ABSTRACT
Umbrella arches belong to the most widely-used reinforcement technics in conventional tunneling, especially for low-depth tunnels (H/D=1 to 5) in unfavorable ground conditions (soils with poor mechanical properties, fractured rocks, etc.) and are installed primarily to increase the safety of the excavation front and to reduce settlements. Due to several constructive advantages, self-drilling hollow bars have been used in tunneling to provide a cost-effective alternative type of umbrella arch: the composite pile roof (CPR). While the conventional steel pipe roof (PR) undergoes essentially bending moments, the CPR relies on the ability of the self-drilling hollow bars to transfer axial forces. In terms of safety, the final result for the tunnel excavation remains the same. The following paper presents an overview of the structural behavior of the above mentioned constructive procedures. The relevant considerations for the design of a CPR will be discussed.
