8/2/2019 GeoPrediction_2011_FCTUNL
1/4
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
8/2/2019 GeoPrediction_2011_FCTUNL
2/4
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]
8/2/2019 GeoPrediction_2011_FCTUNL
3/4
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]
8/2/2019 GeoPrediction_2011_FCTUNL
4/4
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