I am now making review of some PDA tests for 305x305x180kg/m grade 55 steel H-piles. The pile length is about 35m with general geology consisting of fill (8m, gravelly to sandy), alluvium (4m, sandy), grade V weathered soil (sandy)
From the PDA (PAK model) records of 5 piles, it was recorded that the transferred energy ratio (EMX/PE) are about 120% . The testing contractor claimed that such abnormal readings are "due to additional acceleration applied to the hammer ram after rebound". I would like to seek the opinions from any experienced partitioner. Thank you very much!
You need to check the type of driving hammer, whether it is double acting or single acting type. Doble acting hammer has extra energy input in addition to the potential energy (W*H). Next I will check the real drop height of the ram. Very often, the drop height of the hammer is only a visual estimate by the piling operator. The drop height may be 20% higher than the given height of the hammer.
Note that the EMX computation is based on the integral of the force and velocity time from time of impact till pile top velocity returning to zero (just before rebound). Hence, it is not because of rebound.
I will also check the data quality of the force and velocity data since the EMX is the result of the measurements. Of course, one need to check the input quantities to the PDA such as the cross-sectional area and modulus of the pile, calibration factors. If you have the PDA data (X01 or W01 file), you can email to me (firstname.lastname@example.org)and I will be happy to examine the data for you.
If you get a transferred energy that is larger than the stated rated or potential energy from the hammer, you have an error somewhere in your system or the data. You cannot, must not, rely on any of the information or analysis based on your data until you have located that error.
It can be so simple that the information you have on the hammer is just wrong. Ram weight is off, or ram travel or height-of-fall is wrong. (I wish you had provided identifying information on the hammer with your question). Your gage calibration may be off, or the driving is bending the pile (Forces F1 and F2 are different). But then the PDA error signals should have alerted you to this.
The "testing contractor's explanation that the large ratio value is "due to additional acceleration applied to the hammer ram after rebound" is ludicrous. The hammer bounces as a response to the pile head springing back up due to reflections of the force wave in the soil, particularly from the pile toe. If so, the accelerometer at the pile head indicates a "negative" velocity and the transferred energy value starts reducing from that time on. This can sometimes cause concern for having too small values of transferred energy (when the inspector want to apply a specific value of energy ratio and fails to understand the mechanism), it can never cause the transferred energy to increase. It is so basic that the explanation from the "Testing Contractor" is an attempt to pull the wool over your eyes -- hardly professional, he knows better.
I have seen higher than 100% readings before, and they may be correct. Almost always the higher than 100% values come when the hydraulic hammer is operated at less than full stroke. In fact, the lower the hammer setting the more likely this will happen (and conversely, at high hammer settings it is unlikely that this should occur, if as others point out the dynamic data is of good quality). The simple explanation is that many (and perhaps even most) of the hydraulic hammer makers use proximity switches to get an average velocity over a short distance (typically 75 mm). The real velocity at the bottom of the measurement will be higher due to continued acceleration of the fall. In addition the proximity switches are usually placed some distance above the actual impact point (several reasons for this), so there is additional potential energy associated with this extra distance and again the ram continues to accelerate over this distance making the real final impact velocity higher than the "average" velocity over the proximity switch locations. Thus the real kinetic energy is higher than indicated, and since there is good transfer for steel piles the PDA dynamic test result can in fact be higher than that indicated by the hammer energy monitor. Again this is most likely at low strokes where the extra distance below the bottom sensor is a significant fraction of the total reduced stroke.
I might also add that there could be an error in the proximity switches, and of course you have to be careful of the units (if the dynamic test is in kN-m and the hammer monitor is in kip-ft, then the dynamic test to hammer energy may also exceed 100% if you just divide the numbers and do not take into account the different units!!!)