Characteristics Of Soil Liquefaction In The Great Wenchuan Earthquake And Procedures For Gravelly Soil Liquefaction Evaluation

It is remarkably important for seismic resistant design theories and analytical tools improvement to investigate the liquefaction-induced damage and field test in detail. Many significant lessons have been learned from the post-earthquake damage investigation in the past. Field test conducted in the liquefied site is the reliable way to establish a procedure for liquefaction evaluation.The 2008 Wenchuan Earthquake was the most destructive and widely influencing earthquake occurred in China since the 20th century. The intensity and magnitude was larger than those of the Tangshan Earthquake. However,"there was rare occurrence of soil liquefaction"and"the influence of liquefaction was small"are the main opinions in the academe as lack of detailed report of liquefaction. A specific liquefaction investigation team was organized by Institute of Engineering Mechanics, China Earthquake Administration shortly after the great Wenchuan Earthquake. Through months of wide and detailed field investigation, amount to 118 liquefied sites have been investigated, most of which located in the Chengdu plain and Mianyang district Sand boils and water spouts occurred extensively, involving in thousands of mus of farmland, 120 villages, 8 schools and 5 factories, which caused some rural houses, school buildings, manufactory buildings and wells etc. losing function in service. Three main characterizations of liquefaction in the Wenchuan earthquake were: the gravelly soils liquefaction, the liquefaction occurred in seismic intensityⅥ(PGA≈0.05g), and the liquefaction occurred in deep soil layer. Meanwhile, the ground fissures occurred widely in the liquefied zone. It's very important to examine these phenomenon and explore the mechanism for further corresponding study.In general, the gravelly soils has been regarded as non-liquefiable soils for its large grain size and well drainage condition. However, the geological conditions and soil profiles show that the gravelly soils layer distributed extensively in the Chengdu plain, almost 75% to 80% of 118 liquefaction investigated cases were gravelly soils liquefafied sites. Comparing to the limited data of global gravelly soils liquefaction, the gravelly soils liquefied cases in the Wenchuan Earthquake will enrich the global database enormously. The empirical SPT- and CPT- based procedures for sand liquefaction evaluation are not reliable for gravelly soils because these two types of test could not be conducted in the gravelly soils site. Meanwhile, the currently empirical liquefaction evaluating methods were established almostly based on the sand liquefaction data, which also are not reliable for gravelly soils liquefaction assessment. A new approach is strongly demand to be proposed for gravelly soils liquefaction evaluation according to the field test index, because the gravelly soils has been used very commonly in engineering.The main objects and fruits of this paper are:1. To understand the distribution and characterization of soil liquefaction and liquefaction-induced damage in the Wenchuan Earthquake by detailed field investigating, and to learn useful lessons for further study.2. To examine the truth of the liquefaction occurred in seismic intensityⅥ, the gravelly soils liquefaction, and the liquefaction occurred in deep layer soil by field investigating, collecting and analyzing the material, and field testing. To explore the mechanisms of liquefaction-induced ground fissures and liquefaction-induced structure damage.3. To explore the geology background and soil conditions, to obtain the distribution and characteristics of gravelly soils liquefaction in the Wenchuan Earthquake, and to compare the gravelly soils liquefied case in the Wenchuan Earthquake with the case histories in other earthquakes.4. To conduct the Dynamic Penetration Test (DPT) and Shear Wave Velocity Test (Vs) in several decades gravelly soils liquefied sites, and to verify the reliability of currently SPT-, CPT- and Vs- based procedures for liquefaction evaluation by using the gravelly soils measured data.5. To propose the fundamental model for gravelly soils liquefaction assessment based on the field test index and different ground motion parameter.6. To propose the DPT- and Vs- based procedures for gravelly soils liquefaction evaluation, to verify these procedures by using the gravelly soils liquefied case histories in other earthquakes, and to propose suggestions for seismic code amendment especially in gravelly soils liquefaction resistance evaluation.