Ground movements and damage in Satozuka district, Sapporo due to 2018 Hokkaido Eastern Iburi Earthquake

Abstract

This report describes the damage and ground movements observed in the residential area of Satozuka 1-Jo, Sapporo, Japan during the 2018 Eastern Iburi Earthquake as triggered by the liquefaction of pumice sand with which a valley had been filled. The geographical setting and the on-ground and aerial reconnaissance and soil characterization results are reported, leading to a possible scenario of the state of the pumice fill from its formation to the earthquake. This report focuses on the liquefaction disaster in Satozuka 1-Jo and summarizes it as follows: (a) The Satozuka residential area is located at the end of a plateau consisting of pyroclastic flow deposits, pumice sand called Spfl, deriving from the Lake Shikotsu caldera (30 to 40 thousand years ago), and some valleys that formed by erosion. (b) To develop level residential land, the pumice sand ridges were cut and the valleys were filled in with the pumice sand in the 1970s. The gradual loss of drainage capability might have led to a rise in the long-term groundwater level. (c) Liquefaction of the pumice sand fill was triggered and the fluidized fill flowed downwards along the sloped topography. Where the slope angle became steeper, asphalt pavements seem to have been pushed upwards and broken through by the liquefied soil, and the liquefied soil flowed out to the ground. (d) Physical and mechanical tests were conducted on the pumice sand fill, and its characteristics were reported with particular emphasis on its volumetric behaviour upon compaction, wetting, and pre- and post-liquefaction compression. Undrained cyclic triaxial tests showed the low liquefaction resistance of the pumice fill despite its high fines content (around 40%, non-plastic). (e) The laboratory test results and field evidence suggest that the potential key factor of the liquefaction at this site was the loosely compacted pumice sand fill, which easily forms a loose soil structure with macropores even when compacted on the wet side of optimum.