Characterizing Liquefaction Resistance Of Clayey Sand By Shear Wave Velocity

In recent years, the frequent occurrence of devastating earthquake brought great disaster to people’s life and property safety all over the world. Among them, problem of liquefaction is particularly acute. Many cases of clayey sand liquefaction presented in Turkey and Chi-chi earthquake occurred in1999. Until now, most studies focused on liquefaction of clean sand, while research on liquefaction resistance of clayey sand is relatively few due to the limitation of laboratory test, which is an important problem need to be solved. It is necessary to understand the structural and dynamic response characteristics of clayey sand so that the mechanism of liquefaction of clayey sand can be revealed and characterization and evaluation index of liquefaction resistance of calyey sand can be established, which can provide scientific basis for liquefaction resistance of clayey sand.When it comes to the problem of liquefaction of clayey sand, there are no relatively perfect theories and methods can explain its dynamic response for one-sided understanding in the past, which results in defect in the area of discriminating liquefaction susceptibility. It is necessary to understand the dynamic response characteristics of clayey sand so that the mechanism of liquefaction of clayey sand can be revealed, which can provide scientific basis for further development of geotechnical seismic design.This paper concerns the problem of liquefaction of clayey sand. Shear wave velocity is used as the characterization pramater to characterizing the strength of soil. Tests conducted included the observation of microstructure evolution, consolidation test with shear wave velocity measurements, monotonic traxial test and dynamic traxial test. Dynamic geotechnical centrifuge tests and field case study were also conducted and analysised. The influence of evolution of structure of clayey sand on its liquefaction resistance was revealed, and relationship between resistance of liquefaction and shear wave velocity of clayey sand was then developed. The results show that:1. Soil structure and its physical and mechanical properties such as plasticity, permeability, contractivity and so on changed greatly with the increasing of clay content.Sand particles are completely packed by clay particles when a certain amount of clay is reached, meaning that the transformation of soil from "sand-like" to "clay-like" is completed and the plasticity of soil shows obviously. Results of shear wave velocity show that Hardin formula fitting parameters A and n of "sand-like" soil are significantly different from "clay-like" soil. The critical state line can be expressed in the form of straight line in the of space e-log p’under the condition of no grain breaking. The critical state line rotated in the direction of clockwise with the increasing of clay content, which means the dilatancy potential and the critical state soil friction angle of soil is decreasing.2. Relationship of power function between liquefaction resistance and over burden stress corrected shear wave velocity is developed according to theoretical consideration and dynamic traxial tests with shear wave velocity measured. The relationship of CRR-VS1moved upward gradually with the increasing of clay content when the clay content is lower than threshold fine particle content, which is consistent with previous research and provides a quantitative basis for liquefaction strength evaluation of clayey sand by shear wave velocity.3. Geotechnical centrifuge shaking talbe test with shear wave velocity measured was conducted, including soil with different content of clay. Preparation and saturation method of clayey sand ground were proposed. Uniformity of soil model and changes of soil structure during tests were monitored by using shear wave velocity testing technology of bender element. Phenomenon of sand boiling and water spray was respented during tests. Dynamic response like development of shear stress-strain, modulus, damping ratio and excess pore water pressure were analysised systematically through comparasion of clayey sand ground model and clean sand ground model. Similarities and differences of liquefaciton performance of model tests and element tests were found. Earthquake liquefaction analysis method of model ground was put forward, which taken acceleration of shaking table as the input of ground motion. The breakthrough of using shaking table tests data to evaluate liquefaction potential of site was came ture and reliability of CRR-VS1relationship derived from element tests was verified.4. Field cases of Chi-chi earthquake and Turkey Kocaeli earthquake occurred in1999were investigated and data base of seismic liquefaction site of clayey sand was established. Applicability and reliability of CRR-VS1correlation proposed to actual site liquefaciton were analysised. Based on this, liquefaction evaluating method and calculating steps of calyey sand ground under designed earthquake were put forward, and obtaining method of the main parameters was suggested and values of them were also recommended.