Сборник трудов конференции СПбГАСУ ч

Раздел 4. Лабораторные и полевые исследования грунтов и фундаментных конструкций…

References

[1]DEN HAAN E. J., EDIL T. B., 1994.Secondary and tertiary compression of peat. Advances in Understanding and Modelling the Mechanical Behaviour of Peat.

[2]HEAD K.H., 1998: Manual of soil laboratory testing: Volume 3 effective stress tests.

[3]MALINOWSKA E. The strain analysis of selected organic soils considering non-linear flow characteristics. PhD Thesis, Warsaw University of Life Sciences, 2005 [In Polish].

[4]MALINOWSKA E. 2011: Flow-pump technique as a constant velocity method of flow measurement in soft organic soils. Electronic Journal of Polish Agricultural Universities, Civil Engineering Vol. 14, Issue 4, #06.

[5]MALINOWSKA E., SZYMAŃSKI A., SAS W. Determination of water flow characteristics in organic soils by the flow-pump technique. Warsaw University of Life Sciences Press. Scientific Review Engineering and Environmental Sciences Vol. 31 (1), pp. 114-121, 2005 (In Polish).

[6]MALINOWSKA E., SAS W., SZYMAŃSKI A. Nonlinear water flow characteristics describing soil consolidation. Electronic Journal of Polish Agricultural Universities, Civil Engineering Vol. 10, nr 4 # 41, 2007.

[7]MALINOWSKA E., SZYMAŃSKI A., SAS W. 2011: Estimation of flow characteristics in peat. ASTM Inter. Geotechnical Testing Journal, Vol. 34, No. 3, pp. 250-254.

[8]ROWE P.W., BORDEN L., 1966: A new consolidation cell. Geotechnique

[9]SAS W., SZYMAŃSKI A., MALINOWSKA E., NIESIOŁOWSKA A., GABRYŚ K. 2011: Analysis of deformation course in problematic soils under embankment. Geotechnics of Hard Soils – Weak Rocks. Part 1 / ed. by Andreas Anagnostopoulos, Michael Pachakis, Christos Tsatsanifos. – IOS Press & Millpress pp. 1199-1204.

[10]MALINOWSKA E., SAS W., SZYMAŃSKI A. 2013: Analiza wpływu rodzaju obciążenia na odkształcalność podłoża słabonośnego. Budownictwo i Inżynieria Środowiska / Politechnika Białostocka 2013, Vol. 4, nr 1, s. 47-52.

УДК 624.151

В. Soldo (Faculty of Geotehnical Engineering, Varazdin, Croatia) A.A. Aniskin (Polytechnic in Varaždin, Croatia)

EXPERIMENT AND THEORY OF PILE BEARING

CAPACITY IN A COHERENT SOIL

ABSTRACT

An investigation of piles with compressive and tensile load on clay will be shown in this article. Beside the experimental testing of piles in clay and determined laboratory testing (that are adapted to the experiment), determined design procedures, their compare and the valuation of validity are also shown. In the design procedures some factors that are empirically in the literature are set theoretically. Many results of experimental researches on piles drilled in clay are also gathered.

KEY WORDS: drilled piles, clay, experiment, a-method, bearing factor, pile bearing capacity.

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INTRODUCTION

In the literature, there are numerous bearing factor values of bearing capacity on the tip of pile, depending on the author. For example, in the case of undrained

cohesive soil, threw designing the bearing factor Nc is gained which is based on theory and empirically based on laboratoticall investigations. Some authors assume

that the Nc factor depends on numerous factors and it is mostly from 6 to 13. This work shows the theoretical investigation of the bearing factor of the tip of pile.

Many experimental researches for the superficies, that will also be theoretically defined, have been gathered. Laboratory researches that are adapted to the experimental researches have been lead, so that the bearing design with strength parameters c, soil (cohesion and friction angle). The design results will be com-

pared with the experimental (measured).

Experimental researches

In order to establish the piles bearing capacity on the tip and superficies, their relationship and other visual occurrences, an experiment on piles has been lead. The experiment has been lead on two piles, one pile was loaded with compression load and the other one with tensile load. As shown on picture 1.

FIG. 4 Tensile pile testing ,with a press

FIG. 5 Piles pulled out of soil after testing

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FIG. 8 The soil condition after load

The picture 6 shows that the formed soil no the pile is thicker from the tip of the pile, while towards the top of the pile shear occurs on the pile (concrete) and soil contact. The soil has been reviewed after pulling out the pile and after excavation where it has been noticed that soil in the pile tip area with less disturbance transforms to tenuous (loose) soil as shown on picture 8, while the other sample has completely different characteristics along the superficies, its more compact and not loose due to the disturbance as the preceding, picture 8.

Laboratory researches

In purpose of pile investigation, laboratory tests that are shown in the continuation have been carried through:

– undrained clay strength test by the vane shear testing aparature (diameter

12.7mm and slope speed 6 o min ), after numerous test samples was

cu 56 63 kN m2 , therefore the average value cu 60 kN m2 is appropriated.

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Современные геотехнологии в строительстве и их научно-техническое сопровождение

The movement – shear strength relation for the load transfer design method between concrete and clay is lead through (picture 9.). The slope speed by direct shear is approximately equal to the speed of tested piles, in order to lead a suitable design for comparing with the measured values. The slope speed of direct shear and of slope pile till bearing was ≈ 0,5 mm/min.

FIG. 9 Deformation diagram, concrete – clay

The experimental investigation results

The diagram on picture 10 shows the force and pile movement relation for all conducted examples of pile testing: piles tested for tensile strength, piles tested for compressive strength and piles tested for tensile strength that were preliminary tested for compressive strength, as piles tested for compressive strength that were preliminary tested for tensile strength.

FIG. 10 The force – movement diagram for all conducted compressive and tensile pile tests

The lines in the upper part of diagram are the force to movement ratios for tensile load piles, and in the lower part of diagram are the force to movement ratios for compressive load piles. The solid lines indicate the force to movement ratios

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for undisturbed piles that were not previously tested, and the dashed lines indicate the force to movement ratios for undisturbed piles that were previously tested for compressive, respectively tensile strength. The bearing capacity gained on the tensile pile on superficial area is Q f .eksp 20 kN . The bearing capacity gained by ex-

perimental testing of the compressive pile Q f .eksp 33 kN .

The bearing capacity of tensile and compressive piles

The bearing capacity of the compressive pile is defined by the sum of friction between the pile and soil, i.e. on the superficial area and the bearing capacity on the top pile tip.The bearing capacity of the tensile piles is defined by the bearing capacity on the superficial area.

The superficial area bearing capacity.

The friction force, i.e. the superficial bearing capacity is: Qf q f Ap ,

where q f cu is the friction between the pile and soil, and it depends on undrained strength cu and correction factor of undrained strength, in the litera-

ture known as -method. The picture shows the correction factor to undrained strength relation. The diagram is composed by the experimental data from the literature. The two curves and the formula of average amount data are also shown. Using the formulas the average amount of correction factor can be calculated.

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FIG. 12 The correction factor and undrained strength relation

For the described experiment example, the point of undrained strength and correction factor is shown qp cu , which in an interesting way approximately

correction factor is 0,61.

The bearing capacity on the pile tip.

The bearing force on the pile tip is: Qb qb Ab , where qb Nc cu is the bearing pressure on the pile tip, and it depends of the undrained strength cu and bearing factor Nc .

For example, in case of undrained cohesive soil, the bearing capacity factors are gained threw a calculation which is based on a theory and empirically according to the laboratory investigations. Some authors assume that the coefficient Nc depends of others factor: – the depth to diameter ratio L/D (Tavares, 1993.), – the pile ratio D (Robert, 1997.), according to Meyerhoff Nc is from 5-9 and it depends of the number of impacts SPT. The value of bearing capacity coefficients Nc for

clay, according to the literature, is from 6 to 13, but usually value 9 is used.

In the bearing capacity expression Qb qb Ab , where qb Nc cu , the following is concluded: The bearing capacity factor Nc is a ratio of the area on which

the failure takes place (outer torus area) and the tip pile area Ab . The calculation of the assumed area on which the failure takes place:

Assuming that the torus surface begins under the pile tip in angle 45o and finishes that is, closes on the pile superficies with the turn angle 58 of full circle.

The ground plan length of this failure surface is the circumference of the pile tip Do . According to this the torus area can be carried out:

So the rotation surface can be written (torus):

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Раздел 4. Лабораторные и полевые исследования грунтов и фундаментных конструкций…

FIG. 13 Pile with no under ream with the assumed failure surface

FIG. 14 The assumed shear torus surface a) Outer area, b) Lower area

At 14,0866 Do2 .

At last the shear torus area and loaded areas of the horizontal circle area relation can be lead through, i.e. the tip pile area and define with the so called bearing capacity factor Nc :

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Современные геотехнологии в строительстве и их научно-техническое сопровождение

so it may be announced that the bearing capacity factor for the assumed failure area till the vertical i.e. pile: Nc 17,9 .

The failure area assumed till the horizontal. If the failure is assumed till the horizontal, as shown on the next picture, the bearing capacity facto can be computed:

The inner area II. is as before:

The following can be compute:

At 14,9112 R2 , if R Do 2 , At 7,4555 Do2

At last the shear torus area and loaded areas of the horizontal circle area relation can be lead through, i.e.. the tip pile area and define with the so called bearing capacity factor Nc :

so it may be announced that the bearing capacity factor for the assumed failure area till the horizontal i.e. pile Nc 9,50 .

Comment: Assuming that the failure surface closes the angle till the pile, i.e. the vertical, the bearing capacity factor is Nc 17,9, while assuming that the fail-

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Using these bearing capacity factors the bearing capacity pile tip design will be shown.

The design results and comparison with the experiment

The picture17 shows approximately distribution of shear stresses according to the laboratory results that are shown on picture 9.

FIG. 19 The calculation results using the load transfer on tensile pile method

The bearing capacity and experiment results comparison

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Table 2

The calculation results using the load transfer method

The bearing capacity gained on the tensile superficies is Q f .eksp 20 kN .

The bearing capacity gained by experimental testing of the compressive pile is Q f .eksp 33 kN , where from the pile tip bearing capacity is Qb.eksp 13 kN .

CONCLUSION

The authors have shown their investigation on compressive and tensile loaded piles in clay in the article. Beside the testing of bearing capacity of tensile and compressive loaded piles in clay, alternate compressive-tensile and tensilecompressive and visually observed distributions in soil and on the contact of soil and pile, certain conclusions have been lead and determined, also as the gathered data from the literature, the pile bearing capacity expressions have been set. After composing all data a comparison of pile bearing capacity has also been lead through: the results of design and results of experiment, from where it can be seen that the method reliability, in terms of mentioned experimental investigations give very good results. Along with the laboratory investigation, visual observation, load transfer method calculation, bearing capacity on the superficies with set expressions (literature data). The bearing capacity of the tip pile has been distinctly investigated, along with distinct conclusions and bearing capacity coefficients setting.

References

1.Budhu, M., 1999, ˝Soil mechanics and foundations˝, John Wiley & Sons, Inc, New York / Chichester / Weinheim / Brisbane / Singapore /Toronto.

2.Bowles, J.E., 1988, ˝Foundation Analysis and Desing˝, McGraw-Hill Book Company, New York, U.S.A.

3.Coduto, D.P., 1994, ˝Foundation design, Principles and Practices˝, A Paramount Communication Company, Englewood Cliffs, New Jersey.

4.Das, B.M., 1987, ˝Theoretical foundation engineering˝, Department of Civil Engineering & Mechanics, Developments in geotechnical engineering 47, Amsterdam-Oxford-NewYork- Tokyo.

5.Houlsby, G.T. & Martin, C.M., 2003, ˝Undrained bearing capacity factors for conical footings on clay˝, Geotechnique 53, No. 5, pp. 513-520.

6.Kulhawy,F.H. & Mayne, P.W., 1990, ˝Manual on Estimating Soil Properties for Foundation Design˝, EPRI, Cornel University Ithaca, New York.

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