Maintaining Load During a Helical Pile Compression or Tension Test

Performing a load test is the best way to confirm pile capacity. It confirms if the pile has the capacity required to support the load applied. During most tests, the load is increased in time intervals and then held at the required load. These hold periods can be anywhere from 4 minutes, on a “quick” load test, or significantly longer for more long duration load tests that are to measure the amount of long-term creep. To ensure the results of all load tests can be reliably compared, there are standards and specifications in the piling industry. For example, these standards specify the minimum number of increments and the minimum spacing between test pile, reactions/dunnage used to resist the loading. For a compression test, the standard is ASTM D1143 “Standard Test Methods for Deep Foundation Elements Under Static Axial Compressive Load” and for a tension test the standard is ASTM D3689 “Standard Test Methods for Deep Foundation Elements Under Static Axial Tensile Load”. Both standards specify the load magnitude should be kept constant during the interval periods and the load be increased immediately to the next increment following the completion of the previous time interval. Maintaining the magnitude of load is critical to ensure a consistent measurement of pile creep that occurs during each interval.

Let’s consider a load test on a slightly smaller scale than that of a pile for a building. Think about a high school class testing the bridge they have designed. In this example, the load increment is a textbook. Each time the bridge survives one book, another book is added. This slow increase of load is exactly what the ASTM specification requires. Every time the class increases to a higher increment the load is maintained until progressing to the next load increment.

Misinterpretation of this part of the specification can lead to problems with how load tests are conducted. Hydraulic cylinders are good at applying large loads and can be calibrated to directly correlate pressure to load magnitude If pressure is maintained during an interval, the load is maintained. If pressure is allowed to drop, the load decreases. There have been load tests conducted where the cylinder was increased to the correlated pressure required for the interval’s load and then allowed to sit. Any creep or movement of the pile in this style of test results in a lowering of the pressure in the cylinder and thus a lower load being applied to the pile. In the bridge example above, allowing the pressure to drop would be like pulling pages out of the top textbook during the interval slowly reducing load magnitude over time. This is not what is intended.

To maintain load during an interval, the pressure must remain the same. As the pile displaces, this will mean that additional fluid will need to be pumped into the cylinder to hold the pressure. It is not uncommon for the volume of added fluid required to greatly increase as the pile reaches capacity.

Does this difference in testing procedure matter? The answer is yes. The displacement after an increment with continuous creep where the pump was running continuously versus an increment where the pressure dropped off can result in a different pile deflection response. The difference could be whether or not a pile has failed or passed a test. Again, the whole point of the ASTM specs is to allow for load tests to be analyzed and compared in a consistent and reliable manner. While letting the pressure drop off does make the pile appear to perform better, it does not help to ensure the pile has the required capacity.

So, how does an engineer tell if a pile has been correctly tested per the ASTM specification? Interpretation can be difficult with data recorded by hand and limited information. A better way is to require the contractor to maintain the pressure throughout time intervals by either being on site or including it in the test requirements. However, with the increase in use of Data Acquisition (DAQ) systems, this is easier to determine. These systems record the deflection vs load of the pile on a continuous basis (Hubbell’s system is set to record every 30 seconds). Thus, it is possible to see if the deflection increased while the load stayed the same or if the load fell off during the interval. This can be seen in the plot below. The red circles show locations in the plot where the deflection is increasing while the load stays the same; whereas the blue circle shows where the load fell off while the deflection increased.

To summarize, be familiar with the test procedures in the ASTM specifications. Be present during the load test if possible or have a qualified technician on site to monitor the test. Ask questions! Encourage the contractor to invest in a DAQ system to conduct the tests. They are relatively inexpensive and can reduce labor costs.

If you have any questions on load testing, contact your local Chance distributor or reach out to our engineering team at civilconstruction@hubbell.com.