# Predicting Tensile Strength for Prestressed Reinforced Concrete-Driven Piles
## 3 A Case Study: Verification of the Proposed Approach  
This case study is from a thermal-power-plant project in Soc Trang, Vietnam, in 2016. Figure 2 shows Borehole BH6, with a soil profile of the study area; this profile is relatively typical and similar to other boreholes within the footage of the plan. Soil Layer 1 is a loose-medium-dense sand, which is followed by Layers 2 and 3, which are soft-firm-clay weaker soils with much lower SPT numbers. Figure 2: Boring Log BH6 (typical for the study area). The foundations consist of prestressed-reinforcement-concrete square piles with a compressive strength \(f^{\prime}{}_{c}\) of 50 MPa (cylindrical samples). The pile cross-section of 0.5 m \(\times\) 0.5 m, with the details on the pile head and tip, are presented in Figure 3. The pile lengths are 52 m (consisting of three segments: 18 m, 18 m, and 16 m) and 53 m (18 m, 18 m, and 17 m). The hammer used to drive the piles weighs 7.2 tons, with a drop height of 1.8 m; other specifications on the hammer are presented in Table 2.  
For the initial approved design, the reinforcement used for each pile was 15.24 mm-diameter prestressed cables with a tensile strength (\(f_{pu}\)) of 1860 MPa, the value of the prestressing force was \(f_{pi}=0.7f_{pu}=1302\) MPa, and there were four cables for each pile. During pile driving, horizontal cracks were observed on a number of piles. Figure 4 shows the cracks on Pile 12 HA415 and Pile 12 HA174. The pile design, fabrication, and installation processes were revisited and analyzed. It was confirmed that the cause of the damages was not from the quality of the concrete, pile transportation, pile lifting by crane, \begin{table}
\begin{tabular}{l c c} \hline \hline
**Descriptions** & **Units** & **Values** \\ \hline Power stroke & Nm & \(244-122\) \\ Number of stroke & 1/min & \(35-50\) \\ Max. blow of impact & mm & \(4518\) \\ Impact weight & Ton & \(7.2\) \\ Dimensions (L \(\times\) W \(\times\) H) & mm & \(5990\times 1130\times 980\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Specifications of the Hammer (Model HD 72). Figure 3: Pile Details: (**a**) Pile Head with Eight Pre-Stressed Cables and (**b**) Pile Tip. or buckling. Moreover, the impact caused by a clamp used to reduce the effects of pile slenderness during installation was eliminated as a reason for the cracks. The cracks were mainly transverse, which shows that the damages must have come from tensile stress and not from compressive stress [3]. Based on the analysis, the pile design was changed by increasing the number of reinforced cables from four to eight for each pile. The type of cable was unchanged, with a diameter of 15.24 mm, and a prestressing force of 19,200 kg (188.3 kN). The pile tensile strength was calculated using three approaches: (1) not accounting for the concrete tensile strength (\(k\) = 0) as in most standards; (2) accounting for the concrete tensile strength following AASHTO [5] (\(k\) = 0.250; (3) accounting for the concrete tensile strength following the proposed Equation (4) (\(k\) = 0.622). The compressive strength of the concrete used is 50 MPa. The results in Table 3 show that the pile tensile strength is significantly increased when accounting for the tensile strength of the concrete, and the proposed equation gives a tensile strength at least 38% higher than that of AASHTO [5].