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. However, 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, can be complex and must be performed properly for the results to be useful. Therefore, always contact the Office of Construction Administration or the Office of Structural Engineering for assistance before performing a static load test.
The duration of the static load test is generally 36 to 48 hours.
The Office of Construction Administration or the Office of Structural Engineering must be consulted before non-performing the static load test. Also 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). Because the static load test will place a load on the test pile that is twice the Ultimate Bearing Value (UBV), 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 five 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 five 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. Also, 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.
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 for some reason, the second pile is driven as a backup. The dynamic load test on the second pile also 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 testing 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). 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. However, 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, so 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 7 feet (2.1 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.
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 gages or devices capable of accurately determining settlement of the pile to 0.001 inch (0.03 mm) and a calibrated load cell for determining the load applied.
The gages 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 gages should be placed as close to the test pile as possible. Dial gages generally are furnished and they should have sufficient travel to measure up to 2 inches (50 mm). A backup system is required in case of problems with the gages. 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 gage on the hydraulic jack can be used as a backup load measuring system. The pressure gage 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 1/5 the UBV of the pile (R) and 1/10 R for each increment thereafter. Record the dial gage readings for each gage immediately after each load increment is applied; then every 20 minutes. Calculate the average pile settlement from the gage readings. Apply the next load increment after one hour, so long as the most recent settlement reading indicates a settlement less than 0.01 inch (0.3 mm) within the past 20 minutes. If the settlement is 0.01 inch (0.3 mm) or more within a 20 minute period, continue to take readings every 20 minutes until the settlement is less than 0.01 inch (0.3 mm) per 20 minutes, and then apply the next load increment.
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 as when the settlement exceeds 0.03 inch per ton (0.8 mm / 9000 N) for the increment applied. Whenever plunging failure is reached before the total load exceeds 1.5 times the UBV, attempt to apply an additional increment to determine if the pile experiences plunging failure again. If plunging failure is not repeated, apply additional load increments.
If the last load increment has been applied and the pile has not experienced plunging failure, maintain the load for one hour after all measureable settlement has stopped. Continue to make settlement readings every 20 minutes.
After loading is complete, unload the pile in 25 percent decrements in 5 minute intervals. Record the settlement readings at the end of every 5 minute interval, just before removing the next decrement of load. After the entire test load has been removed from the test pile, wait 3 hours and then record the final net settlement reading.
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, except apply the next load increment 15 minutes after measurable settlement has stopped (instead of one hour).
The test load Ultimate Bearing Value (Q) is the maximum capacity of the test pile. To determine Q, it is necessary to plot the settlement of the top of the pile versus the load on the pile. Then, draw a straight line through the zero point and the settlement reading for a test load of 0.2R. 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 Q criterion line.
0.15 inch + 0.008 D ( 3.8 mm + 0.008D )
Equation 506.1 – Settlement Offset
D = diameter of pile (inches or millimeters)
The point where the load-settlement curve from the static load test intersects the criterion line is the test load Ultimate Bearing Value (Q). 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 Q criterion line, then Q is equal to the greatest test load applied (2R).
Figure 506.A - Load-Settlement Curve
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 also require restriking 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 restruck. 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 Structural 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.
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 Structural 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.