Theoretical Research On The Newly Developed Distributed Low-Strain Integrity Test For Deep, Large-Dimensional, And In-Service Pile Foundations

The low-strain test theory of pile foundations has been relatively mature after decades of development.In the past,the low-strain test theory of pile foundations based on the one-dimensional rod model dominated the practical work of low-strain nondestructive testing of pile foundations,and it has become an important theoretical basis for the identification of the pile length and defect location.At present,the theory of lowstrain non-destructive testing of pile foundations has encountered the following three major development bottlenecks: First,as the size of piles continues to increase,the resulting severe high-frequency shear wave interference has become an important obstacle restricting the accurate quality assessment of pile foundations;Second,as the length of piles continues to increase,the non-reflection phenomenon of stress waves due to energy dissipation has become a key factor that makes it difficult to measure long piles in low-strain tests;third,China’s large-scale infrastructure construction began in the1950 s,and these buildings have reached the end of their service nowadays,making the demand for testing in-service pile foundations increasingly strong,although the theory guiding the test of in-service pile foundations is lack.Aiming at handling the above three key bottlenecks,this thesis is entitled "Theoretical research on the newly developed distributed low-strain integrity test for deep,large-dimensional,and in-service pile foundations",focusing on the establishment of low-strain testing in deep & large pile foundations and in-service pile foundations so that the practical testing can be guided,the testing layout can be optimized,and the readability of the signal can be improved.The main findings of this essay can be summarized as:(1)The concept of distributed low-strain testing is proposed,aiming in solving the problem of "non-reflection" of strain waves.Also,a true three-dimensional pile-soil coupling model is established,which enables the simulation of the circumferential transverse wave during the parallel seismic test for the first time.(2)Considering that the partial differential equation is difficult to solve when considering the true three-dimensional effects of stress wave(tangential,radial and longitudinal propagation)and the transverse inertial effect at the same time,the modified Rayleigh-Love rod model is proposed,which has impressively high computational efficiency and the ability to simultaneously simulate the three-dimensional effect of stress wave and the effect of transverse inertia effect.(3)To overcome the difficulties of applying the longitudinal low strain test to inservice pile foundation testing,the distributed torsional low strain test method is proposed.Also,the relevant theories of the torsional distributed low-strain test for the new pile foundation and the in-service pile foundation are respectively established.(4)The establishment of the first theoretical model so far enables the analysis of the three-dimensional effect and the formation mechanics of the high frequency interference during the torsional low strain integrity test.(5)In order to eliminate the high-frequency interference and the mutual interference between upwards and downwards waves during the test of the in-service pile foundations,a workflow for signal noise reduction using FFT low-pass filtering and traveling wave decomposition was proposed.To verify the reliability of the proposed workflow,a largediameter in-service pile-soil interaction model is established.On the basis of this model,the workflow is proved to be extensively reliable and stability.