A Study Into CPT-based Liquefaction Database And Characteristics Of Liquefied Soil Type From 2003 Bachu Earthquake

The liquefaction discriminant methods based on field test indexes are all based on historical seismic liquefaction data.In recent years,pore pressure cone penetration test(CPTU)has been widely used in soil layer classification,soil type identification and liquefaction identification,due to its high accuracy and high stability.It derived a variety of liquefaction discriminant methods and soil classification maps based on CPT index.Chinese code CPT-based liquefaction method is made from data of 1976Tangshan earthquake using discriminant function.It is necessary to verify the applicability of liquefaction discrimination and CPT soil classification maps in other areas.Therefore,testing and constructing historical seismic CPT liquefaction database is great significant for improving liquefaction discriminant method and soil classification map.On February 24,2003,an Ms 6.8 earthquake occurred in the border area between the Bachu and Jiashi of Xinjiang.It appeared in the past 30 years after the Tangshan and Haicheng earthquake,the most large-scale liquefaction phenomenon of sand liquefaction in our country.In 2004,Xinjiang Seismological Bureau carried out drilling survey and standard penetration test in Qiongse disaster area(liquefaction area).In 2009,the Institute of Engineering Mechanics of China Earthquake Administration to Qiongse disaster area again conducted a site survey.The modern multifunctional pore pressure cone penetration test was carried out.A large number of studies show that the penetration number and cone tip resistance index of liquefied sand in Bachu earthquake are too large.The liquefaction method used at home and abroad is often judged as non-liquefaction.The reason of this rare phenomenon is still unknown.In this dissertation,post-earthquake liquefaction investigation data and in situ CPTU test data in 2009 were collected.The Robertson soil classification map was used to re-stratify the CPTU data to test the soil class.The liquefaction layer was selected and four liquefaction discriminant methods were used to discriminate the liquefaction point and non-liquefaction point in Bachu area.A liquefaction database based on CPT and SPT of Bachu earthquake was constructed and discriminant results of liquefaction were analyzed.The main work and results of this thesis include:(1)Consult Bachu earthquake related investigation data.Two in-situ test data were collected.The change of groundwater level,soil type in the same buried depth during two tests were compared.The field CPTU test data for 2009 were representative to liquefaction sites.(2)Based on the field CPTU test data in 2009,Robertson soil classification map was used to stratify the CPTU data of 38 test points.The results of soil test were compared with those in the column chart of drill hole.Compare the corresponding degree of soil types in the two layers.The liquefaction layer was selected by soil type screening,combined with the value of SPT-N blow count and average cone tip resistance q_c.Based on CPTU test data,a database of Bachu seismic liquefaction was constructed.(3)The soil classification results of Robertson soil classification Map were compared with the soil classification results of the field borehole column map in 2009.It is found that Robertson soil classification map has good applicability in Bachu area.(4)The discriminant analysis of the liquefied layer data of each test point in Bachu area was carried out by using the CPT and SPT methods recommended by NCEER.It was found that all four methods judged a large number of liquefaction points as non-liquefaction.This indicates that the mechanical indexes of the liquefied layer(cone tip resistance q_cand SPT-N blow count)are indeed high.(5)The mechanical index of the liquefaction layer in Bachu earthquake is too large,and it is speculated that the unsaturated fine sand overlying the liquefaction layer is easy to form the permeable boundary.The liquefaction phenomenon of less sand blasting but more water spraying occurs in the liquefaction site,a large amount of pore water is discharged,leading to the enhancement of soil consolidation degree.Then,the mechanical index of post-earthquake test must be too large.