Pile Testing: Anomalies, Flaws and Defects

By Joram M. Amir, Ph.D., C.E., D.GE, Piletest.com Ltd.

Pile integrity means strict adherence to the relevant drawings and specifications. This includes the pile geometry (e.g., length, diameter or width, and verticality) as well as continuity and material properties. Since the construction process of piles is practically blind, practitioners realized early on that pile integrity should not be taken for granted. Starting in the early 1960s, the discipline of pile integrity testing has made giant steps in both testing methods and their implementation worldwide. Presently, the prevalent methods are the low-strain impact method (ASTM D5882, Standard Test Method for Low Strain Impact Integrity Testing of Deep Foundations) and the crosshole ultrasonic method (ASTM D6760, Standard Test Method for Integrity Testing of Concrete Deep Foundations by Ultrasonic Crosshole Testing).

In comparison with the widespread advancements in instrumentation, the piling industry has not yet reached a consensus regarding the interpretation of the test results and acceptance criteria. Often, this creates unnecessary friction and even litigation between the parties involved. To avoid these consequences, the parties must first agree about the facts by adopting established definitions to the following three terms that today are often confused: anomaly, flaw, and defect. The following sections suggest such definitions, which are supported by commonly occurring examples.

Definitions

Based on the technical paper by Amir, “Single-Tube Ultrasonic Testing of Pile Integrity,” which was published in the proceedings of the ASCE 2002 Deep Foundation Congress, the following definitions are intended to make a clear distinction between the results of the integrity tests, the physical condition of the pile and the necessary action, if any. The terms were incorporated in ASTM D6760 and can also be visualized using a Venn diagram in which they can be depicted and distinguished.

Anomaly: Any irregular feature in the results from the nondestructive testing (NDT). An anomaly may be due to the testing instrument (e.g., noise), the means used (e.g., access tube debonding), the surrounding soil (e.g., abrupt changes of soil friction) or the pile itself. It is the responsibility of the personnel and/or agency performing the testing to gather and analyze all relevant data and to try to resolve every anomaly.

Flaw: Any deviation from the planned shape or composition of the pile, which does not necessarily detract from the performance of the pile.

Defect: A flaw that, because of either size or location, may detract from the resistance, durability and/or performance of the pile. The geotechnical engineer and the structural engineer are jointly responsible for deciding whether a specific flaw constitutes a defect.

Fig1 - Venn diagram of the five typical zones

EXAMPLES

In the Venn diagram (Fig 1) we can distinguish between five zones related to results of pile integrity tests. The following examples illuminate each of these zones with actual case histories taken from the authors' archives:

I - Anomaly That Is Not a Flaw

An anomaly that is not a flaw, also known as a false positive, will be illustrated using two cases: one from crosshole ultrasonic testing of a barrette and one from low-strain impact testing on a pile.

Fig2 - Ultrasonic test results on a barrette

As shown on the graphic “Ultrasonic test results on a barrette,” (Fig 2) the first arrival time (FAT) is shown in red and the relative energy (RE) in shown in blue. Down to a depth of about 22 metres (72 feet), both curves appear rather regular (i.e., relatively straight lines) with nearly constant values of FAT and RE. However, below this depth and down to about 29.3 metres (96 feet), the FAT gradually decreases from 330 μsec to 100 μsec, doubtlessly an anomaly. A quick questioning onsite revealed the reason; the foundation element was reinforced only in the upper 21 metres (69 feet). Nevertheless, the engineer required that the access tubes be extended all the way to the bottom of the barrette. With no rebar cage to keep them aligned, the access tubes dangled freely and likely moved toward each other. This result had certainly nothing to do with the barrette itself and, accordingly, was not declared a flaw.

The following reflectogram was generated from low-strain impact testing of a driven pile that was 27 metres (88.6 feet) long. From a review of the output (Fig 3), a major anomaly appears at a depth of about 12.5 metres (41 feet). However, after questioning the client about the pile, it was discovered that this pile consisted of two interlocked sections. What appeared as a discontinuity in the reflectogram was actually just the joint between the two sections and was therefore not declared a flaw.

Fig3 - Results of low-strain impact testing on a driven pile

II - Anomaly That Is a Flaw

An anomaly that is a flaw includes “soft bottom” conditions, which occur when the bottom of a drilled shaft constructed using a drilling support fluid (i.e., slurry) is not cleaned properly. The only existing method in which the soft bottom condition is detected and appears as an anomaly is the crosshole ultrasonic logging (CSL) method. With the CSL method, a soft bottom condition manifests as a large increase in the FAT with a corresponding decrease of the RE (Fig 4). Understandably, a soft bottom condition anomaly should be reported as a flaw.

Fig4 - A soft bottom anomaly