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My Perspective on Driven Pile Drivability Studies

This post originally appeared in 2013 on my companion site.

Recently I had a round of correspondence with a county official in Washington state re pile drivability studies and their place in the contract process.  (If you’re looking for some explanation of this, you can find it here).  His question was as follows:

During the bidding process, is the contractor’s sole basis for anticipating the size of the hammer needed the WEAP analysis? Does a contractor rely solely on design pile capacities or does the contractor combine geotechnical boring logs and cross-sections with his expertise? Who will be ultimately responsible that a large enough hammer is considered in the bid and brought to the site, the contractor or the preparer of the design package?

My response was as follows:

First, at this time the WEAP analysis is the best way for contractor and owner alike to determine the size of a hammer (both to make sure it isn’t too small with premature refusal, or too large and excessive pile stresses) necessary to install a certain pile into a certain soil.

It is a common specification requirement for a contractor to furnish a wave equation analysis showing that a given hammer can drive a pile into a given soil profile.  As far as what soil profile is used, that’s a sticky issue in drivability studies.  Personally I always attempt to estimate the ultimate axial pile capacity in preparation of a wave equation analysis.  There are two important issues here.

The first is whether the piles are to be driven to a “tip elevation” specification vs. a blow count specification.  For the former, an independent pile capacity determination is an absolute must.  For the latter, one might be able to use the pile capacities if and only if he or she can successfully “back them out” from the allowable capacities, because the design factors/factors of safety will vary from one job and owner to the next.  Some job specs make that easy, most don’t.

Even if this can be accomplished, there is the second problem: the ultimate capacity of interest to the designer and the one of interest to the pile driver are two different things.  Consider this: the designer wants to know the pile with the lowest capacity/greatest settlement for a given load.  The pile driver wants to know the pile with the highest capacity.  If you use the design values, you may find yourself unable to drive many of the piles on a job or only with great difficulty.  I’m seeing a disturbing trend towards using the ultimate capacity for design and running into drivability problems.

As far as responsibility is concerned, that of course depends upon the structure of the contract documents.  I’ve discussed the contractor’s role; I would like to think that any driven pile design would include some consideration of the drivability of the piles.

Some of the FHWA publications I offer both in print and online (including the Driven Pile Manual) have sample specifications which you may find helpful.

Hope this long diatribe is of assistance.

After this, there’s another way of looking at this problem from an LRFD (load and resistance factor design) standpoint that might further illuminate the problem.  The standard LRFD equation looks like this:

\sum _{i=1}^{n}{\it \gamma}_{{i}}Q_{{i}} \leq \phi\,R_{{n}}

This is fine for design.   With drivability, however, the situation is different; what you want to do is to induce failure and move the pile relative to the soil with each blow.  So perhaps for drivability the equation should be written as follows:

\sum _{i=1}^{n}{\it \gamma}_{{i}}Q_{{i}} \geq \phi\,R_{{n}}

It’s worthy of note that, for AASHTO LRFD (Bridge Design Specifications, 5th Edition)  \phi can run from 0.9 to 1.15, which would in turn force the load applied by the pile hammer upward more than it would if typical design factors are used.  Given the complexity of the loading induced by a hammer during driving, the LRFD equation is generally not employed directly for drivability studies, and the fact that \phi hovers around unity makes the procedure in LRFD very similar to previous practice.

The problem I posed re the hardest pile to drive vs. the lowest capacity pile on the job is still valid, especially with non-transportation type of projects where many piles are driven to support a structure.  When establishing a “standard” pile for capacity, it is still the propensity of the designer to select the lowest expected pile capacity of all the pile/soil profile combinations as opposed to the highest expect pile resistance of all the pile/soil profile combinations necessary for drivability studies.

Put another way, the designer will tend to push the centre of the probability curve lower while the pile driver will tend to push the centre of the probability curve higher.  This is a design process issue not entirely addressed by LRFD, although LRFD can be used to help explain the process.

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