## 3 Design of In-Situ Test  
### Spt  
The SPT method was conducted according to the testing specifications [49] to evaluate the bearing capacity during the engineering survey phase. Figure 2a shows the schematic of the standard penetrator. Nine drilling holes, including SPT-1#, SPT-2#, SPT-3#, SPT-4#, SPT-5#, SPT-6 #, SPT-7#, SPT-8#, and SPT-9# were selected in the area between A-A' and B-B' in Figure 1. During the in-situ SPT, the pit was dug around the drilling hole, and 300 mesh bentonite powder was poured into the pit to make bentonite mud, which is different from the SPT performed in survey phase. The mud was hydrated for 24 h, its average specific gravity was 1.272 g/cm\({}^{3}\), and the viscosity was 15.86 s, as shown  
Figure 1: Schematic diagram of plant area and geological cross-section with the exploratory boreholes.  
in Figure 2b. A protective cylinder with an outer diameter of 127 mm and a length of 3 m was installed at the top of drilling holes to prevent the hole from collapsing. During the SPT test, the standard penetrator was impacted into the tested layer for 15 cm and then impacted into the layer every 10 cm for a total of 30 cm, as shown in Figure 2a. The depth of the tested layer and groundwater, as well as the SPT blow count (SPT-N) of each 10 cm footage, were recorded accordingly three times. Figure 2c shows the SPT in situ.  
There are mainly three methods adopted for calculation of UBC of a single pile based on SPT-N: (1) The JGJ94-2008 method [50]. The ultimate axial pile capacity (\(Q_{uk}\)) consists of the end bearing capacity of the pile (\(Q_{pk}\)) and the shaft friction capacity (\(Q_{sk}\)). The \(Q_{pk}\) is calculated as the product of the pile end area (\(A_{p}\)) and unit end bearing (\(q_{pk}\)). The \(Q_{sk}\) is calculated as the product of the outer pile shaft area (\(A_{s}\)), and the unit skin friction (\(q_{sk}\)). The general formula of ultimate axial pile capacity is given as follows:  
\[Q_{uk}=Q_{sk}+Q_{pk}=u{\sum_{i=1}^{n}q_{sk}l_{i}}+q_{pk}A_{p} \tag{1}\]  
where \(u\) is pile perimeter (m); \(q_{sk}\) is the average unit skin friction (kPa) of soil layer \(i\); \(l_{i}\) is the pile length (m) interfacing with layer \(i\); \(n\) is the number of soil layers along the pile shaft.  
(2) The [PERSON] method [51]. [PERSON] put forward empirical correlations between the SPT-N and the bearing capacity of piles as follows:  
\[Q_{uk}=A_{p}q_{p}+ulq_{s} \tag{2}\]  
\[q_{pk}=40N\frac{L}{D}\leq 400N \tag{3}\]  
\[q_{sk}=2\widetilde{N} \tag{4}\]  
where \(Q_{uk}\) is ultimate axial capacity (kPa); \(A_{p}\) is the cross-sectional area of the pile tip (m\({}^{2}\)); \(q_{pk}\) is ultimate stress (kPa); \(u\) is pile perimeter (m); \(l\) is pile length (m); \(q_{sk}\) is average unit skin resistance  
Figure 2: (**a**) Schematic of penetrator; (**b**) Bentonite mud preparation; (**c**) standard penetration test (SPT) testing.  
(kPa); \(L\) and \(D\) are pile length and pile diameter (m), respectively; \(N\) is average SPT-N value almost \(10D\) above and \(4D\) below pile tip; \(N\) is average SPT (\(N\)) value.  
(3) The Schmertmann method [52]. [PERSON] used the N value to determine the ultimate end bearing capacity and ultimate shaft friction capacity, as shown in Table 2.