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TRANSPORTATION RESEARCH RECORD 1633
Paper No. 98-1059
117
Using Dynamic Pile Testing To Evaluate
Quality and Verify Capacity of Driven Piles
PHILLIP A. WALTON AND STEPHEN L. BORG
Pile testing using a pile driving analyzer is a nondestructive method of
evaluating the support capacity provided by the soil to a pile. The New
York State Department of Transportation’s (NYSDOT) approach to
incorporating this testing procedure into a project is discussed from
design into construction. Specification and contractural responsibilities
are addressed, as are typical applications of the test method. By judiciously using dynamic pile testing, NYSDOT is able to cost-effectively
link the design process to construction. Informed decisions are possible
regarding the field performance of driven piles with respect to pile-soil
support capacity, hammer operation, driving stress, damage to piles during driving, and verification of assumptions made for wave equation
analysis of piles.
The New York State Department of Transportation (NYSDOT)
relies on a spectrum of tools to achieve a cost-effective and safe pile
foundation design. Dynamic pile testing (DPT), also referred to as
the Case Method, is used as a nondestructive quality control on deep
foundations, with its primary purpose to determine the support
capacity of the pile-soil system.
The static pile load test (SPLT) still is perceived by many engineers
as the most reliable method for verifying the capacity (quality) of deep
foundations. An SPLT performed on a pile loaded to failure provides
data about the magnitude and distribution of the ultimate static soil
resistance. However, it is a lengthy process to design and construct a
reaction frame and reference system with telltales. In contrast, the
DPT avoids most of the contractor delays associated with an SPLT
and, consequently, much of the cost involved with verifying the quality of the driven pile foundation. Additionally, it gathers dynamic
information about the hammer-pile system as the pile is driven.
As a result, the pile driving analyzer (PDA) largely has replaced
the need for the SPLT on NYSDOT projects. It is used most often
for the following reasons:
•
•
•
•
•
Verifying capacity during initial driving and after setup;
Measuring pile driving stresses;
Detecting pile damage;
Monitoring hammer performance; and
Verifying wave equation analysis of piles (WEAP) assumptions.
The NYSDOT dynamic pile testing program is operated out of the
main office in Albany. Two PDAs are used to cover work for the
entire state. NYSDOT has used this technology since it was commercially available in 1975 and has owned and operated PDAs since
1978. Testing is conducted exclusively by properly trained engineers
under the guidance of a staff who have maintained a continuity of
PDA experience for more than 20 years.
New York State Department of Transportation, Geotechnical Engineering
Bureau, Building 7, State Campus, 1220 Washington Avenue, Albany, NY
12232-0863.
DESIGN PROCESS
Analysis
NYSDOT uses standard methods in the design of the soil support
capacity of individual piles. For cohesionless soils, borings are taken
with driven samples and an analysis is made by using the sampler
blow count, visual inspection of the samples, and all other available
information about the site. Methods presented by Nordlund (1) are
used to make an estimate of the pile’s static capacity in shaft resistance and end bearing, both during driving and after driving stops.
A check is made to ensure the pile foundation settlement criteria are
satisfied for the structure.
In cohesive soils, the design follows the same general approach
with occasional triaxial test information, except the shear strength
after remolding and regain in strength must be estimated. A method,
usually Tomlinson’s (2), is used to estimate the soil support capacity of the pile at time of driving and after setup. A check is made to
ensure that the estimated settlement of the foundation is within tolerable limits for the structure. In both cases, assumptions are made
regarding the behavior of the soil in its short-term and long-term
support capacity of the chosen pile.
With the pile type chosen and its length estimated, a check is
made for drivability with the WEAP program using hammers typical in New York. For sensitive soils, the WEAP analyses are run for
initial driving conditions and with strength gain of the soil. Should
unusual driving criteria be necessary, notes are placed in the plans
to assist the contractor in selecting a pile driving system.
Specifications
The NYSDOT standard specification has an item for dynamic pile
testing. The use of this item is decided a job-by-job basis. The test
requirement is written into the foundation design report for the project. A note is placed in the contract documents stating the frequency
and circumstances for each test, always with the stipulation that
additional tests may be ordered by the project engineer.
All tests as stated in the contract note are performed by representatives of the state of New York. NYSDOT geotechnical engineering personnel perform the testing and evaluation. This is followed
up immediately with a recommendation to the NYSDOT structures
division.
The standard specification requires the contractor to submit each
combination of proposed driving systems for approval. Only the
approved specific driving systems may be used, not an equivalent.
All piles are driven to the criteria established by NYSDOT, whether
derived from WEAP or PDA. The specification allows for rejection
of pile driving hammers if they do not meet expected operation
standards in the field.
118
Paper No. 98-1059
CHOOSING PROJECTS FOR TESTING
Not all pile installations warrant dynamic pile testing. In NYSDOT,
approximately 20 percent of pile supported structures have dynamic
testing on a representative sample of foundation piles. Most often,
testing programs are incorporated into the contract documents. However, on occasion the pile driving does not progress as anticipated and
thus tests are added during construction. Planned tests are performed
for several reasons, as described in the following sections.
Verifying Capacity During Initial
Driving and After Setup
Some projects are chosen for dynamic testing because the designer
has low confidence in the predicted pile-soil interaction. Some common soil conditions and structure designs that necessitate frequent
dynamic pile testing fall into the following categories.
Questionable Density of Sand and Gravel
NYS uses an analysis for friction pile foundations in sand and gravel
that relies on an estimate of φ, the angle of internal friction, of the
overburden soils. If the soil contains a significant amount of large
gravel, the standard penetration test can be unreliable as a tool for
estimating φ.
Judgment tempering the selection of φ, based on experience and
knowledge of the deposition process, may leave great doubt as to
what is an appropriate value for this parameter.
In the case of H-piles driven in similar deposits, the state of the
effective stress at the pile-soil interface could be highly unpredictable.
Because of these uncertainties in the design parameters, the engineer
may require a dynamic pile test to verify capacity. The strength gain
or setup in sand and gravel may not be a significant factor and a retap
of the piles most likely would be unnecessary.
Cohesive and Silty Soil
In cohesive and silty soil, resistance to the pile hammer during driving can be very low, largely because of the effects of remolding of
the soil and its loss of strength. The rate of strength gain as pore
pressure dissipates is not predicted easily but is estimated on the
basis of experience with similar soil deposits. In many cases, lowcapacity piles can be designed by using conservative values for
the strength of the soil and the adhesion value of the soil to the pile.
These often suffice for an economical design to support the structure
if the loads are small.
A pile encountering little resistance during driving often does not
instill confidence that it has adequate support capacity, although the
designer may have confidence that adequate strength gain will occur
with time. It is wise to provide field testing to verify the soil behavior and pile capacity. In this situation, a designer likely would specify that the pile be tested during the initial drive and subsequently
tested again (the pile is retapped) after a suitable waiting period.
In many cases, the setup time does not have to be long; often
24 hours is adequate to demonstrate sufficient strength gain. On
occasion, a waiting period more than 24 hours long is needed if a
more definitive measurement of capacity is needed. The need for a
TRANSPORTATION RESEARCH RECORD 1633
longer waiting period would be specified in the contract documents
so the contractor could bid accordingly.
Limited Foundation Redundancy with Pile
Capacity Uncertainty
Some lightly loaded substructures have very limited redundancy in
the foundation. A structure with integral abutments (single row of
piles) and pier bents falls into this category. If the integral abutment
piles are driven through variable soils without a well-defined bearing layer that can be reached at a reasonable depth, some check of
the pile capacity is prudent.
Measure Pile Driving Stresses
Piles Subject to Damage
Inspection personnel may be unable to detect pile damage located
below ground surface. Precast concrete piles are particularly susceptible to damage from tensile stresses, especially when the pile is penetrating layers of low end bearing resistance. In situations like this, the
engineer would specify dynamic testing of the first piles driv-en, after
using wave equation analysis to approve the hammer-pile-soil system.
The pile driving analyzer would be used during driving of the first
piles to verify the WEAP assumptions and to check driv-ing stresses
and detect possible damage to the pile during initial driving.
Detecting Pile Damage
Problems detecting pile damage are uncommon largely because of
controls placed on the construction operations specifically to prevent damage. When used to assess possible damage, dynamic pile
testing falls into the unplanned category. Typically, precast concrete
piles can suffer damage if driving is not controlled. Steel H-piles can
break on occasion at the splices because of improper welding procedures. In both cases, the dynamic test equipment may be able to
provide indications that damage had occurred.
Monitoring Hammer Performance
The PDA commonly is used to monitor hammer performance (energy
transferred to the pile top). This can be checked against the transferred
energy calculated for a given blow count and resistance in WEAP.
Preignition in a diesel hammer is when combustion occurs before
the ram strikes the anvil. This condition cannot be detected by general observations. Higher-than-normal combustion pressure develops, causing a cushioning between ram and anvil. The ram will rise
to the desired stroke without the required energy transfer to the pile.
The PDA will detect this condition readily.
Verifying WEAP Assumptions
The PDA should be used in addition to WEAP when an extended
model or complex analyses are available—for example, piles driven
with a follower and open-ended pipe piles.
Walton and Borg
APPLICATIONS
On occasion, when a thin-walled, closed-end pipe pile is driven in
soft to firm silty clay, the resistance to penetration is observed to be
far greater than expected. It has been conjectured that the dynamic
effect at the pile toe during driving creates a dilation-and-contraction
cycle on the soil pores. This dynamic contribution to penetration
resistance will not be present under static loading. The PDA can be
a valuable tool for diagnosing this condition.
On some projects, NYSDOT will conduct an SPLT. On one project for which prestressed concrete piles were used, the SPLT test
was rejected because of poor data. The contractor was given the
option to run the static test again or redrive the pile with dynamic
monitoring. The dynamic pile test was chosen and it resulted in cost
savings and avoided delays.
Frequently, pipe piles will be driven in silty material below
groundwater. To verify capacity at the estimated length, the pile is
restruck the day after it is driven. The silt may be so sensitive to a
dynamic impact that a significant amount of resistance is lost in the
first hammer blow. Often, this resistance is counted on to achieve
the required ultimate pile resistance. In these cases, an observed
blow count is insufficient and dynamic pile testing is needed.
Measurements taken with a PDA can be used to calculate the
resistance contribution from toe bearing versus the pile shaft. For
instance, where the pile toe may end up on steeply sloping rock, it
cannot always be counted on for resistance under a long-term static
loading. The pile could be load rated for just shaft resistance if adequate initial drive and restrike monitoring is done. However, if end
bearing was required with the toe of the pile driven to a steeply sloping rock surface, an SPLT would be more appropriate for capacity
verification.
A common limitation of the PDA is in calculating the ultimate
resistance of a pile in a low-toe-displacement situation. In the case of
a large-diameter concrete cylinder pile, the impact hammer may have
sufficient energy to progress the pile but not enough energy to reach
the toe displacement at which the ultimate end bearing is felt. Thus,
the ultimate resistance of the pile may be significantly underpredicted
and an SPLT test would be required.
INTERPRETATION
The PDA engineer should be well-trained and have access to engineers who are highly experienced in this discipline. It is important
to keep current on the latest trends, technology, and work done in
this field. Achieving good data quality in the field is the most important factor in gaining reliable results. It is important that the test
results agree with the boring information, the pile static analysis,
and the SPLT, if available.
Paper No. 98-1059
119
The Case Pile Wave Analyses Program (CAPWAP) is a computer program that refines the PDA data through wave-matching
techniques. Certain assumptions are made with the PDA about the
dynamic behavior of the soil in calculating the static contribution
to driving resistance. The dynamic contribution to driving resistance is accounted for, and calculations for static shaft and toe
resistance acting on the pile are made.
Most PDA methods for calculating the static pile capacity require
input of an assumed soil damping factor. The magnitude of the
damping factor may significantly affect the calculation for static soil
resistance. Therefore, it is advised in most cases to run CAPWAP
analyses of the data. The current state of practice in NYSDOT is to
perform CAPWAP analyses for all tests.
If the pile is rejected because of inadequate capacity, one of three
things generally will occur:
1. Wait longer with additional restrike test (usually applies to
piles in cohesive soils).
2. Splice and drive the pile deeper with DPT monitoring.
3. Drive additional piles in the footing.
On acceptance of the pile, driving criteria is established for the
remainder of the substructure on the basis of the blow count and
hammer operation of the test pile.
CONCLUSIONS
Dynamic pile tests are an effective tool for ensuring quality deep foundations that are installed by using impact driving methods. They are
an extension of the design process whereby assumptions made about
capacity are verified using measurements taken while driving the pile
into the ground. They have the added ability to monitor pile stress and
to detect damage, as well as to measure the performance of the driving system. Finally, in those cases in which the WEAP computer program cannot appropriately model the hammer-pile system, or in
which soil information is inadequate, dynamic testing in the field
serves to validate the expected performance of the deep foundation.
REFERENCES
1. Nordlund, R. L. Bearing Capacity of Piles in Cohesionless Soils. Journal
of the Soil Mechanics and Foundation Division, ASCE, SM-3, May 1963.
2. Tomlinson, M. F. The Adhesion of Piles Driven in Clay Soils. Proc., 4th
International Conference of Soil Mechanics and Foundation Engineering, London, 1957.
Publication of this paper sponsored by Committee on Foundations of Bridges
and Other Structures.
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