Here is some information I copied directly from the PDA's Help screen which should clarify some of the issues being raised in this thread. Note that there is more than one measure of capacity which can be read from the PDA, depending on the predominant soil type, and whether the pile is a floating pile or predominantly end bearing.
Direct PDA measurements can be conservative or unconservative measures of capacity. As mentioned below, GRL recommends that a CAPWAP analysis be performed on a portion of the data to confirm 1) the "damping factor" that is used to estimate the dynamic portion of capacity and 2) to produce a more refined estimate of capacity in the CAPWAP model.
The CAPWAP program is basically a signal matching program that provides the engineer with a very comprehensive array of pile/hammer/soil variables which can be adjusted to confirm and elaborate the distribution of capacity on the pile.
Set check data should also be examined, as most piles will demonstrate some degree of freeze (though very rarely, at least in this area, soil relaxation can occur, and the true capacity is less than that observed at the end of drive).
Also, be aware that there are other dynamic methods which can be used to determine static capacity besides PDA. The Statnamic test is now used frequently on the east coast. The Statnamic has been extensively tested by the FHWA and various research universities and it is now considered to be at least the equivalent of the PDA in terms of accuracy.
It is usually used for piles (or shafts)that would otherwise require a very large hammer to mobilize capacity. Basically it involves setting a large mass on top of piston full of pelletized jet fuel that sits on top of the pile. The jet fuel is ignited and this mobilizes the pile, pushing it down into the ground. The pile or shaft must be specially instrumented with strain gauges and accelerometers before construction. It is really only cost competitive in situations where the data (capacity and shear strength of the soils) can be applied to confirm the design of a series of shafts to be constructed after it's performance.
There is a lot of PDA information available on GRL's site at
14.7 Capacity Methods
Change (Q1..Q9) using two letter command e.g. "Q3RMX" (Q3 will be RMX)
RSP - Original "Case Method" capacity result (depends on JC).
JC guide: 0.1 - 0.3 SAND; 0.3 - 0.5 SILT; 0.5 - 1.0 CLAY
RP# - RSP with J=.# to get RSP with second J; i.e. RP4 is RSP(J=.4)
RMX - Maximum "Case Method" capacity searches RSP at different T1 times for MAX result,
(depends on JC; should probably NEVER use JC<0.4, unless with static correlation)
NOT INTENDED for piles with bottom in CLAY: JC>0.8 UNUSUAL
RMX(JC=.5) is often a good initial choice (confirm with CAPWAP)
RX# - RMX with J=.# to get RMX with second J; i.e. RX6 is RMX(J=.6)
WDX is WD1 at time (TMX) of RMX; DBX is MAX TOE DISPLACEMENT
RSU - RSP(JC) for high friction cases (early unloading with negative velocity prior to 2L/c).
RU# - RSU with J=.# to get RSU with second J; RU5 is RSU(J=.5)
RA2 - Automatic Method (independent of JC) used for piles with LOW or MODERATE FRICTION.
this is a good first choice if no other information is available (confirm with CAPWAP)
RAU - AUTOMATIC METHOD (independent of JC) used if shaft resistance is very low
(Basically end bearing piles with little or no shaft friction).
SFT - SHAFT FRICTION TOTAL - no correction for damping
(SFR has a CRUDE DAMPING CORRECTION which depends on JC)
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For capacity of NON-UNIFORM PILES, use CAPWAP to determine capacity, NOT Case Method.
- All results to be reviewed & approved by Geotechnical Engineer -
Should compare Case Method result with static load test to determine proper damping factor, or at least Case Method result with CAPWAP to confirm which method and damping factor to use.
14.8. Capacity Evaluation Considerations
For capacity evaluation by the PDA, we ALWAYS recommend using CAPWAP ® analysis to check the PDA result, so really common practice is to use PDA and CAPWAP together which increases reliability compared to PDA alone. For capacity testing, there are a few simple conditions to get help you get the best result.
The PDA test should be a RESTRIKE after a wait following installation. The wait period should be longer for finer grained soils and ideally will be comparable to the wait used for static tests. For silts and clays (and even fine sands) the wait time is often 7 days or more. Since you can do a restrike test with the PDA at any time, if there are several tests with different wait times then they can be often plotted as a straight line on a log-time scale to estimate capacity at other times.
The set per blow must be at least 3 mm per blow (blow count less than 100 blows per foot) to assure that full soil resistance is mobilized (otherwise if set per blow is less or blow count higher then the test has only mobilized part of the total soil resistance, and the PDA result will be only a lower bound estimate as it indicates only the resistance activated). This may often happen as the pile is driven to a low set (high blow count) and then due to capacity increase with time (setup), the set per blow will be even smaller (blow count higher) during restrike due to the capacity increase with time. Possible suggestions include:
This may require in some cases a bigger hammer (larger ram weight) or a higher drop height. Usually a weight of 1 to 2 percent of the desired ultimate test load is sufficient.
In some cases (very low set per blow - high blow count), the end of drive end bearing can be added to restrike shaft resistance to compensate for perhaps not activating the full capacity of the pile and to project a higher total.
In some cases like closed end pile piles, it is possible to increase the pile impedance by filling the steel shell with concrete before the restrike. The increased impedance of pile then causes a higher force input which in turn can overcome higher soil resistances.
For proper correlation of static and dynamic tests, the load test must be run to failure (and the PDA test must cause good permanent pile set per blow as described in point 2 above). PDA usually correlates best with the Davisson limit load method (often regarded as one of the more conservative evaluation methods). If the static load test is a rapid plunging failure, then all failure load interpretations will be similar. If there is continual increase in load with increase in displacement, then the Chin method result, for example, would be much higher than the Davisson limit.
If Davisson is not the method used in evaluation, then a correlation between Davisson and the other method can be established, but the correlation must account for end bearing differences (pile type and soil type at pile bottom) to estimate the other load method result from either the PDA or Davisson result.
The safety factor should relate to the failure definition (a Chin definition of failure from a static load test should require a higher safety factor for the same probability of failure than a Davisson load definition or correlating PDA/CAPWAP result), and to the number of tests. For a given ultimate capacity, a lower safety factor can be assigned if a larger percentage of piles are tested (or alternatively stated, a higher allowable load can be used) since some of the uncertainty is removed by this more extensive testing.
In summary, PDA test should be a restrike with a similar wait time after pile installation, both static test and PDA test must cause soil failure (PDA test must achieve a good set per blow), and static test interpretation method should be by Davisson.
