# Application Study of the High-Strain Direct Dynamic Testing Method
## 3 High-Strain Direct Dynamic Test and Static Load Test
### Comparison between the Results of the Dynamic Load Test and the Static Load Test  
To evaluate the accuracy of high-strain testing using the static load test as a standard, three conditions must be considered. First, both dynamic and static load tests must be conducted on the same test pile and in the same soil conditions. Second, soil resistance must be fully mobilized. Third, the pile's failure modes must be identical in both tests. The results of dynamic and static tests are comparable only if the pile fails to overcome soil resistance in both methods. The four test piles in this study meet these three conditions.  
As previously mentioned, in regions with thick, soft clay, the actual maximum axial force on the pile during a static load test is greater than the designed ultimate bearing capacity because of negative skin friction. The high-strain method is typically used to predict the pile's ultimate bearing capacity. To assess its accuracy, this study compares the actual maximum axial force measured during the static load test with the bearing capacity predicted by the high-strain dynamic test. Table 5 presents the comparison results, showing that the error range between the high-strain method and the static load method is \(-9.5\%\) to \(3.7\%\). This indicates that the high-strain dynamic test is reliable for evaluating pile-bearing  
Figure 12: Axial force curves of the four test piles (**a**) #1–5, (**b**) #2–7, (**c**) #3–9, and (**d**) #4–17.  
capacity. The main reason for the small error is that both tests determine the maximum load according to relevant standards, which ensures sufficient mobilization of soil resistance without damaging the pile material.  
The following potential sources of error exist when using the high-strain dynamic method to predict the ultimate bearing capacity of piles:  
1. Order of Testing: The dynamic load test was conducted before the static load test. Repeated hammering can densify the soil around the pile, potentially leading to an underestimation of the ultimate bearing capacity by the high-strain method and an overestimation by the static load method.
2. Sensor Placement: The segment of the pile near the bottom includes the side resistance up to the sensor \(SE_{N}\). Therefore, the actual end-bearing resistance is less than the calculated value. For more accurate measurement, strain sensors should be placed closer to the actual pile tip.
3. Boundary Effects: As observed in Figure 10, some pile segments located at the boundary between soft and hard soil layers exhibit lower ultimate static resistance compared with other segments. This is due to the change in the direction of forces on the pile side at these boundaries. When calculating dynamic resistance using Equation (10), opposing pressure and tension forces can cancel out, leading to lower resistance values. In practice, increasing the number of measurement points, reducing the spacing between them, and deploying a denser array of strain sensors at soil boundaries can enhance prediction accuracy.  
By addressing these potential errors, the high-strain direct dynamic testing method can be more accurately applied to predict the ultimate bearing capacity of piles.