9kimikrshna journal text

8/12/2019 9kimikrshna Journal Text

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INDIAN GEOTECHNICAL SOCIETY CHENNAI CHAPTER

ABSTRACT: A large scale experimental program using vertical and batter model piles in sand subjected to pull out loads has beencarried out in a model tank of size 1m x 1m x 1m. Mild steel pipes of varying diameters, lengths, and shapes are used as model piles. Apoorly graded river sand having specific gravity G= 2.67, Uniformity coefficient= 3.53, e max = 0.81, e min = 0.54 has been used asfoundation medium. The influence of pile inclination, pile length, diameter, surface characteristics and shape were investigated. It isinferred that net ultimate pullout capacity increases significantly with increase in length to diameter ratio. Pullout capacity also increaseswith increase in diameter. Net ultimate pullout capacity increases with increase in batter angle attains a maximum value and thendecreases. Sand coated piles are found to be resisting more pullout forces than smooth piles. It is also found that for piles of varyingshape but of constant volume, circular pile will resist more uplift force compared to square or rectangular piles. The experimental valuesof net ultimate pullout capacities have been compared with predictions made by available theories.

Pullout Capacity of Model Piles in Sand

K. Kimi Bose 1 and A. Krishnan 2

M.E Student, Department of Civil Engineering, Government College of Technology, Coimbatore [email protected] 2 Asst. Professor, Department of Civil Engineering, Government College of Technology, Coimbatore.

Introduction

When structures are constructed below theground water table or if they are constructed under wateruplift forces are to be applied on the basement ofstructures. Also in the case of transmission line towers,mooring systems for ocean surface or submergedplatforms, tall chimneys etc are usually subjected tooverturning moments due to wind, wave pressure or shipimpact etc. These overturning moments are transferredto structures foundation in the form of compression onsome elements and pullout on others. The type of

foundation usually recommended is a combination ofvertical and batter piles. In this paper the behaviour ofvertical and batter piles under pull out loads has beeninvestigated.

To study the effect of pile inclination, pile length,diameter, shape, surface characteristics and pile tipproperties on uplift capacity of piles, laboratoryexperimental investigation is carried out.

Meyerhof(1973) presented an analysis todetermine the axial pullout resistance of batter piles. Forpile of inclination, with vertical axis , and vertical depthof embedment, D, ignoring pile weight, the pulloutresistance, P u is given as,

( )u u sP 'K tan A= (1)

where,

A s = Embedded pile surface area

o = Average effective overburden pressure= D/2

Ku = Uplift coefficient

= Pilesoil friction angle

Awad and Ayoub (1976) studied about theultimate uplift capacity of vertical and inclined anchors incohesion less soil. An empirical equation was developed

for determining the ultimate uplift capacity of inclinedpiles.

( )cos

p pcos +tan

=

(2)

Where Po = Net ultimate uplift capacity of verticalpile

For vertical piles , o uP =P -W

Hanna and Afram(1986) suggested an analyticalprocedure to evaluate ultimate pullout resistance P as

( )P = Pcos /2 (3)

Chattopadhyay and Pise (1986)proposed a theoretical analysis for predicting the axialuplift capacity of inclined piles, embedded in sand. Theyconcluded that for equal length piles, the ultimate upliftcapacity of inclined pile increases with increase ininclination of pile and decreases after reaching maximumvalue of L at =(15 to 22.5 ). Pise and Sharma (1994)carried out extensive work on uplift behaviour of anchorpiles in sand under axial pulling loads. A comparison wasmade between between the experimental and theoreticaland experimental values. It was concluded that upliftcapacity increases with increase in pile friction angle,depth of embedment and B/d ratio.

Different theories regarding behaviour of pilesunder different loading conditions have been developedover the years. The reliability of the theories can bedemonstrated only by comparison of experimental resultson model or field piles with the theoretical predictions.Full-scale field tests, though highly desirable, aregenerally expensive and difficult to perform. In theabsence of resources and scope of testing prototypesmall scale laboratory model test conducted on piles infoundation medium prepared under controlled conditionmay serve the purpose to some extent. Properly

conducted laboratory tests, with known parameters

KEYWORDS: Pullout capacity, Model piles, Sand bed, Batter Piles, Pile roughness


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STUDENTS PAPER COMPETITION 200950

affecting the soil-pile response under pulling loads wouldprovide information on qualitative and quantitativecontributions of such parameters on ultimate resistanceof piles in the absence of field test results.

Compared to previous studies in this area, this

investigation proposes to consider wider range ofparameters and their effects on the uplift capacity ofpiles.

Experimental Set Up and Model Tests

Testing Programme

Tests under axial pullout have been carried out ontubular mild steel model piles having outer diameter 27.5mm, 33mm, 47mm at different angles with verticalaxis as 0, 10, 20 and 30.The model piles have been

Test Arrangement

Axial pullout loads were applied to the pilesthrough double pulley arrangement The steel loadingframe, movable along the length of chamber with aninverted pulley was used to align the axis of batter pileand wire rope. The non-extensible steel wire rope wasattached to the pile top by bolting. The wire rope wastaken first through an inverted pulley and then over thesecond pulley. Loading pan where dead weights were putfor loading was fixed at the other end of wire as shown in

Figure1. The position of first pulley was fixed according tothe alignment of the wire rope and pile axis as per theinclination of the pile. A long steel flat plate was placedalong the width of the chamber to mount magnetic baseof two dial gauges. Two dial gauges were fixedequidistant from pile axis. The loads were applied bydead weights in the loading pan starting the smallest,with gradual increase in stages. Dial gauge readingswere observed for both dial gauges for each increment ofloading when it becomes stable. Average value ofdisplacement as recorded from both the dial gauges havebeen taken as axial displacement of the pilecorresponding to the pullout load applied.

tested for L/d ratios 8, 16, 24 and 32 and three different

surface roughnesses of piles. All tests have beenconducted in model chamber of size 1000mmx 1000mmx1000mm deep.

The tests were performed in dry localriver sand of unit weight 16.68 kN/m 3; angle of shearingresistance of 38 ; relative density,D r 80%.

The test programme consisted of the followingarrangement.

> Vertical and inclined piles subjected to axial pull.The angle of inclination being 10,20and30 withrespect to vertical direction.

>

Vertical piles of constant length but differentdiameters subjected to vertical pull.

> Vertical and inclined piles of constant length butdifferent surface characteristics subjected tovertical pull.

> Vertical piles of constant volume but differentshapes such as round, square and rectanglesubjected to vertical pull.

Properties of Soil used in the Test

A poorly graded river sand having specific gravityG= 2.67 was used for the tests. The D 10 , D 30 , D 60 of thesoil are 0.17mm,0.25mm and 0.6mm. The coefficient of curvature (Cc) and uniformity coefficient (Cu) of the soilare 0.61 and 3.53. According to Indian StandardClassification System, this soil can be classified as poorlygraded sand with a letter symbol SP.

To determine the density and void ratio of sand,a number of trials have been carried out for varyingheights of fall. It was understood that the height of fall ofsand goes on increasing, the density of sand increases.To verify this, a steel rectangular box of size335mmx235mmx35mm depth was used to pour the sandwith help of hopper. The box was filled with sand fordifferent heights of fall ie, 0cm, 5cm, 10cm, 15cm, 20cm,25cm, 30cm, 35cm, 40cm, 50cm, 60cm.

For every height of fall, corresponding unitweight, void ratio, relative densities were calculated andare shown in Table 1

Height offall (cm)

Unit Weight(kN/m 3) Void Ratio

RelativeDensity in %

0 14.73 0.813 0

5 15.4 0.734 30

10 15.7 0.700 42

15 15.99 0.670 53

20 16.23 0.645 62

25 16.45 0.623 70

30 16.68 0.601 80

35 16.88 0.582 85

40 17.06 0.565 92

50 17.25 0.548 98

60 17.32 0.542 100

Table 1 Height of Fall and Relative Density

Fig 1. Model Pile and Test Set-up


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Pullout Load versus Axial DisplacementDiagrams fo r Variation in Diameter

Figure 7 shows the variation of uplift capacity withdiameter of pile. It is found that as diameter increases theuplift capacity also increases.

Net Ultimate Pullout Capacity Diagrams

The experimental value of net ultimate pulloutcapacity, Pu has been calculated by deducting thecomponent of weight of pile i.e, Wcos from the ultimatepullout capacity, Po. Where W is the weight of model pile.

Variation of net uplift capacity with batter angle

The variation of net ultimate pullout capacity, P uwith respect to the batter angle is shown in Figure10.From this figure it is seen that net ultimate pulloutcapacity increases with increase in , attains maximumvalue and then decreases. The maximum value of P ucorresponds to batter angle 20.

Pullout Load versus Axial DisplacementDiagrams f or Variation in Shape (Figure 8)

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

-0.5 0 0.5 1 1.5 2

AXIAL DISPLA CEMENT(mm )

P U L L O U T L O A D ( k N )

DIA=27.5mm

DIA=33mm

DIA=37mm

Fig 7. Variation of Uplif t Capacity with th e Diameter of Pile

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

-0.5 0 0.5 1 1.5 2 2.5 3

AXIAL DISPL ACEMENT(mm)

P U L L O U T L O A D ( k N )

square

rectangle

round

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

-0.5 0 0.5 1 1.5 2 2.5 3 3.5

AXIAL DISPLACEMENT (mm)

P U L L O U T L O A D

( k N )

=0

=10

=20

=30

Pullout Load versus Axial Displacement

Diagrams for Variation i n Surface Characteristics

Fig 9. Variation of Uplift Capacity for Medium Rough Piles

Fig 8. Variation o f Uplift Capacity wit h th e Shape of Pile.

0

0.05

0.1

0.15

0.2

0.25

0.3

0 5 10 15 20 25 30 35

BATTER ANGLE

n E T U L T I M A T E

P U L L O U T C A P A C I T Y

k N )

L/d=8

L/d=16

L/d=24

L/d=32

Fig 10. Net Uplift Capacity Versus Batter Angl e

Variation of n et ultimate pullout capacity, Puwith diameter of pil e (Figur e 11)

Variation of n et ultimate pullou t capacity, Puwith shape of pi le (Figure 12)

0.128

0.13

0.132

0.134

0.136

0.138

0.14

0.142

0 5 10 15 20 25 30 35 40

DIAMETER OF PILE (mm)

N E T U L T I M A T E

P U L L O U T C A P A C I T Y

( k N )

L/d=24

Fig 11. Net Uplift Capacity Versus Diameter of Pile

0

0.02

0.04

0.06

0.08

0.1

0 0.2 0.4 0.6 0.8LENGTH OF PILE (m)


P U L L O U T

C A P A C I T Y ( k N )

VOLUME=0.003

m3

Fig 12. Net Uplift capacity versus length of pile


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PULLOUT CAPACITY OF MODEL PILES IN SAND 53

Comparison of ResultsThe validity of any experimental work can be

proved only by comparing those results with establishedresults. The experimental results are used to check thevalidity by comparing them with those values calculatedfrom theories proposed by Meyerhof (1973), Awad and

Ayoub(1976), Chattopadhyay and Pise(1986) and Hannaand Afram(1986). These comparisons are presented inFigures 14 to 17. Meyerhofs theory, Hanna and Aframstheory and Awad and Ayoub gives general decreasingtrend for P u values with respect to increasing valueswhich is found to be conflicting with observedexperimental variation. The results from Chattopadhyay

and Pises theory show general trend of initial increaseand then decrease in P u values with respect to increasein -value having maximum value of P u at 20 . Upto60% plus or minus variation has been found from theexperimental Pu values.

Conclusion

> From the laboratory investigations that have beencarried out, the following conclusions are drawn.

> Axial pullout load versus axial displacementdiagrams for batter piles are practically linear atinitial stages of loading and non-linear at later

stages.> The resistances offered by the pile at any axial

displacement increases significantly with increaseroughness of pile i.e, sand coated piles are foundto be resisting more pullout forces than smoothpiles.

> The resistances offered by pile at any axialdisplacement increases significantly with increasein L/d ratio.

> Axial displacement of about 3% to 10% of pilediameter is required to attain the ultimate pulloutcapacity.

>

The net ultimate pullout capacity of a pileincreases significantly with increase in L/d ratio.

> Net ultimate pullout capacity of a pile increaseswith increase in batter angle, , attains maximum

Variation of n et ultimate pullou t capacity, Puwith L/d Ratio (Figure 13)

0

0.05

0.1

0.15

0.2

0.25

0.3

0 5 10 15 20 25 30 35

L/d

N E T U L T I M A T E P U L L O U T C A P

A C I T Y ( k N )

=0 =10 =20 =30

Fig 13. Net Uplift capacity v ersus L/d Ratio

0

0.01

0.02

0.03

0.04

0.05

0.06

0 10 20 30 40

BATTER ANGLE

N E T U L

T I M A T E

P U L L O U T

C A P A C I T Y

MEYERHOF

HANNA and AFRAM

AWAD and AYOUB

CHATTOPADHYAY andPISE

PRESENTEXPERINENTAL

INVESTIGATION

BATTER ANGLE

0

0.02

0.04

0.06

0.08

0.1

0.12

0 10 20 30 40BATTER ANGLE


P U L L O U T


MEYERHOF

HANNA and AFRAM

AWAD and AYOUB

CHATTOPADHYAY

and PISE

PRESENT

EXPERIMENTAL

INVESTIGATION

0

0.05

0.1

0.15

0.2

0.25

0 10 20 30 40

BATTER ANGLE


P U L L O U T


MEYERHOF

HANNA and AFRAM

AWAD and AYOUB


PRESENT EXPERIMENTALINVESTIGATION

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0 10 20 30 40

BATTER ANGLE

N E T U L T I M A T E P U L L O U T C A P A C I T Y

( k N )

MEYERHOF

HANNA and AFRAM

AWAD and AYOUB


PRESENT EXPERIMENTALINVESTIGATION

Fig 17. Net ultim ate pullout capacity - Batter angle forL/d=32

Fig 16. Net ultimate pullo ut capacity - Batter angle for

L/d=24

Fig 15. Net ultimate pullou t capacity - Batter angle forL/d=16

Fig 14. Net ultimate pull out capacity- Batter angle for L /d=8


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value and then decreases. The maximum value ofP u occurs at 20, and it is about 10% to 20%more than the vertical pile capacity.

> Pullout capacity also increases with increase indiameter.

> It is also found that for piles of varying shape butof constant volume, circular pile will resist moreuplift force compared to square or rectangularpiles.

> The variation of net ultimate pullout capacity withbatter angle, by Chattopadhyay and Pise (1986)analysis is qualitatively similar to observedexperimental variation. The theory also predictsmaximum capacity at 20.

> Method of Meyerhof (1973) and Hanna and Afram(1986) Awad and Ayoub(1976) predict decreasingtrend of P u values with increase in -value.

References

A.Awad And Ayoub(1976): Ultimate Uplift Capacity ofVertical and Inclined Piles in Cohesion less Soils.,Proc.5 th conference on Soil Mechanics and FoundationEngineering ,Budafest, pp. 221-226.

Chattopadhyay, B.C. And Pise, P.J.(1986): Axial UpliftCapacity of Inclined Piles, Indian Geotechnical JournalVol.16, No. 3, pp.198-213.

Das, B.M. And Seeley, G.R.(1975): Uplift Capacity ofBuried Model Piles in Sand, Journal of GTE Div.,ASCE ,Vol.10, pp.1091-1094.

Hanna, A.M. And Afram, A.(1986): Pullout Capacity of Single Batter Piles in Sand, Canadian Geotechnical

Journal , Vol.23, No.3, pp.387-392.Ismael, N.F. And Klym, T.W.(1979): Uplift and BearingCapacity of short piers in sand, Journal of GTE Div.,

ASCE , Vol.105, No.5, pp.579-594.

Meyerhof, G.G. And Adams, J.I.(1968): The ultimateuplift capacity of foundation, Canadian GeotechnicalJournal , Vol.5, No.4, pp.225-244.

K.Rajagopal And V.Srihari(1998): ExperimentalInvestigations on Pullout Capacity of Vertical Anchors,Indian Geotechnical Journal, Vol.28(2), pp147-166.

Shanker, K., Basudhar, P. K. And Patra N. R.(2006):

Uplift Capacity of Single Piles Embedded in Sand, IGJ .,Vol. 36. No. 4, pp.334-347.

B.V.R Sharma And P.J Pise.(1994): Uplift Capacity of Anchor Piles in Sand Under Axial Pulling Loads, IndianGeotechnical Journal , Vol.24 (3), pp181-202.

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