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Your understanding of the foundation supporting your structure can make-or-break your project. After all, the worst time to discover an issue with your foundation is after the structure is built and sitting on it. When you're investing piles of money and time in a project, you can't afford any surprises.
That's especially true for commercial, industrial, or municipal construction. There's no room for guesswork on these projects and any foundation issues that are missed in the beginning, will come back later to haunt the structure in a big way.
Take the Ocean Tower in South Padre Island, Texas for example. The idea was to build a 31-story luxury condominium, but the project was halted due to unforeseen foundation issues. Before construction was even completed the structure began to sink and foundations cracked. It turned out the soil was unsuitable for such a tall building.
While hindsight is always 20/20 and we don't know what kind of ground investigations the engineers for that project performed, it highlights the importance of gathering quality data before committing to a given location or design.
That's where the helical pile load test comes in.
A pile load test can tell you a lot about not only the present conditions for a foundation at your jobsite, but also the future implications for that foundation. Had the engineers for the Ocean Tower project appreciated that the ground was inadequate to support such a tall building, they could have either tried to mitigate the soil issues or found a new location for their high-rise condo.
But load tests can do much more than just tell you the axial load capacity of your pile.
They can also give you greater insight into the true soil conditions under your feet. Or provide site-specific data that engineers can use to design a more efficient foundation. And identify problem soils or challenging areas of your job site, enabling you to plan around them.
In this blog post, we'll uncover the true power of a helical pile load test and show you how it can be used to improve all aspects of your foundation. Ready to build some solid knowledge around helical pile load testing? (Sorry, couldn't resist the pun)
Load testing is a process that helps gauge the durability of a helical pile against compressive, shear, or tension forces.
The history of load testing helical piles actually dates all the way back to their very first use at the Maplin Sands Lighthouse in 1838.
At that time, engineers would secure a platform on the helical piles and stack large stones onto it as a rudimentary weight test. A measuring rod was used to check if the piles were being pushed down into the soil due to the weight - this was (and still is) known as checking for 'pile creep'.
Today, modern engineering gives better control, precision, and accuracy in the load testing process.
There are several types of load tests that might be performed on a helical pile, each designed to asses different aspects of pile performance. The most common for helical piles are axial compression, axial tension, and lateral load tests. They test how the pile handles downward pressure, upward pull, and sideways force, respectively.
An engineer will decide which load test to use based on the real-world forces the pile will be subjected to. They'll account for factors like structural loads, soil conditions, and local climate, to determine the best methodology to follow.
Examples of two accepted load test standards are ASTM D1143 for axial compression loads, and ASTM D3689 for axial tension loads. These standards inform how a load test should (ideally) be designed, performed, and interpreted. For the sake of brevity, in this article we'll be looking at what's required to perform an axial compression load test.
As mentioned, the ASTM D1143 standard details how to perform an axial load compression test on a helical pile. Specifically, it contains a few different types of axial load test that will deliver varying degrees of "resolution" on the data.
For example, the standards state the 'Quick Test' can be used to assess pile capacity in just a few hours as loads are applied to the pile, in their words, "relatively quickly". The 'Maintained Test', on the other hand, applies load to the pile for longer periods of time to determine resistance to "pile creep".
The exact test an engineer will decide to use depends on factors like the type of structure to be supported, access restrictions or limitations at test site, and availability of equipment to perform the test. It's also influenced by the kind of data the structural engineers want to see in order to assess the helical pile.
Once the engineer has decided on the type of load test methodology to follow, they'll work with others on the team to create a clear plan for setting up and performing the test.
It takes planning, effort, and expertise, to perform an accurate and successful helical pile load test and interpret the results. While the process is backed by science, it's also a kind of art form when overcoming unexpected situations like this engineer found while performing a load test in rural Brazil with limited equipment.
That's why it's important that regardless of the type of load test being performed, it's critical that it's been done by an experienced team that's familiar with the process and equipped to perform it properly. A poorly-performed load test can be worse than having none at all, as it may return faulty data that leads to faulty assumptions and ultimately design failure.
With that in mind, let's look at how a typical load test is performed. We'll start with the equipment required to perform a load test.
In the same way that different types of load tests exist to achieve different results, the exact equipment used can vary between tests as well. The goal of this post isn't to provide a step-by-step on how to actually perform a load test, but instead to give you an idea of how the overall process works and the benefits it can have for your project.
As such, we're going to describe the straightforward and common equipment that may be used to perform a static axial compression test.
Generally speaking, the equipment that would be used for this kind of load test is:● One or more helical piles installed perfectly vertically, for testing● "Reaction piles", which are temporary piles installed to support test rig● Test rig, which is usually a steel frame made from I-beams and supports test equipment● Hydraulic jack, used to apply downwards pressure on the helical piles to be tested● Various sensors and gauges, to measure and report the results of the test● Laptop computers, to read and record test results
Let's get into more detail about some of that equipment.
Piles to Be Tested (& Install Equipment)
This might be a "no kidding" moment but in order to perform a load test you'll need to have a helical pile to, you know, test.
Again, depending on your specific requirements, there may be only one pile to test or there could be several. Regardless of how many helical piles there are to test, it's incredibly important that each test pile is installed just as carefully as one that's going under a structure. If the pile is installed at an angle or otherwise incorrectly, the accuracy of the entire test will be thrown into the wind.
Speaking of an accurate installation, it's going to take some machinery to put the piles into the ground. Thankfully, helical piles are installed using common construction equipment like excavators and "helical drive" attachments. This can help reduce the cost and difficulty of the load test by making the installation (and removal, if needed) of the helical piles fast and simple.
Some load tests are performed by setting a frame on the test piles, then loading the frame with stacks of solid metal or concrete weights. In this type of test, the pressure of the weights ensures that everything stays in place.
Using a hydraulic jack to exert pressure on the helical pile enables us to have more control over the entire load test. The downside of a hydraulic jack is that by applying pressure downwards on the pile, the jack inevitably wants to lift upwards in response.
To prevent the hydraulic jack from being pushed upwards, we install additional "reaction piles" around the test pile. A steel test rig is then anchored to these reaction piles, which prevent the hydraulic jack from moving while performing the test.
Helical piles are perfect for reaction piles because they can be both installed and removed quickly and easily, leaving no trace behind once gone. Their high resistance to axial tension loads also means they'll be able to resist the force of the hydraulic jack.
When performing an axial compression load test, it's of absolute importance that the force applied to the test pile is perfectly vertical. If the hydraulic jack isn't plumb to the test pile, it will exert a lateral force which can both invalidate the test results and cause damage to the pile. Because load testing applies huge amounts of force, it's important to securely support the load test rig itself.
That's why load test rigs are constructed from heavy duty steel and erected at the site with extreme care. Not only do they need to be tough enough to resist the loads at work during testing, they need to be setup with perfect precision.
One of the big differences between modern helical pile engineers and those of the 1830's isn't so much that they're smarter these days, but that the engineer of today has the power of computers to help out.
Thanks to the sensitivity of modern sensors and instruments, we're able to collect more data than ever on the performance of the test pile. Exactly which sensors and instruments we are are, you guessed it, determined by the type of load test. But, here's some of the common ones...
Load CellsLoad cells are used to measure the axial load or compression force applied to the pile. They're placed at the top of the pile or within the pile cap itself to measure the load.
Strain GaugesStrain gauges are used to measure any deformation or strain in the pile. Attached to the surface of the pile, they'll provide data on how the pile responds to loading.
Soil SensorsVarious sensors might be placed into the soil to measure how it reacts when the pile is loaded. Pore Pressure Sensors, for example, are used to assess the change in soil pore water pressure as loads are applied to the pile.
Settlement Measurement (Pile Creep)There's a number of tools and methods that can determine how much a pile settles (also known as "pile creep") during loads. This could be an electronic sensor or it might be a measuring rod but the idea is the same, to assess how much (if at all) the test pile settles as a result of the loads applied to it.
After the type of load test has been established and the equipment assembled, the only thing left is to hit the field and perform the test.
1: Install Test Piles (& Reaction Piles)
The first thing to do on-site is confirm and mark the test-pile locations, if they're being installed specifically for the load test. If the load test is being used for production helical piles that are already installed, then the specific piles to test will be confirmed.
Once the locations are confirmed, test piles will need to be installed along with reaction piles. Again, if the pile to be tested is a production pile, then only reaction piles will be installed to support the test rig.
2: Assemble Test Rig
The test rig is a sort of skeleton that ties the whole process together. It provides a stable platform to hold the hydraulic jack and is secured to the reaction piles to prevent movement as a result of the pressure applied during the test.
Most load test rigs are constructed from steel I-beams or H-beams which are usually arranged in a square pattern with the test pile in the center. However, other shapes for test rigs such as octagons may be used.
Once the test piles are installed / confirmed, the test rig will be carefully constructed around it and filled with equipment and sensors. After the test is compete, the rig will be disassembled and stored for the next one.
3: Apply Loads to Pile
Before any testing is conducted, final checks should be performed on the test rig and associated equipment to ensure everything is in order and ready for the test. A load test involves immense amounts of pressure being applied to many different components, so it's important to have a safe and focused team.
When everything is in order, the engineer in charge will use the hydraulic jack to apply loads on the top of the pile. The amount of force applied, and for how long, is determined by the test methodology being used.
Some tests entail rapidly loading, unloading, then loading again, the helical pile. Others may load the pile slowly, but for a much longer period of time. Each test has its own use-cases and limitations, which is why it's key to engage with a qualified contractor who uses the right method for the right scenario.
As loads are applied to the pile, all the various sensors and gauges attached to the pile and in the ground will feed information to a computer. This information is stored for later review and assessment.
4: Data Assessment & Report
Once the test is complete, the engineer will have stacks of performance data to review and draw conclusions from. An engineer will take the information from the load test and turn it into a report that details, among other things, the ultimate load capacity and performance of the pile.
While it might seem like a lot of hassle and time to perform a load test simply to confirm its' load capacity, a quality load test can actually tell you quite a bit more than that. In fact, when used to the fullest, a load test can help you build a more efficient and economical structure.
An obvious benefit of having a helical pile load test performed is that you can guarantee the soil and pile are able to safely meet design loads. Much like the Ocean Tower example from the beginning of this article, not understanding the real-world performance of your soil and foundation could lead to trouble later.
We also don't want to make it sound like a load test is the only way to confirm the capacity of a helical pile. Installation torque, for example, is a common method of confirming load capacity by using the empirical relationship between the torque it takes to install a helical pile and that pile's capacity.
However, because of the large amounts of data recorded, a load test can do more for your foundation than just provide the ultimate capacity. Here's just a few...
Value Engineer an Efficient Foundation
For some, the term "value engineering" conjures images of cost-cutting and finding ways to do things "cheaper". We'd argue that vision of value engineering has been fostered by cheap companies who use it as a nicer way to say that they cut corners.
When used correctly, value engineering isn't just about cutting costs. It's about maximizing value for your construction project. Saving money is definitely part of that, but maximizing value can also mean finding efficiencies or reducing timelines.
Because a load test produces a large amount of quality data regarding soil and foundation performance, a good structural engineer can use that data to make informed decisions about design elements.
For example, a load test could indicate the soil is more cohesive than originally thought and design loads can be reached with a smaller helical pile. Or, maybe the overall number of helical piles can be reduced. Without the load test, those opportunities could be missed.
On the other hand, what if the load test uncovered unexpected soil conditions that require the piles to extend deeper than originally thought? If discovered in advance, the engineer can adjust the design and plan for a deeper pile. Imagine you only found out you needed longer piles on the first day of install. There could be days, maybe weeks, of delays while the foundation is re-engineered and new material is shipped to site.
You can see, then, that value engineering takes many forms beyond reducing costs. When done correctly, it identifies as many possible areas for improvement and ensures your foundation installation runs as smoothly as possible.
Assess Site-Specific Torque-to-Capacity Correlations
As we mentioned earlier, one of the methods that can be used to determine the load capacity of a helical pile is installation torque. We don't have time to dive into the science of installation torque right now, but here's the short-and-sweet explanation:
In order to apply the rotational force needed to install a helical pile, we use a piece of equipment known as a Helical Anchor Drive. It uses hydraulic force to drive a gear motor that turns the helical pile into the ground, in a sense acting like a massive power drill. The "size" of the anchor drive is determined by how much torque it can exert.
As the anchor drive turns a helical pile into the ground, the soil offers resistance which the drive must overcome to continue advancing the pile. Loose and poorly-consolidated soil offers less resistance than dense and well-consolidated soil. Typically, as the pile goes deeper, the soil becomes more consolidated and resistance increases.
We can measure the resistance of the soil on the pile in terms of the torque that's required to continue turning the pile deeper into the ground. More torque required to turn the pile (usually) indicates the soil is offering more resistance.
As it turns out, the amount of torque that's required to install a helical pile has an empirical relationship to the capacity of the pile. This relationship was first described in 1989 and led to capacity to torque correlations (CTC) being defined and a formula established to calculate them.
All an engineer needs to do is multiply the empirical torque factor for the pile with an average installation torque figure. The resulting number will be the ultimate load the pile can resist. The equation goes like this:
Qt = Kt x T
Qt = ultimate pile capacity, Kt = empirical torque factor, T = average installation torque
This calculation tells us the minimum torque that has to be exerted on each pile during installation in order to achieve design loads. During installation our crews will digitally monitor installation torque in real-time in order to verify the pile's performance.
However, in order to truly unlock the power of installation torque, you need to combine it with a helical pile load test.
One of the downsides to installation torque, and why it can't be solely relied on as a way to confirm pile capacity, is because the established capacity to torque correlations (CTC) may not be appropriate for the soil at your jobsite.
Current CTC have been determined by various researchers using real-world experiments to derive the performance data needed to create reliable equations. However, it's impossible for researchers to test every type of soil composition to establish CTC for every possible condition.
The problem comes in when the soil at your jobsite reacts to helical piles in a way that hasn't been studied in a lab setting yet. Thus, the existing CTC may not provide an accurate picture of the installation torque your piles will need to reach.
In these instances, a helical pile load test can produce valuable data regarding the actual soil conditions at your site. This data can in turn be used by an engineer to develop CTC that are specific to your site, resulting in a higher degree of confidence in your installation torque. It still shouldn't be the only way to confirm pile capacity, but it offers a rapid method to assess the quality and accuracy of pile installation in the field.
Identify Unexpected Changes in Soil Composition
The soil beneath our feet is still a great mystery to engineers and scientists, especially in the field of deep foundations. One of the big reasons for this mystery is the difficulty in actually observing what's going on in the soil. We can't build a telescope that looks into the ground, unfortunately.
Of course there are ways to study soil conditions and this is where geotechnical engineers come into the picture. Using their specialized tools and instruments they can take measurements of the soil and paint a picture of what's going on under the surface. The information they collect helps structural engineers make informed choices regarding foundation choice and design.
Even though they have powerful tools and plenty of knowledge, their abilities are ultimately limited by the nature of soil itself. Here's an example...
Let's say you're building in a location that hasn't seen development, so there's little information regarding soil conditions. You hire a geotech to perform a ground investigation and their report indicates supportive layers of soil are found 35 to 50 feet down. The helical piles are installed to that depth and all of them reach the specified installation torque in order to meet design capacity.
What you don't know is the geotech couldn't see the layer of supportive soil around 35 to 50 feet down is actually resting on top of a loose and poorly consolidated layer. Like a thick crust sitting on a pie made of pudding.
Even though the all piles reached installation torque because they anchored in the dense layer of soil, when loads are applied they may "punch through" the crust and sink into the pudding layer (note that "pudding layer" isn't necessarily a technical term!)
Performing a helical pile load test complements a geotech report because it can help overcome the limitations of the observations an engineer can make about the soil. It reveals how your soil actually interacts with helical piles and, based on that, our team can devise the best solution.
Provides Confidence in Foundation Performance
One of the biggest advantages of doing a helical pile load test is peace of mind. By confirming the pile can carry the required weight at your specific site, you can be confident it will continue to perform as long as the structure stands. No surprises, no future headaches, no safety concerns.
That's why even though the process for performing a pile load test can be involved and takes time to do correctly, the results are well worth it. The data collected can improve every aspect of the foundation process from engineering to installation. It eliminates much of the guesswork that can be unavoidable with deep foundation work and brings peace of mind to you and your clients.
Discovering foundation issues after construction is a costly and often irreversible problem. This is particularly important to note for commercial, industrial, or municipal construction, where foundation issues could have serious public safety implications.
The Ocean Tower in South Padre Island serves as a stark reminder this, as it sank into the ground during construction and was demolished before completion due to a failure to identify unsuitable soil conditions.
Thankfully you don't have to go into a project blind to what's happening in your soil.
The helical pile load test is a powerful tool that goes beyond guaranteeing load capacity and performance. These tests provide valuable insights into soil conditions and equip our engineers with the data needed to design better foundations while identifying potential problem areas on the job site.
Here's the main things to remember about helical pile load testing:
Our intention at the beginning of this post was to provide an in-depth look into the science behind helical pile load testing and uncover at least some of the reasons why you would undertake it.
There's no doubt it's an involved process that can involve your investment in both time and money. With that said, few good things in this world come without a little investment. When it comes to load testing a helical pile, you'll see your investment returned many times over in a variety of ways.
Not the least of which is being able to sleep soundly at night knowing the foundation on your project isn't going to sink into the ground like it did for a condo tower in South Padre Island, Texas.
If you're interested in helical pile load testing services, our experienced team at VersaPile is ready to help you. With extensive experience and key expert partners in the industry, we help you achieve a high level of confidence in your foundation. Get in touch with us today!
Ile Des Chenes, MB
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