| Pore water pressure is the core component of Terzaghi K effective stress principle and a key parameter of soil mechanics,which can effectively reflect the soil stress state,instability process and mechanism of foundation and geotechnical structure.Miniature pore water pressure transducer for measuring pore water pressure is different from other pressure transducers,which has complex structures such as porous stone and cavity,and takes the change of pore fluid stress between soil particles as the test object,so that the accuracy and reliability of the transducer test have always been an important topic in geotechnical engineering.The centrifuge model test is known as the most advanced physical test method in the 21st century,which can reproduce the stress state and deformation mechanism of dozens to hundreds of meters thick soil with 1m thickness and small scale model.However,due to the similarity ratio,centrifuge model test has special test conditions such as high frequency vibration,viscous liquid,high pressure load and strong electromagnetic environment,which puts forward strict requirements for the transducers.Since the shutdown of the transducer PDCR-81 in 2010,the global centrifuge laboratory was caught in the dilemma of lacking the supply of the transducers.Besides,the existing transducers in China are mostly suitable for static test,and generally lack of performance in dynamic measurement,response accuracy and long-term stability.Therefore,the research and development of domestic high frequency response miniature transducer is an imperative issue.The existing Specification for geotechnical centrifuge model test techniques(DL/T5102-2013)clearly gives the saturation requirements and operation steps of pore water pressure transducers.However,there is no clear method and index requirement for the calibration method and technical parameters of the transducer.At present,pore water pressure transducer calibration devices at home and abroad are mostly transformed from triaxial cell,mainly focusing on static performance calibration.Recently,in LEAP-2015international parallel dynamic centrifuge liquefaction model test carried out by the United States,Britain,China and other countries,the results of pore water pressure test data and liquefaction threshold show significant dispersion.Therefore,many scholars have raised great doubts about the accuracy and reliability of pore water pressure test,and stressed that a reasonable analysis should be given in combination with the differences of pore water pressure test technology.In view of the late development of pore water pressure test level under centrifuge model test,many domestic scholars mainly focus on the unit model test for the analysis and research of the effecting factors of pore water pressure.Based on the urgent needs of the development of accuracy and reliability technology of pore water pressure test in centrifuge model test,this paper focuses on the key issues such as the research and development of pore water pressure dynamic response calibration device,and a novel domestic high-performance miniature transducer that can replace the international transducer PDCR-81,the analysis of effect factors and specification suggestions evaluation of pore water pressure test under centrifuge model test,and the construction of a novel pore water pressure increment model.The main achievements and innovative works are as follows:1.Based on the working principle of UC Davis calibration device,a novel pore water pressure dynamic response calibration device is developed,and its dynamic pressure uniformity,high-range pressure-bearing sealing characteristics and arbitrary pressure wave control were verified.The evaluation indexes and calibration methods of pore water pressure transducers suitable for centrifuge model test are established and the differences in frequency response,accuracy and stability of three types of the representative miniature pore pressure transducer are revealed.The variation of the dynamic test accuracy of the transducers with the saturation time according to the saturation method of specification was discussed.The results show that the amplitude attenuation and response lag of three types of the transducers are different.The effects of three saturation methods of specification vacuum stirring,continuous vacuum and normal atmospheric pressure on the saturation rate of the transducers were compared.The saturation time of 9h,16h and 4d are used as the evaluation criterion for the reliability and accuracy of dynamic test.2.The structure design,material selection and common fault forms of various typical miniature transducers at home and abroad were analyzed,a novel domestic high performance transducer DSP-II was developed,the advantages and characteristics of its design principle,structure composition and key technologies are expounded.Taking the international’standard’transducer PDCR-81 as the reference,a serious of static and dynamic calibration tests were carried out by using the calibration device to evaluate the linearity,response time,amplitude error and other indicators of the DSP-II.The results show that the test results of DSP-II and PDCR-81 are highly consistent,and can meet the requirements of conventional dynamic centrifuge model test with frequency within200 Hz.The static and dynamic centrifuge model tests of pure water and saturated sand models were carried out by using the centrifuge shaking table test system.The test accuracy,frequency response,stability and other performance of the transducer DSP-II were verified,and the measurement performance of domestic transducer DSP-II was proved to meets the international’standard’transducer PDCR-81.3.The effecting factors and reliability of the miniature transducer were analyzed in centrifuge model test environment,and the parallel tests and calibration methods for dry sand,saturated sand and pure liquid models were designed and established.The influencing characteristics and response errors of the reliability of the transducer test were analyzed under the single conditions,such as centrifugal acceleration,viscosity coefficient and layout direction.The results show that when the centrifugal acceleration is from 0 to 50g,the amplitude error of the transducers in each direction of dry sand model is less than 1.0 k Pa,and the influence can be ignored.When the viscosity coefficient is from 1 to 100cst,the response rate and resolution of the transducers show a downward trend.Different layout directions have significant effects of pore water pressure,in which the pore water pressure curve oscillates seriously with parallel placement under the influence of dynamic shear stress,while the pore water pressure ratio more than 1.04 with vertical placement under the influence of pore water pressure seepage,and the perpendicular placement is the best layout direction.The influence of transducer response in strong electromagnetic environment can be ignored.The proposed calibration methods and effect factors for centrifuge modeling provide supplementary provision and data support for the revision of the specification,and provide guidance for the construction of major scientific and technological infrastructure in China.4.Based on the basic idea of Ishibashi multiplication of various factors,the key factors such as effective stress,dynamic shear stress ratio and vibration cycle times were mainly considered,a pore water pressure incremental model suitable for the centrifuge model test is proposed.Using the test data of the domestic miniature transducer DSP-II,the applicability and reliability of the new pore pressure incremental model is verified.The results show that the pore water pressure increment model can well describe the pore water pressure development process of saturated sand and liquefaction threshold under different seismic loads,relative densities,soil depths and other conditions.The liquefaction resistance and calculation parameters of saturated sand with different densities are given.With the decrease of liquefaction resistance of saturated sand,the initial liquefaction trigger vibration cycle times N increases gradually.With the increase of relative density D_rof saturated sand,the liquefaction resistance also increases.The liquefaction resistance prediction datas calculated by the model are basically consistent with the measured datas,which can better predict the liquefaction resistance point of saturated sand in centrifuge model test. |
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