Identification Of Soil Liquefaction Based On Strong Ground Motion Records

Liquefaction is a common kind of disaster caused by major earthquakes. Specialattentions should be paid to this field in seismic engineering. China is a country withvast territory which is easily subjected to serious liquefaction disaster. The research insoil liquefaction has been an important topic in seismic engineering in China. Duringearthquakes, it is the main content in real-time earthquake disaster reduction to quicklyidentify whether the soil liquefaction occurs and its extent based on strong motionrecords.At present, there are five methods available to identify liquefaction, i.e., Miyajimamethod, Suzuki method, Kostadinov and Yamazaki method, HHT based method and themethod based on frequency decrease rate. In this paper, these five methods forliquefaction detection are systematically compared first. For each method, parametersintroduced in calculation and the liquefaction characteristics of ground motions areanalyzed. Then, some improvements to existing liquefaction identification methods aresuggested and two methods for liquefaction identification are proposed based on surfacestrong ground motions and borehole ground motions respectively.In case that only surface ground motions are available, the decrease rate ofinstantaneous frequency is used to identify the liquefaction of soil. First, theinstantaneous frequency of strong ground motion is calculated by using time-varyingARMA model. At the same time, the intensity envelope is used to describe thenon-stationary characteristics in intensity of ground motions. Corresponding to the riseand decline of the strength envelope, the mean values of instantaneous frequency forthese two segments are calculated separately which are respectively taken as the initialvalue and the final value of instantaneous frequency. Then the time during which theinstantaneous frequency decrease from the initial value to the final value and therelative proportion of frequency decrease are used to identify whether soil liquefactionoccurs in this site.In case that borehole records are available, the method based on systemidentification is used. In this method, borehole (base rock) record is used as the inputand the surface ground motion record is used as the response (output) with the soilbetween these two records as the structure (system). Modal frequency of the system canbe calculated according to the input-output pair by using system identification methods. Finally, whether liquefaction occurs in this site can be identified according to the modalfrequency.In this paper, some strong ground motion records are selected to verify theproposed methods. Results show that the two methods are reliable and robust. Finallythe sites which record the strong ground motions from the March11,2011Tohokuearthquake are identified and those identified to be liquefaction stations are marked inthe map which is in consistent of the result of disaster survey after this earthquake.At last the deficiency of present research are remarked briefly and the potentialdirection for future research is prospected...