In Integrity Evaluation of New and Existing Foundations
By Farid Moradi and Hamed Layssi, FPrimeC Solutions Inc.,
and Ignacio Zulaga, geotecnia.ONLINE
Evaluating the load bearing capacity of existing and newly constructed deep foundations and piles has been of interest to engineers and piling contractors for years.
Various testing solutions have been developed over the decades to determine the static load bearing capacity as well as dynamic load testing of piles (ASTM D4945, 2017). However, the main emphasis of these tests is on the global response of the piles. Since this group of tests are generally expensive and difficult to deploy, their application is limited to critical elements.
Certain defects in pile materials, quality of workmanship and errors in pile construction can negatively impact the load bearing capacity of these elements. Therefore, quality control of pile elements is key during the design and construction processes.
Traditional methods for evaluating pile integrity involve massive excavation of surrounding soil and extraction of continuous core samples. This practice is extremely difficult for testing existing structures. In the case of new construction, such intrusive methods could increase the cost of the quality control and impact the construction timeline.
Non-destructive test (NDT) solutions for evaluating the quality of piles were developed in the 1960s. Electronic developments significantly helped non-destructive pile integrity tests become widely and globally available to the geotechnical engineering market. Modern electronic and computer techniques allow the processing of signals that facilitate the subsequent presentation and interpretation of the results. These NDT solutions have helped engineers gain additional knowledge about the quality and integrity of existing and new foundations. NDT solutions help reveal potential defects that might have happened during pile construction (in the case of cast-in-place piles) or transportation and installation (in the case of precast piles).
Among NDT solutions for piles, the pile integrity method (ASTM D5882, 2016) is certainly the one with the highest economic return. The test is easy to perform and requires minimal preparation. However, interpretation of test results require engineering knowledge, information about the soil profile and an understanding about soil-pile interaction. The test method is less accurate for piles of large diameters or when dealing with very large length to diameter aspect ratios (>30).
Crosshole Sonic Logging (CSL) (ASTM D6760, 2016) has been developed to address some of these challenges. CSL works on the concept of ultrasonic pulse velocity that is measured along the pile length. To perform the test, access tubes should be installed in the steel case ahead of pile construction. While CSL provides critical information about the quality of pile material at different depths, it does not provide information about potential defects outside of the steel case.
Thermal integrity profiling has gained popularity among engineers over the past few years. The test uses temperature history, which is collected at different depths, to study the strength gain across the pile length. The test is easy to implement and provides information about the entire cross section of the pile (inside and outside the steel cage). The practice has been standardized as ASTM D7949 (2014).
Parallel seismic tests have been used for evaluating the unknown length of existing piles and deep foundations. The procedure has been described by the ACI 228.2R guideline.
Quality control of deep foundations
New construction
Quality control of new construction is an integral component for successful construction of deep foundations. Quality related issues with material and workmanship or defects that can occur during placement of concrete or installation of driven shafts can impact the load bearing capacity of the piles and affect the performance. Non-destructive tests have long been utilized by engineers for this purpose, mainly because alternative methods such as removing soil, visual inspection or extracting cores are relatively expensive, create delays in construction timelines and might adversely impact the quality and integrity of the elements.
Existing structures
Existing infrastructure was designed and built in accordance with older design codes and construction practices and guidelines. Reusing these foundations is considered a time and money saving practice. However, this requires obtaining critical information about these elements. Access to structural designs and drawings for these elements is usually very difficult, if not impossible. Engineers need to obtain information about the load bearing capacity as well as integrity of these elements before using them as part of new construction.
Non-destructive quality control for new deep foundations
Different methods have been developed to evaluate the quality of newly built deep foundations. Apart from general fresh concrete tests such as a slump test, air content test and taking concrete cylinder samples, various NDTs have been developed to evaluate the integrity of piles. These tests help identify and quantify integrity and quality related issues in deep foundations. Depending on the type of deep foundation, and surrounding soil condition, a proper NDT method can be deployed to obtain critical information about the safety and reliability of the piles.
Among existing methods, acoustic methods based on propagation of stress waves have been widely used for evaluating integrity and consistency of pile materials and structures. Low strain pile integrity tests (PIT) and CSL are two widely accepted methods in this category. Over the past few years, other test methods based on temperature monitoring (referred to as Thermal Integrity Testing) of piles have gained popularity. These tests help identify and quantify the location and extent of defects in deep foundations. Like any other test, these methods have certain limitations that need to be considered.
Low strain impact integrity testing
Low strain pile integrity testing, known as PIT, is the most widely used NDT method for the evaluation of deep foundations. PIT provides a cost-effective and easy to deploy test for rapid assessment of integrity in piles and deep foundations. PIT was developed based on the concept of impact-echo and is customized for slender structural elements. In this test, stress waves and compression mode of deformation are used to obtain information about the location of anomalies and defects within pile elements. Fig. 1a schematically shows a pile integrity test on a sound pile and defected pile.
PIT uses stress waves generated by a hand-held hammer strike over the pile head. A motion transducer placed on the pile head records echoes (reflections) from the pile toe or other internal defects and anomalies. The recorded signal is amplified, digitalized and used for data interpretation and analysis.
PIT can be used for cast-in-place concrete piles, driven piles, steel tube filled concrete piles, augured piles, drilled shafts, structural columns and timber piles. PIT is not recommended for integrity testing of steel sheet piles, H-piles or unfilled steel tube piles because the dominant mode of vibration changes from compression to bending. The main features of the PIT can be described as:
- Pile preparation is relatively easy.
- The process of testing and data collection is rapid and inexpensive.
- PIT can identify major defects (location of defects and severity).
Some of the main disadvantages of the test can be summarized as:
- Interpretation of test results requires experience.
- PIT cannot be conducted over the pile cap.
- PIT is sensitive to pile length (L) to pile diameter (D) ratio. The test results might be unreliable for piles with an L/D ratio over 30. Surrounding soil friction and change of dominant mode of vibration from compression to bending affects the PIT results (Massoudi et al, 2004).
- Piles with highly variable cross sections or multiple discontinuities/anomalies may be difficult to evaluate using PIT.
Ultrasonic crosshole testing
Ultrasonic Crosshole Testing (or CSL) is the most common test for integrity evaluation of large diameter cast-in-place shafts. This test is an extended form of the ultrasonic pulse velocity test and provides information about homogeneity and integrity of concrete material over the pile profile. Fig. 2 schematically shows a general setup for CSL. This method includes two ultrasonic transducers (i.e. one emitter transducer; one receiver transducer).
The concept behind this method is supported by the ultrasonic pulse velocity (UPV) concept. UPV measures the transmission time of stress waves between emitter and receiver transducers. This travel time can be converted to wave velocity if the stress wave trajectory between emitter and receiver probes is known. The wave velocity is directly correlated with the quality of material. Poor and/or damaged concrete has a lower wave velocity when compared to sound concrete. Table 1 shows the relationship between wave velocity and quality of concrete (Saint-Pierre et al, 2016).
To perform a CSL test, a number of vertical boreholes (tubes) are made during concrete placement using parallel metal or plastic (PVC) tubes. The recommended number and configuration of boreholes depends on pile diameter. Fig. 3 schematically shows the configuration of boreholes for piles with different diameters. This figure has been adapted by ASTM standard as ASTM D6760 (2016).
The boreholes should be filled with water to ensure there is constant contact between the device probes and surrounding area. Both transducers (probes) are pulled upward at the same rate and the transmission time between the two probes is measured at each level. If a significant change in transmission time (or wave velocity) is observed, it might be correlated to internal anomalies of defects. The tomography concept using two probes is applied to identify the extent of the defects at a specified depth (ASTM D6760, 2016).
The advantages of deploying a CSL test in integrity assessment of new piles include:
- Ultrasonic crosshole testing is an ideal test for evaluation of large diameter piles.
- There is no limit on the shaft length.
- Provides precise information about the location and the extent of defects.
- Soil profile does not impact the test results.
- Easy data interpretation.
The CSL method has some disadvantages:
- This method is expensive and is not fast enough for condition assessment of piles.
- Boreholes should be installed before placement of concrete. Boreholes might break or experience damage during construction that results in inconclusive test results.
- Stress waves cover areas between emitter and receiver probes. Therefore, larger diameter piles need a number of boreholes to achieve conclusive results.
- The test does not provide information about concrete sections outside the steel cage.
- This test is not commonly used for existing piles and deep foundations because it usually needs pre-construction preparation (i.e. boreholes need to be installed before concrete placement).
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, micropiles, 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. Fig. 4 shows a typical temperature history of normal concrete over the pile depth.
This technology includes a number of temperature sensors along pile length, connected by wire to a data acquisition system. The sensor holding the cables is attached to longitudinal steel rebar of the reinforcing cage. Each sensor logs the temperature of its surrounding area in a specified time interval to track and record temperature variations and the evolution of concrete strength. The temperature at any location of deep foundations depends on the pile size and diameter, concrete mix design, concrete cover thickness outside of the reinforcing cage and other factors.
The temperature history is compared with a reference graph for integrity evaluation. It is obvious that TIP measurements colder than the reference graph indicate a lack of sufficient cement which can be caused by necking, soil inclusion or poor-quality concrete; while warmer than normal measurements are indicative of an increase in cross-sectional dimensions (Mullins and Kranc, 2007). TIP has several advantages compared to previously described methods:
- Can be used to evaluate the portion of concrete outside the steel cage.
- 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:
- Can only be used for integrity testing of new piles.
- Measurements need a reference graph for comparing recorded logs for integrity evaluation.
- Wires and sensors may be damaged during installation and concrete placement.
- 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.
Non-destructive quality control for existing deep foundations
The majority of existing NDT solutions are designed to address integrity evaluation in new construction. When it comes to existing piles, engineers and researchers need tools that can help gain information about the quality of materials. This is challenging because in most scenarios, no information is available about the quality of materials used. In some projects, no structural drawing is available to verify the load bearing capacity of the piles.
Another major issue regarding integrity evaluation in existing piles is that access to these elements is often difficult if not impossible. This becomes an even bigger problem when dealing with a group of piles covered by a pile cap.
Engineers and researchers have tried using different techniques to evaluate the quality and integrity of existing foundations.
Pile Integrity Test (ASTM D 5882)
When pile heads are accessible, a low strain impact integrity test might be used to obtain information about the unknown pile length and the integrity of pile elements. However, implementing a PIT on existing piles is often challenging. Access to the pile head is the first challenge. In most cases the pile head, even if accessible, provides little room to place a motion transducer on the surface and apply vertical impacts. Another challenge is estimating the speed of the wave. One practical method is to evaluate the pulse velocity speed using other NDT methods. A special setup which involves installation of two motion sensors along the pile at certain spacing can be used to measure the wave velocity in piles.
Parallel Seismic Testing (ACI 228.2R)
Even though Parallel Seismic Testing has not been standardized, this method is used for integrity evaluation of existing piles and unknown foundations. A borehole is drilled close to the concrete pile, longer than the pile length. Boreholes are lined with a plastic or metal tube and filled with water for coupling between the device transducer and surrounding surface.
The signal receiver transducers placed at the bottom of the tube, move upward with a constant speed. At each level, the pile head or pile cap is hit by a hand-held hammer impact. The generated stress waves travel through the pile length and surrounding soil before reaching the receiver probe. Fig. 5 schematically shows a parallel seismic setup for testing piles and deep foundations.
The stress waves are recorded at each level and a stack graph of time against depth is recorded. Each recorded signal is plotted in sequence. The first arrival time of the wave from each depth is marked on a single graph and connected to each other. In sound and continuous foundations, this slope can be used to determine an unknown depth of deep foundations (sudden change in the slope). The pile toe depth is determined when the transit time of the first wave increases and an inflection is apparent in the line linking first arrival points. The concept of “inflation of linking line” can be used to find out the locations of internal anomalies in deep foundations. The transmit time and stress wave velocity changes at the location of anomalies and defects.
Parallel seismic testing has been introduced in the ACI 228.2R (2013). Parallel seismic testing provides useful information about the depth of existing deep foundations, as well as the location of potential defects within the pile profile.
However, the test has certain limitations:
- This method is expensive and requires drilling boreholes parallel to existing piles.
- Parallel seismic testing is commonly used for estimation of pile length, not pile integrity. The stress wave generated by hammer impact travels through the pile and surrounding soil. The soil profile may significantly affect stress waves, resulting in a poor or misleading signal for data interpretation.
- Accurate data interpretation requires knowledge about soil profile.
- The borehole should be longer than pile length. This makes drilling challenging when there is no knowledge about the pile length.
Concluding remarks
Alternative methods, such as removing soil, extracting core samples and visual examination are generally expensive, provide little information about other locations and create significant delays in the decision-making process and/or in the construction timeline. In addition, intrusive methods such as extracting cores might negatively impact the safety and reliability of an existing pile.
Non-destructive testing has widely been used for quality control of deep foundations in new construction, as well as forensic evaluation and condition assessment of existing piles. NDT methods provide cost-effective and easy to deploy tools for the evaluation of integrity in piles and deep foundations.
Some codes and guidelines strongly recommend performing integrity tests of deep foundations either for new construction or for comprehensive evaluation of existing structures. Some other codes and guidelines consider integrity testing as an optional quality control test. However, these codes and guidelines authorize engineers and designers to increase the superstructure loads should the integrity of piles have been verified through NDT methods.
Low strain pile integrity testing, ultrasonic crosshole testing, thermal integrity profiling and parallel seismic testing are widely used for quality control of new construction and integrity assessment of existing deep foundations. Among these methods, PIT is very cost-effective, fast and easy to implement for both new construction and existing structures. It is recommended to evaluate integrity initially by the PIT method, then proceed to use advanced non-destructive integrity testing methods if PIT results are not conclusive or PIT shows an integrity issue. Application of the PIT method for existing structures is more complicated because access to pile heads is often limited due to the presence of superstructures or pile caps.
Moreover, it is recommended that non-destructive integrity testing be performed by a trained technician in order to collect reliable data during field work. Interpretation of integrity test data requires basic knowledge on the concept of non-destructive integrity testing. Complementary information (i.e. knowledge of the construction record, design documents, soil profile, concrete mix design, compressive strength) help to interpret field data and analyze test results more accurately.
References
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