Development Of A New Dynamic Calibration Device For Pore Water Pressure Transducer And Performance Evaluation

The pore water pressure is one of the fundamental physical parameters that characterize the mechanics behavior and damage degree of soil,and improving the accuracy and reliability of the pore water pressure transducers is a core issues of the measurement technique for dynamic centrifuge modeling.It was found that the failure rate of the sensors in the dynamic centrifugal modeling at home and abroad showed that the failure probability of the pore water pressure transducers was the top of all measurement sensors,even if the failure rate of pore pressure water pressure sensors reached 70%~80% in single tests,which directly led to the failure of the dynamic centrifuge modelling.The calibration of the pore water pressure transducer before and after test is an effective method to guarantee the accuracy and reliability of pore water pressure measurement.However,a lot of the existing calibration devices are transformed by the common triaxial cell or by using pressure tanks,and they only have the capability of static calibration and cablibration environment with the actual working environment difference,which can't meet the requirements of dynamic centrifuge modelling.The pore water pressure transducer is a special purpose of precision sensors,and the manufacturers are very limited in the global scope.The type of pore water pressure transducers are different of structure,material,size,and so on,which can't provide dynamaic calibration data of the transducer,so it's a huge limit to develop of the dynamic centrifuge modelling technique and the analysis of the test data,which causes a lot of problems.This paper focuses on the development of a new dynamic calibration device for pore water pressure transducer,the performance evaluation and improvement method of pore water pressure gauge.The calibration device for the dynamic performance of the pore water pressure transducer is taken as the main sample with UC Davis.The first set of calibrating device for calibrating the static and dynamic performance of the pore water pressure gauge is designed and developed in our country.It can reproduce the real pore water pressure accumulation waveform in situ.Based on the new calibration device,a series of experiments are carried out to demonstrate the performance of the new calibration device and the static and dynamic pressure load generation and waveform control capability of the DPCM-700.Three kinds of international representative pore water pressure meter are selected,the method of dynamic performance calibration and evaluation is set up,and the suggestion for improving the design of sensor structure is put forward,and the results of the analysis of the test data are verified with the numerical simulation.The main work and results achieved are as follows:1.The paper sums up the development process of the pore water pressure transducer in the progress of static and dynamic performance measurement,points out the working characteristics and the key common problems of the pore water pressure transducer in the dynamic centrifugal modelling environment.The development of calibration devices for pore water pressure gauges at home and abroad is summarized,and the structural composition,functional characteristics and deficiencies of several calibration devices representing the development level at home and abroad are preliminarily compared.2.The working principle and structural characteristics of the existing differential type of pore water pressure transducer that differential-type and steel-wire and piezoresistive.The environmental characteristics of dynamic centrifugal test and the key difficulties of measuring pore water pressure gauge are analyzed.The performance parameters and structural differences of Druck,Keller and Hrct pore water pressure gauges were preliminarily compared.According to the centrifugal model test data of the University of California,Davis,it is proved that there are serious differences between different types and brands of pore water pressure gauges in dynamic measurement results.3.The design structure and function parameters of the UC Davis calibration device are analyzed,and the defects in the control method,the waveform generation,the frequency response and the calibration range are analyzed,and the modern mechanical assembly and numerical simulation are used.The overall design idea and scheme of the new pore water pressure gauge calibration device of DPCM-700 are put forward,and the specific component parameters and processing methods are designed,and the first set of calibration devices for static and dynamic performance is developed in China.4.With the new calibration device of DPCM-700 as the research platform,a series of tests are designed to evaluate its static and dynamic calibration performance respectively,including pressure resistance,air tightness,gas pressure transfer into water pressure uniformity and applicable range.The influence of the design structure and main parameters of the pore water pressure transducer,including the size of the porous stone,the material and the saturation state,is discussed through the contrast of the static and dynamic response performance of the three type of sensors.5.The test results show that the new calibration device has good pressure and air tightness,the gas pressure transfer into water pressure uniformity,the response speed meets the demand of the dynamic centrifugal test,and the accumulative waveform of the pore water pressure of soil liquefaction can be recovered.Under the condition that the porous stone is fully cleaned and saturated,the static and dynamic calibration results of three kinds of pore water pressure transducers are consistent with each other,and all of them show good linearity and stability.However,under the condition of incomplete saturation and cleaning of porous stone,the dynamic measurement performance of pore water pressure transducer can have obvious difference,and the calibration curve shows significant hysteresis characteristic.It is also indicated that the core issuse of the improved design of the pore water pressure transducer is at the heart of porous stone,and the cleaning and saturation of the porous stone before testing is very necessary.6.Using ANSYS CFX fluid mechanics software,the porous media and sensor internal cavity flow are established.The influence of permeability,size,saturation and internal cavity on pore water pressure transducer measurement performance is discussed.The simulation results verify the analysis conclusion of the calibration test data,and further indicate that the thickness,saturation and inner cavity gap of the porous stone are the main parameters that affect the dynamic response of the pore water pressure gauge.