Numerical Study On Two-level Vibration Isolation Measures For Micro-vibration Control On Environmental Vibrations Generated By Vehicle Loads

As the city expands in size and road construction extends everywhere,the impact of nearby vibrations caused by road vehicle loads becomes more and more apparent,especially for high-precision instruments and equipment such as quantum instruments,lithography machines,precision processing machine tools,etc.Even a very slight vibration will have a great impact on the working accuracy.It is necessary to predict the level of environmental vibration caused by vehicle traffic and perform vibration reduction and isolation treatment to meet the micro-vibration(?m/s level)control requirements of high-precision equipment.A proposed high-precision laboratory needs to meet the VC-E level(?3.2?m / s)micro-vibration control requirements.It is necessary to evaluate the environmental vibration level of the vehicle load on an existing road and a planned road in the surrounding area and propose a targeted vibration control measure.Drill holes to obtain undisturbed soil samples from the laboratory site and conduct advanced geotechnical tests to obtain the dynamic parameters of soil.A three-dimensional dynamic finite element analysis model including the above two roads,the foundation pit of the laboratory equipment platform and the foundation was established to simulate the environmental vibration level caused by the existing road vehicle load and was compared with the field test results to verify.Then,the environmental vibration level of the two roads with simultaneous vehicle loads is predicted,and two-stage vibration isolation measures of "row pile + group pile foundation" and "air spring + concrete pedestal" are specifically proposed.Adding the pile isolation barrier and the pile group foundation to the above finite element model,the output vibration velocity at the top of the pile group foundation cap after the first-level vibration isolation is obtained.And then substituted into the theoretical formula of vibration isolation rate of the second-level measures,the output vibration velocity of the top su of the concrete pedestal was finally obtained,and made it meet the limit requirement of 3.2?m/s.The vibration isolation effect of the above two-level measures is compared and analyzed.In particular,the parameter analysis of the row pile barrier is carried out.Finally,main research contents and conclusions are as follows:(1)Take the undisturbed soil samples of the laboratory site through a thin-walled borrower,and measure the basic physical and mechanical properties of the soil samples,and obtain the dynamic parameters such as the elastic modulus,Poisson's ratio,and damping ratio of the soil samples through the GDS resonance column.(2)The on-site actual measurement of the environmental vibration level of the proposed site caused by the loading of existing road vehicles was carried out,and the measured results were analyzed in 1/3 octave bands.The foundation pit and the existing and proposed roads,equipment platforms and foundations were established.The three-dimensional dynamic FEM of the foundation simulates the environmental vibration level caused by a load of existing road vehicles and is compared with the field test results to verify.Then,the vibration level of the foundation pit equipment platform when two roads are loaded with vehicles is predicted.The results show that if no vibration isolation measures taken,the vibration level of the equipment platform will significantly exceed the VC-E limit requirements.(3)Two-stage vibration isolation measures of "row pile + group pile foundation" and "air spring + concrete pedestal" are proposed.In the FEM,add the pile isolation barrier and the group pile foundation located in the equipment foundation pit to obtain the output vibration velocity of the top surface of the group pile foundation cap after the first-level vibration isolation.Then the parameters of pile barrier such as pile length,pile spacing,row number and row spacing were analyzed.The results show that the spacing,length and number of rows are the main factors that affect the vibration isolation effect.Increasing the number of rows and the length of the piles and reducing the spacing of the piles are beneficial to improve the vibration isolation effect.In general,the vertical vibration isolation effect of the row pile barrier is better than that of the horizontal direction.The vertical vibration isolation rate is about 92.7% ? 99.1%,and the horizontal vibration isolation rate is about 23.3% ? 87.6%.(4)For the second-level vibration isolation measures,the formula for calculating the vibration isolation rate of the air spring-supported concrete pedestal system is introduced,and the vibration velocity on the top of the bearing cap after the first-level vibration isolation is input to calculate the output vibration velocity after the second-level vibration.The results of two-stage vibration isolation are calculated and analyzed.The results show that if only the first-level vibration isolation is performed,the high-amplitude vibration speed in the low-frequency band will exceed 3.2?m/s.