pile foundation 2

8/17/2019 Pile Foundation 2

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NPTEL – ADVANCED FOUNDATION ENGINEERING-I

Module 8

(Lecture 33)

PILE FOUNDATIONS

Topics

1.1

PILE-DRIVING FORMULAS

1.2

NEGATIVE SKIN FRICTION

Clay Fill over Granular Soil

Granular Soil Fill over Clay

1.3 GROUP PILES

1.4

GROUP EFFICIENCY


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PILE-DRIVING FORMULAS

To develop the desired load-carrying capacity, a point bearing pile must penetrate the

dense soil layer sufficiently or have sufficient contact with a layer of rock. Thisrequirement cannot always be satisfied by driving a pile to a predetermined depth

because soil profiles vary. For that reason, several equations have been developed to

calculate the ultimate capacity of a pile during driving. These dynamic equations arewidely used in the field to determine whether the pile as reached satisfactory bearing

value at the predetermined depth. One of the earliest of these dynamic equations-

commonly referred to as the Engineering News Record (ENR) formula-is derived fromthe work-energy theory. That is,

Energy imparted by the hammer per blow = (pile resistance) (penetration per hammer blow)

According to the ENR formula, the pile resistance is the ultimate load , expressed as = ℎ+ [8.118]Where

= ℎ ℎ ( , . 4 ) ℎ = ℎℎ ℎ = ℎ = The pile penetration, S , is usually based on the average value obtained from the last few

driving blows. In the equations’ original form, the following values of C were

recommended.

For drop hammers: C = 1 in. (if the units of S and h are in inches)

For steam hammers: C = 0.1 in. (if the units of S and h are in inches)

Also, a factor of safety, = 6, was recommended to estimate the allowable pilecapacity. Note that, for single- and double-acting hammers, the term ℎ can be replaced


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by (where = ℎ = ℎ).Thus

= + [8.119]The ENR formula has been revised several times over the years, and other pile-drivingformulas also have been suggested. Some of them are tabulated in table 1.

The maximum stress developed on a pile during the driving operation can be estimated

from the pile-driving formulas presented in table 11. To illustrate, we use the modified

ENR formula:

= ℎ+ +2+ In this equation, S equals the average penetration per hammer blow, which can also be

expressed as

= 1 [8.120]Where

ℎ = ℎ ℎ Table 11 Pile-Driving formulas

Name Formula

Modified ENR

formula = ℎ + + 2 + Where

= ℎ = 0.1 . , ℎ ℎ ℎ = ℎ ℎ = ℎ ℎ Typical values for E


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Single- and double-acting hammers 0.7-0.85

Diesel hammers 0.8-0.9

Drop hammers 0.7-0.9

Typical values for n

Cast iron hammer and concrete pile (without cap) 0.4-0.5

Wood cushion on steel piles 0.3-0.4

Wooden pile 0.25-0.3

Michigan State

HighwayCommission

formula (1965)

= 1.25 + + 2 +

Where

= ′ ℎ ( −. ) = ℎ = 0.1 . A factor of safety of 6 is recommended.

Danish formula(Olson and Flaate,

1967) =

+ 2

Where


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= ℎ = ℎ

=

ℎ

= ℎ = ℎ

Pacific Coast

Uniform Building

Code formula

(International

Conference of

Building Officials,1982)

= () + + +

The value of n should be 0.25 for steel piles and 0.1 a for all other

piles. A factor of safety of 4 is generally recommended.

Janbu’s formula(Janbu, 1953)

= ′

Where

′ = 1 + 1 + ′ = 0.75 + 0.14

′ =

2

Gates formula

(Gates, 1957) = ( − ) If is in , then S is . = 27, = 1, is in − .


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If is in , then S is in mm, = 104.5, = 2.4, is in −

= 0.75 for drop hammer;

= 0.85 for all other hammer

Use a factor of safety of 3.

Navy-McKay

formula = 1+0.3

Use a factor of safety of 6.

Thus

=

ℎ(1/)+0.1+2

+ [8.121]

Different values of N may be assumed for a given hammer and pile and calculated.The driving stress can then be calculated for each value of N and / .

= 100 2 The weight of the pile is

=

100 2144

(80

)(150

/

3) = 8.33

If the weight of the cap is 0.67 , = 8.33 + 0.67 = 9 Again, from for an 11B3 hammer,

Rated energy= 19.2 − = = ℎ Weight of ram= 5 Assume that the hammer efficiency is 0.85 and that

= 0.35. Substituting these values

in equation (121) yields

= (0.85)(19.2×2)1+0.1 5+(0.35)2(9)5+9 = 85.371+0.1 Now the following table can be prepared:


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() (2) / (/2) 0 0 100 0

2 142.3 100 1.42

4 243.9 100 2.44

6 320.1 100 3.20

8 379.4 100 3.79

10 426.9 100 4.27

12 465.7 100 4.66

20 569.1 100 5.69

Both the number of hammer blows per inch and the stress can now be plotted in a graph,

as shown in figure 8.47. If such a curve is prepared, the number of blows per inch of pile

penetration corresponding to the allowable pile-driving stress can be easily determined.

Figure 8.47

Actual driving stresses in wooden piles are limited to about 0.7 . Similarly, for concreteand steel piles, driving stresses are limited to about 0.6 ′ 0.85 , respectively.


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In most cases, wooden piles are driven with hammer energy of less than 45 − (≈ 60 ∙ ). Driving resistances are limited are limited mostly to 4-5 blows per inch of pile penetration. For concrete and steel piles, the usual N values are 6-8 and 12-14,

respectively.

Example 11

A precast concrete pile 12 .× 12 . in cross sections in driven by a hammer. Given: ℎ = 30 − = 0.8 ℎ = 7.5 ℎ = 80 = 0.4 ℎ = 550 = 3× 106 /2 Number of blow for last 1 in. of penetration = 8

Estimate the allowable pile capacity by the

a. Modified ENR formula (use = 6) b. Danish formula (use

= 4)

c.

Gates formula (use = 3) Solution

Part a

= (0.8)(30×12 − .)18+0.1

×7.5+(0.4)2(12.55)

7.5+12.55= 607

= = 6076 ≈ 101 Part b

= + 2

Use = 3× 106 /2.


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2 = (0.8)(30×12)(80×12)2(12×12)3×106

1000 /2 = 0.566 .

=

(0.8)(30×12)18+0.566

≈417

= 4174 ≈ 104 Part c

= ( − ) = 2 7 (0.8)(30) [1−18] ≈ 252 = 2523 = 84 NEGATIVE SKIN FRICTION

Negative skin friction is a downward drag force exerted on the pile by the soilsurrounding it. This action can occur under conditions such as the following:

1. If a fill of clay soil is placed over a granular soil layer into which a pile is driven,the fill will gradually consolidate. This consolidation process will exert a

downward drag force on the pile (figure 8.48a) during the period of

consolidation.2. If a fill of granular soil is placed over a layer of soft clay, as shown in figure 8.

48b, it will induce the process of consolidation in the clay layer and thus exert a

downward drag on the pile.3. Lowering of the water table will increase the vertical effective stress on the soil at

any depth, which will induce consolidation settlement in clay. If a pile is located

in the clay layer, it will be subjected to a downward drag force.

In some cases, the downward drag force may be excessive and cause foundation failure.

This section outlines two tentative methods for the calculation of negative skin friction.


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Figure 8.48 Negative skin friction

Clay Fill over Granular Soil (Figure 8.48a)

Similar to the method presented in section 12, the negative (downward) skin stress onthe pile is

= ′ ′ [8.122] Where

′=

ℎ

=

= 1

−

′ = ℎ = ′ ′ = ℎ = − ≈ 0.5− 0.7 Hence the total downward drag force, , on a pile is = ∫ ( ′ ′ ) = ′ ′ 2 2 0 [8.123]Where

= ℎℎ ℎ If the fill is above the water table, the effective unit weight, ′ , should be replaced by themoist unit weight.


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Granular Soil Fill over Clay (figure 8.48b)

In this case, the evidence indicates that the negative skin stress on the pile may exist from = 0 = 1, which is referred to as the neutral depth (see Vesic, 1977, pp. 25-26, fordiscussion). The neutral depth may be given as (Bowles, 1982):

1 = (− )1 − 2 + ′ ′ − 2′ ′ [8.124]Where

′ ′ = ℎ ℎ ℎ , For end-bearing piles, the neutral depth may be assumed to be located at the pile tip (that

is, 1 = − ).Once the value of 1 is determined, the downward drag force is obtained in the followingmanner. The unit negative skin friction at any depth from = 0 = 1 is = ′′ [8.125]Where

′ = = 1 − ′ = ′ + ′

= 0.5

−0.7

= ∫ = ∫ ′ ′ ′ 1010 = ( ′ ′ )1 + 12 ′ ′ 2 [8.126] If the soil and the fill are above the water table, the effective unit weights should be

replaced by moist unit weights. In some cases, the piles can be coated with bitumen in the

downdrag zone to avoid this problem. Baligh et al. (1978) summarized the results ofseveral field tests that were conducted to evaluate the effectiveness of bitumen coating in

reducing the negative skin friction. Their results are presented in table 12.

A limited number of case studies on negative skin friction is available in the literature.

Bjerrum et al. (1969) reported monitoring of downdrag force on a test pile at Sorenga in

the harbor of Oslo, Norway (noted as pile G in the original paper). This was alsodiscussed by Wong and The (1995) in terms of the pile being driven to bedrock at 40 m.

Figure 8.49 a shows the soil profile and the pile. Wong and The (1995) estimated the

following:


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Figure 8.49 Negative skin friction on a pile in the harbor of Oslo, Norway [based onBjerrum et al., (1969); and Wong and The (1995)]

: ℎ, = 16 /3 ℎ, ( ) = 18.5 /3 So

′ = 18.5− 9.81 = 8.69/3 = 13 :′ ≈ 0.22 ℎ, ′ = 19 − 9.81 = 9.19/3

:

= 40

, = 500 Thus, the maximum downdrag force on the pile can be estimated from equation. (126).

Since it is a point bearing pile, the magnitude of 1 = 27, so = ()(′ )[ × 2 + (13 − 2) ′ ](1) + 12 ′ (′ )2


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Or

= ( × 0.5)(0.22)[(16×2) + (8.69 × 11)](27) + (27)2(×0.5)(9.19)(0.22)2 = 2348 The measured value of maximum

was about 2500

(figure 8. 49b), which is in

good agreement with the calculated value.

Table 12 Summary of Case Studies of Bitumen-Coated Piles(After Baligh et al.

(1978))

Downward drag Test loadings

Case number 1 2 3 4 5 6 7

Soil type Fill,sand, and

clay

Fill andsilty clay

Fill andclay

Sandand

silty

clay

Siltyclay

Siltyclay

Sandfill,

clay,

and peat

Pile type

Pile crosssection (mm)

Cast-in-

place

concrete

= 530 Steel

pipe

= 300

Steel

pipe

= 500

Steel

pile

= 760

6 RC

piles

300× 300

6 RC

piles

300× 300

Precast

concrete

380

× 450

Length incontact with

settling soil

(m)

25 26 40 25 7-17 9-16 24

Installation

method

Predriven

casing

Enlarged

tip and

slurry

Enlarge

tip and

casing

Driving Driving Driving Driving

Bitumen

coating

Type (pen

25° )Coating

thickness

(mm)

20/30

10

80/100

1.2

80/10

1.2

60/70

1.5

60/70

1.2

80-100RC-0

cutback

1.2

43special

grade

10


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Measured

shaft

resistance

Uncoated pile (ton)

Coated pile(ton)

Coating

effectiveness

(%)

70-80

5-7

92

120

10

92

300

15

95

180

3

98

31-40

10-33

30-80

31-40

20-42

30-80

160

Predicted

downdrag

Coated pile

(ton)

Coating

Effectiveness(%)

0.1

100

2-11

91-98

5

98

0-23

87-100

Example 12

Refer to figure 8. 48a; = 3. The pile is circular in cross section with a diameter of0.5 m. For the fill that is above the water table, = 17.2 /3 = 36°.Determine the total drag force. Use = 0.7.Solution

From equation. (123),

= ′ 2 2

= (0.5

)= 1.57 ′ = 1− = 1− 36° = 0.41

= (0.7)(36) = 25.2° = (1.57)(0.41)(17.2)(3)2 25.22 = 23.4


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Example 13

Refer to figure 8. 48b. Here,

= 2

,

= 0.305

,

= 16.5

/

3,

= 34°

, = 17.2 /3

, = 20. The water table coincides with the topof the clay layer. Determine the downward drag force. Assume = 0.6 .Solution

The depth of the neutral plane in given in equation (124) as

1 = − 1 − 2 + ′ − 2 ′ Note that ′ in equation (124) has been replaced by because the fill is above the watertable, so

1 = (20−2)1 (20−2)2 + (16.5)(2)(17.2−9.81) − (2)(16.5)(2)(17.2−9.81) 1 = 242.41 − 8.93; 1 = 11.75 Now, referring to equation (126), we have

= ( ′ )1 + 12′ ′ 2

=

(0.305) = 0.958

′ = 1− 34° = 0.44 = (0.958)(0.44)(16.5)(2)[(0.6 × 34)](11.75) +

(11.75)2(0.958)(0.44)(17.2−9.81)[ (0.6×34)]2

= 60.78 + 79.97 = 140.75 GROUP PILES

GROUP EFFICIENCY

In many cases, piles are used in groups, as shown in figure 8.50, to transmit the structural

load to the soil. A pile cap is constructed over group piles. The pile cap can be contact

with the ground, as in most cases (figure 8.50a), or well above the ground, as in the caseof offshore platforms (figure 8.50b).


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Figure 8.50 Pile groups

Determining the load-bearing capacity of group piles is extremely complicated and has

not yet been fully resolved. When the piles are placed close to each other, a reasonableassumption is that the stresses transmitted by the piles to the soil will overlap (figure 8.50c), reducing the load-bearing capacity of the piles. Ideally, the piles in a group should

be spaced so that the load-bearing capacity of the group should not be less than the sumof the bearing capacity of the individual piles. In practice, the minimum center-to-center

pile spacing,

,

2.5

, and in ordinary situations, is actually about 3

−3.5

.

The efficiency of the load-bearing capacity of a group pile may be defined as

= ( ) [8.127]Where

=


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() = − ℎ = − ℎ ℎ ℎ Many structural engineers use a simplified analysis to obtain the group efficiency for

friction piles, particularly in sand. This type of analysis can be explained with the aid offigure 8. 50a. Depending on their spacing within the group, the piles may act in one of

two ways: (1) as a block with dimensions × × , or (2) as individual piles. If the piles act as a block, the frictional capacity is ≈ (). [Note: = perimeter ofthe cross section of block = 2(1 + 2 − 2) + 4 , = average unit frictionalresistance.] Similarly, for each pile acting individually, ≈ . (Note: = perimeter of the cross section of each pile.) Thus

= ( ) = [2(1+2−2)+4]12 = 2(1+2−2)+412 [8.128] Hence

() = 2(1+2−2)+4 12 [8.129]From equation (129), if the center-to-center spacing, , s large enough, > 1. In thatcase, the piles will behave as individual piles. Thus, in practice, if < 1,() = And, if

≥1,

() = There are several other equations like equation (129) for the group efficiency of friction

piles. Some of these are given in table 13.

Feld (1943) suggested a method by which the load capacity of individual piles (friction)

in a group embedded in sand could be assigned. According to this method, the ultimatecapacity of a pile is reduced by one-sixteenth by each adjacent diagonal or row pile. The

technique can be explained by referring to figure 8.51, which shows the plan of a group

pile. For pile type A, there are eight adjacent piles; for pile type B, there are five adjacent

piles; and for pile type C, there are three adjacent piles. Now the following table can be prepared:


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Figure 8.51 Feld’s method for estimation of group capacity of friction piles

Table 13 Equations for Group Efficiency of Friction Piles

Converse-Labarre equation = 1 − (1 − 1)2 + (2 − 1)19012 ℎ () = −1(/)

Los Angles Group Action

equation = 1− 12 [1(2 − 1)] + 2(1 − 1)

+ √ 2(1 − 1)(2 − 1)] Seiler and Keeney equation

(Seiler and Keeney, 1944) = �1 − 111

7(2 − 1) 1 +

2

−2

1 + 2 − 1+0.3

1 + 2 ℎ Pile type No. of

Piles

No. of adjacent

piles/pile

Reduction

factor for each pile

Ultimate

capacity

A 1 81

−8

16

0.5

B 4 51− 5

16

2.75 C 4 3

1− 316

3.25


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6.5 = ()

(No. of piles)(

) (reduction factor)

= Hence

= ( ) = 6.5 9 = 72% Figure 8.52 shows a comparison of field test results in clay with the theoretical group

efficiency calculated from the Converse-Labarre equation (table 13). Reported by Brand

et al. (1972), these tests were conducted in soil for which the details are given in figure 8.

7 from chapter 3. Other test details includeℎ = 6 = 150 = 2 × 2 ℎ = 1.5 ℎ

Figure 8.52 Variation of group efficiency with / (after Brand et al., 1972)


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Pile tests were conducted with and without a cap (free-standing group). Note that for/ ≥ 2, the magnitude of was greater than 1.0. Also for similar values of / thegroup efficiency was greater with the pile cap than without the cap. Figure 8.53 shows

the pile group settlement at various stages of the load test.

Figure 8.53 Variation of group pile settlement at various stages of load (after Brand et al.,

1972)

Figure 8.54 Variation of efficiency of pile group in sand (based on Kishida and

Meyerhof, 1965)


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Figure 8.55 Behavior of low-set ad high-set pile groups in terms of average skin friction

(based on Liu et al., 1985)


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Figure 8. 55 (Continued)

Figure 8.54 shows the variation of group efficiency () 3 × 3 group pile in sand(Kishida and Meyerhof, 1965). It can be seen that, for loose and medium sands, themagnitude of group efficiency is larger than one. This is primarily due to the

densification of sand surrounding the pile.

Liu et al. (1985) reported the results of field tests on 58 pile groups and 23 single piles

embedded in granular soil. Test details include

ℎ, = 8 − 23 , = 125 − 330 =

,

= 2

−6

Figure 8. 55 shows the behavior of 3 × 3 pile groups with low-set and high-set pile caps

in terms of average skin friction, . Figure 8.56 shows the variation of average skinfriction based on the location of a pile in the group.

Figure 8.56 Average skin friction based on pile location (based on Liu et al., 1985)


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Based on eh experimental observations of the behavior of group piles in sand to date, the

following general conclusions may be drawn.

1. For driven group piles in sand with ≥ 3,() may be taken to be

, which includes the frictional and the point bearing capacities of individual

piles.2. For bored group piles in sand at conventional spacings ( ≈ 3),() may betaken to be 23 3

4 (frictional and point bearing capacities of individual

piles).

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