## 5 Fabrication of Testing Instruments  
In the in situ tests, the soil nails were instrumented with vibrating wire strain gauges (JMZX-416A), which were attached to the steel tendon of each soil nail. JMZX-416A was applied to measure the stress of stressed reinforcement in reinforced concrete structures, the measuring range and sensitivity of which are 200 MPa and 0.1 MPa, respectively. Readings were obtained with and stored on a data logger (JMZX-3001). During the installation of the strain gauges, great caution was undertaken to protect the gauges and ensure their survivability. The fixed strain gauge consists of four parts: sensor, connecting rod, wire, and plug. Details of the strain gauge are illustrated in Figure 4.  
Before gauge installation, their initial readings were brought to the prescribed range. Gauges were then mounted onto the blocks used for fixation and wrapped with a plastic cover to isolate and protect them from subsequent grouting. A typical installation procedure of the strain gauge is specified as follows. First, the steel bar is cut into the predesigned size and length. Then, the fixed part of the strain gauge, as shown in Figure 4, is welded to the steel bar. It is noted that the overall length of soil nail remains unchanged after the installation of the strain gauge. Afterward, wires are arranged along the length of the soil nail and fixed on the soil nail with waterproof tape. Finally, wires are collected at the top of the slope and connected to data loggers. It is worth noting that care must be exercised during welding to avoid sensor damage, and the part where the sensor and the connecting rod are combined should be covered with a wet cloth and moisturized constantly to avoid overheating. Afterwards, fiber optic sensors are fixed onto the soil nail according to the circuit of U. The soil nails, with centralizers at a 2.0 m interval, are placed in the boreholes, followed by two-stage grouting (i.e., gravity and pressure grouting). Figure 5 shows details of the instrumented soil nails. Different from soil nails, each anchor (25 mm ribbed high yield steel bar) was instrumented with a vibrating wire load cell (MJ-101) at the head to monitor the anchor force, with the exception of strain gauges. Figure 6 presents the details of a load cell. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Soil Nail/** & **Depth (m)** & **Length** & **Bonded** & **Spacing** \\
**Anchor** & **Depth (m)** & **Length (m)** & **Length (m)** & **(m)** \\ \hline
1 & 1.20 & 9.00 & - & 1.50 \\
2 & 2.70 & 12.00 & 12.0/9.5 & 1.50 \\
3 & 4.20 & 9.00 & - & 1.50 \\
4 & 5.70 & 7.00 & - & 1.50 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Design parameters of composite soil nailed retaining structure. Figure 4: The details of the strain gauge. Due to the anchor tendons being made of steel bars instead of steel strands or wires, the main difference between soil nail and anchor is the processing of the unbonded part. Depending on the designed unbonded length, after grease was spread, a PVC-corrugated pipe was used to wrap the steel reinforcement bar. The connection between the unbonded and the bonded part was fastened with fine steel wires. Figure 7 presents the manufactured testing anchors. To investigate the mechanical behavior of composite soil nailing, a series of tests was carried out under different conditions. The measured results from the testing profiles of C1, C4, and C5 were analyzed to study the effects of two important characteristics, namely, prestressing force of the anchor and unbonded length of the anchor, on the soil nail force. For the first parameter, it is worth noting that loss of prestressing force should be avoided if possible. For the second parameter, it can be expressed in terms of the length ratio \(\rm{Lu/L}\), where \(\rm{Lu}\) is the length of unbonded part and \(\rm{L}\) is the entire length of the anchor. Testing results of profiles C1, C4, and C5 were simplified with No.1, No.4, and No.5 in the following analyses.