A Study Into Liquefaction Discrimination Methods Based On Surveys Of The Bachu Earthquake

Earthquake disaster prevention is currently one of the most direct andmost effective means to mitigate the earthquake damages to engineeringprojects. The first step of the liquefaction damage prevention procedures is topredict and discriminate the soil liquefaction potential in a site of engineeringproject. A reasonable and reliable method to discriminate soil liquefaction isimportant to earthquake disaster prevention and an engineering project'sbudget, and is a major subject of interest in soil dynamics and earthquakeengineering, widely studied by academic and engineering communities alike.In February24,2003, an earthquake with a magnitude of Ms6.8struckthe Bachu-Jiashi area in the Xinjiang Uygur Autonomous Region of China. Itis the most serious earthquake by casualties and property losses in Xinjiangsince the People's Republic was founded in1949. The soil of the disaster site,which is located in Kashgar Area along the Yarkand River valley, is composedof thick layers of silty sand, fine sandy silts, silty soil and silty clay formed inthe Holocene-late Pleistocene periods. The groundwater depth is low.Considerable seismic geological hazards, including soil liquefaction, groundcracking, and river steep bank slumping, occurred in areas where the intensityscale was VII, VIII, IX, respectively. The extent of soil liquefaction in theBachu Earthquake was on par with the1975Haicheng Earthquake in LiaoningProvince, and embodied the most notable occurrence of sand liquefaction inChina since the1976Tangshan Great Earthquake. The Bachu Earthquaketherefore provides a unique opportunity for soil liquefaction studies.This paper, based on the soil liquefaction surveys in the BachuEarthquake, obtains new basic data, enriches and supplements China's in-situexperiment records of soil liquefaction. With the new data, it validates thefeasibility of soil liquefaction discrimination methods in practical use at homeand abroad. It obtains new knowledge from the research, proposing newacademic questions and answering them by means of theoretical analysis.With the new experimental data it unveils a new discrimination method forsoil liquefaction that is feasible for Xinjiang.The main tasks and achievements of this paper are:1. It completes a detailed survey of the Bachu Earthquake soilliquefaction site, obtaining experimental data with three in-situ test measures,which enriches the SPT data volume for China's liquefaction sites by40%.Upon this survey, China's seismic studies community has for the first time obtained the CPT data of liquefaction sites which complies with internationalstandards, as well as a set of in-situ experimental data which complies withthe SPT, CPT and Vs specifications.2. It compares records of previous cases of earthquake-induced soilliquefaction in China with the experimental data, and analyzes the geologicalconditions of Bachu's liquefaction sites. It researches the external and internalcauses of Bachu's earthquake-induced liquefaction, including magnitude scale,duration, graduation of liquefied soil, density, deposit conditions, etc.Meanwhile, it points out the liquefaction features of the Bachu Earthquake.3. It discovers the fact that the test by means of the CPT liquefactiondiscrimination method which is recognized by China's seismic studiescommunity and the National Standard since two decades ago shows signs ofanomaly as soil depth changes: This method goes at odds with the Bachu dataand with similar international qualitative methods. In essence, it confuses therelation of sand layer depth and liquefaction potential. Also, existingliquefaction discrimination methods both from home and abroad prove correctqualitatively on the Bachu earthquake, but obviously exaggerates its hazardquantitatively, and are not applicable to the case of Xinjiang.4. Underground water level and sand layer depth are two importantparameters to determine the occurrence of soil liquefaction. This paperproposes a theoretical solution to the relations of these two parameters withliquefaction potential. It reveals generic laws of the influence of these twobasic parameters on the liquefaction potential, and proves the exceptions ofChina's standard CPT liquefaction critical curve to be a qualitative error,providing a new way to judge the feasibility of other experience-basedliquefaction discrimination formulae.5. In response to the fact that liquefaction discrimination methods inexistence at home and abroad are not applicable to the Bachu case, itconstructs a new liquefaction discrimination model which complies with theSPT and CPT specifications, and, based on the Bachu data and this paper'stheoretical findings, proposes two new liquefaction discrimination formulaethat are applicable to Xinjiang.