Static load tests are performed on piles to determine the accuracy of dynamic load test results and to determine if the capacity of the pile being tested has increased or decreased after it has set in the ground for some period of time. The intent of performing the static load test is to potentially reduce the driven length of pile by fine-tuning the pile capacity determination. There must be a substantial amount of piling at a structure to justify the expense of a static load test. Generally, 10,000 feet of piling (all of the same size and ultimate bearing value) is the amount required before specifying a static load test.
Static load tests are not performed very often and can be complex and must be performed properly for the results to be useful. Always contact the Office of Construction Administration or the Office of Geotechnical Engineering for assistance before performing a static load test.
With the 2013 C&MS, the Department adopted the ASTM Quick Load Test method for static load testing. The duration of the static load test using this method is generally 8 to 10 hours. Using the previous method, the test could take longer than 24 hours. The other significant change with the 2013 C&MS is that the Contractor is responsible for taking the readings. Previously, the Department was responsible for taking readings during the test.
The Office of Construction Administration or the Office of Geotechnical Engineering must be consulted before non-performing the static load test. Consult either of these two offices to determine if a subsequent static load test should be performed.
Static load tests are not performed on piles driven to refusal on bedrock.
Most static load tests are performed on cast-in-place piles (also called pipe piles or tube piles). The static load test will place a load on the test pile that is twice the Ultimate Bearing Value (UBV); therefore, the test pile may need a thicker wall than that required by 507.06 or the Contractor will have to fill the pile with concrete and allow the concrete to cure for 5 days.
For test piles, if the wall thickness is less than the required wall thickness given by the equation below, then the Contractor must fill the pile with concrete and allow the concrete to cure for 5 days before performing the static load test.
t = Shell wall thickness (inches)
R = Ultimate Bearing Value, UBV (pounds)
D = Diameter of pile (inches)
If the test pile wall thickness is equal to or greater than the wall thickness given by the above equation, the Contractor can perform the static load test 72 hours (3 days) after he has finished driving the test piles and anchor piles. Generally, piles with a diameter of 16 inches or greater will meet the minimum wall thickness requirement for the test pile. Depending on the UBV, 12-inch and 14-inch piles may not be available with the minimum wall thickness for test piles. These piles will have to be filled with concrete before the static load test.
The test pile can be, and usually is, a production pile. The test pile must be vertical. Sometimes people are concerned about “failing” a production pile. This should not be a concern. In fact, it is desirable to “fail” the test pile as when this happens, we determine the true capacity of the pile and achieve the most value from the test. In the case where a test pile, which is also a production pile, “fails” during the static load test at a capacity lower than required, the solution is usually as simple as splicing on more pile length and driving the pile deeper.
The standard plan note for a static load test requires the Contractor to drive four piles, not including the anchor piles. The first two driven piles are test piles. Each one is driven to the required UBV as determined by the dynamic load test. This means both of these piles are dynamic load tested (that is one dynamic load test item). Do not overdrive these first two piles. Perform the static load test on one of these two piles. If the first pile is not suitable to use for the static load test, the second pile is driven as a backup. The dynamic load test on the second pile gives additional data that can be used to interpret the static load test results. The third and fourth piles are driven to reduced blow counts 75 and 85 percent of the driving criteria. For example, if the driving criteria from the dynamic load test is 40 blows per foot (bpf), then the third pile is driven to 30 bpf (0.75 × 40), and the fourth pile is driven to 34 bpf (0.85 × 40). It is important that the third and fourth piles be shorter than the test pile, so the person interpreting the test data can make an accurate evaluation. The exact method of determining shorter length is not critical. They can also be driven to 75 and 85 percent of the length, or some other lengths shorter than the test pile. The third and fourth piles are also dynamic load tested (one more dynamic load test item).
In some soil, piles will gain capacity with time. This is called pile set-up or sometimes, pile freeze. The reduced capacity piles are driven to see if the pile will gain enough capacity with time to meet the Ultimate Bearing Value requirement. If there is some increase in capacity, but not enough to meet the UBV, then we can still estimate the appropriate driving criteria if we know the amount of pile set-up. To do this, we need to know the capacity at the end of the initial pile driving, which we can determine from the dynamic load test. This is why we dynamic load test the reduced capacity piles.
Production piles can be used as anchor piles. Vertical piles are preferred for the anchor piles, but the Contractor may use battered piles for the anchor piles as long as the battered piles are symmetric around the test pile. The Contractor determines the number of piles to use as anchor piles and the required length of penetration. Anchor piles must be at least 8 feet (2.5 m) from the test pile, measured from center-to-center. If the anchor piles are also production piles, they may need to be re-driven to the required driving criteria after the static load test, depending on the test results.
Generally, the test pile should be driven before the anchor piles, but this is not required. If the anchor piles were driven first and there was a problem with the first test pile so that the backup test pile had to be tested, then the Contractor would have to drive additional anchor piles around the backup test pile.
No other production piles are to be driven until after the results of the static load test have been interpreted. However, the Engineer can allow the Contractor to proceed with pile driving at his own risk, with the understanding that the Department will not pay for piling driven deeper than required based on an evaluation of the static load test results.
The Contractor must wait at least 72 hours (3 days) after driving the test pile and anchor piles before applying the test load. If the Contractor has filled the test pile with concrete because it does not meet the minimum wall thickness requirement, then the Contractor must wait 5 days before applying the test load. The plan notes may require a longer waiting period.
The test pile should be cut off as near to the ground as practical and the jack placed along the axis of the pile with full bearing on the required load cell and bearing plate.
The Contractor must furnish a set of gauges or devices capable of accurately determining settlement of the pile to 0.001 inch (0.025 mm) and a calibrated load cell for determining the load applied. Dial gauges must have graduations every 0.01 inch (0.25 mm) or less.
The gauges used to measure the settlement of the pile should be placed opposite each other and should be placed at the sides of the pile. They should be supported from posts or fixed objects. The post or fixed objects are to be independent of the test load set-up and at least 4 feet (1.25 m) away from the test pile. However, the gauges should be placed as close to the test pile as possible. Dial gauges are furnished and should have sufficient travel to measure up to 2 inches (50 mm). A backup system is required in case of problems with the gauges. The backup system usually consists of a ruler applied to the test pile, with a mirror and string line for measuring settlement.
The primary means of determining the applied load is a calibrated load cell. The pressure gauge on the hydraulic jack can be used as a backup load measuring system. The pressure gauge alone is not accurate enough for the static load test. Also, if the jack should bind up, the hydraulic pressure would increase while the load transmitted to the pile would not necessarily increase.
The load is to be applied in increments consisting of a first increment of 1/10 the UBV of the pile (R). The Contractor records the dial gauge readings for each gauge 1, 2, 4, 8, and 15 minutes after each load increment is applied. Calculate the average pile settlement from the gauge readings. Apply the next load increment after the 15 minute reading.
Maintain the test load during each load increment. Due to settlement of the pile, the load and pressure in the jack may decrease with time. The Contractor should run the hydraulic pump as necessary to maintain the load on the pile.
Continue to increase the test load until the load is twice the UBV or the pile reaches plunging failure. Plunging failure is defined when continuous jacking is required to maintain the test load.
After loading is complete or plunging failure is reached, unload the pile in five equal decrements in 15 minute intervals. Record the settlement readings at 1 and 15 minutes after each load decrement. After the entire test load has been removed from the test pile, record the settlement after 1 and 15 minutes.
If it is necessary to remove and reapply the load, such as a problem with the jack or load cell, reapply the test load using the same procedure used to apply the initial loads.
The test load Ultimate Bearing Value (Qf) is the maximum capacity of the test pile. To determine Qf, it is necessary to plot the settlement of the top of the pile versus the load on the pile. Draw a straight line through the zero point and the theoretical elastic settlement of the pile using the equation in the specification. This line represents the elastic compression of the pile (the distance that the pile compresses under the test load). Draw another line parallel to the first, but offset by the distance given in Equation 506.1. The second line is called the Davisson criterion line.
0.15 inch + 0.008 D ( 3.8 mm + 0.008D )
D = Diameter of pile (inches or millimeters)
The point where the load-settlement curve from the static load test intersects the Davisson criterion line is the test load Ultimate Bearing Value (Qf). An example from an actual static load test is shown in Figure 506.A.
In some cases, the slope of the unloading portion of the curve can be used to determine the elastic compression of the pile.
If the load-settlement curve does not intersect the Davisson criterion line, then Qf is equal to the greatest test load applied (2R).
The standard plan note for a static load test requires the Contractor to restrike the test pile and the two reduced capacity piles. The note may require re-striking the backup test pile. The restrikes are performed at least 7 days after the piles were driven. Each restrike test consists of dynamically testing two piles and determining the capacity of the piles. Pile set-up is the increase in capacity for a pile from the time it was initially driven to the time it was re-struck. Ideally, the capacity from the restrike test on the test pile should be close to the results of the static load test.
Either the Office of Construction Administration or the Office of Geotechnical Engineering will review the pile driving logs, the static load test results, the dynamic load test results, and the restrike test results, and then recommend the driving criteria for the production piles. Wait for the recommended driving criteria before letting the Contractor drive the rest of the production piles. However, the Engineer can allow the Contractor to proceed with pile driving at his own risk, with the understanding that the Department will not pay for piling driven deeper than required based on the recommended driving criteria after the evaluation of the static load test results.
The static load test results will apply to the same type and size of piling, driven with the same type of hammer, to approximately the same depth, with similar driving characteristics as the test pile.
The pile hammer used for driving the test pile shall be used for driving all piles represented by the static load test. If the Contractor subsequently finds it necessary to use a different size and type of hammer, the Office of Geotechnical Engineering or the Office of Construction Administration will determine if an additional static load test is required; any such additional static load test shall be completed at no additional cost to the Department.
Documentation for the static load test consists of the driving logs of the test piles and reduced capacity piles, the dynamic load test report, the restrike test report, and the load-settlement readings from the static load test. Identify the piles using the pile numbering system on the plans if the piles tested are production piles.