We discuss the mechanical characteristics of underground structures that are subjected to vertical oscillations. Under such circumstances − expected when an earthquake rupture occurs very close to highly developed urban areas − the conventional seismic analysis, which mainly considers horizontal shear oscillations, may not be fully valid. Our analysis, based on the elastic wave theory, shows the effects of vertical disturbances and the generation mechanism of the unique damage patterns observed on the occasion of the 1995 Hyogo-ken Nanbu (Kobe) Earthquake.
Wir diskutieren ueber die mechanischen Eigenschaften der unterirdischen Strukturen, die sich vertikalen Schwingungen aussetzen. Unter solchen Umstanden − erwartet, wann ein Erdbebenabbruch sehr nah an hochentwickelten stadtischen Bereichen geschieht − kann die herkömmliche seismische Analyse, die sich hauptsachlich horizontale Schubbewegungen ueberlegt, möglicherweise nicht völlig gueltig sein. Unsere Analyse, basiert auf der elastischen Wellentheorie, zeigt die Effekte der vertikalen Bewegungen und den Erzeugungsmechanismus der einzigartigen Beschadigungsmuster, die anlaßlich des 1995 Kobe Erdbebens beobachtet wurden.
Nous discutons les caracteristiques mecaniques des structures souterraines qui s'exposent aux oscillations verticales. Dans ces conditions − s'attendent quand une rupture de tremblement de terre se produit tout près des secteurs urbains fortement developpes − l'analyse seismique conventionnelle, qui considère principalement des oscillations horizontales de cisaillement, peut ne pas être entièrement efficace. Notre analyse, qui est basee sur la theorie des vagues elastiques, montre les effets des mouvements verticaux et le mecanisme de generation des modèles uniques de dommages, que nous avons observes à l'occasion du tremblement de terre 1995 de Kobe.
Earthquakes rarely cause failure of underground structures such as mines and tunnels. However, when the epicentre of an earthquake is located close to a highly developed urban area, high frequency components of the associated seismic waves may interact with the structures in that area before attenuation. In such circumstances, the dynamic structural behaviour may be different from the one expected from the conventional analyses that mainly take account of the effects of low-frequency horizontal (shear) oscillations, and consequently disastrous damage may be caused, even to underground structures. We have found two possible examples, at depth and near the surface, on the occasion of the 1995 Hyogo-ken Nanbu (Kobe), Japan, Earthquake:
the collapse of the Daikai Underground Station in the city of Kobe; and
the damage to the Bantaki Tunnel in the mountains near Kobe (Figure 1)1–8.
At the Daikai Station, situated some 20km from the epicentre of the earthquake, the reinforced concrete columns supporting the roof at midspan (central columns) failed catastrophically, and the roof slab dropped almost onto the train tracks. The collapse of over 20 central columns induced subsidence of maximum 2.5m of the street above, with substantial settlement over an area of 100m by 20m. Aside from this column-collapse-induced movement, no distinct evidence of permanent ground deformation by other causes such as liquefaction was found at the site. This failure is the first case of severe earthquake-induced damage to a modern underground facility due to the reasons other than fault displacement or instability near the portal1–3.
