Free-field Seismic Liquefaction Testing With Centrifuge Shaking Table And Liquefaction Evaluation By Shear Wave Velocity Method

The urbanization process of China has developed rapidly recently, and our country is on high earthquake zone, the earthquake liquefaction caused a lot of loss of people’s lives and property. It is of great significance to deeply probe into the mechanism of earthquake liquefaction, and put forward a set of effective liquefaction evaluation method. The cycle liquefaction shear stress method proposed by Seed firstly get cycle liquefaction shear stress of the Fujian sand through dynamic triaxial test based on the method of equivalent cyclic shear stress, then it’s compared with actual seismic shear stress to determine whether it’s to liquefy. The cyclic liquefaction shear stress method proposed by Seed is simple and clear, but there is big error in evaluation result because of disturbing influence when sampling.The super gravity centrifuge is used in simulation of site liquefaction more widely as it is able to accomplish simulation to prototype in the gravitational field to make the stress state of each point in the model and prototype have very good correspondence and approximately achieve stress and strain characteristics of model material. First, the author conducted some tests, such as shear test,(dynamic) triaxial test, to get soil static characteristic parameter, then the author made a free-field model whose relative density is40%with Fujian sand and performed five dynamic loading liquefaction tests in ZJU-400geotechnical centrifuge shaking table, got the development and variation rules of acceleration, excess pore-water pressure (EPWP) with time at different soil depth for a few of seismic vibrations, as well as the relationship between soil acceleration, settlement and EPWP increasing, which help improving our comprehension of interaction of soil acceleration, settlement and pore-water during earthquake. White noise was also used in the centrifuge shaking table test to obtain the variation of natural frequency of soil with shaking by frequency analysis. At the last, based on the work above, the method of earthquake liquefaction evaluation through shear wave velocity based on initial liquefaction proposed by Institute of Geotechnical and Engineering of Zhejiang University is applied in free-field liquefaction evaluation test on the centrifuge shaking table, the author proposed a new method to calculate earthquake shear stress based on field acceleration, the final result did show the method is satisfactory. The results are as follows:the site amplification depends on a combination of soil softening and EPWP increasing, liquefaction destroy connection strength and structural stability of soil grain, post-liquefaction soil fabric isn’t stable and is easy to liquefy again; effect of shear stress after peak on EPWP increasing can’t be neglected; EPWP increasing and soil settlement is result of external load shift from soil grain to pore-water, and then to soil grain; site soil natural frequency increases when consolidation is finished after liquefaction, it’s reasonable to determine CRR (Cyclic Resistance Ratio) curve based on shear wave method, and the method of earthquake liquefaction shear stress calculation based on shear beam proposed by Institute of Geotechnical and Engineering of Zhejiang University is more reasonable than previous work.