Capacidad Axial de Pilotes Simples

8/12/2019 Capacidad Axial de Pilotes Simples

1/32

8. Axial Capacity

of Single Piles

CIV42491998 Dr . J .P. Seid el

Mod ified by J.K . K odik ara, 2001


2/32

Methods

Pile dr ivin g fo rm ulae Static lo ad test Dyn am ic or Statnam ic load tes t Stat ic form ulae


3/32

Pile driving formulae e.g. Hiley for m ula (Energ y balance)

Q = e .W.h .

F (s et + tc / 2) R u = w ork ing lo ad, W=weigh t of th e

hamm er, h= height o f the ham m er dro p(str ok e), F=facto r o f s afety

tc= elast ic (tem po rary) co m press ion e = eff ic iency

F s

tc

R u


4/32

Static Load Test

Plung ing fai lure

Load to speci f iedcon trac t requi rement

What is thefai lure load?

Davissons Method But ler and HoyChins Method Br inch Hansonetc. etc.

What i s the d is t r ibut ionof resis tance?

Approx imate methodsIns t rumenta t ion

Load

Deflect ion


5/32

Dynamic and Statnamic

Testing Methods Rapid al ternat ives to s ta t ic tes t ing Cheaper Separate dyn am ic resis tanc e Correlat ion


6/32


7/32

Base ResistanceQ b = A b [c b N c + P ob (N q - 1) + 0.5 gB N g + P ob ]

m inus w eight o f p i le, W p

but W p A b .P ob

and as L >>B , 0.5 gB N g 0, N q - 1 N qQ

b


8/32

Shaft Resistance

Due to co hes ion or f r ic t ion

Cohesive co m po nent : Q sc = A s . a . c s

Fric t ion al com pon ent : Q s f = A s .K P os tan d

P o sK.P os

Q s = Q sc + Q s f = A s [ a .c s + K P os tan d ]

As


9/32

Total Pile Resistance

Q u = Q b + Q s

Q u = Ab [c b N c +P ob N q ] + A s [ a .c s +K P o tan d ]

How do w e com pute Q u w hen sh af t res is tanc ealon g th e pi le is varying ?


10/32

Mobilization

Shaft

2 - 5mm

Base

10 - 20% diam

Total

Sett lement

L o a

d


11/32

Piles in Clay

Q u = A b [c b N c +P ob N q ] + A s [ a .c s +K P o tan d ]


Q u = A b c b N c + A s a .c s


Q u = A

b P

ob N

q + A

s K P

o tan d

Q u = A b c b N c + A s a .c s

Q u = A b P ob N q + A s K P os tan d

Undrained

Drained / Effectiv e


12/32

Driven Piles in Clay

2.0

1.5

1.0

0.5

0 10 20 30 40 50 60r a

Du

vo

Average curve for sensitiveamarine clay

Average curve for clays of low-medium sensitivity


13/32

Driven Piles in Clay

300

250

200

150

100

50

01 5 10 50 100 500 1000

Time after driving in days

B e a r

i n g c a p a c

i t y

i n

k N 200 x 215mm conrete

(Gothenberg)

300 x 150mm tapered timber (Drammen)

150mm (8 in) steel tube (San Francisco)300 x 125mm I-Beam

(Gothenberg)

30

25

20

15

10

5

B e a r

i n g c a p a c

i t y

i n

t o n s


14/32

Nc Parameter

N c

Compare Skemptons N c for sh al low foun dations

N c = 5(1+0.2B /L)(1+0.2D/ B )

10

9

8

7

6

50 1 2 3 4 5

L/d B

B e n

d i n g c a p a c i

t y f a c t o r

N c


15/32

Adhesion Factor, a

50 100 150 200 250

1000 2000 3000 4000 5000

2.0

1.5

1.0

0.5

0

Figures denote penetration ratio =Depth of penetration in clay

Pile diameter Key:Steel tube pilesPrecast concrete

pilesDesign curve for

penetration ratio >

49 49

49 56 13 15

17 27 33

4010

5815

38

3327 39

44

44 39

1917

19

13

35 44 A d h e s

i o n

f a c t o r

Undrained shear strength (c ) lb/ft 2u

Undrained shear strength (c ) kN/m 2u

20

1.0

0.8

0.6

0.4

0.2

0 100 200

Average Undrained Shear Strength, c , kPau

R e d u c t

i o n

F a c

t o r ,

a

A us t. Piling Cod e,A S159 (1978)


16/32

Bored Piles in Clay

Skemptons recommendations for sideres is tance

=0.45 fo r c u 215 kPa

N c is l im ited to 9. A reduct io n factor is app l ied to accou nt for

lik ely fis su rin g (I.e., Q b = A b c b N c )


17/32

Soil disturbance

samp l ing at tem pts to es tabl ish in-s i tus t rength values

so i l is fai led/remo uld ed b y dr iv ing o rdr i l l ing

pile ins tal lat ion caus es su bs tant iald is turbance

bo red p i les : potent ial loosening driven p i les : p robable densif icat ion


18/32

Scale effects

Laboratory samp les or in -s i tu tes tsinvo lve sm al l volum es o f so i l

Fai lure of so i l aroun d pi les inv olves m uchlarger so i l volum es

If so i l is f iss ured, the sam ple may n ot b erepresentat ive


19/32

Q u = A b [c b N c +P ob N q ] + A s [ a .c s +K P os tan d ]

Piles in Sand

Q u = A b [c b N c +P ob N q ] + A s [ a .c s +K P os tan d ]

Q u = A

b P

ob N

q ] + A

s K P

os tan d ]


20/32

Overburden Stress P ob

Q u = A b P ob N q ] + A s K P os tan d ]

Meyerhof Method : P ob = gz

Vesic Method : cr i t ical depth , z c

for z < z c : P ob = gz for z > z c : P ob = gz c

z c /d is a function o f f after installation - see gr aph p. 24


21/32

Critical Depth (z c)

L

zc

vc

W.T.

d

20

15

10

5

028 33 38 43

f

z

/ d

c


22/32

B earing Fac to r, N q

N q i s a func t ion o f : f r ic t ion ang le , fN q i s a func t ion o f :


Wh at affects f ? In-s i tu dens i ty Part icle p ro pert ies Ins tallat ion pro cedure

N q de term ined f rom graphs app ropr ia te

to each par ticular m ethod

Total end bear ing m ay also b e lim ited:

Meyerho f : Q b < A

b 50N

q tan f

B eware if f is pre- or po st-ins tal lat ion:

Layered s oi ls :N q m ay b e reduc ed i f penetrat ion

ins uffic ient . e .g. Meyerh of (p 21)


23/32

Nq factor (Berezantzevs Method)

1000

100

1025 30 35 40 45

f

Nq

If D/B


24/32

Overburden Stress P os


Meyerhof Method : P os = gz mid

Vesic Method : cr i t ical depth , z c

for z m id < z c : P ob = gz for z m id > z c : P ob = gz c

z c /d is a function o f f after installation - see gr aph p. 24


25/32

Lateral s tres s param eter, K

A func t ion of K o no rm ally co nso l idated o r ov ercon sol idated -

see Ku lhawy p roper t ies m anual see recom m endations by Das, K ulh awy (p26)

A func t ion o f ins tal lat ion driv en p iles (ful l , part ial d isplacem ent) bo red p i les augercast pi les screwed pi les


26/32

Das (1990) recommends the following values for K / K o :

Pile Type K / K o

Bored or Jetted piles 1

Low-displacement, driven piles 1 to 1.4

High-displacement, driven piles 1 to 1.8

Kulhawy (1984) makes the following similar recommendations:

Pile Type K / K o

Jetted piles 1/2 to 2/3

Drilled shaft, cast-in-place 2/3 to 1Driven pile, small displacement 3/4 to 5/4

Driven pile, large displacement 1 to 2


27/32

K.tan d

The K and tan d values are of ten co m binedinto a s ingle func t ion

see p 28 for Vesic values f rom Poulos andDavis


28/32

Pile-so il fr ic t ion ang le, d

A func t ion of f See values b y B rom s and K ulhaw y (p26) A fu nc t ion o f pi le m ater ial

steel, co nc rete, t im ber A func t ion of p i le roughn ess

precast conc rete Cast-in-place con crete


29/32

Pile-soil friction angleBroms (1966) suggests the following

Pile Material / f '

Steel d

Concrete 0.75

Timber 0.66

Kulhawy (1984)

Pile Material / f ' Typical analogy

Rough concrete 1.0 Cast-in-place

Smooth concrete 0.8 to 1.0 Precast

Rough steel 0.7 to 0.9 Corrugated

Smooth steel 0.5 to 0.7 Coated

Timber 0.8 to 0.9 Pressure-treated


30/32

Example Driven precast co nc rete pi le 350mm squ are Uniform dense sand (f = 40 o ; g = 21kN/m 3)

Water tabl e at 1m Pile leng th 15m Check end bearing with Vesic and Meyerho f Metho ds Pi le is driven on 2m furth er into a very d ense layer f = 44 o ; g = 21.7 kN/m 3 Com pu te m od if ied capaci ty using Meyerhof


31/32

Example Bo red pi le 900mm d iam eter Uniform medium dense sand (f = 35 o ; g = 19.5kN/m 3)

Water tabl e at 1m Pile leng th 20m Check sh aft capacity with Vesic and Meyerho f Metho ds By co m parsion, check capaci ty of 550mm d iam eter

scr ewed pile


32/32

Lateral load o n sin g le p i le

Calcu lat ion of u l t im ate lateral resis tance(refer webs ite/han do uts fo r detai ls)

Lateral pi le def lect io n (us e us e su bg radereact ion m etho d, p-y analysis)

Roc k s oc keted p ile (us e ro ck et , Carter etal . 1992 m etho d )

Capacidad Axial de Pilotes Simples

Documents

Transcript of Capacidad Axial de Pilotes Simples