foundation ii

7/25/2019 Foundation II

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Bahir Dar University Faculty of Engineering Department of Civil Engineering

1

Lecture Note (Amsalu G.) Academic Year: 2005/2006

CH PTER TWO

PILE FOUND TION

1. Design a pile foundation for the following soil profile and column load at foundation level.

Solution: The following assumptions are made for the soil data to compute individual pile

capacity

Kpa19.6)3/30*2tan(*1*2*17*2

1tan

2

1

1

skh

Note ve value as there may be consolidation

negative skin friction

Kpa75.332

150*45.0

2

qc u

2

Kpa5.672

300*45.0

2

qc

u

3

Individula pile capacity:

Consider the depth of imbedment to be below 10m, specifically at 13m (arbitrary), and pile

diameter of 400mm, thusSkin Friction

1(Kpa)=-6.19 2(Kpa)=33.75 3(Kpa)=68 4(Kpa)=

L1(m)=2 L2(m)=8 L3(m)=3 L4(m)=

Perimeter =pD = 1.26Qalls(KN)= 281.32

Bearing pressure

Qub(KN)= CNc* Base area=(9*C)*Base area

Qub(KN)= 169.65 Qallb(KN)= 84.82

Total Single Pile Capacity (From soil)

Qall(KN)= Qalls + Qallb = 366.15

Perimet*2)FS(

LLLLL

55443322

11all

Q

Fi l l

C = 0 , = 3 0 = 1 7 . 0 K N / m

3

+ 0 . 0 m

- 2 . 0 m

St i f f c l ay

q = 1 5 0 K P a ,

= 1 7 . 0 K N / m- 1 0 . 0 m3u

- 2 0 . 0 mu 3

- 3 0 . 0 m

F i g u r e a : S o i l D a t au lt

B a s a l t

q = 1 0 ,0 0 0 K P a

St i f f c l ay

q = 3 0 0 K P a ,

= 1 9 . 0 K N / m

o

3

D e n s e S a n d

= 3 0

= 1 9 . 0 K N / m

M = 3 00 K N mx

yM = 2 00 K N m

4 0 0 m m

600m

m x

y

F i g u r e b : L o a d D a t a

P = 2 0 0 0 K N

Use

Concrete C-30

Steel S-300

F i l l

C = 0 , = 3 0

= 1 7 . 0 K N / m3

+ 0 . 0 m

- 2 . 0 m

S t i f f c l a y

q = 1 5 0 K P a ,

= 1 7 . 0 K N / m- 1 0 . 0 m3u

- 2 0 . 0 m

S t i f f c l a y

q = 3 0 0 K P a ,

= 1 9 . 0 K N / mu 3

D e n s e S a n d

= 3 0

= 1 9 . 0 K N / m3

- 3 0 . 0 m

o

B a s a l t

q = 1 0 , 0 0 0 K P au lt

A s s u m e

/3

K = 1 . 0

A s s u m e

A s s u m e

s

P

1

2

3q

b u


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2


From concrete capacity: fcd=0.67fck/1.5=13.4MPa

Pall= Area*fcd= p(0.2)2*13400=1683.9KN; Hence soil strength governs. Pall=366.15KN

Number and spacing of piles

It can be easily estimated (from load and single pile capacity) that using 8 piles under a pile cap

is reasonable. A pile spacing of 3d can be adopted (EBCS 7). Thus s=3d=3*400=1200mm. The

pile cap shall be extended by 500mm (=300+d/2) from centerlines of exterior piles. Thus pilearrangement will be as shown below.

Now we can determine the load carried by each pile.

2222222

2

y

2

x

i

2.88m6.08x;14.4m6.08.14y

x

xM

y

yM

n

PP

P =2000+pile cap weight (assuming 500mm thickness)=2127KN

88.2

200x

4.14

300y

8

2127P

i

Pi x y Value Pi x y Value

PA1 -0.6 1.8 261.7 PC1 -0.6 -0.6 211.7

PA2 0.6 1.8 345.0 PC2 0.6 -0.6 295.0

PB1 -0.6 0.6 236.7 PD1 -0.6 -1.8 186.7

PB2 0.6 0.6 320.0 PD2 0.6 -1.8 270.0 Thus Pmax=345KN < Pall=366.15KN OK! (8 piles would be enough)

Check pile group capacity

Pile group capacity is considered for the right figure.

Thus Pug=fsuLi P +qbu Ab- i hi APall g= fsallLi P +qball Ab- i hi A

First lets evaluate qbu:

qbu= CNc = 150*9 = 1350Kpa

qball=1350/2=675Kpa

Group perimeter, P =2*(1.6+4) =11.2m;

2127KNP5374.5KN1452.84320.02507.34P

)6.1*4(*3*198*172*17)6.1*4(*6752.11*2

3*67.58*33.752*6.19-P

appliedug

ug

Design of Pile Cap

Try a thickness D = 500mm

d = D-100-75 = 325mm (See Fig 2.16)

a.Check wide beam shear:

i) along the long direction (Sec L-L)

1189.17KPa0.325*2.2

850.26

db

P v

26.850PP400

175PPP

w

w

B2B1A2A1w

KN

;501f0.3ctdall

v 1.28MPa)5.1(

f0.35f0.005;Assume

c

ck

ctd

0 .6

0 .4

0. 5

0. 5

1 .2

1 .2

1 .2

1 .20 .5 0 .5

y

x

1 2

A

B

C

D

4 .6

2 .2

0 .4

0 .61

2

0 .2

0 .2

1 .2

1 .2 1 .2 1 .20 .20 .2

CD

B A

4 m

1.6m

4 .61 .2 0 .6

1 .2

0 .5

1

0 .4

2

D

C

1 .2

0 .5

2 .2

0 .5

y

1 .2

x

0 .5

B

A

dd

LL

S

S


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3


D.increase480Kpawall

vv

Let D = 750mm d=750-100-75 = 575mm

OK!KPa6.4790.575*2.2

606.7

7.606PPP

allw

A2A1w

vv

KN

Therefore use pile cap which is 750mm thick

ii) along the short direction (Sec S-S)

OK!KPa42.330.575*4

76.88

88.76PPPP400

25P

allw

2D2C2B2Aw

vv

KN

b.Check punching shear:

Critical section is at d/2 from face of support (see fig right!)

It can be shown that about 7.24% support is contributed

from each pile reaction to resist punching. Thus,

600mmdand775mmDuseTherefore

OK!765.4KPa6.0*1.20.12

P

2020.65KNPPPP100

101272P

775mmDUseD!Increase800Kpa5010.5f

12.8290575*1.1750.9752

P

2050KNPPPP100

24.71272P

PPPP100

24.7PP

pu

pu

2C1C2B1Bpu

ctdpall,

pu

pu

2C1C2B1Bpu

2C1C2B1Bappliedpu

v

v

Kpav

Reinforcement

a) In the long direction:

Critical section is at the face of the column

300,-Sand30-Cwithand64.360.6*2.2

1063.11

bd

MK

mKN11.10633.0*PP5.1*PPM

22

u

m

B2B1A2A1max

Km= 36.64

K s= 4.08 As(mm2)= 7224.01

# 24= 16.0 Use 16bars in the long direction; better to use more bars directly above the piles.

b) In the short direction:

mKN70.12300.1*PPPPMD2C2B2A2max

Km= 27.26

K s= 3.99 As(mm2)= 8189.514

# 24= 18.1

Use 18bars in the short direction; better to use more bars directly above the piles.

0 .6

0 .4

0 .5 d

C

B

1 2

0 .5 d

0 . 4 + d = 9 7 5 m m

0.6+

d=11

75m

m

4 .61 .2 0 .6

1 .2

0 .5

1

0 .4

2

D

C

1 .2

0 .5

2 .2

0 .5

y

1 .2

x

0 .5

B

A

dd

LL

S

S


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4


Design of Pile shaft

Asmin=0.008Ac =1005mm2. # 16 = 5. Use 6 16 as it is circular shaft and a minimum of 6

bars are needed. Use spiral tie with a pich of

4 .6

2 .2

0 .5

1 .2

1 .2

1 .2

0 .5

0 .5 1 .2 0 .5

a

b

Se c t ion A -A

a 1 8 2 4 , L = 3 2 6 0

6 0 06 0 0

2 0 6 0

6 3 5

6 3 5

16

24,L

=

5730

b

4460

4 24

5 24

5 24

4 2 4

8

24

8

24

foundation ii

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