 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 K_xz (= _xx/_zz) and K_yz (= _yy/_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, K_xz 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.On a conduit une série d'analyses numériques tridimensionnelles élasto-plastiques de consolidation couplée pour étudier les effets du rapport de rigidité (n = E _h/E _v) et du coefficient initial de pression des terres au repos (K₀) sur les déformations et les mécanismes de transfert de contraintes dû à un creusage de tunnel à face ouverte idéalisé (i.e., « New Austrian Tunnelling Method » ou NATM) à une longueur donnée non soutenue. Lorsque n augmente, de petites zones de déformation plastique en extension sont mobilisées à la couronne et au radier pour un K₀ donné. Ceci conduit à un creux de tassement de surface en travers plus étroit et plus profond. Par ailleurs, pour un n donné, plus le K₀ initial est faible, plus le creux de tassement calculé est étroit et profond. Les mécanismes de transfert des contraintes tridimensionnelles à la face d'avancement du tunnel se manifestent par le changement dans les contraintes normales et les contraintes de cisaillement induites autour du tunnel. Durant la progression de l'excavation du tunnel, la réduction maximale de la contrainte normale totale dans les directions latérale (_xx) et verticale (展开▼