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Proceedings of the 18

th

International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013, volume 6, 2016

The history of cyclic loadings applied to foundations, whether

calculated or measured, comprises a succession of variable loads

with an irregular amplitude and somewhat random distribution.

However, the cyclic tests that are feasible to run in the laboratory

on material samples have normally been designed as series of

cycles with a regular amplitude and constant period. A software

application called "Cascade", built to transform a single random

series of cyclic loads into a succession of ordered, constant-

amplitude series, was developed within the scope of this project.

This software employs cycle counting methods, of the "rain

flow" type (ASTM E 1049-85, NF A03-406, 1993). The concept

of damage, as intended by Miner, is applied herein to estimate

material damage on the basis of S-N type curves (also so-called

"Woelher curves"), as experimentally obtained by bringing to

failure the samples subjected to series of cycles with a constant

stress amplitude.

8.2.2

The SOLCYP project's experimental base

Pile tests were conducted at two experimental sites in northern

France. The first campaign took place at MERVILLE, where the

stiff and very highly overconsolidated Flanders clay was present

as of a depth of 3 m. Ten test piles were installed, featuring four

closed ended driven tubular metal piles, four CFA-type bored

piles, and two screwed piles. All piles were 13-m long with

diameters of either 406 mm (for the driven piles) or 420 mm

(bored piles). They underwent standard incremental static

loading tests, rapid monotonic loading tests and series of cyclic

tests including high-amplitude tests leading to failure at a small

number of cycles, and lastly low-amplitude tests run until 10,000

cycles. All loading modes were applied (tension, compression,

one-way, two-way). The main results were published in Benzaria

et al.

(2012, 2013a).

The second site, at LOON-PLAGE near Dunkirk, contained

dense sands. Two driven piles were set up along with five bored

CFA piles, as both types featured the same characteristics as

those at MERVILLE but with different lengths (10.5 m for the

driven piles and 8 m for the bored piles). The loading program

was also similar (Benzaria

et al.

, 2013b).

Many test series on instrumented model piles were completed

in both Fontainebleau sand and Speswhite clay. These tests were

conducted in the centrifuge at the Nantes-based IFSTTAR lab

(formerly LCPC). The objective targeted was to establish cyclic

stability diagrams in both types of reference soils (i.e. sands and

clays) and for both types of piles under study (driven and bored)

by considering a wide array of initial conditions (density,

consistency, consolidation) and loading modes. The first results

were the subject of publications (Guefresh

et al.

, 2012; Puech

et

al.

, 2013); this type of approach led to confirming the

representativeness of data acquired during

in situ

tests and then

extending their range of validity.

A third experimental approach consisted of performing tests

on very heavily instrumented model piles in the large calibration

chamber at the 3S-R Laboratory in Grenoble. These tests in

Fontainebleau sand, carried out in collaboration with Imperial

College London, yielded some outstanding information on the

mobilization of friction at the soil-pile interface and how it

evolves with changes in intensity and number of cycles (e.g. see

Tsuha

et al.

, 2012; Silva

et al.

, 2013).

8.2.3

Responses of piles to cyclic loadings

Figure 17 illustrates the type of behavior observed at the

MERVILLE site with highly overconsolidated clay. This figure

depicts the load-displacement relationship at the head of the

bored F2 pile loaded in compression. The ultimate load in

compression Q

uc

, as measured via a standard static test on pile

F1 (which is identical to F2), equaled 900 kN. The F2 pile,

which did not undergo any preliminary loading, was initially

subjected to a series of three cyclic loadings exceeding 3,000

cycles. The first two series (CC1 and CC2) did not cause any

significant permanent pile displacement. The third one (CC3),

characterized by a maximum loading Q

max

on the order of

800 kN, however generated fairly sizable permanent

displacements (nearly 20 mm). The test was stopped after 3,000

cycles and followed by a rapid static loading (CR1), which

indicated a post-cyclic capacity of 900 kN. Next, seven series of

cycles were applied. Series CC4 through CC7 did not bring

about any permanent pile head displacement upon completing

1,000 cycles per series. (Let's note that these tests were

arbitrarily separated to allow for visualization.) Tests CC8

through CC10, which once again reached a maximum force of

800 kN, generated permanent displacements that quickly began

to accumulate (each series was run for fewer than 100 cycles).

Post-cyclic capacity remained on the order of 900 kN (tests CR2

through CR4).

Fig. 17: Force-displacement relations at the head during

one-way compression tests on the MERVILLE F2 bored pile

(according to Benzaria

et al.

, 2013a)

This type of behavior was found during all tests carried out at

Merville, regardless of either pile type (bored, screwed, driven)

or loading mode (pure compression, pure tension, alternating

tension/compression). In sum:

-

A critical operating threshold is in place;

-

For a maximum load Q

max

below the given threshold, the pile

is stable (no significant permanent displacement, constant

cyclic stiffness) even with a high number of cycles (N>1000);

-

Once this threshold is reached, permanent displacements are

generated, and cyclic failure quickly ensues, typically in fewer

than 100 cycles;

-

The threshold is high in the regime of one-way loadings

(80%-90% of Q

us

) but decreases in the regime of two-way

loadings (see Fig. 19);

-

The post-cyclic capacity is not affected by prior cyclic

loadings.

The behavior observed at the LOON-PLAGE dense sand site

is altogether different. Figure 18 shows the response of two

identical bored F4 and F5 piles. The F4 pile underwent a

standard static loading test, yielding an ultimate reference load

Q

uc

=1,100 kN. The F5 pile was subjected to a cyclic loading

characterized by a Q

max

value of ~0.62 Q

uc

. The pile very quickly

accumulated permanent displacements (3% relative displacement

after just 14 cycles). The test was halted and the cyclic amplitude

substantially reduced (Q

max

~0.35 Q

uc

). The pile continued to

accumulate displacements (14 mm over 5,000 cycles).

From a general perspective, these observations were noted:

-

Bored piles are highly sensitive to cyclic loadings;

-

The post-cyclic capacity was significantly affected by the

cyclic loadings;

-

The cyclic failure criteria in compression need to be defined

in terms of tolerable displacements. An analysis of all tests

performed has led to defining the cyclic failure for a 3%

relative displacement.

Volume 6 - Page 90