Abstract

The Kumamoto earthquake of April 2016 produced two foreshocks of moment magnitude 6.0 and 6.2 and a mainshock of 7.0, which should have been followed by widespread and intense soil liquefaction. A Geotechnical Extreme Events Reconnaissance team (GEER) led by Professor Rob Kayen of UC Berkley was dispatched to the Kumamoto Plain--which is in Kumamoto Prefecture, the southern main island of Japan--immediately following the earthquake. The Japanese and U.S. engineers in the GEER team observed mostly minor and sporadic liquefaction, which was unexpected as the local site geology, known soil stratigraphy, and intensity of the seismic loading made the Kumamoto Plain ripe for soil liquefaction. The paucity and limited scale of liquefaction shows a clear gap in our understanding of liquefaction in areas with volcanic soils. This study is a direct response to the GEER team's preliminary findings regarding the lack of significant liquefaction. An extensive literature review was conducted on the Kumamoto Plain and its volcanic soil. The liquefaction of the 2016 Kumamoto Earthquake was also researched, and several sites were selected for further analysis. Four sites were analyzed with SPT, CPT, and laboratory testing during the spring of 2017. A slope stability analysis and undisturbed testing were performed for specific sites. The results of the analysis show a general over-prediction of SPT and CPT methods when determining liquefaction hazard. The Youd et al. (2001) NCEES method was the most consistent and accurate in determining liquefaction. The soils in the area including sands and gravels had high levels of fines, plasticity, and organic matter due to the weathering of volcanic ash and pyroclastic material. The volcanically derived coarse-grained soils may also have exhibited some crushability, which gave lower resistance readings. Filled river channels had the worst liquefaction with natural levees and the Kumamoto flood plains having only minor liquefaction. Publicly available boring logs rarely showed laboratory test data of bore holes which led to a general inaccurate soil classification. Boring logs were also not updated with laboratory classifications and data. Undisturbed cyclic triaxial testing of soils at one site showed that volcanic soils had relatively high resistance to soil liquefaction, though drying of samples may have compromised the results. Embankment cracking at one test location was calculated a lateral spread and a seismic slope failure along the pyroclastic flow deposit.

Degree

College and Department

Ira A. Fulton College of Engineering and Technology; Civil and Environmental Engineering

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