## 3 Low-Strain Integrity Testing
### The SE Method
However, these minor defects can seriously affect the structural integrity of the pile. In a previous study, [PERSON] et al. [63] demonstrated that only cross-sectional reductions exceeding 30% could be reliably identified. [PERSON] et al. [15] showed that the minimum detectable defect size ranged from 8% to 69%, depending on the soil stiffness based on finite element simulations. Meanwhile, using an improved defect size assessment formula, the maximum detectable shaft L/D ratio varied between 10 and 32 reflections. Notably, [PERSON] [64] achieved enhanced sensitivity using CCWT, successfully identifying subtle defects through characteristic phase angle variations in wavelet coefficients at predicted arrival times. Beyond its limited sensitivity to minor defects, the SE method exhibits the following limitations [16; 65; 66; 67]:  
1. L/D Ratio Constraints: For piles with L/D ratios exceeding 30, pile tip reflections become undetectable due to stress wave energy dissipation into the surrounding soil. 2. Pile Cap Interference: The impedance contrast between pile caps and shafts creates complex signal patterns, obscuring defect signatures through wave reflections and pile-soil interactions. 3. Multiple Defect Limitations: Deeper defects in multiple-defected piles often remain undetected, compromising length evaluation accuracy. 4. Horizontal Defect Sensitivity: Defects in the horizontal direction are often difficult to assess. 5. Wave Speed Dependency: The accuracy of defect length or depth depends on the assumed wave speed. 6. Phase Ambiguity: Low-strain data cannot reliably distinguish whether phase variations originate from defects or other wave propagation effects. Beyond technical limitations, the SE method also faces challenges in field implementation due to a lack of standards and unclear operating specifications. For example, ASTM D5882 does not explicitly specify the impact of sensor installation height on signal quality, and in field operations, sensors often experience poor coupling due to uneven pile tops. These testing uncertainties caused by unclear specifications will result in a reduction in the validity of the data [51]. Moreover, inconsistencies between standards may introduce systematic biases in test results. ASTM D5882 is only applicable to low-strain impact integrity testing and does not set strict upper limits on pile type, pile length, or pile diameter. It specifies minimum requirements for testing equipment. JGJ 106 provides recommended ranges for pile length and diameter, and clearly specifies requirements for sensor frequency response, vibrator hammer mass, and other parameters. In ASTM D5882, only requires \"at least three effective impacts exhibiting consistent waveforms\", and no details regarding pile top treatment are specified. However, JGJ 106 specifies that the pile cap must be chiseled flat, polished, and free of loose mortar; each pile must undergo at least two effective impacts, and auxiliary verification testing (core drilling or ultrasonic transmission) must be conducted to confirm defects. Therefore, when multiple standards are applied concurrently to the same project, supplementary technical conditions must be established to unify equipment specifications, testing procedures, and defect grading standards to avoid systematic deviations in testing results.