Forepoling pre-support application in the shallow tunneling through weathered granite

Abstract

Tunnel face stability and ground movements induced by excavation are of significant concern in shallow tunneling through weak ground. In conjunction with other support systems, various pre-support measures are applied to reinforce the ground prior to excavation for ensuring tunnel crown stability and construction safety in general. Among them, forepoling is the simplest form of umbrella arch pre-support method which is widely used in tunneling application. Until now, very few studies addressed the effect of forepoling design parameters on crown stability of shallow tunnel constructed in weathered rock and no study is available for weathered granite. This leads to conservative and uneconomical designs of basic forepoling design parameters. This study aims at quantitative understanding through numerical parametric analysis regarding the behavior of forepoling pre-support design parameters that affect the crown stability of a shallow tunnel constructed in weathered granite. The effect of three design parameters - forepole length, installation angle and spacing on crown stability were investigated by using 3D finite element analysis program RS3 by Rocscience; using Pahang-Selangor raw water transfer tunnel project as a reference case. These parameters were then optimized considering total displacement and induced major principal stress at tunnel crown as evaluation indicator of crown stability. The results show that, the total displacement at tunnel crown increases by 0.64% when the forepole length was increased from 5 m to 9 m. The induced major principal stress at tunnel crown decreases by 1.02% when the length is varied from 3 m to 8 m. The total displacement at tunnel crown decreases by 0.23% when the installation angle of forepole is varied from 5° to 9°. From this study, it was found that length of 3 and 4 m; with angle of 5° to7°, and spacing of 300 mm are optimum forepoling design parameters under given tunneling condition.