Pile Integrity Testing (PIT), also known as low-strain impact integrity testing, is a non-destructive method used to evaluate the continuity, consistency and approximate physical dimensions of deep foundation elements. The test is applicable to driven concrete piles, cast-in-place piles, drilled shafts, augered piles and other deep foundation members that can respond to a low-strain impact.
Reference:
ASTM D 5882-07, Standard Test Method for Low Strain Impact Integrity Testing of Deep Foundations.
What Does PIT Evaluate?
PIT provides information that can assist in assessing:
- Continuity of the pile.
- Approximate changes in cross-sectional area.
- Consistency of pile material.
- Approximate pile length, where a clear toe reflection is obtained.
- Major impedance changes caused by defects or geometric variations.
However, PIT does not determine pile load-carrying capacity. The results are approximate and must not be used as the sole basis for pile acceptance or rejection.
Principle of the Test
A low-strain impact is applied, normally at the pile head, using a handheld hammer or similar device. The resulting stress wave travels through the pile and reflections occur when the wave encounters a change in pile impedance.
Pile impedance is related to the material stiffness, cross-sectional area and wave speed. Therefore, a significant reduction or increase in pile cross-section, material discontinuity or other anomaly can produce a reflection before the expected pile toe response.
The standard identifies two principal methods:
1. Pulse Echo Method (PEM)
Pile head velocity is measured and evaluated as a function of time. In most applications, this method provides sufficient information for integrity evaluation.
2. Transient Response Method (TRM)
Both pile head motion and impact force are measured. The data are generally evaluated in the frequency domain.
For TRM, force measurement is mandatory, whereas force measurement is optional for PEM.
Test Equipment and Technical Requirements
The basic test arrangement consists of:
- A low-strain impact hammer.
- An accelerometer, velocity transducer or equivalent motion sensor.
- Data acquisition and processing equipment.
- Graphical display and permanent data storage.
The impact pulse should generally be less than 1 ms in duration and should not cause local damage to the pile. The impact should be applied axially.
For accelerometer-based measurements, the sensor axis must be parallel to the pile axis. The accelerometer should be linear to at least 50 g and calibrated within 5% accuracy over the applicable measurement range.
The following image illustrates the typical arrangement: impact device and accelerometer at the pile head, connected to recording and data-reduction equipment, followed by graphical output and permanent data storage.
Pile Head Preparation and Test Timing
For cast-in-place concrete piles and concrete-filled pipe piles, testing should be carried out:
- No sooner than 7 days after casting, or
- After the concrete reaches at least 75% of its design strength,
whichever occurs earlier.
The pile head must be:
- Accessible and above water.
- Free from loose concrete, soil and foreign material.
- Exposed to sound concrete where contamination or defective material exists.
- Locally ground to provide a smooth surface where necessary.
Proper pile head preparation is critical because poor sensor contact or impact conditions can directly affect the test signal.
Sensor and Impact Location
The motion sensor should normally be placed near the centre of the pile head and firmly attached using a suitable bonding material.
Important requirements include:
- Sensor sensitivity axis aligned with the pile axis.
- Impact applied within 300 mm of the motion sensor.
- For piles larger than 500 mm diameter, measurements should be taken at a minimum of three locations to assess localized conditions near the pile head.
Test Procedure
The testing sequence generally involves the following:
- Record relevant pile and project information.
- Prepare the pile head and attach the motion sensor firmly.
- Apply several low-strain impacts.
- Record individual impact responses.
- Average suitable records from at least three impacts.
- Apply necessary signal amplification.
- Evaluate the averaged record for pile integrity.
The system should also be capable of storing individual records and averaging multiple impacts where required.
A useful field check: test the system on a rod of known length (L) and known wave speed (c, e.g., steel) and confirm the reflection returns at the expected time 2L/c, within 2%.
Data Quality Checks
Repeatability is one of the most important requirements in PIT.
The operator should verify that:
- Successive impact records are consistent.
- The sensor remains firmly attached.
- The measurement system is not overloaded.
- The impact is applied consistently on sound concrete.
- Faulty or overloaded records are rejected.
If the records are not repeatable, the data should not be used until the cause is identified and corrected.
Interpretation of PIT Results
In time-domain analysis using PEM, the interpretation is based primarily on changes in velocity response following the initial impact.
Generally:
- A reflection having the same sign as the input pulse indicates a relative decrease in impedance.
- A reflection having the opposite sign indicates a relative increase in impedance.
These reflections may indicate changes in pile geometry or material properties. Comparison with other piles of similar construction at the same site is recommended because interpretation is qualitative and requires engineering judgment.
A relatively uniform pile typically produces a response consistent with gradual impedance changes and soil friction, whereas a strong early reflection may indicate a major impedance change associated with a significant anomaly.
Limitations of PIT
PIT has inherent limitations that must be considered during interpretation.
The method may be inconclusive when:
- A crack or discontinuity completely reflects the stress wave, preventing evaluation below that location.
- The pile has highly variable cross-sections or multiple discontinuities.
- Soil response cannot be clearly separated from pile response.
- High shaft friction or pile length significantly attenuates the toe reflection.
- The pile toe reflection is not visible.
- The pile is rigidly connected to a footing or superstructure.
The method is also generally unsuitable for steel sheet piles, H-piles and unfilled steel pipe piles. Importantly, PIT may not identify every imperfection; it is primarily useful for identifying significant defects within the effective test length.
Information to Include in the PIT Report
A technically useful PIT report should include:
| Category | Typical Information |
| Project details | Project identification and location |
| Pile details | Pile ID, type, diameter and length |
| Construction data | Casting/installation date, concrete placement method and as-built geometry |
| Reinforcement | Reinforcement and casing details, where applicable |
| Ground conditions | Relevant soil stratigraphy |
| Construction observations | Interruptions, unusual events and suspected construction issues |
| Test details | Test date, equipment, sensor location and test procedure |
| Results | Velocity records and force records, where applicable |
| Analysis parameters | Assumed wave speed, pile length and data-evaluation method |
| Interpretation | Comments on pile integrity and identified anomalies |
The report should also identify the test method and explain relevant factors, such as soil conditions or installation procedures, that may influence the measured response.
Key Engineering Takeaway
PIT is a fast and valuable quality-control tool for detecting major continuity defects and impedance variations in piles, but it is not a direct measurement of pile capacity or a complete substitute for engineering evaluation.
A suspected anomaly should be assessed together with construction records, bore logs, concreting data, installation observations and, where necessary, supplementary testing. Final acceptance should therefore be based on an integrated engineering assessment rather than on the PIT trace alone.
In practice, the reliability of PIT depends heavily on three factors:
- Proper pile head preparation,
- Repeatable high-quality signals, and
- Competent interpretation of the reflected wave response.
