LIQUEFACTION CHARACTERISTICS OF A GRAVELLY FILL LIQUEFIED DURING THE 1995 HYOGO-KEN NANBU EARTHQUAKE

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During the Hyogo-ken Nanbu Earthquake which occurred on January 17,1995,liquefaction occurred extensively throughout the Hanshin area (around Kobe and Osaka in Japan). On Port Island, about 20 m in thickness of a manmade fill consisting of gravelly sand, which is a type of weathered granite widely distributed in the Rokko mountain range (locally called Masado), was liquefied. In order to investigate the cause of the liquefaction of the Masado layer, a series of liquefaction tests were performed on high-quality undisturbed gravelly samples recovered from this hydraulic fill on Port Island using the in-situ freezing sampling method, about five months after the earthquake. The reliquefaction potential and the settlement of the hydraulic fill soil following the liquefaction were also investigated. Based on the laboratory test results obtained in this study and field observations made after the earthquake, the following can be concluded. 1) The mean diameter, D_<50>, of Masado fill is in the range of 1.7 to 3.7 mm, the maximum diameter is about 37.5 to 101.6 mm, and it has a large amount of gravel (almost more than 50%). Based on these physical properties, the Masado fill can quite reasonably be classified as gravel. The dry density ranges between 1.7 and 2.0 g/cm^3. This value is nearly equal to that of natural gravel deposits, and is much larger than the maximum dry density of dense Niigata sand and Toyoura sand. 2) In spite of its large dry density and gravel content, the liquefaction strength of Masado fill was found to be 0.15 to 0.23,which is very low, and nearly equal to that of Toyoura sand with a relative density of about 70%. The liquefaction strength is almost consistent with that of clean sands obtained from undrained cyclic triaxial test indicated by Yoshimi et al. (1989) for the same N_1-value. This result suggests that the liquefaction strength of Masado fill obtained in cyclic triaxial test can be roughly determined by the simplified procedure using the N_1-value. 3) Considering the test results, which indicated that the artificial Masado fill has a large particle size but low liquefaction strength, the range of the particle size of the soil whose liquefaction potential should be investigated as described in some of the design standards, is hoped to be reviewed. 4) A correlation between the post liquefaction volumetric strain (ε_v) and the maximum shear strain (γ_<max>) of the undisturbed Masado samples was found in laboratory tests. Based on the correlation between γ_<max> and ε_v, the total settlement of the Masado fill layer at the sampling site was estimated to be about 32 cm, this value is almost consistent with the observation results (30 to 40 cm), as reported by Tokimatsu et al. (1996). 5) The effects of the strain history (liquefaction) on the liquefaction strength of Masado fill was known to be negligibly small.
1997-09-15

LIQUEFACTION CHARACTERISTICS OF A GRAVELLY FILL LIQUEFIED DURING THE 1995 HYOGO-KEN NANBU EARTHQUAKE

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