GeoPrediction_2011_FCTUNL

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Universidade Nova de Lisboa

Faculdade de Cincias e Tecnologia

Geo Prediction 2011

The Geo-Institute of the American Society of Civil Engineers

Name e-mail Status

Joo Miguel Rosa Pereira Mirante [email protected] Undergraduate

Ricardo Filipe Amaral Teixeira [email protected] Undergraduate

Lisbon, 2011-02-24


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GeoPrediction 2011 Geo-Institute of the American Society of Civil EngineersFaculdade de Cincias e Tecnologia Universidade Nova de Lisboa.

1

INTRODUCTION

Piles are usually used as the foundations

of structures in which the loads are either

very high, or the upper soil layer has very

weak resistance characteristics. They

transfer the load to the soil in a combination

of two ways, by skin friction capacity

(floating piles) and point capacity (end-

bearing piles). They can also be classified

by installation method (driven, cast-in-place,

auger, etc.), and come in a variety of

materials (steel, concrete, timber, etc.) andgeometry (circular, rectangular, H, etc.).

The pile in study is an open-ended steel

pipe, with a length of 160ft, a diameter of

18in, a wall thickness of 0,375in, and was

driven through 14ft of water into a clay

deposit and tested a week later.

2 PREDICTION METHODAs the soil is clayey and underwater, it

was assumed that it is saturated, which isproven in the saturation ratio of the samples,

leading to an undrained analysis.

To determine the point capacity, the

Meyerhoff (1976), Vesic (1977) and

Nottingham and Schmertmann (1975)

methods were used.

Meyerhoff uses the following expression:

9in which Ap represents the entire cross

section area, because, in clay, a plug isformed for a hollow pipe pile and cu is the

undrained shear strength at the base of the

pile.

Vesic uses this expression:

= + = 1+23

in which c is the cohesion of the bearinglayer, q is the vertical effective stress at the

pile tip, Nc*

and Nq*

are the bearing capacity

factors modified for deep foundations and

K0 is the coefficient of earth pressure at rest,

which was calculated from Bowles (1997)

(p41, eq. 2-21 and 2-21a) for an average

IP=48. Ir=200, from Bowles (1997) p. 894,

was assumed because the soil is a clay.

Nottingham and Schmertmann use the

following correlation from CPT data:

= 2 in which qc1 and qc2 are minimum

averages of qc values in the influence zones

of 4D below the pile tip and 8D above it, R1

is a reduction factor evaluated from

Gunaratne (2006) (table 6.3), and depends

on cu, and R2 is 1.0 for the electrical cone.

To determine the unit skin friction, the

following expression was used (Gunaratne,

2006):

= In which cu is the undrained shear

strength and is an adhesion factor that was

obtained from table 6.2 from Gunaratne,

(2006), API (1984) and Semple and Rigden

(1984). The undrained shear strength was

determined from the laboratory results and a

correlation method with the CPT results

obtained from Bowles (1997) (Chapter 3-

11.1: 172-177).

According to Bowles, the undrained

shear strength can be obtained by the conebearing resistance by the bearing capacity

equation that is as follows:

=

where qT=adjusted total tip resistence

(corrected by the measured pore pressure

and the area ratio); p0=overburden pressure;

GeoPrediction 2011

Joo Miguel Rosa Pereira Mirante, Lisboa, Portugal, [email protected]

Ricardo Filipe Amaral Teixeira, Lisboa, Portugal, [email protected]


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GeoPrediction 2011 Geo-Institute of the American Society of Civil EngineersFaculdade de Cincias e Tecnologia Universidade Nova de Lisboa.

NkT=cone factor, that depends on the

plasticity index.

The calculation of the adhesion factor was

done by three methods, as stated. Gunaratne

cited Peck (1974) that indicated (from 1 to

0.5) depending on the undrained strength.

API (1984) and Semple and Rigden (1984)

proposed two different methods to reach for driven piles in clay. Both methods

depend only on cu.

Unit skin friction fs was then calculated

and from it was reached the skin friction

capacity Rs by:

=

Where is the perimeter of the pilesection,

is the coordinate axis along the

pile and is the length of the pile. Since and cu were obtained from three and two

different method, respectively, it was

obtained six different results of skin friction

capacity.

The soil was divided into two layers, an

upper layer with cu800psf, because

of the significant differences in unit weight

and cohesion.

3 RESULTSThe soils saturated unit weight was

calculated from the laboratory test results.An average of the wet unit weight, becausethe samples were saturated, was done forboth layers.

Table 1 - Unit weight

Unit weight [lb/ft3]

Upper layer 102.8

Lower layer 113.4

In order to determine the value and

relation between the undrained shear

strength and depth, a graph was plotted, with

the help of Excel, and a linear trendline was

applied.

Figure 1 - cu vs depth

In fig. 1, the blue dots are the cu valuesfrom the laboratory, the green are from the

CPT correlation and the red line represents

cu=800psf. It is clear, in fig.1, the good

match between the lab and field results and

the relation of cu vs depth with a high R2

value.

Although this is a floating pile because it

does not reach the bedrock, it was decided to

calculate its point capacity, and the results

are presented in table 2.

Table 2 - Point Capacity

Meyerhoff Vesic N & S

Pp [tf] 17.65 26.91 21.05

average 21.87

The six different results of skin friction

capacity are plotted versus depth in the chart

presented next.

depth = 0.0721Cu + 14

R = 0.8747

0

20

40

60

80

100

120

140

160

180

0 500 1000 1500 2000 2500 3000

Depth[ft]

Cu [psf]


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GeoPrediction 2011 Geo-Institute of the American Society of Civil Engineers

Faculdade de Cincias e Tecnologia Universidade Nova de Lisboa.

Figure 2 - Skin friction capacity

A summarization of the total skin friction

capacity is presented in table 3:

Table 3 - Total skin friction capacity

Skin capacity

Method Rs (tf)

lab test

Peck 370.64

API 255.72

Semple 275.18

CPT

Peck 408.81

API 425.06

Semple 519.28

average 375.78

4 DISCUSSIONBy analyzing fig. 2, one can observe a

similar evolution of Ps from the different

methods in depth. All the methods were

used because they were considered reliable,

and the similarity of results certifies the

results. Thus, it was decided to make an

average of the six results to reach the skin

friction capacity of the pile, as presented in

table 3.

5 CONCLUSIONSIt was concluded that the skin friction

capacity is the most important component of

the total capacity of the pile, as assumed.

The total axial capacity of the pile is:

= 397.65

REFERENCES

Meyerhoff, G.G., 1976, Bearing capacity andsettlement of pile foundations, Journal ofGeotechnical Engineering, ASCE, 102(GT3):197227.

Vesic, A.S., 1977, Design of Pile Foundations,National Cooperative Highway ResearchProgram, Synthesis of Practice, No. 42,Transportation Research Board, Washington, DC.

Nottingham, L. and Schmertmann, J., 1975, An Investigation of Pile Design Procedures, FinalReport D629 to Florida Department ofTransportation, Department of Civil Engineering,University of Florida.

Gunaratne, M., 2006, The Foundation EngineeringHandbook, Chapter 6: 235-254, Taylor & FrancisGroup, Boca Raton, Florida.

API, 1984, API recommended practice for planning,designing and construction of fixed offshoreplatforms, 15th Ed., API RP2A, AmericanPetroleum institute.

Semple, R.M., Rigden, W.J., 1984, Shaft capacity ofdriven pipe piles in clay, Proceedings ofSymposium on Analysis and Design of PileFoundations, ASCE, edited by J.R. Meyer.

Bowles, J.E., 1997, Foundation Analysis and Design,McGraw-Hill, Singapore.

Peck, R.B., 1974, Foundation Engineering, 2nd ed.,John Wiley, New York.

0

20

40

60

80

100

120

140

160

180

0 200 400 600

Depth(ft)

Rs (tsf)

Rs (Peck) - lab test

Rs (API) - lab test

Rs (Semple and Ridgen)

- lab test

Rs (Peck) - CPT

Rs (API) - CPT

Rs (Semple and Ridgen)

- CPT

GeoPrediction_2011_FCTUNL

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