How to interpret a PIT test: wave reflection, impedance, and anomaly identification in piles

What the PIT test measures, and what it does not measure

PIT, an acronym for Pile Integrity Test, is the low-strain integrity test applied to piles and other deep foundation elements. The correct translation of the term is pile integrity test: the "pile" in the original name means pile, not stack. This vocabulary detail matters because interpreting the test depends on understanding exactly what is being measured.

The test evaluates the physical continuity of the shaft. It indicates whether there is a relevant variation in section, concreting discontinuity, soil inclusion, transverse crack, or abrupt material change along the length. PIT does not measure load capacity, does not measure settlement, and does not replace a load test. For load capacity, there are the dynamic PDA test and the static load test, governed by ABNT NBR 16903:2020.

In Brazil, the requirement for integrity tests comes from ABNT NBR 6122:2019, with Amendment 1:2022, which deals with the design and execution of foundations and defines in which situations integrity control must be performed, with emphasis on bored piles executed under stabilizing fluid.

The physics of the test: wave propagation and reflection

The test is based on the theory of one-dimensional stress wave propagation. A low-energy impact, applied to the top of the pile with an instrumented hand hammer, generates a compression wave that travels down the shaft. An accelerometer fixed to the top records the velocity response over time.

The wave propagation velocity in the material is given by:

c = square root of (E divided by ρ)

where E is the modulus of elasticity of the material and ρ is the specific mass (density) of the concrete. For common foundation concretes, after adequate curing, the velocity is typically between 3,000 m/s and 4,000 m/s.

The correct formulation of impedance

The mechanical impedance of the section is the parameter that governs wave reflection. It is defined as:

Z = (E · A) divided by c, equivalent to Z = ρ · c · A

where A is the cross-sectional area, E the modulus of elasticity, ρ the specific mass, and c the propagation velocity. That is, impedance is proportional to the cross-sectional area and the material properties. It is not obtained by dividing the product of these quantities by density, a formulation that frequently circulates in technical texts and is incorrect.

Whenever the wave encounters a variation in impedance, part of the energy is reflected back to the top and part continues to travel. It is this reflected portion that the accelerometer records and that the engineer interprets.

How to read the reflection signal

The practical rule follows directly from the physics of the problem:

• Reduction in impedance, such as shaft necking, concreting defect, or soil inclusion: generates reflection that increases the particle velocity at the top, i.e., a peak with the same polarity as the initial impact.
• Increase in impedance, such as section enlargement, concrete bulb, or rock socket: generates reflection with inverted polarity relative to the impact.
• Free pile toe: is the largest possible reduction in impedance because the section ends. It therefore produces a strong reflection with the same polarity as the impact, at the time corresponding to the element's length.

How to calculate the depth of the event

The depth of any reflection results from a simple relationship because the wave travels a round trip:

L = (c · t) divided by 2

With the adopted wave velocity for concrete and the time between impact and reflection, the depth of the event is obtained. Knowing the design length of the pile, the engineer calculates the expected time for the toe reflection and uses this marker as a reference for reading the entire record. A reflection that appears well before this time warrants investigation.

An essential caveat: the adopted wave velocity directly changes the calculated depth. Young concrete, different mix design, or incomplete curing reduce the velocity and distort the depth scale. It was precisely this sensitivity that motivated Geoteste's experimental research on wave velocity, reinforcement, and concrete strength.

Typical patterns in the PIT record

Observed pattern in the record	Wave behavior	Physical hypothesis to investigate
Intermediate reflection with the same polarity as the impact	Local reduction in impedance	Necking, concreting defect, soil inclusion, or transverse crack
Intermediate reflection with inverted polarity	Local increase in impedance	Section enlargement, concrete overconsumption, boulder, or socketed section
Clear toe reflection, at the expected time	Wave travels the intact shaft and returns	Continuity compatible with the designed length
Absent toe reflection	Energy dissipated before return	High side friction, very long or slender pile, or severe upstream discontinuity
Toe reflection before the expected time	Wave path shorter than designed	Executed length shorter than predicted or shaft sectioning
Smooth and regular signal decay	Progressive attenuation due to side friction	Typical behavior of a continuous pile in resistant soil

Limitations that must be included in the report

Reading the PIT graph with technical honesty means recognizing the scope of the method:

• The relationship between length and diameter limits the test. In very long or slender piles, the impact energy may not return from the toe, and the absence of toe reflection ceases to be conclusive.
• Gradual reductions in section, distributed over several meters, produce discrete signals and may go unnoticed.
• A necking followed by enlargement generates overlapping reflections and often renders the section inconclusive.
• The test evaluates the shaft, not the base. Poor bottom cleaning rarely appears unequivocally in the record.
• Top preparation is crucial. Deteriorated concrete, an irregular surface, or poor accelerometer coupling produce noise that can be confused with an anomaly.

Anomaly in the graph is not synonymous with defect

This is the point that separates good practice from bad practice. The PIT records impedance variations, and not every impedance variation is a defect. Section changes foreseen in the design, splices, sections with metallic casings, variations in surrounding soil stiffness, and differences in concrete age produce legitimate reflections.

The correct interpretation cross-references the record with the executive design, the construction log, the volume of concrete consumed, the geotechnical profile, and the construction process employed. When doubt remains, the correct procedure is complementary investigation, with retesting after adequate top preparation, parallel tests such as cross-hole sonic logging in large-diameter piles that have tubes installed, core extraction, or exploratory excavation in the suspect section. The complete guide is in anomaly detected in PIT: what to do now.

Good field practices that prevent erroneous reports

• Prepare the pile top: sound concrete, flat and clean surface, without reinforcement interfering with the reading point.
• Respect the minimum curing age. Very young concrete has a lower wave velocity and generates scale error.
• Repeat the test at least three times, moving the accelerometer, to separate signal from random noise.
• Record the design length, diameter, construction process, and concreting date of each element.
• Adopt a wave velocity consistent with the concrete used, documenting the value used in the report.

Where PIT fits into the test plan

PIT is the scanning tool: fast, low cost per element, capable of covering a large sample of the work. It answers the question of how the pile was executed. The question of how much the pile supports is answered by other tests. The complete comparison between the three methods is in PIT, PDA, or PCE: which test to specify.

Geoteste technical support

Geoteste performs and interprets pile integrity tests throughout Brazil, integrating PIT into the construction's technological control plan along with dynamic tests, static load tests, and instrumentation. Learn about our PIT integrity test service or speak with our technical team to structure the test plan for your project.

Technical references

• ABNT NBR 6122:2019, with Amendment 1:2022. Design and execution of foundations. Rio de Janeiro: ABNT.
• ABNT NBR 13208:2007. Piles: dynamic load test. Rio de Janeiro: ABNT.
• ABNT NBR 16903:2020. Soil: static load test on deep foundations. Rio de Janeiro: ABNT.
• LIANG, L.; RAUSCHE, F. Quality assessment procedure and classifications of cast-in-place shaft using low strain dynamic test. Proceedings of the Deep Foundations Institute 36th Annual Conference on Deep Foundations, Boston, 2011, p. 553-562.
• GEOTESTE. Experimental study of the influence of reinforcement and concrete strength on wave propagation velocity in integrity tests. 2024.