# Field Test Study on Vertical Bearing Characteristics of Coal Rock
## 2 Materials and Methods
### Field Tests
#### 2.2.3 Bi-Directional Load Test  
The test piles are manual digging piles. The geometric dimensions of the two test piles are shown in Figure 6. The design pile diameter is 2.0 m, and the thickness of the reinforced concrete for the wall supporting during the digging process is 0.15 m, so the outer diameter of the reinforced concrete supporting wall is 2.3 m. The concrete is not poured to the top of the hole, but at a certain depth below the top level, so the length of the pile is less than the depth of the hole. The TP1 test pile is 6.8 m long and the hole depth is 7.5 m; the TP2 test pile is 4.0 m long and the hole depth is 7.7 m.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**No.** & **Initial Load (kPa)** & **Load Increment (kPa)** & **Max Load (kPa)** \\ \hline PLT1 & 280 & 140 & 2100 \\ PLT2 & 280 & 140 & 1960 \\ PLT3 & 280 & 140 & 2240 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Loading conditions of plate load tests.  
Figure 6: Geometric dimensions of test piles.  
The bi-directional load test is used for the vertical compressive bearing capacity testing, as shown in Figure 7. The load cell is placed at the bottom of the pile. During the test, the load cell is loaded from the hydraulic pressure controller on the ground through the oil pipe. The bottom plate of the load cell is loaded downward to test the bearing capacity of the pile tip; the top plate of the load cell is loaded upward to test the bearing capacity of the pile side. The load applied is calculated by the pressure gauge at the hydraulic pressure controller. The downward displacement of the bottom plate and the upward displacement of the top plate are transmitted to the ground surface by the telltale and then measured by the displacement indicator [18; 19; 20; 21; 27].  
The bottom plate of the load cell is a circular steel plate with a diameter of 1800 mm and a thickness of 30 mm, as shown in Figure 8. When the pile hole is excavated to the design elevation and before the load cell lowers down, the cement mortar is use to level the bottom of the pile hole, and that ensures good contact between the bottom plate of the load cell and the coal rock.  
The maintained load test method is adopted. At each level of loading, the displacement is measured at 5 min, 15 min, 30 min, 45 min, 60 min and then every 30 min thereafter. The displacement convergence criteria are as follows: The displacement increment per hour does not exceed 0.1 mm/h, and it occurs twice in a row (starting from 30 min, and for a  
Figure 8: Load cell of the bi-directional load test.  
Figure 7: Sketch of bi-directional load test.  
consecutive 90 min). At a certain level of loading, if the displacement increment of the upper plate or the lower plate is greater than 5 times the displacement increment of the previous level of loading, and if the total displacement exceeds 40 mm, it can be considered that the ultimate bearing capacity has been reached, and the loading process shall be terminated. When unloading, the unloading amount of each level is twice the loading amount. The load of each level is maintained for 1 h, and the displacement is measured at 15 min, 30 min and 60 min, respectively. After unloading to zero, the displacement is measured for 3 h at 15 min, 30 min and every 30 min thereafter [27].  
The loading conditions of the test piles TP1 and TP2 are shown in Table 2. The loading amount of the first level is 1600 kN, and the load increment of each level is 800 kN. Both of the two test piles failed when they were loaded to the 11 level, and the maximum loading value was 9600 kN for both.