Discussion on the load bearing capacity of ground and the stability of tunnel is presented. We derive a simplified solution of a circular lined-tunnel in an elastic ground. The solution results in a better understanding of characteristic curves. Use of DEM allows us to consider effects of possible yield of the overstressed zone around the tunnel and inherent discontinuity of ground on the curves.

Diskussion auf der Last Tragfahigkeit Bodens und die Stabilitat des Tunnels wird dargestellt. Wir leiten eine vereinfachte Lösung eines kreisförmigen Ausbautunnels in einem elastischen Boden ab. Die Lösung ergibt ein besseres Verstandnis der Kennlinien. Um Gebrauch DEM erlaubt uns, zu betrachten Effekte des möglichen Ergebnisses der ueberbetonten Zone den Tunnel und zugehörige Unstimmigkeit des Bodens auf den Kurven.

La discussion sur la portance de charge de la terre et de la stabilite du tunnel est presentee. Nous derivons une solution simplifiee d'un revêtement-tunnel circulaire dans une terre elastique. La solution a comme consequence une meilleure comprehension des courbes caracteristiques. L'utilisation de DEM nous permet de considerer des effets du rendement possible de la zone surchargee autour du tunnel et de la discontinuite inherente de la terre sur les courbes.

INTRODUCTION

Active mobilization of bearing capacity of rock mass surrounding an underground opening characterizes the NATM (New Austrian Tunneling Method) concept of supporting. A significant number of investigations for the concept of the tunneling support system have been reported. (e.g. Kovari, K., 1994[1]). In the first part of this paper theoretical relations between support pressure and tunnel wall displacement are briefly discussed. We derive a simplified solution of a circular lined-tunnel in an elastic ground. This solution correctly models the loading conditions and the ground-lining interaction around the tunnel.

The elastic theory of the stress distribution around tunnels demonstrates that the deviatoric stresses are maximum at the periphery of the excavation. Therefore, the rock mass may yield in the overstressed zone surrounding the excavation. Very often, the rock mass is far from being continua and consists essentially of two constituents, intact and discontinuities. Discussion here must include the dependence of the bearing capacity of ground on the yielded zone and discontinuity of ground. We have carried out numerical simulations for a discontinuous ground with Distinct Element Modeling. The ground reaction curves by the analyses are examined with the theoretical solution mentioned above.

ELASTIC ANALYSIS

Figure 1 shows an analytical model of a circular tunnel lining.

(Equation in full paper)

The loading conditions imply that the tunnel opening has been excavated and supported before or after the load corresponding to the free-field stresses is applied. Similar assumptions have been formulated in literatures (e.g. Einstein, H.H., 1979[3]). These two loading conditions are called as 'external loading' and 'excavation unloading'. The solution for the lining in radial direction as follows:

(Equation in full paper)

ELASTO-PLASTIC BEHAVIOR OF GROUND

Figure 3(a) shows a uni-axial compression curve (σc: the uni-axial compression strength) and its normalized relations (σ0/ σ0 *~u0/u0*).

This content is only available via PDF.
You can access this article if you purchase or spend a download.