Construction and installation of concrete piles and deep foundations is a challenging task. Problems can occur during the process, which in turn results in defects in pile components. Pile formation problems, Concrete faults, and reinforcement installation issues are among the most common problems. The resulting defects can negatively impact the durability and mechanical performance of the pile. Pile Integrity Test is a critical step towards quality control of deep foundations and piles. This article will briefly review test methods for evaluating pile integrity.
Deep Foundations and Piles - Overview
Concrete piles and drilled shafts are an important category of foundations. Despite their relatively high cost, they become necessary when we want to transfer the loads of a a heavy superstructure (bridge, high rise building, etc.) to the lower layers of soil. The special geometry of these elements (such as their length, diameter, and presence of steel cage) make certain challenges during the construction process. Drilling, stabilizing the walls, the amount of water used, as well as concrete placement problems are among these challenges. These can lead to defects such as discontinuity (cracks, voids), or sudden change of cross section (bulging, necking) in pile elements, refereed to as Pile Integrity. Since pile elements are generally buried under ground, the Quality Control and Quality Assurance of these elements becomes a challenging issue. Different intrusive and non-intrusive methods have been developed over the past decades to help engineers with easy, reliable and cost-effective methods for the evaluation of quality and consistency in these elements. Most of the time, the pile head is the only accessible area for inspection and testing.
What is Pile Integrity ?
In general, Pile Integrity refers to certain characteristics of deep foundations and piles such as:
- Physical Dimensions of Pile (Length or Cross-Section);
- Continuity of Pile (presence of Voids or Major Cracks); and
- Consistency of the Pile Material.
What is Pile Integrity Test?
According to the ASTM D5882, Pile Integrity Test refers to qualitative evaluation of the physical dimensions, continuity of a pile, and consistency of the pile material. Several methods have been developed over the past few decades in order to evaluate the integrity of piles. The selection of test method depends on many parameters, including (but not limited to):
- Pile Dimensions (cross section, depth)
- Pile Type
- Strata / Soil Condition
As we discussed in the previous sections, access to pile element is often limited to the pile head area. Pile Integrity can be evaluated through three major non-destructive testing solutions:
- Low-Strain Pile Integrity Test
- Crosshole Sonic Logging
- Thermal Integrity Test
Each testing method has its own advantages and limitations. The following sections will briefly present and discuss these test methods:
1- Low Strain Pile Integrity Test
Pile Integrity Test (PIT), or "low strain impact integrity testing of deep foundation" is a widely used non-destructive test method for the evaluation of pile quality, and integrity. The test can also be used to estimate the unknown length of existing piles and foundations. In the low strain impact integrity testing, the response of the pile to an impact on the head of the pile shaft is determined by a high precision transducer mounted on the pile head. The transducer can either be an accelerator, or a velocity sensor. The test standard allows two different procedures to obtain acceleration and force information: 1) Pulse-Echo Method (PEM) or Sonic-Echo (SE) and 2) The Transient Response or Impulse-Response (IR).
How to Perform Low Strain Pile Integrity Testing?
The first step in collecting reliable measurements in the low strain pile testing is Preparing Pile Head or simply Surface Preparation. Any type of contamination should be removed (using a grinder) to reach to solid and sound concrete surface. The pile head surface should be accessible, above water, and clean of loose concrete, soil or other foreign materials resulting from construction. The motion transducer is mounted on the pile head. A suitable Coupling (bonding) Material should be used to attach the transducer to the pile head. An impact source (usually a hand-held hammer) is used for Strike Pile Head; the impact should be applied Axially with the pile. Motion transducer should be capable of detecting and recording the reflected echos over the pile top. Acceleration or velocity transducers can be used for this purpose. In the case of using accelerometers, the device should be capable of integrating test results and displaying velocity.
What Information Does Low Strain Pile Integrity Testing Provide?
The low strain pile integrity test results can be used to study the following:
- Changes in the Pile Cross Section (Necking or Bulging)
- Discontinuity in Piles (Voids, major cracks)
- Consistency of Pile Materials (Concrete quality, segregation, honeycombing, soil inclusion)
- Estimate Pile Depth (Note: only possible when the pile toe response is super clear)
Limitations of Low Strain Pile Integrity Testing
The low strain pile testing can provide an indication of soundness of pile materials and integrity; however, the test has certain limitations:
- Low strain pile testing does not provide pile bearing capacity
- The test cannot be used over pile caps.
- This test is not effective in piles with highly variable cross sections
2- Crosshole Sonic Logging
Ultrasonic Crosshole Sonic Logging provides information about the homogeneity and integrity of concrete. The method can be used for quality control of concrete piles. This method overcomes the limitation of low strain impact integrity testing. vertical holes are created using tubes at the time of pile construction (at least two). The tubes are filled with water. An acoustic wave emitter transducer is lowered to the bottom of one tube; while another acoustic wave receiving transducer is placed at the bottom of second tube. Both transducers are pulled upward at the same rate. The signals are analyzed and integrity profile of the pile is developed. an ultrasonic profile. The test procedure is standardized as ASTM D6760
Cross Hole testing can be used to determine the location of defects, as well as identifying the extent of the defects. The test can be done on larger diameter piles.
Requires installation of tubes during pile construction. Data recording and analysis might be expensive. Access to the tip of the wall is needed for most applications.
3- Thermal Integrity Profiling
Thermal Integrity Profiling (TIP) uses the temperature variation of cement paste of concrete for integrity evaluation of piles and deep foundations. This method covers a wide range of piles and deep foundations including drilled shafts, bored piles, micro-piles, augured cast-in-place piles, continuous flight augured piles, drilled displacement piles and more. The concept behind this technology is to record temperature changes and history during the curing time of cement. This temperature can be correlated to the strength gain of concrete and integrity of piles and deep foundations. TIP has several advantages compared to previously described methods:
- Thermal Integrity Profiling can be used to evaluate the portion of concrete outside the steel cage.
- Thermal Integrity Profiling provides real-time data on pile quality, which can shorten the construction timeline.
- Data interpretation is relatively easy.
TIP measurement has some limitation for integrity testing of piles and deep foundation including:
- Thermal Integrity Profiling can only be used for integrity testing of new piles.
- Thermal Integrity Profiling requires a reference graph for comparing recorded logs for integrity evaluation.
- TIP wires and sensors may be damaged during installation and concrete placement.
- TIP measurement is a comparative method for integrity evaluation. Change in mix design may result in a huge difference when compared to the reference graph even though the concrete pile is sound.
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