The Preliminary Exploration On The Specification Of DFOS-based Pile Testing

Pile testing is an indispensable work to ensure the quality of the pile. As a new testing technology, the distributed fiber optic sensing (DFOS) technology has been continuously applied and popularized in pile testing over the past decade. The DFOS technology is increasingly approved by many engineering units, therefore a large number of engineering experience have been accumulated.In order to standardize the technology application in the pile testing, it is vary necessary to write the pile technical specifications of DFOS testing. On the basis of the domestic and foreign relevant results, as well as more than 10 years of experience in DFOS testing of the research group, the technical specification of the DFOS testing technology is inisial formed.This paper, systematically summarizes the principles, methods, specifications scope of the DFOS testing technology, proposes the type,, performance, laying and installation of sensing cable,, explains the data processing and pile internal force analysis.. The main results of paper are as follows:(1) Advantages and disadvantages of common pile testing technology are analyzed. Advantage, application status and development trend of pile DFOS testing technology based on Brillouin scattering light sensing principle are described.; Importance of writing the pile technical specifications of DFOS testing is illustrated.(2) Three kinds of pile DFOS testing based on BOTDR, BOTDA, BOFDA are compared from the aspects of the principles, demodulators and technical indicators.(3) The test principle of DFOS testing technology to detect the pile is described; the pile foundation bearing characteristics under vertical loads and horizontal loads is analyzed; the deformation of the pile by using load transfer method is analyzed; according to the test results of fiber strain, the formula of axial force, pile compression deformation, each soil friction resistance, tip resistance, bending moment and deflection are given.(4) The types performance and usable range of sensing fiber optic in pile testing are described; The selection criteria of pile DFOS testing is given; sensing fiber calibration methods are proposed; several commonly used sensing cables through trial are calibrated.(5) A variety of layout processes of sensing fiber are put forward aimed at their own characteristics of piles, precast pile and steel pipe piles. Layout scheme and process of temperature compensation sensor are proposed under the condition of the need for temperature compensation.(6) The pile DFOS testing system is analyzed and built. The quality evaluation atandard of sensing fiber is put forward.(7) A data processing method of pile testing based on DFOS technology is proposed. The common pile testing curve is analyzed and the common law of the pile bearing capacity transferring is sumed up. Combined with geological data and construction records, the main factors that affect the integrity of the pile foundation are analyzed and the change rule of testing curves which related to pile foundation quality problems are drawn.(8) According to the research and practice results of pile DFOS testing and referring to The People’s Republic of China industry standard" Technical Specification of Pile Testing ", the preliminary specification of pile DFOS testing is completed.