How site averaging of PET can come to the rescue of PIT in challenging conditions.

As a pile integrity tester, the following points summarize key considerations when analyzing low-strain test results, particularly for deep piles in soft soil with high length-to-diameter (L/D) ratios.

Diagnostic Challenges

• High L/D Ratio & Soft Soil: Testing piles with high L/D ratios (e.g., L/D > 30) in very soft soil significantly complicates interpretation. The strong soil-pile interaction effects and signal attenuation often make it difficult to obtain reliable data for the full pile length.

• Unrealistic Toe Expectation: In these challenging conditions, expecting to see a clear, defined reflection from the pile toe is often unrealistic. The energy dissipation and signal distortion may mask the toe reflection.

See the example below

The flaw in the pile makes it difficult to see the pile toe to understand the pile length

Distinguishing Systemic Soil Effects from Pile Defects

• Benefit of Averaging: The primary diagnostic benefit of averaging the test results across multiple piles on the site is to distinguish between localized pile defects and consistent, site-wide features.

  • Systemic Features: Features that appear consistently at the same depth across almost all tested piles (e.g., an impedance change at ~8m) are highly likely to be the result of the pile penetrating a stiffer soil layer (a soil-pile impedance contrast) rather than a geometric defect unique to an individual pile.

  • Localized Anomalies: True pile geometry effects or defects are typically more localized and non-uniform in occurrence.

In the example bove a site averaging of all piles on the site has shown the following result

Showing the average of all piles on site highlights a common flaw caused by changing soil conditions.

• Complex Reflection Polarity: The interaction between actual pile geometry and the surrounding soil is complex. When penetrating a tough layer:

  • The reduction in cross-section (a "step") and the influence of the stiffer soil layer affect the reflected wave in opposite directions.

  • Therefore, the actual reflection polarity (up or down) caused by a stiff stratum is not always a classic inverted reflection, but is influenced by the combination of a potential hole size reduction and the increased confinement.

Professional Interpretation Approach

• Partial Diagnosis: In situations with relatively high L/D ratios and strong soil-interaction effects, the most appropriate and conservative approach is to provide a partial diagnosis of pile integrity.

• Clarity and Confidence: Clearly state the depth range over which the results can be interpreted with high confidence, and refrain from attempting to draw definitive conclusions about the integrity of the full pile length or the status of the bottom part when data is unreliable.

  • In the case above, it is better to give an analysis of the top of the pile, clearly stating that the status of the bottom part is unknown.