Effect of Post-Liquefaction Long Shaking on Roads and Buried Pipes During the 2011 Great East Japan Earthquake

Abstract eng:
The 2011 Great East Japan Earthquake, a megathrust earthquake with a magnitude of Mw=9.0, occurred in the Pacific Ocean. The rupture plane of this quake was about 450 km in length and 200 km in width. The duration of the failure of the rupture plane was extremely long, about 160 seconds. Soil liquefaction occurred in wide area including Tokyo Bay area. Many houses, river dikes and other structures severely settled or uplifted by liquefaction. Not only the settlement and uplift of the structures but also remarkable damage of buckling of roads, disconnection of buried pipes and the shear failure of sewage manholes occurred. This was the first experience that these remarkable damages were observed in the liquefied ground though many past earthquakes caused liquefaction frequently in Japan. As many seismic records were obtained in the Tokyo Bay area, the authors collected many accelerograms at liquefied and not liquefied sites and estimated time histories of velocity, displacement and non-stationary spectra. Among them, accelerograms recorded at K-NET Inage in Chiba where boiled sand was observed are very important because the liquefaction time can be judged from the recorded waves. In Inage’s wave, the amplitude of acceleration decreased suddenly at around 126 sec., which means liquefaction occurred at this moment. Then large amplitudes of velocity and displacement continued after 126 sec. for more than two minutes with a predominant period of 3 to 4 seconds. Such amplitudes of velocity and displacement were not induced at not liquefied sites. The long-period shaking of liquefied ground, which was a kind of sloshing, was recorded by several inhabitants on video. Slow cyclic horizontal movement at an amplitude of about 15 cm was seen in a video taken two minutes after the peak acceleration in Urayasu City. During the investigation of damaged area, strange thrusts and heavings of footways and alleys were seen at many sites. So, it seemed that some boundaries beside the footways and alleys, such as banks of old sea walls and elevated bridges, caused the thrusts or heavings due to the sloshing of liquefied ground. A lot of sewage, water and gas pipes were deformed, cracked, broken and meandered, and joints were sheared or disconnected. Many sewage manholes were cracked and sheared in the horizontal direction. So, it was concluded that the large horizontal displacement of liquefied ground had to have caused large cyclic compressional and tensile stress to the pipes in horizontal direction, resulting in the disconnection of the pipe joints and the shear failure of the manholes.

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