＞Archives＞Vol 39 No 4 (Dec, 2016)
As construction and reclamation activities become larger and more widespread the distribution of man-made strata is rapidly extending especially in developed countries. Such strata are often more likely to suffer geohazards or to contain harmful substances than natural strata. In particular, geological disasters are often associated with the unconformity between artificial and natural strata (known in Japan as the Jinji Unconformity) with highly consolidated strata beneath and less consolidated above. The flow of liquids within man-made strata is often less simple than in natural strata and a specific flow system tends to develop at the unconformity. The Great East Japan Earthquake on March 11, 2011 (Magnitude: M7.9, Epicenter: longitude 38°06.22 and latitude 142°51.62 and 24km in depth) affected the Kanto Plain near Tokyo. Large-scale liquefaction fluidization caused by the seismic vibrations was essentially confined to areas with man-made strata.The present study focused on land reclaimed by hydraulic filling at Hinode, a suburb of Itako City, which had been used subsequently for urban development and suffered significant damage. Cored boreholes and observation wells were made to assess evidence of liquefaction. Continuous cores were halved longitudinally and lacquer peels were made and examined using X-ray photograms. Samples taken from liquefaction zones, liquefaction-fluidization zones and non-liquefaction zones were subjected to grain size analysis. A large peel was made from the outcrop in a dewatered trench section.In the Hinode area, the Jinji Unconformity is relatively deep in the centre thinning towards each margin of the reclaimed area, essentially forming an irregular basin shape into which the man-made strata were emplaced. The reclaimed area is dominated by fine sand and silt which is generally more susceptible to liquefaction and fluidization. Therefore it is not surprising that sand eruptions caused by the earthquake of 11th March 2011 demonstrate widespread liquefaction-fluidization. In the northern and eastern parts of the reclaimed area, where man-made strata, directly overlie the natural Sarawa Sand and Mud Formation and in areas underlain only by natural strata there were relatively few sand eruptions. Although a higher density of ground investigation and sampling points, particularly in a ring about half way between the central area and margins, would be needed to refine these observations and conclusions but it is already clear that the variations laterally and vertically in the distribution of the mobilized layers occurs at a scale of less than 1 meter. This calls into question current Japanese practice for assessing liquefaction and fluidization potential which relies on N-value SPT tests at 1 meter intervals. This needs to be reconsidered.
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