Liquefaction of housing sites caused by the 2007 Niigataken Chuetsu-oki Earthquake

Hirofumi TOYOTA

The Niigata-ken Chuetsu-oki Earthquake occurred on the offshore of Jo-Chuetsu area on 16 July 2007. It has been reported that many housing site and houses were damaged by liquefaction. The damage was remarkable along Sabaishi River in Kashiwazaki city and Kariwa in Kariwa village. Therefore, the detailed study about a cause of the liquefaction was carried out based on the investigation of damaged area due to liquefaction during the Chuetsu-oki Earthquake.
First of all, the hearing and site investigation were conducted to examine the characteristics of damage in Hashibacho. In addition, the degree of liquefaction risk was estimated by using the result of the Swedish weight sounding test. Swedish weight sounding test is an in-situ test where penetration resistance by load and rotation is used, and it has an advantage of excellent penetration ability among the other simple sounding tests. This time, it rotated by human power and the ground water level was estimated by the rod observation after the testing.
Next, the cyclic direct shear test was performed to examine the liquefaction mechanism and dynamic properties of the soils comparing with the results of Toyoura sand. Stress and deformation conditions close to the real ground are able to realized using the cyclic direct shear test because horizontal cyclic shear stress was applied to the specimen under plane strain condition. In addition, it has an advantage of simple testing method and small amount of sample.
The damage of liquefaction was remarkable in a toe of dune slope, which is boundary between dune and alluvial soft clay or the former river channel. There is no difference in judgment of liquefaction between plane and slope ground. Therefore, new liquefaction standard was examined by applying an initial shear stress induced by slope to the specimen.
The results obtained in the study are follows:
1) The damage was concentrated along the former river channel whose ground is soft and has high ground water level in Hashibacho.
2) The peak of N value over 10 appeared in the ground lower than 2 m and the peak appeared around 5 m depths in the other area.
3) The sample obtained in Kariwa was similar to Toyoura sand in the respects of physical properties and liquefaction characteristics, and it was easy to liquefy.
4) If the stress amplitude is enough large such as double amplitude occurs in the slope, the slope is suffered by the greater risk of liquefaction.
5) If the stress amplitude is enough small such as cyclic stress is applied in one side, the slope is safer than level ground in the respect of liquefaction.

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