(1) Loading magnitude (*Fl*)

Before the formal loading of the test pile, it was necessary to conduct a pre-loading experiment on the test pile to examine the stability of the loading system and check whether the various original components can operate normally, and then the formal loading of the test pile could only be carried out after the debugging. According to Chinese specification (GB/T 50152-2012) [21], the load size during the pre-loading was about 10% of the estimated ultimate load (0.1*Fu*, *Fu* = 60 kN), and it was loaded three times in stages, and 2 kN in a single stage. The loading experiment was loaded from 0 kN until the test pile was damaged after the pre-loading.

(2) Loading step length (Δ*F*)

The loading step length (Δ*F*) in the loading process of the test pile should be determined by the loading magnitude (*Fl*), ultimate bearing capacity (*F*max), and loading rate (*Vl*). The loading step lengths of each level before and after the cracking of the test pile should be about 5% *Fu* and 10% *Fu*, respectively. When the load reached 90% *Fu*, the loading step length of each stage was loaded at 5% *Fu* until the test pile was damaged. Therefore, before the test pile cracked, the loading step length of the jack 1, jack 2, and jack 3 (Δ*F*1, Δ*F*2, Δ*F*3) in the loaded section were designed to be 0.5 kN, 1.0 kN, and 1.5 kN, and increased to 1.0 kN (Δ*F*<sup>1</sup> ), 2.0 kN (Δ*F*<sup>2</sup> ), and 3.0 kN (Δ*F*<sup>3</sup> ) after the cracking of the test pile. When the load reached 90%*Fu*, Δ*F*1, Δ*F*2, and Δ*F*<sup>3</sup> were designed to be 0.5 kN, 1.0 kN, and 1.5 kN until the end of the test.

(3) Loading rate (*Vl*), loading time (*tl*), and holding time (*th*)

Referring to the loading method of indoor concrete members [22,23], it was found that the reasonable loading rate (*Vl*) in the loading process is 10~30 N/s. In order to avoid possible adverse effects caused by excessive loading rate, the loading rates of jack 1, jack 2, and jack 3 proposed in this paper are 10 N/s, 20 N/s, and 30 N/s, respectively. The loading time (*tl*) is determined by the loading step length (Δ*F*) and the loading rate (*Vl*), and the single-stage loading times before and after cracking was determined to be 50 s and 100 s, respectively. When the load reached 90% *Fu*, the single-stage loading time was 50 s. After the thrust load was applied, the loading state was maintained for 100 s (*th*) to ensure that the applied trapezoidal load was fully transferred to the test pile, the data of all monitoring elements were stable, and then the next loading step was performed. The loading method is illustrated in Figure 8.

**Figure 8.** Loading design of the test pile.

#### 2.4.3. Data Acquisition System

The data acquisition system mainly contains the percentage meter, strain gauge, pressure sensor, DH3818 static strain tester, and a laptop computer equipped with DHDAS dynamic signal acquisition system. The percentage indicator was applied to obtain the lateral displacement of the test pile, and a total of six percentage indicators (DIs) were arranged along the loaded section of the test pile in this experiment, which were 0 cm, 13 cm, 26 cm, 39 cm, 52 cm, and 65 cm from the pile top, respectively, as illustrated in Figure 9. The strain gauges on the steel bars were applied to measure the steel bars' strains of the test pile and the type was BE120-3AA. It was arranged in two rows of 18 places on the tensile steel bars and the structural steel bars of the test pile, which were 9.5 cm, 21.5 cm, 33.5 cm, 47.5 cm, 57.5 cm, 70.0 cm, 80.5 cm, 91.5 cm, and 102.5 cm from the pile

top, respectively. The concrete strain gauges were applied to measure the concrete strains of the test pile and the type was BX120-50AA. It was arranged in two rows of 10 places on Side B of the test pile (CSG-1,CSG-2), which were 47.5 cm and 65.0 cm from the pile top, respectively (Figure 7). The pressure sensors were arranged between the screw jacks and the test pile to obtain the applied loads. The type was CZLYB-1A and the measuring range was 50 kN.

**Figure 9.** Arrangement of percentage meters.

### *2.5. Experimental Procedures*

Figure 10 shows the main steps of this test. In Step 1, the steel strain gauges were posted according to the steel strain gauge arrangement scheme shown in Figure 7. Then the tension steel, structural steel, and the hoop steel were tied into a steel cage according to Figure 2b. The mold was made with wood boards according to the pile size and finally the pile concrete was poured and cured for 28 days. Step 2 was to set up the experimental system, including fixing the steel support, placing the test pile, laying screw jacks and pressure sensors, and connecting strain gauges and strain tester. Step 3 was to load the test pile with trapezoidal load by referring to the loading method in Section 2.4.2. Step 4 was to collect experimental data, including the pile displacement, residual bearing capacity, steel strain and crack development. Step 5 was to dismantle the experimental system and end the experiment.

**Figure 10.** Experimental procedures.
