Webmaster's note: This letter is reproduced at the
request of Dr. Mark Svinkin, and
does not necessarily reflect the views of the webmaster, vulcanhammer.net
or The Wave Equation Page for Piling.
June 25, 2001
Dr. Michael Holloway
Chair
DFI Committee on Deep Foundations Testing and Implementation
Dear Mike:
You wrote in Fulcrum magazine that "The
committee's objective is to demystify testing methods and their
applications in foundation design and construction practice". In
the frame of this objective, I am sending you my opinion regarding the
project on application of dynamic testing results for rewriting AASHTO
Deep Foundation Specifications for the year 2001, the NCHRP Project 24-17,
"LRFD Deep Foundation Design". The basic ideas of the project
are presented in Paikowsky and Stenersen (2000).
A LRFD was originated in the USSR in 1920s with peak usage in 1960s. As any other method, the LRFD has its advantages and disadvantages. The LRFD is applicable to calculations of steel and concrete structures. Due to tolerance of plastic deformations, the LRFD provides more economical design solutions of certain types of structures. For example, plastic design is ideally suited to continuous beams and frames, and is not normally used with single-span beams. While the LRFD allows for cracks in concrete structures, such approach is not acceptable for some concrete structures. That was one of the reasons why the LRFD was not used for foundations design in the Soviet Union.
There is no doubt that learning Russian experience
with the LRFD would be very beneficial for the Project. According to
Paikowsky and Stenersen (2000), there are references only to Russian
initial publication on the subject. So, Russian experience was not
actually learned.
Applicability of the LRFD to bridge structures does
not mean that this method is appropriate for calculation of bridge
foundations. Nevertheless, there are different opinions regarding the
LRFD concept. This approach may be accepted or not, but without doubt,
good quality data would have to be used for consideration of the
application of LRFD concept to foundations. It is important to reveal
true resistance factors for deep foundation design. However, the
resistance factors obtained in the Project are founded on wrong
databases. These databases use comparison of pile capacity values that
are incompatible from the point of verification of pile dynamic testing
and analysis.
The Project is founded on PD/LT 2000 Database made for comparison of pile capacities obtained with dynamic methods and static loading tests. The principles, and the major part of this Database, were created for the Research Project, Paikowsky et al. (1994), where one more dynamic formula was suggested for determination of pile capacity. There are problems with PD/LT 2000 Database in evaluation of accuracy of Dynamic Formulas and Dynamic Testing.
Determination of pile capacity by dynamic formulas is
the oldest and most frequently used method. There is a great number of
dynamic formulas available with different degrees of reliability.
Dynamic formulas have been criticized in many publications.
Unsatisfactory prediction in pile capacity by dynamic formulas is well
characterized in FHWA Manual for Design and Construction of Driven Pile
Foundations, Hannigan et al.(1996): "Unfortunately, dynamic
formulas have fundamental weakness in that they do not adequately model
the dynamics of the hammer-pile impact, the influence of axial pile
stiffness, or soil response. Dynamic formulas have also proven
unreliable in determining pile capacity in many circumstances. Their
continued use is not recommended on significant projects".
However, there is an attempt to breathe new life into
dynamic formulas. Paikowsky and Chernauskas (1992), Paikowsky et al.
(1994) and Paikowsky and Stenersen (2000) have suggested one more energy
approach using dynamic measurements for the capacity evaluation of
driven piles. Liang and Zhou (1997) have concluded regarding this
method: "Although the delivered energy is much more exactly
evaluated, this method still suffers similar drawbacks of ENR".
Authors of a new dynamic formula used the ratio, also
called index K, of the static load test capacity to the predicted
capacity or vise versa to evaluate performance of the Energy Approach
and dynamic testing. However, such a ratio is irrelevant for
verification of dynamic formulas and dynamic testing (DT) results for
two reasons: first, dynamic testing methods yield pile capacity only for
the time of testing (Rausche et al. 1985), and second, the pile capacity
from static load test (SLT) is considered as a constant value which is a
major error.
Paikowsky et al. (1994) and Paikowsky and Stenersen
(2000) use an assumption that accuracy of Dynamic Formulas are
independent of the time between DT and SLT. However, SLT as well as DT
yields the pile capacity at the time of testing (Svinkin et al. 1994;
Svinkin 1997, 1998; Svinkin and Woods, 1998). In mentioned papers, by
way of illustration, results of DT and SLT are shown for two identical
cylindrical, 1372 mm x 127 mm, prestressed concrete piles, TP1 and
TP2. Each of the piles TP1 and TP2 was tested 2, 9 and 22 days after the
end of initial driving. The difference was that three restrikes were
made for TP1 and three SLTs were made for TP2. Pile capacity from three
SLTs was a function of time as well as pile capacity obtained from DT.
These tested data help to explain the causes of unsatisfactory
prediction in pile capacity by dynamic formulas. Dynamic formulas using
maximum energy, pile set and maximum displacement from DT do not take
into account the time between SLT and DT. In the case of a few SLTs made
on one pile, like three SLTs performed on pile TP2, what would be the
reliability of pile capacity prediction by the energy approach methods?
Which SLT should be taken for comparison? Currently, there are no
answers to these questions. Nevertheless, Paikowsky and Stenersen (2000)
assert that the Energy Approach Formula is ideal for construction and
better than Signal Matching technique, e.g. CAPWAP. There is no
theoretical and experimental confirmation of such conclusions. It is
necessary to utilize other appropriate way for comparison of results of
the Energy Approach Formula and DT which use data from the same dynamic
measurements. Such comparison was made in the frames of preparation of
FHWA-GRL Database. The results obtained were very poor and confirmed
that the Energy Approach with dynamic measurements cannot yield reliable
prediction of pile capacity. Statistical analysis itself cannot reveal
good results and replace engineering judgment if comparison of measured
pile capacities is incorrect.
Dynamic testing followed by a signal matching
procedure has obvious advantages in determining pile capacity at any
time after pile installation. Since dynamic testing is often used to
replace the static loading tests, it is important to ascertain the
adequacy of both SLT and DT. Paikowsky et al. (1994) and Paikowsky and
Stenersen (2000) use a wrong approach for comparison of DT and SLT.
Design methods predict pile capacity as the long term
capacity after soil consolidation around the pile is complete.
Independently of the time elapsed between the driving of the test pile
and the static loading test, the ratio of the predicted ultimate load to
the measured ultimate load from static loading test is used for
approximate evaluation of the reliability of design methods, Briaud and
Tucker (1988). According to the traditional approach, the main criterion
for assessment of the pile capacity prediction based on dynamic
measurements is the ratio of capacities obtained by dynamic and static
tests or vice versa.
It is necessary to point out that a ratio of DT/SLT
or vice versa, taken for arbitrary time between compared tests, is not a
verification of dynamic testing results. It is well-known that dynamic
testing methods yield the real static capacity of piles at the time
of testing, Rausche et al. (1985). This is not a predicted value.
Moreover, the static capacity from SLT is considered as a unique
standard for assessment of dynamic testing results. Unfortunately, that
is a major error. As a matter of fact, pile capacity from Static
Loading Tests is a function of time and the so-called actual static
capacity from SLT is not a constant value. As it was shown before, SLT,
as well as DT, yields a different pile capacity depending on the time of
testing, as measured after pile installation.
For a few separate piles, it is possible to find
published information regarding the time between static and dynamic
tests. However, for the general case of assessment of reliability of the
DT, the ratio of restrikes to SLT results has been considered for
various pile types, soil conditions and times of testing lumped
together, Svinkin (2000, 2001). What is the real meaning of such
mixture? Nobody knows. It is not a verification of dynamic testing at
restrikes and it is not assessment of real setup factor because
everything is lumped together without taking into account the time
between different tests. Such a comparison of the pile capacities from
SLT and DT is invalid for piles driven in soils with time-dependent
properties because the soil properties at the time of DT do not
correspond to the soil properties at the time of SLT i.e. soil
consolidation is taken into account for restrikes using the DT but is
not in the SLT.
Static Loading Tests and Dynamic Testing present
different ways of determining pile capacity at various times after pile
installation. The adequacy of SLT and DT have to be confirmed by proper
correlation of time. Due to the consolidation phenomenon in soils,
comparison of SLT and DT can only be made for tests performed
immediately one after another. In practice, it is sometimes difficult to
make two immediately successive tests, but nonetheless the time
difference between both comparable tests should not exceed 1-2 days
during which soil setup changes only slightly.
Also, it is necessary to point out that assertion
about independence of soil damping of soil type is incorrect. The
damping coefficient in sandy soil is substantially less than the damping
coefficient in clayey soils, but the latter is close to the damping
coefficient in saturated sandy soils, Svinkin (1995a, 1995b, 1996a,
1996b, 1997).
So, PD/LT 2000 Database in the Project has no common
and engineering sense and its conclusions are misleading. It is clear
that an AASHTO mandatory document based on false assumptions and
misleading results would be a disaster for geotechnical community. I
believe that situation with the Project "LRFD Deep Foundation
Design" should be considered by the Special Committee of Congress.
My opinion has received support from participants of
prestigious conferences, papers reviewers, authors of discussion papers, Journal
of Geotechnical and Geoenvironmental Engineering, and Ground
Engineering Magazine. Some of my publications regarding
determination of pile capacity by dynamic methods you can find by
clicking here.
Best regards,
Mark Svinkin
References
- Briaud, J.L. and L.M. Tucker 1988. Measured and predicted axial response of 98 piles. Journal of Geotechnical Engineering, ASCE, 114(9): 984-1001.
- Hannigan, P.J., Goble, G.G., Thendean, G., Likins,
G.E. and Rausche, F. 1996. Design and construction of driven pile
foundations. Workshop manual, Publication No. FHWA-HI-97-014.
- Liang R.Y. & J. Zhou 1997. Probability Method
Applied to Dynamic Pile-Driving Control. Journal of Geotechnical
Engineering, ASCE, 123(2): 137-144.
- Rausche, F., G.G. Goble & G. Likins 1985.
Dynamic determination of pile capacity. Journal of Geotechnical
Engineering, ASCE, 1985, 111(3): 367-383.
- Paikowsky S.G. and Chernauskas L.R. 1992. Energy
approach for capacity evaluation of driven piles. F. Barends
(ed.), Proceedings of Fourth International Conference on the
Application of Stress-Wave Theory to Piles, A.A. Balkema, The
Hague, 595-601.
- Paikowsky S.G., Regan J.E., and McDonnell J.J.
1994. A simplified field method for capacity evaluation of driven
piles. Publication No. FHWA-RD-94-042.
- Paikowsky S.G. and Stenersen, K.L. 2000. Keynote
lecture: The performance of the dynamic methods, their controlling
parameters and deep foundation specifications. Proc. Conf. on
Application of Stress-Wave Theory to Piles, Sao Paulo, Brazil, A.
A. Balkema: 281-304.
- Svinkin, M.R., C.M. Morgano & M. Morvant 1994.
Pile capacity as a function of time in clayey and sandy soils. Proc.
Fifth Inter. Conf. and Exhibition on Piling and Deep Foundations,
Bruges, 13-15 June: 1.11.1-1.11.8. Rotterdam: Balkema.
- Svinkin, M.R. 1995a. Pile-soil dynamic system with
variable damping. Proc. 13th International Modal Analysis
Conference, IMAC-XIII, Beyond the Modal Analysis, Nashville, 13-16
February, 1: 240-247, Bethel, Connecticut: SEM.
- Svinkin, M.R. 1995b. Soil damping in saturated
sandy soils for determining capacity of piles by wave equation
analysis. Proc. DFI Annual Member's Conference, Charleston, South
Carolina, 16-18 October: 199-216, Englewood Cliffs: DFI.
- Svinkin, M.R. 1996a. Discussion of 'Setup and
relaxation in glacial sand' by York et al., Journal of Geotechnical
Engineering, ASCE, 122(4): 319-321.
- Svinkin, M.R. 1996b. Soil damping in wave equation
analysis of pile capacity. In F. Townsend, M. Hussein &
M. McVay (eds.), Proc. Fifth Inter. Conf. on the Application of
Stress-Wave Theory to Piles, Orlando, 11-13 September: 128-143,
Gainesville: University of Florida.
- Svinkin, M.R. 1997. Time-Dependent Capacity of
Piles in Clayey Soils by Dynamic Methods. Proc. XIVth Inter. Conf.
on Soil Mechanics and Foundation Engineering, Hamburg, 6-12 September,
2: 1045-1048, Rotterdam: Balkema.
- Svinkin, M.R. & R.D. Woods 1998. Accuracy of
determining pile capacity by dynamic methods. Proc. Seventh Inter.
Conf. and Exhibition on Piling and Deep Foundations, Vienna, 15-17
June: 1.2.1-1.2.8, Rickmansworth: Westrade Group Ltd.
- Svinkin, M.R. 1998. Discussion of 'Probability
method applied to dynamic pile-driving control' by Liang & Zhou, Journal
of Geotechnical and Geoenvironmental Engineering, ASCE, 122(4):
319-321.
- Svinkin, M.R. 2000. Time effect in determining pile
capacity by dynamic methods. Proc. Conf. on Application of
Stress-Wave Theory to Piles, Sao Paulo, Brazil, A. A. Balkema:
35-40.
- Svinkin, M.R. 2001. Pitfalls in wave equation
analysis of pile capacity. Proc. Inter. Conf. on Computer Methods
and Advances in Geomechanics, A.A. Balkema, V. 2, pp. 1495-1499.
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