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Experimental Study and Design
Senior Assistant, University of Zagreb, Geotechnical Faculty Varazdin , Croatia Geokod, d.o.o., Zagreb, Croatia Hrvoje Babić Student, University of Zagreb, Geotechnical Faculty Varazdin , Croatia |
ABSTRACT
The work shows the investigation of compressive and tensile loaded piles in soil. The work shows the experimental testing of piles in clay and specific laboratorial investigations (which are adjusted to the experiment), as it also shows specific design procedures, their compares and value grade. In the design procedures the algorithms are set, the bearing capacity terms compute and the programs programmed, and the results compared with the experimental. Except for the algorithms, specific relations based on many results gathered from experimental investigations on drilled piles in clay are used.
Keywords: Keywords: piles, tension piles, analysis, design
INTRODUCTION
There are various values of bearing capacity factors, proposed by various authors, which are used in tip pile bearing capacity design analysis. For example, for the case of undrained cohesive soil, threw the design the bearing capacity factor that is based on the theory and empirically based on the laboratorial investigation. Some authors assume that the bearing capacity factor for undrained methods depends of more facts. In this work the theoretical exploration of the bearing capacity factor for the pile tip in cu and cf soil is shown, and for the superficial area many experimental explorations, that will also be theoretically defined, have been gathered. Laboratorial explorations that are adjusted to the experimental are made, so that the bearing design with soils 
(cohesion and friction angle) and soils 
. The pile design in clay with soils is also shown with setting and integrating the shear strength on slide surfaces. Specific results of visual observations on tested piles will be shown. The design results will be compared with the experimental (measured).
Handling Curvilinear Coordinates
An experiment on piles for determining the pile bearing capacity on the tip and superficial area, and their relation and other visual occurrences, has been led. The experiment was lead on two piles, one pile was loaded with compressive load and then on tensile load, Figure 1.
The diagram in Figure 2 shows the force an movement relation for all lead examples of pile testing: tensile loaded piles, compressive loaded piles, and tensile loaded piles that have been previously been loaded with compressive load also as the compressive loaded piles that was previously loaded with tensile load.
The lines in the upper part of the diagram are the force to movement relations for compressive loaded piles, and in the lower part are the force to movement relations for tensile loaded piles.
The solid lines show the force to movement relations for undisturbed piles, that were not previously tested, and the dashed lines also show the force to movement relations for piles that were previously tested on compressive, and then on tensile load.
Figure 1. (a) The pile load scheme, (b) The force to movement relation diagram for all lead testings of compressive loaded and tensile loaded piles.
The bearing capacity gained on the tensile loaded piles is . The bearing capacity gained on the tensile loaded piles threw experimental testing is .
Figure 2(a). The formed soil on the pile after pulling it out, (b) The post-load condition
The Figure 2 (a) shows that the formed soil on the pile is thicker toward the pile tip, while toward the pile head the shear on the pile (concrete) and soil contact occurred.
After pulling out the pile from the soil and digging, the following has been noticed: In the pile tip area was the soil that transforms from the condition with less disturbance to the loose condition as shown on Figure 2 (b), while the other sample has completely different properties in the superficial area; its more compact, its not loose while disturbed in differ of the previous one, Figure 2b).
THE LABORATORY SOIL TESTING
In purpose of investigation the pile, laboratorial investigation shown in the continuation have been led:
- The undrained clay strength has been tested with the vane shear (diameter 12.7 mm and slope speed ), and after several tested samples it was , therefore the accepter average value was .
- The movement – shear strength ratio testing among the clay and concrete has been led. The testing speed for direct shear is approximately equal to the tested piles speed, so that a specific design for comparison with measured results could be led. The direct shear test speed was ( 0,5 mm/min.
- The strength parameters were tested on three samples and there average value was approximately: cohesion and friction angle .
THE PILE BEARING CAPACITY
The pile bearing capacity c, ( soil.The literature mostly shows the bearing capacity terms using the so-called bearing factors. An example of shear strength integration on an assumed formed failure envelope of logarithmic spiral has been made. After setting the shown algorithm, a software design has been made.
Figure 3. Pile, failure zone, logarithmic spiral
By defining the initial diameter of logarithmic spiral radius , the depth z on the logarithmic slide surface, the radial distance from the pile center till the observed point on the slide urface , the normal stress on the slide surface curve , the shear stress , the differential area and radius at the turn angle, the multiple of the differential and shear strength on the curve is:
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(1) |
As the result, the pile bearing force is 
:
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(2) |
Pile bearing capacity in soil. Bearing capacity the tip. The bearing force on the pile tip is: , wherefore is the bearing pressure on the tip pile, and it depends of the undrained strength and bearing capacity .
For example of a case of undrained cohesive soil, bearing factor capacities that are based on theory and empirically based on laboratorial investigations are gained. Some authors assume that the Nc coefficient depends of other facts i.e: - the depth diameter ratio L/D (Tavares, 1993.), - the pile diameter D (Robert, 1997.), according to Meyerhof is 5-9 and it depends of the number of impacts SPT. According to the literature the bearing capacity for clay is from 6 till 13, but usually 9 is used.
Figure 4. Pile, assumed failure surface
The design of assumed area where failure takes place:
Assuming that the torus failure surface begins under the pile tip in angle of and ends, that is closes on the piles superficial area with angle turn of of a full circle. The plan length of this failure surface is the piles tip perimeter . Based on this the torus are can be carried out.
The rotation surface area can be carried out (torus):
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(3) |
The total assumed shear according to Figure 4 (b) is . The assumed surface till the horizontal according to Figure 4 (c) is. According to this, the term for pile tip bearing capacity can be written:
, till the vertical or assuming that the failure occurs till the horizontal.
The bearing capacity for compressive piles is defined as a sum of the bearing capacity of friction between the pile and soil, that is the superficial area, and the bearing capacity of the pile tip, while the bearing capacity of tensile piles is defined only as the bearing capacity of superficial area.
The superficies bearing capacity. The friction force, that is, the superficies bearing capacity is: , wherefore is the friction among the pile and soil, and depends on the undrained strength and correction factor of undrained strength, in literature known as the -method. A factor correction and undrained strength ratio diagram in shown in the Figure 5. The diagram is composed of experimental investigation literature data. Two curves and formula of average value data are shown. The average value of the factor correction can be calculated using the term.
Figure 5. The factor correction and undrained strength relation
For the shown example, a point of undrained strength and factor correction is shown, which noticeably approximately equals to the shown curves that have the term: and , and for the experimental example the correction factor is .
THE DESIGN AND EXPERIMENTAL RESULTS
The comparison of the bearing capacity design and experiment results
Table 1. The bearing capacity pile results (for the average soil parameters)
Note: Beside the conclusion that the results depend of exact laboratorial soil testings, the following can be added. The design has been carried out using the dimensions of the incorporate pile. The gathered design results all minor then the experimental. If the facts that are noticed after the experiment are used (for example the formed soil on the superficies area, which changes the pile diameter and likewise) that is, the slide surface as shown on Figure 2, more precise results can be gathered comparing to the experimental.
CONCLUSION
The authors showed their work of investigation of compressive and tensile loaded piles in clay. Beside the investigation of the pile bearing capacity of tensile and compressive loaded piles, visually observed displaces in soil and soil – pile contact, specific conclusions have been gathered also as for literature data. For the complicated designs as integration of shear strength threw the surface failure and others, computer programs have been designed. After leading the data through, a comparison of certain conclusions have been led through and other gathered literature data have gathered. For the complicated designs as integration of shear strength on a certain failure surface, computer programs have been programmed. After leading all these data, a comparison of pile bearing capacity has been made: the design results and experiment results, and mostly depend of the accuracy of laboratorial investigations of soil parameters, where the parameter soil design relation is shown. The parameter test speed is adjusted to the pile experiment that took place. On this experiment example, the exact results, also as less sensitive is the undrained strength method, while the soil model with Mohr-Columbovim parameters gives a greater sensitivity. Exept for the influence of the laboratory testes to the design results, it can be added that accurate design results are gathered by using the observed formations on the experiment, as for example the formed slide surface. The reliability of each method is also notable.
REFERENCES
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