Single Pile Dynamic Stiffness

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    11 99 SSIINNGG LLEE PPIILLEE

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    19.0 INTRODUCTION

    We will commence our consideration of the dynamic behaviour of pilefoundations by reviewing information from field tests on single piles. This will befollowed by consideration of the dynamic stiffness of single piles in this chapter,after which the dynamic response of pile groups will be covered in Chapter 20; asthere are many similarities between the material presented in this chapter and thatin the next there are some forward references to Chapter 20.

    19.1 Dynamic tests on prototype scale piles and pile groups.

    19.1.1 Blaney and O'NeillBlaney and O'Neill (1986a and b) describe a field investigation of the forced

    vibration response of a steel tube pile driven into a deposit of clay. The pile hada 273 mm in outer diameter and 254.5 mm internal diameter and penetrated 13.4m into the clay. The details of the test set up and brief soil profile information isgiven in Figure 19.1. By means of a vibrator attached to the extension of the piletube above the ground surface it was possible to apply a sinusoidal excitation at

    various frequencies to the pile. The response of the pile at a range of fixed force

    amplitudes and a sweep of frequencies is shown in Figure 19.2. Of significance

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    is the normalised dynamic lateral displacement profile with depth being similar tothe static profile.

    Note that the natural frequency decreases slightly after the 600 lb sweep, a

    possible explanation is the formation of a gap adjacent to the pile shaft near theground surface.

    Figure 19.1 Details of the pile-mass system and soil profile investigated byBlaney and ONeil (after Blaney and ONeil (1985)) (1 foot (1) 305 mm).

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    Figure 19.2 Dynamic and static response of the Blaney and ONeil test pile(after Blaney and ONeil (1985)).

    19.1.2 Jennings et al Jennings et al (1985 and 1986) report on dynamic tests on a pair of 450mmdiameter piles driven into saturated silty sands. The spacing between the piles issuch that the interaction between them is assumed to be negligible, hence thiscase history is discussed in this chapter and not the next which deals with pilegroups. Details of the soil profile are reproduced in Figure 19.3 and the details of the piles in Figure 19.4. Self-weight was sufficient to for the pile shells topenetrate to a depth of 5m, subsequent gentle tapping was all that was requiredto get them to the required depth of 6.75m. After cleaning out, reinforcement

    was placed in the shells and concrete poured. Dynamic and slow cyclic loads were applied to the piles. Cyclic loads were applied by means of a jack mounted

    between the piles 1.35 m above ground level, and the dynamic loads with a shak-

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    Figure 19.3 Field data for the Central Laboratories twin pile test (after Jenningset al (1985)).

    Figure 19.4 Details of the Central Laboratories test piles (after Jennings et al(1984)).

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    Figure 19.5 Response of the Central Laboratories piles: upper: cyclic loaddeformation loops, lower: resonance curve (after Jennings et al (1984)).

    ing machine mounted on top of one of the piles. Initial testing involved dynamicshaking of the west pile. This was followed by slow cyclic loading at a rate of about one cycle per hour with the following sequence of maximum loads percycle in each direction: 10kN, 20kN, 40kN (2 cycles), 80kN (2 cycles), 120kN (2cycles), 160kN (2 cycles), 200kN (2 cycles).

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    Figure 19.5a has the force deflection results for the slow cyclic loading andFigure 19.5b has the resonance curve for the dynamic test on the single pile. It isclear that, as with the Blaney and O'Neill pile, there is a distinct natural frequency

    when the pile is loaded dynamically at low levels of excitation. It was apparent

    from ground surface observations during the loading that high pore waterpressures were generated adjacent to the pile shafts. When piles are embedded insaturated sands gaping cannot occur along the pile shaft but there is,nevertheless, a dynamic degradation in the pile performance because of areduction in stiffness of the silty sand as a consequence of the build-up in pore

    water pressure.

    Figure 19.6 Moment profiles during slow cyclic loading of the CentralLaboratories test piles (after Jennings et al (1984)).

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    In attempts to match the observed moment profiles, shown in Figure 19.6, a Winkler model with k increasing uniformly with depth was adopted, i.e. k = n hz. The moment profiles, determined from the strain gauge readings, were the basisfor the inference, using a Winkler spring approach, that towards the bottom of

    the piles nh 12 but near the ground surface a value of 6 is more appropriate. This was interpreted as evidence for softening of the upper sand layers due to thedevelopment of positive pore pressures.

    19.1.3 Sadon et alField experiments were conducted at a site in Auckland with driven steel tubepiles embedded in Auckland residual clay. Site and soil profile details are givenby Sadon et al (2010) and Sadon (2011). The first batch of tests used aneccentric mass shaking machine to excite the foundations with sinusoidaloscillations at a range of frequencies. Although successful there are limitationsto this approach for the following reasons. First, a given level of excitation

    force cannot be obtained until the shaker frequency has been increased fromzero to the frequency required to generate the required force. Second, theresponse of the system is measured under steady state excitation at a fixedfrequency. In this way what is obtained from the use of a shaking machine isnot representative of what happens during earthquake excitation.

    An alternative is the use of snap-back testing. This test is simpler than using aneccentric mass shaking machine. It gives the response of the system to oneimpulsive excitation instead of continuous excitation; it is more representativeof what occurs during an earthquake. An added bonus is the static load-deflection curve obtained during pull-back phase of the test. The initial pull-

    back can generate a force of comparable magnitude to the maximum force thatcan be produced by the shaking machine used.

    Figure 19.7 shows the pile head with the eccentric mass shaker in place set-upfor steady state sinusoidal excitation (top) and for snap-back testing (bottom).

    Figure 19.8 gives frequency curves for the piles before and after high levelsinusoidal excitation. These are obtained from the small amount of out of balance in the shaking machines; there is no added mass. Before high levelshaking the natural frequency is largest; after the high level shaking the systemstiffness has deteriorated so the indicated natural frequency is reduced. Since

    the piles are embedded in clay the main reason for the reduction in lateralstiffness is likely to be the opening of a gap near the top of the pile shaftbetween the surrounding soil.

    Figure 19.9 shows hysteresis loops recorded during high level sinusoidalexcitation. A line defining the lateral stiffness of the pile head obtained using the small strain shear modulus of the soil is plotted. It is seen that hysteresisloops recorded during forced vibration at +- 10 kN indicate that a smaller soilmodulus is operational. Furthermore, when the forcing amplitude is +- 60 kN,then the operational stiffness is about one third to one quarter of that definedby the small strain shear modulus of the soil.

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    Figure 19.7 Pile with eccentric mass shaking machine attached andinstrumentation installed. Top: Set-up for sinusoidal excitation; bottom: set-up for snap-back testing (the eccentric mass shaker now provides pile-head mass).

    Figures 19.10, 11 and 12 give results of a series of snap-back tests on a givenpile. First, Figure 19.10 gives the load-deformation curves generated during thepull-back phase. Figure 19.11 gives hysteresis loops during the vibrationfollowing the snap release. During pull-back the load applied to the pile ismeasured directly through the load cell shown in the bottom part of Figure19.7. During the free vibration the loads are calculated from the recordedstrains in the strain gauges on the pile shaft above the ground surface.

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