Except for H-piles driven to bedrock, bearing piles are driven to a required blow count to ensure that they have the ultimate bearing value shown on the plans. However, the relationship between the blow count and ultimate bearing value is dependent on many variables, including the pile type, material and length; pile hammer energy and performance; and soil properties. The simple formulas used in the past considered only one of these variables – pile hammer energy. Dynamic load testing measures the energy going into the pile from the hammer and accounts for many of the different variables to estimate the capacity of the pile with each blow of the pile hammer. Therefore, the Department uses dynamic load testing to determine the required blow count for the ultimate bearing value on every project that includes driving bearing piles.
The methods used in dynamic load testing (also called PDA testing after the name of the equipment - Pile Driving Analyzer) were developed in Ohio beginning in the 1960’s. The Ohio Department of Transportation was involved early in its development, and began to apply the results of the research in their construction projects around 1968. Since 2002, the Department has used dynamic load testing for all pile driving (except piles driven to bedrock) and discontinued the use of the simple formula used previously.
Each dynamic load test consists of dynamically testing a minimum of two piles. If there are piles of different size, shape, or capacity, it will be necessary to perform dynamic load testing for each of these differing sizes, shapes, or capacities and there should be additional pay items in the contract to reimburse the Contractor for performing these tests.
Dynamic load testing is accomplished by connecting two sets of gages to a pile. One set of gages measures the strain in the pile at the top. The other gage measures the acceleration of the pile at the top. These gauges are then connected to a computer called a Pile Driving Analyzer (PDA). The PDA converts the measured strain and acceleration into force and velocity. By analyzing the way the force and velocity change with time, the PDA estimates the pile capacity for each blow of the pile hammer. After the PDA is connected to the pile, the Contractor begins driving the pile.
Once the dynamic load testing begins, the driving of the pile continues until the required ultimate bearing capacity is achieved. At this time the blow count, blows/ft (blows/meter), is noted. It is also necessary to record the stroke height of the hammer. In addition to performing dynamic load testing, the Contractor is also required to perform a Case Pile Wave Analysis Program (CAPWAP) on one of the piles tested. The CAPWAP analysis is a more refined method of analysis that takes into account the properties of the different soil layers. The results of the dynamic load testing and CAPWAP are then used to establish the driving criteria required to achieve the ultimate bearing capacity for the remaining piles represented by this test. Immediately after the dynamic load test has been completed the personnel performing the testing must inform the Engineer of the required driving criteria. This will include both the blow count and the stroke height. Within 48 hours after completing the test, the Engineer is to be given a written report with the results.
Prior to allowing the test to begin, the personnel performing the test must supply the Engineer with a copy of a certificate showing that they have a current Advanced PDA Certification in the PDCA/FQA Examination for high-strain dynamic load testing. An example is shown below.
Example PDA Testing Certificate
If the designers suspect that the capacity of the pile could increase or decrease after it has been in the ground for some period of time, an additional test called a restrike could be specified. If a restrike is specified, the plans specify the minimum elapsed time from when the pile was driven until the time of the restrike. This waiting period could be anywhere from a day to a week of more. Each restrike test consists of dynamically testing a minimum of two piles, the same as the dynamic load test.
When a restrike is specified it is very important that, during the waiting period, the pile to be tested should not be disturbed in any manner until the PDA is properly hooked up and the test is ready to begin. This is due to the fact that disturbing the pile can cause the pile to partially or completely lose any change in capacity it has acquired during the waiting period.
The pile hammer used to restrike must be the same hammer used to perform the initial dynamic load test on the pile and must be thoroughly warmed up by applying at least 20 blows to another pile other than the pile being tested immediately before the test begins. When the test begins the first few blows are used to determine the capacity of the pile. Any results obtained after the first few blows have occurred will result in the pile returning to the capacity it had obtained prior to the required waiting period.
1. File a copy of the field technician’s certificate showing that they have a current Advanced PDA Certification in the PDCA/FQA Examination for high-strain dynamic load testing.
2. Document the initial driving criteria received immediately after the dynamic load test is performed.
3. Receive a formal report within 48 hours including the information required in 523.04 A-D. Submit a copy of the report to the Office of Structural Engineering.