An undrained upper bound solution for the face stability of tunnels reinforced by micropiles

Abstract

Tunnel in difficult soils may require procedures to prevent tunnel face failures. Face stabilization can be achieved by the installation of some structural elements. This paper presents an analysis of face stability of shallow tunnels in undrained soils reinforced by an umbrella of subhorizontal micropiles. Upper bound solutions for two dimensional plane strain conditions are given including the effect of micropiles. The micropile umbrella is embedded in the soil and supported on the tunnel lining. The kinematically admissible collapse mechanism defined to calculate the upper bound solution includes the action provided by a subhorizontal micropile at limiting conditions. The solutions are given in practical dimensionless charts which are useful to quantify easily the effect of the umbrella of micropiles. The plots provide a simple procedure to design the umbrella. The most relevant properties defining the umbrella are grouped into a single dimensionless coefficient which includes the yielding conditions and the geometry of the micropiles as well as the distance between them.

Tunnel in difficult soils may require procedures to prevent tunnel face failures. Face stabilization can be achieved by the installation of some structural elements. This paper presents an analysis of face stability of shallow tunnels in undrained soils reinforced by an umbrella of subhorizontal micropiles. Upper bound solutions for two dimensional plane strain conditions are given including the effect of micropiles. The micropile umbrella is embedded in the soil and supported on the tunnel lining. The kinematically admissible collapse mechanism defined to calculate the upper bound solution includes the action provided by a subhorizontal micropile at limiting conditions. The solutions are given in practical dimensionless charts which are useful to quantify easily the effect of the umbrella of micropiles. The plots provide a simple procedure to design the umbrella. The most relevant properties defining the umbrella are grouped into a single dimensionless coefficient which includes the yielding conditions and the geometry of the micropiles as well as the distance between them.