Quality Control of Drilled Shafts - Pile Integrity

Pile Integrity refers (ASTM D 5882) 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.

Certain pile construction practices can lead to defects in drilled shafts. These defects can be major voids, poor quality concrete, geometric errors, and entrapped slurry or ground water. These defects may negatively impact the the load bearing capacity and load distribution properties. 

Pile Construction Methods

There are different methods of drilling, and different types of drilling liquid. No matter what procedure is selected by design engineers, it is necessary to make sure no voids, or poor patches form in the concrete pile. The following summarized the 3 main methods in pile construction:

• Dry Method: In this procedure, the borehole is excavated, and backfilled directly with concrete.
• Casing Method: In this procedure, a temporary casing is used to support the borehole, prevent caving, and prevent intrusion of groundwater before placement of concrete.
• Direct Slurry Displacement: In this method, drilling fluid is used to stabilize the borehole during excavation. The liquid is then displaced by placing concrete under the fluid through tremie pipe.

One should also make sure that the properties and characteristics of the fresh concrete meets the requirement for the specific drilling method. Guide to Tremie Concrete for Deep Foundations (DFI, 2018) provides the specifications of concrete materials.

Most Common Integrity Problems of Concrete Piles

Why We Need Quality Control Tests?!

Every construction site is unique: depending on the soil type, and subsurface condition, a suitable construction practice is selected. No matter what method is used, the main issue is that remains the same: both drilling and concerting problems happen within the shaft, far from the curious eyes of contractor or the inspector, making it very difficult to verify the quality, and integrity of drilled shafts. Integrity problems are commonly categorized based on the place in the construction cycle at which the defect occurs (Source: Michael W. O’Neill, TRB, 1991):

• Defects arising from general construction problems,
• Defects arising from drilling problems,
• Defects arising from casing management problems,
• Defects arising from slurry management problems, and
• Defects resulting from design deficiencies.

Quality Control of Drilled Shafts || Pile Integrity Tests

The process of quality control for this group of elements is very much through indirect measurement of other parameters, i.e. resistance of pile to driving or drilling, or other non-destructive testing techniques (NDT). NDT methods can shed light on this hidden part of concrete piles. (Learn more: NDT methods of Pile Integrity Testing)

Access to pile element is often limited to the pile head area. Therefore the test either have to performed on pile head, or certain features need to be implemented during pile construction. In this article, we will present the most common NDT methods for evaluation of pile integrity:

• Low-Strain Pile Integrity Test
• Crosshole Sonic Logging
• Thermal Integrity Test

Each testing method has certain advantages and limitations. The following sections will briefly present and discuss these test methods.

1- Low Strain Pile Integrity Test - PIT

Low strain impact integrity testing of deep foundation (ASTM D 5882) 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).

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 - CSL

Ultrasonic Crosshole Sonic Logging (ASTM D 6760) 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.

Applications

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.

Limitations and Disadvantages

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 - TIP

Thermal Integrity Profiling (ASTM D 7949) 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.

Applications and Advantages

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.

Limitations and Disadvantages

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.

Concluding Remarks

• Pile construction practices might result in hidden defects in the pile element, within the concrete shaft, or on/around the pile toe, and pile head.
• Pile defects can be identified as major air voids, discontinuity in the pile (no concrete passing through steel cage), or concrete mixed with groundwater, slurry, or soil, creating poor quality, low strength patches within the concrete shaft.
• Using proper methods to assess the integrity of drilled shaft is an integral part of the pile construction, and should be considered in the quality control and quality assurance process.
• Low Strain Pile Integrity Test provides a cost-effective and easy tool for assessing major voids and defects; however, it becomes less effective for larger diameters, and longer shafts.
• Crosshole sonic is an excellent method for assessing concrete quality along the shaft. It can help locate and quantify defects, however, the test cannot provide information about critical areas beyond the steel cage, mainly because testing tubes are pre-installed inside the cage.
• Thermal Integrity Profiling is more expensive in terms of equipment and setup, but can provide critical information about the quality and integrity of concrete inside and outside the steel cage.