Y. Kikuchi, T. Mizutani & Y. Morikawa Port & Airport Research Institute, Yokosuka, Japan
T. Sato
Kumamoto University, Kumamoto, Japan
ABSTRACT: The mechanism of plugging phenomenon at the toe of vertically loaded open-ended piles was observed in this study. The behavior of the surrounding ground at the pile toe is discussed based on the observation of the movement of iron particles, which were mixed with sand to form layers in the model ground, extracted from visualized X-ray CT data. In addition, the movement of sand particles was extracted using PIV (Particle Image Velocimetry) method. The CT images of the experimental results showed that the condition of wedge formation below the open-ended pile was clearly different from that below the closed-ended pile. Although the penetration resistance of the open-ended pile and closed-ended pile was similar, the movement of soil inside the open-ended pile was not stopped but was restricted, as shown by intermittent increase and decrease in penetration resistance during pile penetration.
1 INTRODUCTION
For more than 50 years, steel pipe piles were used for port facilities, because of its flexural capacity and ease of pile driving. During these decades, diameters and embedded lengths of steel pipe piles were dramati-cally changed. Such changes may affect the plugging behavior of the pile.
The mechanism of plugging phenomenon at the toe of vertically loaded open-ended piles was observed in this study. Three series of static penetration exper-iments with model piles were conducted by using a micro-focus X-ray CT scanner (Kikuchi et al. 2006).
And a series of large scale model pile penetration experiments was conducted in a model sandy ground to investigate the bearing capacity of open-ended piles.
The model piles used in this study were open-ended piles and closed-ended piles.
The behavior of the surrounding ground at the pile toe is discussed based on the observation of the move-ment of iron particles, which were mixed with sand to form layers in the model ground, extracted from visualized X-ray CT data. In addition, the movement of sand particles was extracted using PIV (Particle Image Velocimetry) method. From large scale pile pen-etration experiments, the periodic behavior of bearing resistance of open-ended piles after plug formation was discussed. Finally we concluded an expected plugging mechanism of open-ended piles.
2 VISUALIZATION OF THE PLUGGING PHENOMENON
The first series of experiments was conducted to grasp the plugging phenomenon. The piles used in this series
were open-ended stainless steel piles with 16 mm in outer diameter and 80 mm in length and the thickness of the pile wall was 0.3 mm. The container used was made of acrylic resin with 85 mm in inner diameter and 160 mm in height. The model ground was prepared with dry Toyoura sand (D50= 0.2 mm, Uc= 1.6). The thickness of the ground was 150 mm. Relative densi-ties of the ground were set to 5, 70, and 98%. The pile was penetrated into the ground at a rate of 1 mm/min.
The pile penetration experiment was conducted out-side the X-ray CT scanner room. The penetration resistance and depth were measured at the pile top.
When the pile had penetrated to about 30 and 60 mm, the load was released and the container was moved into the CT room, and X-ray CT scanning was performed.
The relation between penetration resistance and depth is shown in Figure 1. Penetration resistance increases as the relative density of the model ground increases.
Figure 2 shows vertical sections from selected CT images to obtain views through the central axis of the pile at each depth. The white lines are the pile, the gray area is sandy ground, and the black part in the pile is air. The top of the CT images is the ground surface. It was observed that the ground surface, where the inner pipe pile was located, slid down with the pile in the test case involving low density ground (Case 1, Dr= 5%).
Although it is guessed that the plug occurred at the pile toe, penetration resistance did not increase. In the test case involving high density ground (Case 3, Dr= 98%), penetration resistance was relatively high although the ground surface did not slide down with the pile. From these results, it is realized that the occur-rence of ground invasion phenomenon into the pipe pile toe depends on the balance of ground reaction and
Figure 1. Relationship between penetration resistance and depth.
Figure 2. Vertical section of CT images.
frictional resistance of the pile inside and the weight of the soil. Therefore, the increment of resistance and appearance of the plugging phenomenon do not have a one-to-one correspondence. In other words, there are cases in which resistance occurred without the appearance of plugging phenomenon.
Figure 3. Penetration test in CT chamber.
3 GROUND BEHAVIOR AROUND
THE PILE TOE
Referring to the previous test results, a series of detailed pile penetration experiments were conducted.
New penetration apparatus was made to improve test accuracy. The dimensions of the model piles were 15 mm in diameter, 40 mm in length, and 1 mm in thickness for open-ended piles. The pile was made of aluminum. The container was made of acrylic resin, with 100 mm in inner diameter, and 440 mm in height. The sand used for the ground was Toyoura sand (D50= 0.2 mm, Uc= 1.6).The model ground was 270 mm high with 65% relative density and prepared by air pluviation method. An overburden pressure of 2.5 kPa was applied by stainless steel balls (diameter:
2 mm).
In order to investigate the movement of the ground from X-ray CT results, a layer of iron particles (diame-ter: 0.3 mm) was used. The pile was penetrated into the ground from the ground surface at a rate of 1 mm/min.
The entire pile penetration experiment was conducted in the micro-focus X-ray CT scanner chamber, as shown in Figure 3. When the piles had penetrated to about 35 mm and 70 mm, pile penetration was stopped, the load was released, and extension rods were added.
To obtain test data, pile penetration was stopped at penetration intervals of 3 mm, and X-ray CT scanning was performed.
The relationship between penetration resistance and depth is shown in Figure 4. Small drops of resistance in each 3 mm intervals were observed in each relation-ship, because penetration was stopped to perform CT scanning. As a distinctive feature of the bearing capac-ity of open-ended pile, penetration resistance does not occur at the beginning of penetration, and penetration resistance decreases and increases in the middle of penetration. The increment of penetration resistance in both cases was almost equal after about 35 mm of penetration depth. This means that sufficient plugging of the open-ended pile may have developed. With the open-ended pile, resistance decreased and increased in the course of penetration at about 55 to 60 mm of penetration depth due to corresponding changes in the plugging effect. In other words, these results suggest that full plugging is not continuous but a plug is formed and broken repeatedly during pile penetration.
Figure 4. Relationship between penetration resistance and depth.
Figure 5 shows the movement of the particles during the pile penetration, depth of which was from 42 mm to 81 mm, with points and lines as extracted from the CT images. The points are the relative positions of the par-ticles for pile at each 3-mm step of penetration, and the lines are the particle routes. In the case of closed-ended pile, the particles below the pile showed a tendency to be pushed out to the outside of the pile toe. A clear wedge was constructed at this area, and the soil was unable to intrude there. Some of the particles below the pile were caught at the surface of the wedge, and some were discharged to the side of the pile at the edge and then moved along the pile. Because the wedge unified with the penetrating pile, the relative movement of soil at the surface of the wedge was greatly different. This implies that a shear zone may develop at the wedge surface. On the other hand, the particles below the pile toe were able to move upward and penetrate into the pile. The particles outside the pile were pushed out to the outside of the pile toe.
4 DEFORMATION ANALYSIS USING PIV In order to examine in detail the ground behavior, PIV method was applied to the CT images. This series of experiments was conducted so as to focus on the evolution process of the plugging phenomenon.
In this series, the model pile was set up in the model ground at the initial penetration depth of 50 mm. The container and the loading device in this series of exper-iment were the same as the previous case. The pile used in this series was open-ended with 32 mm in diameter, 140 mm in length, and 1.5 mm in thickness. The pile was made of aluminum.
Figure 5. Relative movement of particular particles with respect to the pile at each 3 mm penetration. Traces of the particles started from 42 mm of pile penetration. Par-ticles presented were located from one to two times the diameter beneath the pile tip in the beginning of the trace and they moved upward with penetration of the pile. Plots were observed positions and connecting lines showed the movement of each particle.
Figure 6. Grain size distribution curve of Souma sand #4.
The sand used was Souma sand #4 (D50= 0.7 mm, Uc= 1.6). The density of the soil particles of Souma sand #4 is equal to 2.644 g/cm3, maximum and min-imum void ratios are 0.634 and 0.970 respectively.
Figure 6 shows the grain size distribution of Souma sand #4.
A larger-diameter model pile and larger-diameter sand were used to observe the ground behavior of the inner pipe pile by PIV in this series of experiments.
The model ground was 270 mm high with 65% rela-tive density and prepared by air pluviation method.
An overburden pressure of 2.5 kPa was applied by stainless steel balls (diameter: 2 mm). The pile was penetrated into the ground at a rate of 1 mm/min from
Figure 7. Relationship between penetration resistance and depth.
50 mm to 98 mm in depth. The entire pile penetration experiment was conducted in the micro-focus X-ray CT scanner chamber.
The relationship between penetration resistance and depth is shown in Figure 7. Resistance occurred in the early stages of penetration in this experiment, because soil had been packed in the pile at the start of penetration.
In the PIV analysis, one pixel of the CT image was 0.1 mm square. The size of one element for the PIV analysis was 1.5 mm square, and the reference frame size was 4.5 mm.
The vectors of ground displacement that were mea-sured by the PIV method are shown in Figure 8.
The displacement vectors presented were measured between each 3 mm of penetration. The numbers shown at the top of each figure are penetration depths for each figure. The pile is shown as two white lines and gradations show the displacement of the ground.
As it is difficult to recognize the deformation of the ground in this figure in detail, major displacements were presented by arrows in the figure.
The soil inside and below the pile moved downward when the penetration depth was from 0 to 3 mm. This is because the soil inside the pile during in the initial state created frictional resistance and made a plug. When the penetration depth increased slightly, the rate of change of resistance went down immediately at the penetration depth from 3 to 6 mm. Low rates of resistance incre-ment were observed at the penetration depth from 3 to 33 mm. Movements of the soil inside and below the pile were small at this penetration depth. The transient process of plugging effect occurred at this stage. Rel-atively large movement of the soil inside the pile was observed at the penetration depth from 18 to 21 mm.
But little movement was observed in the next stage of penetration. It was confirmed that repeated production and destruction of plug occurred at these steps. The rate of resistance increment rose again after 33 mm
Figure 8. Images of ground displacements from CT images using PIV method. Black arrows show major direction and the amount of displacement of the ground during the 3 mm penetration. The penetration depth is shown above each picture.
of penetration. The displacement of the soil inside and below the pile got larger at penetration depth from 30 to 36 mm; in particular, a downward movement of the soil existed and maintained at the penetration depth from 36 to 42 mm. A sufficient plugging effect occurred at this step. In this way, the relationship between penetra-tion resistance and ground deformapenetra-tion was observed by using PIV method.
5 PENETRATION EXPERIMENT OF
OPEN-ENDED PILES IN MODEL GROUND To examine the plugging effect, a series of penetration experiments of open-ended pile in model ground was performed (Mizutani et al. 2003).
The model ground was made of Souma sand #4.
Physical properties of Souma are presented in the pre-vious section. Dried Souma sand was pluviated into the container, which was 6 m in length, 3 m in width and 3 m in depth, through a pipe with diameter of 3 cm.
The height of the sand fall was kept 1 m above the sur-face during sample preparation. The relative density of the model ground was about 40%.
After the sample preparation was completed, model piles were driven into the ground. The model pile, with 20 cm diameter and about 2 m long, was made of acrylic resin. The model pile could be used as both
Figure 9. Vertical cross section of the model pile used in experiments.
Figure 10. The relationship between the depth and the penetration resistance.
the closed-ended pile and the open-ended pile through a removable bottom plate, as shown in Figure 9.
The model piles were driven into the model ground statically at a speed of 20 mm/min. Penetration resis-tance at the head of piles and height change of the surface of the ground inside open-ended piles were measured continuously during the penetration of model piles.
Figure 10 shows the relationship between the depth and the penetration resistance of model piles. In case of the closed-ended pile, the penetration resistance increased immediately after the onset of the pile driv-ing, while the penetration resistance of the open-ended
Figure 11. Cyclic changing of the penetration resistance of the open-ended pile compared with the height change of the inside ground.
pile increased gradually. After the penetration depth reached 800 mm, a remarkable change of the penetra-tion resistance of the open-ended pile appeared, that is, the penetration resistance increased and decreased periodically.
One cycle of this periodically changing penetration resistance was enlarged and displayed in Figure 11, comparing with the height change of the inside ground surface of the open-ended pile. The cyclic behavior included four phases as follows:
1) A sudden reduction of the penetration resistance took place at about 1250 mm in depth. At that moment, the height change of the inside ground surface indicated as ‘H ’ in Figure 11, came to a standstill.
2) The penetration resistance increased rapidly, while the height of the inside ground surface was standing at about 475 mm.
3) From 1260 mm to 1300 mm in depth, the pene-tration resistance stopped to increase, and kept a constant value. In the meantime, H increased grad-ually, however, the increment of H was less than the increment of the pile penetration depth.
4) After the depth exceeded 1300 mm, the penetration resistance resumed increasing. At this stage, the increment of H was equal to the increment of the depth, that is, the inside soil and the open-ended pile itself penetrated into the model ground as one body.
Thus, open-ended pile could not continuously remain under a fully-plugged condition, and intermit-tent plugging was observed. This sort of phenomenon has been already reported by Hight et al. (1996) who conducted another type of model tests. In their investi-gation, submerged sand columns were pushed up from their base inside steel pipe piles using a rigid platen, and the load-displacement relationships for the sand plug were obtained by monitoring the load on the platen and its displacement. It is noteworthy that the identical behavior was observed in different types of model tests.
Figure 12. Plugging mechanism of open-ended pile.
6 PLUGGING MECHANISM
OF OPEN-ENDED PILES
From these observations, expected plugging mecha-nism is presented in Figure 12 (Kikuchi et al. 2008).
The ground below the pile toe is deformed by pile pen-etration. The deformed and dilated soil intrudes inside the pile and friction is produced between the pile and the intruding soil. If the inner friction resistance and self weight balance with the bearing resistance of the ground below the pile toe, a plug is produced. Then, the area below the pile is compacted to form a soil wedge.
However, if the bearing resistance of the ground below the pile overcomes the resistance of the inner friction, the plug is destroyed and the wedge and the ground underneath intrude into the pile interior. This genera-tion and destrucgenera-tion of the plug is repeated during the penetration of open-ended piles.
7 CONCLUSIONS
In this study, the behavior of surrounding ground around a pile toe was discussed based on static pene-tration test. During the penepene-tration test, the movement of the ground was observed using a micro-focus X-ray CT scanner. The CT images of the experimental results showed that the condition of wedge formation below
open-ended pile was clearly different from that below closed-ended pile. Although the penetration resistance of the open-ended pile and closed-ended pile was sim-ilar, the movement of soil inside the open-ended pile was not stopped but restricted, as shown by intermittent increase and decrease in penetration resistance during pile penetration. As a result, a plugging mechanism was proposed in Figure 12.
REFERENCES
Hight, D. W., Lawrence, D. M., Farquhar, G. B., Milligan, G. W. E., Gue, S. S. and Potts, D. M. 1996. “Evidence for scale effects in the end bearing capacity of open-ended piles in sand.” Proc. of the 28th Annual Offshore Tech.
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Kikuchi Y, Mizutani T, Nagatome T and Hata T. 2006. “Study on applicability of micro-focus X-ray CT scanner for geomaterials”. Technical Note of the Port and Airport Research Institute, No.1125 (in Japanese).
KikuchiY, Sato T, and MorikawaY. 2008. “Observation of the plugging phenomenon in a vertically loaded open-ended pile”. Technical Note of the Port and Airport Research Institute, No.1177. (In Japanese)
Mizutani T, Kikuchi Y and Taguchi H. 2003. “Cone pen-etration tests for the examination of plugging effect of open-ended piles.” Proc. of IC on Foundations, BGA, 655–664.
Soil-Foundation-Structure Interaction – Orense, Chouw & Pender (eds)
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