Evaluation Methods Of Parallel Seismic Testing For Foundation Piles

The quality of pile foundation engineering directly influences the safety and reliability of building or structures.Conventional methods used for quality inspection of pile foundations currently,such as sonic echo and impulse response methods,have limited capabilities for existing piles and piles under construction with upper structure.Due to difficulties of information identification for pile shaft,it is hard to give quantitative judgements for the integrity of the pile foundation in general.The Parallel Seismic Test(PST)method is a borehole test method for detecting the integrity and depth of foundations.However,the relevant theoretical research of PST method can not meet the needs of practical engineering testing.The function expansion of this dynamic testing method and the principle of its test and analysis were studied systematically.Based on Fermat's principle,a series of formulas for determinating the depth and integrity of pile shaft and detecting the effect of enlarged bottom of the belled pile with PST method were derived.The finite element(FE)simulation and model piles and field test were carried out to verify the correctness and validity of the theories proposed in this thesis.The concluding remarks of this study are summarized as follows.(1)This paper presented the application of the first arrival of PS waves to detect the quality of pile foundation,for PP waves often suffering the surrounding environment interference and be difficult to determine the first arrivals accurately and reliably.The results of simplified theoretical models and FE calculations for intact piles and defective piles verified the effectiveness of PS waves in the detection of the depth and integrity of pile foundation.(2)The applicability and influencing factors of various existing methods for determing the depth of pile tip were analysed and compared through the study of the characteristics of the time-versus-depth(t-d)relationships of the first arrival PP and PS waves.The results of simplified theoretical models and FE simulations show that the scope of application of the correction method of two straight line intersection and the inflection point method are wider than previous methods.In addition to the inflection point method,the depth of borehole are required to ensure accuracy of test for the other several methods.That is,the base of the borehole should be located below the pile tip for a vertical distance of at least five times the distance between the pile and the borehole.(3)Based on the shortest duration theory of the first arrival waves,simplified theoretical models were established for several types of low-speed defective pile.A method of determining the position,length and degree of the pile defect was put forward.The results of FE simulations and field test indicate that the quantitative identification of the low-speed pile defect section could be realized with the method proposed this thesis.(4)A simplified theoretical model for the t-d relationships of the first arrival waves received by the probe in borehole close to the belled pile in homogeneous soil was deduced,and an approach to test the effcet of enlarged bottom of the belled pile was developed.The calculation results of FE for belled piles in saturated and unsaturated homogeneous soils and test results of model piles verified the reliability and accuracy of the method for determining positions of enlarged bottom and pile tip.(5)On the basis of Fermat's principle,simplified theoretical derivation for the t–d relationships of the first arrivals of PS waves in two representative layered soil conditions were developed,and the application of PST method in layered soil was explored in this thesis.The main conclusions are as follows.i)To ensure the reliability of test,it is necessary to know about the characteristics of layered soil surrounding the piles and P-wave and S-wave velocities before conducting PST.ii)When the first arrival of PP waves in the saturated layered soil with shallow groundwater are discussed,the feature of layered soil could be ignored and regarded as homogenous.For the PS waves,one should pay attention to the impact of soil layer,no matter the depth of the groundwater level.iii)It is not easy to identify the defect of the pile reliably if layered soil and pile defect appear in the same depth range.(6)The SR-SW/PST detector and the corresponding collection and analysis software were developed jointly with relevant parties,which could be used for PST method and down hole method.Signals received by the detector are stable and clear.Analysis results of model pile test verified the reliability of the instrument.It is worth of popularizing PST method in engineering.