Research On Pore Water Pressure Build-Up For Sand Model Under Shaking

Granular material out of rock and mineral such as silts,sands and gravels are commonly found as bases of various buildings and structures.Under seismic loading such as earthquake,blasting and mechanical dynamics,it is very easy for these granular materials under saturated condition to be liquefied.Liquefication are always related to the build-up of residual excess pore water pressure.Accumulation of pore pressure can lead to decreasing of effective stress in the soil,which thus causes reduction in stiffness and strength of soils.Overall,the change of pore water pressure inside the granular material will act as an indicator to predict the strength of soil during the seismic loading.Thus,this thesis conducted a series of experiments to study seismic pore water pressure using shaking table.The generation of pore water pressure of during seismic loading can be seen as a result of the interaction between granular skeleton and pore water.Using the micro structure of granular assembly to account for generation of pore water pressure has been a meaningful step for the study.According to coefficient of uniformity of testing sand,granular assembly rules in the macro respective have been proposed for both even and uneven sized particle sand,which are then be employed to analyze the build-up of pore water pressure.The main conclusions are:When the coefficient of uniformity is small,which means that the sand particles are basically uniform in size,granular initial assembly structures present both stable and unstable assembly structure.If the seismic loading is minor,stable and unstable assembly structure is dominantly contributed to the build-up of pore water pressure,if the seismic loading gets violent,stable assembly structure starts playing its part.When the coefficient of uniformity gets larger,granular initial assembly structure is unstable in general,liquefaction is liable to happen in theory.Subsequent study has used PFC^2D to simulate the making of sand testing model by the wet sedimentation method,including even sized sand testing model and uneven sized sand testing model.According to the particle contact band in the numerical model and taking even sized particle for example,the general rules of particle assembly have been concluded,and the rationality of pore pressure analysis is verified to some extent.Modes for presenting seismic pore water pressure is described,from which it can derive variableN/N_L to fit pore water pressure ratio generation curves of 4 testing sand before liquefication that are S-shaped.S-shaped pore water pressure ratio generation curves can be divided into three phases,and it can be determined that the second phase is the most representative one of all three phases to describe pore water pressure build-up.The relationship of cyclic ratioN/N_L and accumulating rate of pore water pressure indicates that the testing sands can be listed in descending order by accumulating rate of pore water pressureA large quantity of datum from shaking table tests are adopted for classification and regression model,which have been developed by using PSO-SVM algorithm.In classification model,the result shows that the prediction accuracy is 100%with binary classification and 83.3%with three classification,which severs the purpose.In regression model,the coefficient of determinant is 0.95,which is satisfactory.In addition,the sensitive analysis that involves regression model mentioned above has been applied to understand the contribution of input factors to pore water pressure.