Vulcan’s Blow Count Specifications

The durability and longevity of Vulcan pile hammer is something that is seldom replicated in just about any other manufactured product.  Since pile driving is self-destructive on the equipment, this is a remarkable achievement, but it should be tempered by the fact that it’s possible to render a Vulcan hammer inoperable by the way it’s used.  There are many things that can make this happen–inadequate or nonexistent hammer cushion material or lubrication to mention two–but the one thing that Vulcan decided to include in its warranty was the blow count specification.

Recording the blow count–the number of hammer blows per inch, foot or metre of pile advance–is virtually universal on pile driving jobs.  The dynamic formulae basically translated blow count into pile capacity.  While anyone familiar with pile dynamics understands that blow count is a crude measure of the response of a pile to impact, including a blow count specification is a good first measure of both the advance of the hammer and also how much energy is being returned to the hammer, which is a case of hammer damage.

BLOWCNT
Blow count-resistance graph, developed by Vulcan in the early 1990’s as part of its effort to educate state and federal agencies on the basics of pile driving. As the blow count increases, the amount of SRD (soil resistance to driving) increases, but at a progressively slower rate. This indicates that simply increasing the blow count is a “diminishing returns” proposition, destructive for hammer and pile alike.

High blow counts indicate that more and more of the energy was going back into the hammer rather than into the pile, thus increasing the danger of hammer damage.  They also indicate that pile top stresses increase with higher blow counts, as the movement of the pile to mitigate the maximum impact force decreases.  Thus high blow counts just to get the pile to tip elevation without considering changes in hammer or basic drivability considerations is a losing proposition.

Starting in the late 1970’s, Vulcan voided the warranty on its hammers if the blow count exceeded 120 blows/foot.  It’s interesting to note that Vulcan never made its specification in blows/inch.  This was true for its onshore hammers; however, for its offshore hammers it was forced by circumstance to increase the hammer refusal criterion as follows:

BLOW COUNT SPECIFICATION 1

Vulcan hammers are designed to withstand a continuous driving resistance of 120 blows/foot (400 blows/meter). In addition to this, Vulcan hammers will withstand refusal driving resistance of 300 blows/foot (1000 blows/meter) for five (5) consecutive feet (1500mm) of penetration. Any resistances experienced in excess of these are beyond rated capacity and will void the warranty. This definition is not an exclusive definition of excess of rated capacity and other criteria may apply.

1 Specification applies to all Vulcan offshore hammers, not just those listed in this catalog.

This was drawn from the API RP 2A specification, which was discussed relative to pile stick-up.  An elevated refusal blow count specification was justified by two things.  First, the offshore hammers were more robustly built than the Warrington-Vulcan hammers which made the company famous, as they were derived from the Super-Vulcan hammers.  Second, the remoteness of offshore job sites made high blow counts a necessity, as bringing a larger hammer to the job was frequently impractical.  (Improved methods of drivability predictability lessened the possibility of this happening.)

Blow count limiting warranty specifications are not an absolute method to prevent hammer abuse, but they’re a good start, and Vulcan used them to the advantage of itself, its end users and the owners of the projects where Vulcan hammers were used.

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