SYNOPSIS

INTRODUCTION
An experimental study of the load-settlement behaviour of a model pile in sand, in a calibration chamber (CC), and subjected to static and dynamic loading was performed at Ghent University by the main author. The goal was to clari- fy, at least qualitatively, the fundamental mechanism of cyclic pile-soil interaction behaviour, focussing on the pile shaft friction mobilisation. Major previous suggestions, Ina (1992) - fig. 1 a,b,c, were mainly related to cohesive soils. The tests described have refer to pure dry quartz sands. o Tests with no failure for N > 10,000 McAnoy et al.(1982) Δ F  Test with failure at N = 564 ● Tests of Puesch(1982) - no failure x Stevens(1978) - no settlement Stevens(1978) - continuing settlement → F  Stevens(1978) - plunging failure ▼ F  Poulos(1981) - model tests, failure after 3-180 cycles工F  Karlsrud et al.(1986) - failure after 100 cycles (マalminium pile : d=19 mm, 1=374 mm jacked into sandy clay in CC T=28 s (b) steel pile : d=191 mm, 1=11.3 m driven into NC silty clay T = 120 s
EQUIPMENT AND TEST PROCEDURE
Fig. 2 shows the setup of the pile loading test. CPT tests are also carried out on identical sand specimens in the CC. Each sand specimen has a 1,4 m effective diameter instru¬ mented ring system and 2.1 m effective height. The ring system was removed in the lateron tests, because of hand- ling difficulties. The specimen is vertically loaded by 4 oil jacks on the rigid top plate. This overburden pressure is kept constant throughout the test. The model pile or penetrometer is jacked by means of a loading frame mounted on a reaction beam. The steel top plate has 5 holes : one at centre for pile loa- ding test or cone penetration test. There are also 4 side holes located as shown in this figure. Dilatometer tests and some cone penetration tests were performed out of the centre. The sand specimens were prepared by a carefull layered tamping method using the undercompaction principle. An amount of sand, equivalent to 0.11 to 0.12 m of the heightfor each layer, is discharged as evenly as possible through a flexible tube. The tube has a set of perforated plates in its tip so that the sand drops on a previous layer in loose condition irrespectively of the drop height. After flattening smoothly the surface of the layer, it is tamped in a severely standardised way and procedure by a rod of 9 kg and diameter 0,30 m. The achieved relative density of the overall sample becomes about 65 % and is very repro- ducible. In order to identify the initial state of the specimen, pene¬ tration tests, CPT ; DMT ; and LSCT, were conducted in the chamber. The results also provide information of the representative strength and deformation properties of the specimen. The test conditions are assembled in table 1. No notable influence on the results of different location, diffe¬ rent side boundary, different rate and loading history was observed.
Some cone penetration test results are shown in fig. 3. The cone resistance q, is quickly developed and reaches some plateau in between 0.4-1.6 m of depth. Those con¬ stant q e values correspond very well to the indicated relati¬ ve density, and ranges between 11-14 MPa. The local skin friction f, value also corresponds to the expected values but tends to increase slightly with depth. The discontinuities at 1 m of depth are due to the stopping of the test for installing a second CPT tube. Dilatometer test results are assembled in fig. 4. The indexes proposed for assessing a deposit are plotted from the test results. The peak TXC-shear angle of the sand at the test condition ' was φ = 39° and the maximum shear modulus at very low strain Gmxx = 80 MPa. Regarding the initial state of the specimen, the uniformity of the specimen can be considered as good both in vertical and lateral direction. However, the stiffness of the speci¬ men is likely to slightly increase with depth. The derived lateral stress coefficient at rest Ko implies that the speci¬ men is apparently only in a lightly overconsolidated conditi- on, perhaps due to sand tamping in the rigid boundary calibration chamber. Based on many correlations in literatu- re between penetration test results and soil properties, the original strength and deformation properties of the speci- men could easily be evaluated for this pure sand, fig. 5.
.The model test pile, table 2, carried five load cells at equal interdistance, from top to top. The diameter ratio of cham- ber to pile became about 40. For the "smooth" pile (S), the stainless steel was left bare ; for the "rough" (R) pile, the shaft was coated with coarse sand.
Regarding the test pile installation and testing procedure three parameters were considered : pile shaft surface roughness (S or R), pile installation method (displacement Qmax (D) or buried (ND), fig. 6) and the cyclic load level Qua (fig. 7). (a) (b) Non-displacement pile  (burried pile) pile setup on the sand (2) initial deposit with pile (3) pressurizing (o'... = 100 kPa) and stabilizing (4) (5) pile losding : - initial static loading (lst day) - cyclic loding (2nd day) - final static loading (immediatelyafter cyclic loading) All loading tests were performed in a load-controled mode. The cyclic loading wave period was set to 6 sec. The summary of the performed pile loading test parameters is given in table 3. For the interpretation of the test results, the residual loc¬ ked stresses due to pile installation, especially in the case of displacement piles, were measured before each loading test. Such residual stresses at the pile tip varied typically around 20 kPa for SD, 10 kPa for RD and about 5 kPa for SND piles.
The unit toe resistance q, at s/d = 10 %, 30 % and 100 % are not surprisingsly very much in accordance with the relevant q, values. On the other hand, the q, value for non¬ displacement (buried) pile still heavily depends on the settlement level even after the conventional s/d = 30 % - value. These observations coincide with many experimen¬ tal test results both in model and prototype piles. The rough piles show a peak value for the unit shaft resis- tance at about 10 % of the relative settlement, whereas it is not clear at all for the smooth piles, due to interface slip. Intuitively, the observed pile shaft behaviour is similar to the interface simple shear test results when implementing different roughnesses of the shaft. For the RD piles, a clear peak appears at much larger displacement with much higher shear resistance than for SD piles. For SD piles, the unit shaft resistance is abruptly flattened due to the inter- face slip.
TEST RESULTS OF THE CYCLIC LOADING ON THE MO-
DEL PILES
Referring to the fig. 7b and the table 3 data, the most interesting picture of the cyclic load settlement behaviour is coming from the analysis of the cycle-by-cycle pile head displacements (δ cko), The (δ cyc) one-cycle-displacement (fig. 7b) as a function of the "cyclic stiffness" of the model-pile soil system. From the results on fig. 10, it can be derived that in our tests this "cyclic stiffness" is equivalent to about 25 kN/mm, which is in agreement with the initial static pile load-settlement stiffness (fig. 8 and 9). For RD plies, the cycle by cycle displacement (fig. 11) steadily decreases with cycles up to a certain number of cycles depending on the applied cyclic load level. If the applied cyclic load is considerably large, the cycle-by-cycle displa- cement only over a few cycles slightly decreases, whereaf- ter it increases drastically. This figure strongly reflects a soll "creep" behaviour for a pile behaviour under one-way cyclic loading, in clay soils. As expected, a very different behaviour comes out from the SD pile tests (fig. 12). Although a drop of the cycle-by- cycle displacement occurs occasionaly at the beginning of the loading, it mostly increases steadily. If the pile behaviour is predominantly governed by the lateral stresses acting on the pile shaft, we can anticipate some changes of the shaft resistance during cyclic loading (fig. 13 a,b). The figures indicate the unit shaft resistance in the upper half of the pile, varying with the accumulated displacement. Under a highenough cyclic loadlevel, the unit shaft resis- tance for RD-piles increases up to 4-5 mm of overall settle¬ ment, then gradually decreases with increasing displace¬ ment. On the contrary, under a lower cyclic load, the unit shaft resistance drops clearly at the beginning depending on the cyclic load level, followed by a slight recovery, the shaft resistance becoming almost constant at the end. The SD-piles' unit shaft resistance drops in all cases parti- culary at the beginning of the loading, then tends towards or stable position. Both the unit toe resistance and the unity shaft resistance before cyclic loading and after cyclic loading are compared in figure 14. Regarding the unit base capacity, almost no influence of static preloading for RD piles is measured, while q, is only slightly degraded for SD piles. On the contrary q, increases dramatically for buried piles, because of strong settlement dependency of q, and some compaction effect underneath the pile toe in this type of pile installation. With respect to the unit shaft resistance, the ratio of resistance before and after cyclic loading depends on the applied cyclic load level on the RD pile. Under a large cyclic load level the static f , is finally degraded. On the contrary, applying a small cyclic load level on RD piles is preferable to gain shaft resistance. For SD piles, the unit shaft resistance is severly degraded in all cases. The combination of the particle re-orientation and some interfa- ce slip finally destroys the SD pile shaft capacity.
CONCLUSIONS
There is an analogy between the simple qualitative displa- cement model pile behaviour under one-way cyclic loading in sand and soil "creep" behaviour. Similar behaviour trends as wellknown for RD piles in clayey soils are such behaviour can be attributed to the internal change of the sand fabric in the immediate surrounding of the model pile, during cyclic load regardless of the pile roughness and installation method, the cyclic "critical" load corresponding to the turning point of the δω diagram under a one-way cyclic loading, ranges from 70 to 80 % of the static con ­ ventional ultimate capacity. This observation for sands also coincides with the previous cyclic pile test results in clayey soils.