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Letter Concerning Dynamic Methods

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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