Experiment on Seismic Disaster Characteristics of Underground Cable

Abstract eng:
For protection from unexpected large-scale disasters, NTT Group has been tackling various measures based on the basic anti-disaster policies of “Enhancing communication network reliability,” “Securing critical communication,” and “Restoring service early.” NTT's underground facilities have been improved and developed according to the results of facility damage analysis conducted at every great earthquake. After the Niigata Earthquake in 1964, conduit joints were improved with priority to measures against ground liquefaction. This improvement was proved effective by later great earthquakes. According to past reports about underground cable damage, a conduit was fractured and disengaged by local shear due to land subsidence at a quick change of stratum or the crossing of the conduit over another buried item. This damaged a cable inside the conduit by side pressure or shearing force. At the Niigata chuetsu Earthquake in 2004 and Niigata Chuetsu-oki Earthquake in 2007, road surface collapses at banking sections produced tensile force on entire cables and caused optical transmission losses and the bracings of manholes cut cables. We experimentally verified external force to work on a cable in a conduit after conduit damage by a ground change and the mechanism of damaging the cable. In the experiment, the standard conduit, manhole, and cable securing methods were modeled and the cable damage, optical transmission loss, and conductor strain after the forced disengagement of a conduit were checked. This paper introduces typical cases of disaster verified in this experiment. As introduced here, we also experimentally confirmed the basic characteristics about faults on the communication service level when the assumed seismic conduit-damaging tensile force and bending were applied to an optical fiber cable.

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