Inspection And Reconstruction Of The CPT-based Liquefaction Database From 1976 Tangshan Earthquake

At present,liquefaction discrimination methods which are widely used both at home and abroad are established by the previous liquefaction database,and with the update of the database,the methods are constantly improved.In recent years,with the development of CPT,piezocone penetration test(CPTU)with its advantages of high precision,high stability and high repeatability are favored by many researchers and engineers.Meanwhile numerous classification charts of soil behaviors and liquefaction discrimination methods are developed,since then CPTU are widely used in soil layers division,soil classification and liquefaction evaluation.But related studies show that the prediction success ratios of the NCEER recommended CPT liquefaction discrimination methods are extremely low,and the results of Robertson CPT-based soil behavior-type chart are different in different areas,so to build and test CPT liquefaction database which are of great significance to improve the methods and charts.The 1976 Tangshan earthquake resulted in widespread liquefaction.China academy of railway sciences and other institutes conducted single bridge probe CPT test on these liquefied and non-liquefied sites in 1977-1978,and established the CPT-based Chinese code methods by using these data.However,the single-bridge CPT measured only specific penetration resistance,and is not conformity with international standards.In order to enrich this data defect,Institute of Engineering Mechanics,CEA and Southeast University retested 18 sites using piezocone penetration test(CPTU)in 2007.For the data of the second test,Moss and Boulanger provided the CPT liquefaction database of Tangshan earthquake successively.However,since the two tests were separated by 30 years,Tangshan has undergone reconstruction,and the urban construction has changed with each passing day since the reform and opening up.The changes of groundwater level and the repositioning errors of test points will affect the representativeness of the CPTU data.In this thesis,the liquefaction investigation data of Tangshan earthquake and in-situ test data of twice tests were collected and compared.After comparison,the liquefaction layers were selected in the correct sites,the data of wrong sites were removed.The liquefaction database of Tangshan earthquake based on the two test data was established respectively.The correct data was analyzed and inspected by CPT-based Chinese code methods and Robertson liquefaction discriminant methods which was recommended by NCEER expert group.The main contents and achievements of this dissertation include:(1)The related liquefaction damage information was consulted and twice in-suit testing data were collected.The consistency of site mechanical properties were determined by comparing the variation trend of the two static penetration data along depth and the variation trend of the number of penetration hits with depth.By comparing soil types obtained from Robertson CPT-based soil behavior-type chart with borings in first test,the liquefaction layers were selected in the correct sites,the data of wrong sites were removed,and the Tangshan earthquake liquefaction database based on the two test data is given respectively by using the correct sites data.(2)The combination of twice test data and liquefaction damage information after the earthquake,using four liquefaction discrimination methods to Lutai L1 and L2 test sites to has carried on the liquefaction discrimination test,close the distance but the differences between seismic damage larger phenomenon has made the analysis and speculation,suppose L1 test point is lighter because of damage to liquefiable layer of plastic index is higher.In addition,based on the analysis of two CPT test data in Lutai area,it is concluded that the soil index I_cis more reliable than the friction ratio R_f.(3)Comparing the soil classification results of Robertson CPT-based soil behavior-type chart with the borehole histogram and laboratory geotechnical experiment results,it is found that Robertson CPT-based soil behavior-type chart has good applicability in Tangshan and Lutai area.(4)The discriminant analysis of the CPT liquefaction database of Tangshan earthquake was carried out by using the CPT-based Chinese code methods and Robertson liquefaction discriminant methods.It was found that liquefaction discrimination result for the first database is good,while liquefaction discrimination result for the second database is poor,which shows that potential of soil liquefaction had changed significantly after 30 years.Therefore,the liquefaction database based on CPTU test has low reliability,and has little help for future improvement of liquefaction discrimination methods.But by decomposing the first test data p_sinto q_cand f_s,and also using the CPT-based Chinese code methods and Robertson liquefaction discriminant methods,the q_cand f_sdata obtained by decomposition were compared with the previous discriminant results,and it was found that the decomposed data based on the first test was more representative than the data from the second test.It can be used as the CPT liquefaction database of Tangshan earthquake in the future to provide data support for the improvement of CPT liquefaction discriminant methods.