## 2 Engineering Description
### Testing Instrumentation and Layout  
The testing instrumentation of pile driving mainly consists of the FBG strain sensors, FBG angle sensor, and optical data collector. After the pile was impacted by the pile hammer, the pile vibration was measured by the sensors embedded in the body of the pile at the three measurement points. Only two types of transducers were used for the pile's response measurements, and all transducers used were based on the FBG technique, which were suitable for monitoring of port engineering. The FBG strain sensors used in the testing were designed to monitor civil infrastructures by the Dalian University of Technology [16; 17]. Prior to applying the FBG strain sensors in practical engineering, calibration tests were conducted on a steel plate to verify the performance of all the FBG strain sensors. An FBG strain sensor and a bare FBG sensor were mounted directly on the plates. Then, the tensile test in the steel plate was carried out in a uniaxial testing machine (shown in Figure 4). The steel plate was loaded continuously from 0 kN to 20 kN. The calibration test result of the FBG strain sensor is plotted in Figure 5, showing the relationship between the Bragg wavelength shift of the FBG strain sensor and the strain variation of the bare FBG sensor. The bare FBG and the FBG strain sensor agree well with each other during this load test, and the coefficient of linear association is greater than 0.9999. Figure 5 shows that the strain sensitivity of the FBG strain sensor is 4.33 pm/\(\mu\)e, indicating that the FBG strain sensor is sensitive to the strain variation. The test results demonstrate  
Figure 3: Transducers and test setup in the field. that the FBG strain sensor has good performance on host materials with a large Young's modulus, such as steel. The uniaxial FBG strain sensor has a range of \(\pm 1000\)\(\mu\epsilon\), as shown in Figure 6a. The FBG obliquity sensor, which has a range of \(\pm 10^{\circ}\) and a resolution of \(0.1^{\circ}\), is used to monitor the pile's obliquity, as shown in Figure 6b. The two kinds of sensors are all attached to the reinforcement bars (Figure 6c,d), then the concrete is poured. The strain and angle signals from the FBG sensors are digitized by the optical digital spectrum analyzer. The propagation speed of the stress wave in the concrete pile is generally high when performing pile driving. Therefore, a four-channel portable dynamic optic signal acquisition and analyzer system (SM 130) was used to acquire the time domain strain and angle data of the response points. The SM130 optic digital spectrum analyzer used in the testing is manufactured by Micron Optics, Inc. and had the following parameters: precision, 24 bits; measurement range: 1510 nm-1590 nm; and maximum sampling rate, 1000 Hz/channel. This equipment is highly suitable for structural vibration measurements with FBG sensors in situ. Figure 4: Performance testing of the FBG strain sensor. Figure 5: Result of the calibration test. When conducting the mechanical performance testing of a driven pile, the frequency sampling rate was set to 1000 Hz, and the sampling length included the entire pile driving process, approximately 850 s. The test setup is shown in Figure 7. The FBG strain transducers and FBG angle transducers were all embedded into the pile body during the prefabrication in the factory. Based on the design, three FBG strain sensors were embedded into the pile body near the head, middle, and tip, while one FBG angle sensor was embedded near the head. The installation of transducers is shown in Figure 8. In our testing, one pile was monitored, and three strain signals and one angle signal were collected in the testing. Meanwhile, three cases of testing for the driven pile were carried out, which were the time when the pile was set flat on the ship before driving, the time when the pile was set vertically for driving and the time during pile driving. Mechanical performance testing was conducted at three measurement points in each precast prestressed concrete pile (Figure 8).