# Experimental Study on Deformation Monitoring of Bored Pile Based on BOTDR
## 3 Bored Pile Field Test
### Monitoring the Deformation of Pile  
The following is the process of monitoring the bored pile deformation using the distributed optical fiber technology:  
1. Optical fiber selection. There are two kinds of optical fiber to choose from: single-mode optical fiber and GFRP optical fiber. Although the single-mode optical fiber has the best coordination effect with the measured object's deformation, it is easy to be wound and broken. The GFRP optical fiber is not only good in coordination with deformation of the measured object, but also not easy to damage and easy to layout. It can be seen that the single-mode optical fiber is relatively fragile, with poor internal protection effect and easy to damage. Since it is difficult to arrange it into the measured object, it is not suitable for field application practice. It can only be \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Soil Layer** & **Soil Name** & **Thickness** & **Density** & **Elastic Modulus** & **Cohesion** & **Internal Friction Angle** \\
**Numbering** & **Soil Name** & **(m)** & **(p/cm\({}^{3}\))** & **E (MPa)** & **C (kPa)** & **\(\varphi\) (\({}^{\circ}\))** \\ \hline (1) & Plain fill & 3.55 & 1.90 & 18.5 & 30.9 & 24.6 \\ (2) & Clay & 2.58 & 1.97 & 38.5 & 46.6 & 16.6 \\ (3) & Silty clay & 2.89 & 1.89 & 29.5 & 31.1 & 21.6 \\ (4) & Silt & 3.45 & 1.88 & 41.0 & 6.2 & 26.8 \\ (5) & Silty sand & 4.91 & 1.93 & 43.0 & 3.8 & 29.4 \\ (6) & Silty clay & 6.44 & 1.89 & 23.0 & 30.9 & 20.4 \\ (7)-1 & Silty clay & 3.08 & 1.88 & 21.5 & 30 & 19.7 \\  & Silty clay & & & & & \\ (7)-2 & and silt & 3.55 & 1.91 & 41.0 & 3.9 & 28.2 \\  & interbed & & & & & \\ (8) & Silty sand & — & 1.93 & 42.0 & 3.8 & 29.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Physical and mechanical properties of soil. used for scientific research and surface monitoring of some simple structures without too many protective measures. Designated paste can be well bonded GFRP fiber by reinforcing cage in piles in the main steel profiles, through the prestress can well reflect the deformation of the pile body; its reinforcement structure can have a very good protection effect on the fiber core, and it can greatly improve the optical fiber tensile, bending, bending capacity. The fiber at the joint of bored piles and pile head is not easily damaged, and the convenience of fiber has great applicability in engineering practice. Therefore, GFRP fiber was used for monitoring in this test. 2. Optical fiber layout. Figure 1 is the schematic diagram of optical fiber and field test. Glass fiber reinforced plastic (GFRP) optical fiber can be seen from Figure 1(a). The layout of optical fiber is shown in Figure 1(b). The main parameters of the optical time domain strain measuring instrument are that: Optical fiber type is GFRP, Maximum dynamic range is 15 dB, Strain distance resolution is 1 m. Strain measurement accuracy is \(\pm\)50 \(\mu\varepsilon\); strain test range is \(-\)15,000\(\sim\)+15,000 \(\mu\varepsilon\). The test range is 1 km, and maximum sampling resolution is 0.05 m.
3. For the laying of optical fiber sensor, the symmetrical U-shaped layout is fixed, and the optical fiber sensor can be laid on the main reinforcement of the steel cage before the optical fiber can be connected to the bored pile through concrete pouring. The steel cages are lowered and welded in turn. 4. Loading and monitoring. After 30 days, the top of the pile is subjected to the loading stages, the loading and unloading conditions are shown in Table 2, including 1094 kN, 1641 kN, 2188 kN, which are selected as processing objects. The photos of field test are shown in Figure 2. Figure 2a is the process of fixing the optical fiber, Figure 2b is the U-shaped layout diagram of the optical fiber, Figure 2c is the field static load diagram, and Figure 2d is BOTDR instrument. Figure 1: Optical fiber and schematic diagram of field test. (**a**) GFRP optical fiber; (**b**) Schematic diagram of field test. Figure 2: Photos of field test.