Title: Analysis of High Stresses in Tunnel Boring Machine Joints: Experimental Study and Theoretical Justification
Author(s): Cosimo Iasiello and Alejandro Pérez Caldentey
Publication: Structural Journal
Appears on pages(s): 55-69
Keywords: concentrated loads; confined concrete; D-regions; experimental program; joints; tunnel boring machine (TBM) tunnel design
Engineering practice in the design of tunnels built with boring machines (TBMs) results in joints between segments transmitting high pressures during the service life of the structure. These pressures often exceed two and three times the design compressive strength of concrete, which are, more or less, the limits allowed by modern concrete standards for maximum pressure in a concrete block subjected to a concentrated load. This practice is based on experimental results which show that joints can function at higher stresses in the confined conditions that occur in the joints between tunnel segments. The fact that standards do not seem to acknowledge the empirical evidence of this practice, and the fact that
current practice overrules standards, may be attributed to normal practice where tunnels are designed by engineers specializing in geotechnics while concrete standards are written by engineers specializing in structural design and to the fact that communication
between these two groups is often poor. The aim of this paper is to present the study of a real case and to document and analyze tests carried out, taking this example as a reference, to try to explain the adequate behavior of tunnel structures, despite going beyond code-of-practice limits. For this, an ad-hoc experimental campaign was carried out representing the actual geometry of the tunnel lining and simulating varying degrees of longitudinal confinement by varying the degree of geometrical confinement through the use of different bearing plate widths. The results show that although the failure load exceeds the standards’ recommendations, the adequate joint behavior can be justified by the experimental evidence which clearly shows that the limit on the maximum compressive stress proposed in current Standards is overly conservative. Even so, doubts remain regarding whether these conditions are really met at the region close to the ring ends.