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. 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. 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 (named after 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 gauges to a pile. One set of gauges
measures the strain in the pile at the top, while the other gauge 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 necessary to record the stroke height
of the hammer. In addition to performing
dynamic load testing, the Contractor is required to perform signal matching
analysis of the dynamic test data on at least one of the piles tested. The Contractor must perform the test
according to ASTM D 4945.
The signal matching analysis is a more refined analysis method that takes into
account the properties of the different soil layers. The results of the dynamic load testing and
signal matching analysis 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 an Advanced Master or Expert Level
Certification in High Strain Dynamic Pile Testing (HSDPT)
from either the Pile Driving Contractors
Association (PDCA) or Foundation QA. The Foundation QA examination was the
original certification program, but the Department is changing to the PDCA certification program.
Both certification programs are currently acceptable. A sample certificate is shown below. The Engineer can also check a person’s
certification status on the following website:
www.pdaproficiencytest.com
Figure 523.A
– 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 or 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. 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 will result in the pile returning to the capacity it obtained prior to
the required waiting period.
1. File a copy of the field technician’s certificate which
shows they have a current Advanced Master or Expert Level Certification in High
Strain Dynamic Pile Testing from the PDCA or FQA.
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 Geotechnical Engineering.