The aim of this study is to clarify the mechanism of the progressive excessive deformation observed in real underground RC box culverts of about 30 years of age. It was found by the site-inspection, monitoring and the destructive testing that the excessive deflection of top slabs for the culverts, which is almost 10 times the design estimated value, accompanies the out-of-plane shear failure. It is also computationally investigated that the coupling of subsidence of the backfill soil and the combined creep and shrinkage of concrete after cracking is closely associated with the delayed shear failure found in the culvert in service. In order to prove the delayed shear failure under higher sustained loads, the timedependent shear crack propagation was reproduced in the laboratory test and the computational approach used in this study was examined.

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Long-term excessive deformation of underground RC box culverts in service was monitored over 20 years and its mechanism is analytically discussed in this study. The long-term excessive deformation possibly attributes to synergy effects accompanying delayed shear failure of RC slabs subjected to vertical soil pressures and the time-dependent creep-shrinkage of structural concrete. Special attention is directed to the delayed shear cracking which was actually found in real underground box culverts over the service life. The delayed shear crack is experimentally reproduced in the laboratory as well as the multi-scale computational simulation.