Pile testing in marine environments
The installation of concrete piles and drilled shafts can be a complex and challenging task on land. Problems can arise which result in defects that affect the performance and the lifespan of a pile. In a marine environment, construction is even more difficult because of accessibility issues, which is why pile integrity testing should be included from the beginning of the production timeline.
“The earlier the better to allow the construction team to correct deficiencies in a cost-effective manner,” said Paul Bullock, chief engineer at the Loadtest office of Fugro USA Land, Inc. “Difficult site conditions are everyone’s problem, not just the contractor’s. The goal is to provide a reliable foundation in a timely, cost-effective manner.”
The crux of pile testing in marine foundations, says Bullock, is that design procedures are semi-empirical, based on theoretical analysis of complex pile-soil interaction behaviour, and are modified by research calibrations and local experience.
“The possibility of unforeseen site conditions is significant, and a full characterization of the soil mass remains to be verified during construction. The pile installation process is a more thorough site investigation than the work done for design purposes,” he said.
Brent Robinson, the vice-president of Pile Dynamics, Inc. concurred, “Integrity testing is a test for quality of installation for new construction – and a test of length and condition for existing marine projects that are to be reused, expanded or improved.” Testing helps to predict how the structure’s foundation will perform in a marine environment as wind, waves and saltwater can cause unexpected changes to the pile’s cross-section or cracks in concrete.
Methods for testing marine piles
Integrity tests specifically look for the structural adequacy of each foundation element tested such as cross-section, depth and whether the reinforcement cage has sufficient cover.
For driven and helical piles, high-strain dynamic testing is the most common test for new construction. A Pile Driving Analyzer® (PDA) is used to measure force and velocity waves within the pile during impact by a heavy hammer. The hammer strikes the pile and the data from the sensors is recorded and used by the analyst running the test to evaluate structural integrity and geotechnical resistance in the field.
The dynamic tests are then interpreted in detail by engineers who have the technical expertise. The test results can be used to fine-tune and economize the pile design. During construction for marine projects, the PDA checks the effect of site variability on design assumptions and provides quality assurance for the installed piles.
Blow counts and pile behaviour provide immediate feedback during the construction of driven pile foundations that is not available for drilled piles, which are often large and each may replace 10 or more driven piles. Proper construction is critical to the performance of drilled piles, and there is a wider array of testing options. For instance, sonic or mechanical calipers are often used to verify the shape of an excavation prior to placing concrete.
The sonar caliper has become a preferred testing method for quality control and quality assurance for deep foundations. It uses a rotating sonar probe to give engineers the ability to view the pile shape in a marine environment. The team can see a 360-degree plan view of the shaft, which allows them to create a 3D model of the walls. With real-time display results, the sonar caliper is an efficient technology used to spot sidewall irregularities and estimate hole volume before concreting.
Frequently used to determine the length of existing piles and foundations, low-strain pile integrity testing, also known as a pile integrity test (PIT), entails striking the pile top with a hand-held hammer as an accelerometer records the pile’s subsequent small movements. The time record of the pile movement can be interpreted for changes in the pile’s cross-section and stiffness with depth.
Thermal Integrity Profiling (TIP) is used as an integrity test for drilled concrete deep foundations. The test evaluates the pile down the length of the cage using the heat generated by the curing cement to assess quality of the concrete in the drilled shaft and the shape of the cross-section. For TIP, sensors passively record the temperature of the hydrating concrete, capturing the temperature changes at determined time intervals, typically every 15 minutes. An analyst looks for zones that are cooler than expected to identify potential integrity issues.
Crosshole sonic logging (CSL) tests the integrity of the concrete using sound transmission. During a CSL test, transmitters and receivers are lowered into pre-installed tubes or coreholes. The arrival time of the sound wave data is interpreted for the speed of transmission, which indicates the concrete stiffness and integrity with depth.
The bi-directional test method, developed by Jorj O. Osterberg, PhD, is performed using a high-capacity loading device built into a driven or drilled pile. As pressure is applied, the device works by loading in two directions – upward against upper skin friction and downward against base resistance. The pile provides the entire test load and no overhead reaction frame is required, a distinct advantage in a marine environment. It is possible to test every pile using the bi-directional test method, but the decision must be made ahead of time so the cell can be installed.
An alternative during the foundation construction phase, the RIM-cell is a more current technology that provides bi-directional loading for confirmation of pile capacity and integrity for drilled piles. The RIM-cell has a donut shape to permit normal concreting procedures and uses grout to restore the integrity of the pile after applying the proof load.
Although testing has come a long way and contractors can proceed with confidence on marine projects when the test results are favourable, there is still room for improvement.
“Geophysics will be the next big thing that will change the way we build on land and in marine environments,” said Bullock. “It’s not the challenges that we find that cause problems; it’s the things that we miss.” He believes that geophysics will present a more accurate picture of the soil conditions, resulting in more economical projects.
For instance, he recently worked on a bridge which had a relic riverbed that crossed the site and included soft clay and weak silt.
“If you just poked holes around the site, then mostly you would [find] a layer of sand, but geophysical testing showed the weaker soil.” If that riverbed had been found during construction, the contractor would have been required to change equipment to handle much longer piles. The seismic testing prevented project delay and cost overrun.
Leave testing to the experts
Although some testing can be done in-house, Robinson and Bullock agree that safe, professionally executed and interpreted testing is best left in the hands of experts.
“Personnel qualified in collecting the data should perform the test in conjunction with personnel qualified in interpreting the results,” said Robinson. “Some projects rely on specialty testing firms contracted through the owner’s team or the contractor’s team.”
Bullock advocates for independent third-party testing to avoid a conflict of interest and stresses that the testing firm and personnel should be qualified and experienced.
“Testers should be cognizant of the tendency to find problems that validate their existence. A good result is just as important as a bad one,” he said.
Contractors may resist testing because it can delay production timelines, but a delay at the beginning of a project will be less costly than fixing structural problems well into the project. Some tests’ results are available in real-time, while other results will be sent a few days after the data collection and interpretation are complete.
As with all construction projects, safety is paramount. Most of the tests can be performed from barges, cofferdams and watercraft.
“A hazard assessment should be completed based on site conditions at the time of testing, with proper protective equipment and safeguards determined from that assessment,” said Robinson.
What should a contractor do if the test results seem off? Get a second opinion to validate test results – and use more than one test. None of the current tests are infallible, nor should any of them be used alone to reject a pile. It is also important to look at the test results in the context of project information such as installation records and soil borings. When in doubt, consult industry guidelines for interpreting test results.
Bullock cautions against creating an adversarial work environment when parties don’t agree about test results.
“Avoid the tendency to lawyer-up or hire competing experts with competing interests,” he said. “Anything can be done badly, but we also have to believe the data results when they are properly obtained. The best projects usually include a resolution team that meets every week to resolve issues as they arise.” 