savings. Lacking testing, the pile design must be more conservative
and thus more expensive. With testing, many codes allow lower
factors of safety to be used for allowable stress designs (or higher
resistance factors for LRFD) resulting in shorter piles, or fewer
piles, reducing pile material costs which is the main expense of
the foundation. As shown in Figure 1, the Ohio Department of
Transportation (ODOT) tracked driven pile expenses over a sixyear
period.* By dynamically testing typically two piles per foundation,
significantly higher LRFD resistance factors (ODOT uses
φ = 0.70; for D/L = 3 the equivalent factor of safety is 1.96) were
used for friction piles (piles not driven to rock) compared with a
design governed by dynamic formula (φ = 0.40; equivalent factor
of safety is 3.44). The resulting savings can be estimated at 43 percent
while the testing cost was only 2.5 percent of pile costs, a small
price to pay for such significant savings.
The PDA also assists in evaluating driving procedures.
Dynamic measurements allow the engineer to assess driving
stresses during installation. This is particularly important for
concrete piles where tension and compression stresses are critical.
Knowing the concrete pile driving stresses allows the engineer to
adjust the installation procedures, such as limiting hammer stroke
height or changing pile cushion thicknesses to keep stresses under
rational limits and reduce the risk of structural damage to the pile.
Procedures can similarly be developed for steel pile projects where
high compression stresses can be problematic. The PDA is used
to detect if damage occurred on any pile exhibiting unusual blow
counts or terminating at significantly different lengths.
PDA testing information can be evaluated both for capacity
and for installation procedures. Such knowledge is invaluable
for a satisfactory final foundation installation that appropriately
services its supported structure. Dynamic testing provides
facts and reduces the potential risk of unsatisfactory
foundation performance. t
Garland Likins, P.E., hs been involved with dynamic testing since
1971 and was president of Pile Dynamics, Inc. from 1977 to 2014 In
that capacity, he directed the research and development of transducers,
real-time processing equipment and software analysis programs for
deep foundations. Likins remains a principal and senior consultant
for PDI. He has a B.S. and a M.S. in civil engineering from Case
Western Reserve University in Cleveland, Ohio and has achieved
“Expert” level on the PDCA/PDI Dynamic Measurement and
Analysis Proficiency Test. He is a member of the American Society of
Civil Engineers, the American Society for Testing and Materials, the
Deep Foundations Institute, the Pile Driving Contractors Association
and ADSC. Likins is a frequent lecturer and has more than 100
papers published in journals and conference proceedings. He is a
licensed professional engineer in Ohio.
* Peter Narsavage, 2011 PDCA DICEP program, Orlando, Florida
FEATURE
For larger projects,
dynamic testing
programs can optimize
the foundation.
84 | ISSUE 6 2017