Three-dimensional Shape Measurement And Test Of Pile Foundations Based On Brillouin Optical Fiber Sensing

In the past three decades,optical fiber sensing technology has been rapidly developed,and Brillouin scattering based distributed optical fiber sensing technology has obvious advantages compared with other sensing technologies,making it one of the most outstanding and potential optical fiber sensing technologies in today's society.With the continuous enhancement of Brillouin optical fiber sensor in spatial resolution and sensing distance,it has been widely used in aerospace,road monitoring,pile foundation detection and other fields.Time domain Brillouin optical fiber sensing technology includes two research directions:Brillouin optical time domain analysis(BOTDA)and Brillouin Optical Time Domain Reflectometry(BOTDR).In this paper,three-dimensional shape measurement and test of pile foundations based on Brillouin optical fiber sensing are studied.It mainly obtains the strain distribution information through the distributed measurement of the Brillouin frequency shift of the sensing fiber,and combines the corresponding algorithm to reproduce the shape characteristics of the measurement object.The specific research content is as follows:Firstly,the basic theory of Brillouin fiber sensing is described,and the principle of differential pulse pair technique to improve the spatial resolution of BOTDA system is explained.In the theoretical study of space curve,the parallel transmission frame algorithm is derived,which provides a theoretical basis for the subsequent curve reconstruction.A new type of three-dimensional shape sensor was designed,combined with the built DPP-BOTDA system to measure the Brillouin frequency shift and strain distribution,and the parallel transmission frame algorithm was used to restore the four spatial shapes designed.The shape reduction effect is defined by the distal error,in the restoration results of the four shapes,the distal error is the lowest 4.73%and the highest is 26.3%.The BOTDR system is used for the pile foundation inspection of offshore wind turbines.The distributed strain data of two test piles under different horizontal loads are collected,and their maximum strain values under different loads are linearly fitted and analyzed.The fitted R~2are 0.9905 and 0.9981 respectively,which proves that the horizontal bearing capacity of the first pile reaches 1000k N,and the second pile is 700k N.Finally,a displacement calculation method based on distributed strain data is used to obtain the horizontal displacement distribution and pile deformation of the pipe pile under different loads.