Centrifuge testing of cross passages in shallow tunnels: challenges and lessons learned

This study investigates the redistribution of hoop and longitudinal stresses around cross-passage (CP) openings in shallow tunnel linings using centrifuge-scale physical modelling. Two instrumented model tunnels (parent tunnels, PT) were tested: a baseline tunnel with no opening (PT-X) and a tunnel with an opening of 50 mm dia. (PT-50). Linear strain gauges were installed at the springlines and crown of the CP in both hoop and longitudinal directions, and tests were conducted at 50 and 75 g over three cover-to-diameter (C/D) ratios under saturated soil conditions. Measured hoop strains near the springlines increased nearly linearly with C/D, rising by factors of 3.5, 2.6, and 1.78 for C/D = 0.5, 1.0 and 1.5, respectively, when comparing PT-50 to PT-X. Converting hoop strains to hoop forces and bending moments revealed that hoop forces at the springlines are amplified by 1.74–1.87 times in the presence of a CP, while bending moments remain anomalously small. Longitudinal strains at the CP springlines and crown were generally low, consistent with published numerical studies, but several longitudinal gauges recorded high readings, especially at the crown of the CP. These elevated values may be attributable to the curved tunnel geometry, installation effects, gauge misalignment, and bonding variability. Finally, this study offers a holistic set of practical recommendations for future physical modelling of CPs, addressing both instrumentation optimisation and geotechnical model design. These guidelines are intended to help future studies overcome the limitations identified here and close the identified research gaps.