Study On Long Term Settlement Of Silty Clay In Hangzhou Area During Operation Period

In order to solve the problem of urban traffic congestion,a lot of metro tunnels are built in our country.The metro tunnels built in the eastern soft clay area will have great settlement under the vibration load by vehicle,which will cause the cracks of lining segment,the settlement of upper structure and other problems.In order to study the long-term settlement of metro tunnels in soft clay areas,the dynamic characteristics of clay in Hangzhou were studied by indoor triaxial test firstly.Secondly,the microstructure of the soil before and after the triaxial test was qualitatively and quantitatively analyzed by electron microscopy scanning test and PCAS image processing technique.The relationship between macroscopic deformation and micro-pore variation is expounded.Based on the results of dynamic test,numerical modeling and the empirical formula of settlement,the settlement caused by cumulative plastic deformation of the soil and the settlement caused by the accumulated pore pressure dissipation are calculated respectively.Some conclusions about the long-term settlement of clay in Hangzhou and metro operation period are obtained:(1)The dynamic strain of saturated clay in Hangzhou is stable developmental type.Under the anamic laod,the cumulative strain curve shows a stable state after rapid development with vibration times developing.When the confining pressure and vibration frequency are constant,under the same vibration frequency the dynamic strain increases with the increasing of dynamic load,and finally develops stably.And the dynamic pore stress increases with the dynamic stress increasing under different dynamic stress.(2)The load acting on the rail cause by metro train is simplified and analyzed,and the vibration load curve is obtained.(3)The long-term settlement of Hangzhou metro tunnels in the operation period is divided into two parts: long-term settlement caused by cumulative plastic deformation and long-term settlement caused by accumulated pore pressure dissipation.During the first five years,the total settlement in the operation period is developing rapidly.After the operation for ten years,the total settlement still increases with the increasing of time,but the growth rate is very slow regarded as stable state.At this time,the cumulative settlement caused by plastic deformation accounted for more than 85% of the total settlement.(4)Through analyzing SEM photos,it can be found that the soil micro particles size,arrangement,mutual position and shape will change with the changing of dynamic load.The effect caused by cyclic load frequecy and the dynamic stress amplitude on the micro unit of soil structure is very obvious,and has great regularity.And the regularity corresponds to the regularity of strain development of soil samples tseted by indoor dynamic triaxial test.