Beam-spring structural analysis for the design of a tunnel pre-reinforcement support system

Various methods are used to stabilize tunnel faces during construction through weak ground and fault zones, and at shallow depths. They include multiple-drift excavation method, reduction of advance length, and auxiliary pre-reinforcement support systems. Pre-reinforcement support systems, such as the large-diameter steel-pipe-reinforced umbrella arching method (SPRUAM), have been widely used, especially in unfavorable conditions. However, owing to the absence of standardized guidelines, their design currently depends on the judgment of engineers and case histories. While there have been some systematic and theoretical model studies on the quantitative design of such supports, a practical investigation of the same has not been undertaken. Thus, our study suggests a simplified theoretical beam-spring structural analysis model for optimizing some important design parameters of the SPRUAM. On the basis of this structural model, we developed simple but robust finite element software that can process various design conditions and variables of the SPRUAM, such as ground condition, tunnel section, primary support characteristics, and the dimensions of the steel pipes. We verified the reliability of the developed software by comparing its output with that of commercial structural analysis software. We also conducted case studies on three representative ground conditions - weathered rock, highly weathered rock, and weathered soil - to evaluate the practical applicability of the beam-spring structural analysis model. Our findings showed that the developed model was reliable in estimating the quantity of steel pipes needed for the SPRUAM at an early stage of the design.