Numerical Investigation of Tunnel Face Stability Using Forepoling or Fiberglass Nails

Pre-support of tunnel excavation faces using fiberglass nails or forepoling umbrellas aims to improve face stability in cases where an unsupported excavation face will develop uncontrollably large face extrusion, leading to face instability. The paper presents the results of a large set of parametric 3D numerical analyses of tunnel face excavation by the Finite Element Code Simulia Abaqus, using various degrees of reinforcement by fiberglass nails or forepoling umbrellas. The analyses use the average face extrusion as a measure of face stability, considering that face instability is associated with large face extrusions while the safety factor against face instability can be correlated with lower face extrusions in case of pre-supported tunnel faces. The results of the analyses are normalized and a set of semi-empirical formulae and design graphs are produced to calculate the safety factor of supported tunnel faces against instability and other useful quantities in tunnel design (average face extrusion, volume loss and deconfinement coefficient) as a function of ground strength, overburden depth and amount of face reinforcement. The analyses show that tunnel face reinforcement with FG nails is much more effective and less costly in securing face stability than the use of forepoling umbrellas. It is shown that even a coarse grid of FG nails can achieve better results than very heavy forepoling, and the difference in effectiveness is more pronounced in weaker ground and or deeper tunnels.