Pile driving formulas don’t estimate static capacity!

I have written recently about pile driving formulas, and the importance of correlating these with the results of PDA testing and Capwap analysis. Fundamentally we are accepting that PDA/Capwap is the most reliable reference method for pile driving acceptance. When a pile is driven there are broadly two types of resistance - (1) static resistance from pile shaft and toe that provide (long-term) structural support, and (2) dynamic resistance which is the result of the (rapid) motion of the pile into the ground. This dynamic resistance is only present during each hammer impact, and cannot be considered when evaluating the geotechnical capacity of the pile. The combined the static and dynamic resistances is called the pile driving resistance.

The PDA estimates the static component of driving resistance using the Case Method formula - by varying the Case damping factor, Jc, more or less of the driving resistance is attributed to static capacity - at Jc=0, all driving resistance is attributed to static capacity, and as Jc increases to 1 and beyond, less resistance is assigned to static and more to dynamic resistance.

Capwap has a similar function - ie by the wave matching process to separate the static and dynamic resistance components and moreover to assess the distribution of the resistance along the shaft and toe.

Pile driving formulas have no intrinsic ability to separate the static and dynamic resistances. They simply calculate the pile driving resistance. It is our job as engineers to determine static capacity by downrating the pile driving resistance. This is what we are doing when we are correlating a pile driving formula with PDA/Capwap.

Now it stands to reason that in easy driving (when sets are high) that there is more motion-related resistance than when sets are low (hard driving). The implication is that the correlation between pile driving formulas and PDA/Capwap is not a constant, but is variable - and the correction factor increases as set increases. The variable correction factor is what I call the Dynamic Reduction Function (DRF) - which I have posted about before.

I’ll leave you with PDA resistance-time graphs for a pile tested at 5.0mm set/blow (left) and 2.7mm set/blow (right). The graphs each show the driving resistance-time response (blue) and the Case Method static capacity-time response (red). The difference between the two curves is the dynamic resistance component. You can see that at the start of each curve, the resistance is dominated by dynamic resistance (when the impact first occurs and velocities are highest). Static resistance progressively increases and then stabilizes at a maximum value, whereas dynamic resistance decreases as the pile slows down (and eventually rebounds). At 5mm set, the static resistance is 72% of the maximum driving resistance. As the set reduces to 2.7mm/blow the static resistance increases to 89% of the maximum driving resistance.

…and for you PDA testers - you will find that the Hiley Formula pile driving resistance (PDA quantity QUS with appropriate SET) is typically very similar to the Case Method estimate of pile driving resistance (RX0 ie RMX with Jc=0).

That’s enough for now - I will expand further on this topic in future posts.

#geotechnicalengineering #pile #piles #piledriving #pilemonitoring #piledrivingmonitoring #piledrivingformulas #PDA #CAPWAP #PDM #PDM3 #FoundationQA #ConstructionQA