In unstable ground, the tunnel can be pre-supported by driving spiles and forepoles into the crown and walls ahead of the excavation face with a small inclination angle upward. In Sweden, the nomenclature for this type of pre-support is "spiling". Spiling, which is a temporary support, is frequently used in Swedish tunnels. However, there is a lack of guidelines and international standardization for the design of spiling. This paper explores the design of spiling in unstable ground with focus on tunnels excavated in the Nordic countries with generally hard rock masses. Based on a benchmark, case and literature study review, example of guidelines and starting points in the design of spiling have been compiled. Cases, with different types of designed and installed spiling, are presented in the paper, followed by a discussion of when, and for what rock conditions, to use rebar spiles, pipe spiles, or self-drilling spiles to achieve a safe excavation progress. Analytical design methodologies used in Swedish cases are presented, including beam models for spiling sections between supporting arches or the face. Cases where numerical modelling have been used in the design of spiling are also described. The findings presented in this paper shows a lack of guidelines on how to design spiling. Future research work linked to model uncertainty, local arching effect and quality assessment is suggested, which will be beneficial for tunnel engineers in designing spiling in the future.
KEYWORDS
Spile, forepole, design, analytical, numerical
