## 1 Introduction  
PCC pile is developed using a new technique combining the advantages of pre-stressed concrete (PC) pile, drilled pile, and steel pile for ground improvement of highway and railway engineering [1]. Compared with other rigid concrete pile such as square pile, the PCC pile is simple in conformation, fast to construct, less concrete and more cost-efficient [2]. It has been widely used for pile foundation in China, it has yielded great social and economic benefits. [PERSON] and many other researchers have carried out a series of studies on PCC pile, including studying its bearing behavior under vertical and horizontal loads, its characteristics of negative friction, the squeezing soil effect and so on [3; 4; 5; 6; 7; 8]. The deformation characteristics of PCC pile are important for optimum design and construction. Now, the standard for measuring the strain of pile is based upon electronic gauges; this method has the disadvantages of difficult layout, low survival rate, poor stability, and low precision. Furthermore, this method cannot measure the deformation of the pile accurately and comprehensively. In order to obtain the deformation of PCC pile, a new measuring method should be used to monitor the strain. Distributed Optical Fiber Sensor (DOFS) technologies have been responsible for tremendous advances in strain measurement. The optical fiber is both a sensor and a transmission channel along the entire length of the fiber [9]. DOFS technologies have been developed for monitoring geotechnical engineering, such as Optical Frequency Domain Reflectometer (OFDR), Fiber Bragg Grating (FBG), Brillouin Optical Time Domain Reflectometer (BOTDR), Brillouin Optical Time Domain Analysis (BOTDA), Brillouin Optical Frequency Domain Analysis (BOFDA), Brillouin Optical Correlation Domain Analysis (BOCDA), and so on. Compared with conventional sensors, the fiber optic sensor has benefits including resistance to corrosion, high precision, and small size. Among these technologies, the BOTDR, BOTDA, BOFDA, and BOCDA technologies are fully distributed, while FBG technology is quasi-distributed. The spatial resolutions are: OFDR (1 mm in the sensing range of 50-70 m), BOTDR (1 m in the sensing range of 50 km), BOTDA (2 cm in the sensing range of 2 km and 2 m in the sensing range of 150 km), BOFDA (20 cm in the sensing range of 3 km), BOCDA (7 cm in the sensing range of 5 km). The strain precisions are OFDR (1 \(\mu\epsilon\)), BOTDR (40 \(\mu\epsilon\)), BOTDA (20 \(\mu\epsilon\)), BOFDA (2 \(\mu\epsilon\)), BOCDA (10 \(\mu\epsilon\)), FBG (1 \(\mu\epsilon\)). The OFDR is a type of DOFS technologies which has shorter measuring time, higher sensitivity, and higher spatial resolution than other frequently-used DOFS technologies like BOTDR, BOTDA, BOFDA, BOCDA, and so on [10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23]. OFDR technology was developed by [PERSON] in 1981 [24]. The basic principle of OFDR is shown in Figure 1. The continuous light emitted by a tunable light source is divided into two parts: one is sent out into the test fiber, producing Rayleigh backscattering light, which passes through the coupler as signal light, the light is coupled in the photodetector; the other is reflected back to the coupler as a reference light, and the reference light is coupled in the photodetector. In the spectrum analyzer, the mixed signal of reference light and backward Rayleigh scattering light from the photo detector are measured. The optical frequency of the backward Rayleigh scattering light at different position of the optical fiber is different because the laser light source of OFDR technology is tunable to that light source. The position information of the optical fiber can be obtained by detecting the light frequency, while the strain/temperature information of the optical fiber can be achieved by comparing the spectrum of reference and signal lights respectively. Figure 1: Basical principle of OFDR technology [24].