## 1 Introduction
The optical backscatter reflectometry (OBR) instrument is based on OFDR technology, manufactured by LUNA company. It has successfully been applied in different areas for strain and temperature measurement. [PERSON] et al. [25] used OBR to monitor the mechanical behavior of a representative-scale reinforced concrete structural element. It was found that the strain measured by OBR was consistent with both the experimental data obtained from conventional vibrating wire gauges and the theoretical predicted strain. [PERSON] et al. [26] measured the transient temperature inside a single channel of a wash-coated monolith reactor using OBR. The capability of OFDR technology in temperature monitoring has been tested successfully. [PERSON] et al. [27] bonded optical fibers to the surface of a concrete slab. The OBR was then used to measure the strain of the concrete under load. The result verified the potential of OFDR in Structural Health Monitoring (SHM). [PERSON] et al. [28] embedded a distributed fiber-optic sensing sensor within a composite joint, and used OBR to monitor the strain. The monitoring results agreed well with the results of Finite Element Analysis. [PERSON] et al. [29] used FBG, OBR, and strain gauges to measure the blades of a 13.5 m wind turbine blade prototype. The results were compared, and showed that OBR could define sensors every 10 \(\upmu\)m. The resolution and accuracy of OBR presented a strong inverse dependency of the defined sensors. [PERSON] et al. [30] monitored crack initiation and propagation in the glue line by FBG and OBR respectively. The experimental results showed that the great advantage of the OBR was that the entire length of fiber could be used for sensing strain, thus providing better spatial resolution compared to conventional FBG arrays. The feasibility of OFDR in aeronautical construction and structural engineering has been verified. However, the OFDR in strain and temperature measurement of geotechnical engineering has not been widely reported on to date. The deformation of concrete pile is very important in the refinement of geotechnical engineering. Compared with aeronautical materials such as steel, aluminum, and so on, concrete material has rough surface; as such, the bonding of optical fiber is difficult. Its roughness makes it hard to maintain fastness. When compared with concrete structures such as slab, girder, and so on, the pile is buried in soil where the optical fiber needs protection due to the friction and pressure from the surrounding soil. The application of OFDR to concrete pile is worthy of further study. The deformation of pile is important for foundation. In order to measure the deformation of PCC pile, a small-scale model test of a single PCC pile was carried out. OFDR technology was used to measure the strain of PCC pile. The method and process of laying the optical fiber, applying vertical loads, dealing with data, etc. are introduced in detail. The strain curves were analyzed according to the experimental situation, a numerical model was established by COMSOL Multiphysics, and the experimental results and numerical results are compared. Finally, the potential for the application of OFDR to PCC pile and PCC energy pile in situ was discussed.