deep foundations students 2006

8/8/2019 Deep Foundations Students 2006

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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 1

DEEP FOUNDATIONSDEEP FOUNDATIONS

D. A. CameronRock and Soil Mechanics 2006

NOTE: all photos are from UC at Davishttp://cgpr.ce.vt.edu/photo_album_for_geotech/GeoPhoto.html


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Why go deep?

[A] Near surface soils inadequateweak relative to applied loadserodible

watercourses, scour of soil

[B] Load orientationlateral loading raked piles

uplift loading - anchors

[C ] Settlement concerns


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Types of Deep Foundations

1. Driven PilesM ATERIA LS

- wood, precast concrete, steelS ECTIO NS

- octagons, solid circles, rings, H-sectionsLIMITATIO NS

Vibrations due to driving? Head room?

Deep foundations usually L/ B > 5L = pile length, B = dia. or breadth of pile


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DRIVEN

PILING


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2. Bored Concrete Piles

Large diameter?

Increased base diameter? underreamed

Ex cavation support?

Bentonite slurryLimited practical depth

Soil restrictions


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Bored Pile

1. Shaft


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2. Base enlargement tool


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3. Reo cage


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4. Concreting/ bentonite slurry displacement


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Bentonite slurry

Weak soil,high W T Concretedisplacesfluid


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3. Other

Driven cast in-situ piles driven tube pile, filled with concrete

Continuous flight augur piles hollow augur string

concrete slurry inserted through tip asstring withdrawn

E tc, etc,etc


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Reference http://www.keller-ge.co.uk/index.html

Cast in-situ piling


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Dry mi x concrete plugcan be usedin place of steel cap


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C asing may be withdrawn


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PILE LOAD CAPACITY

Capacity dependent on construction

relaxation of field soil stresses?

less contact with side soil, less supportBentonite slurry used?

slippery side contact ( smeared )

Stress rela xation e xpected for DISPL ACEMENT P ILES


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NON-DISPLACEMENT PILE

Soil is removedThe e xcavation may or may not besupported

DISPLACEMENT PILE

Soil is displaced within the adjoiningsoil mass

Displaced volume } pile volume


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SITE INVESTIGATION FOR PILING

1. Soil strength and stiffness

2. Soil chemical analysis corrosion

3. Possible obstructions to installation

4. Potential for damage to adjoining

structure due to ground heave

5. Vib rat ion sVib rat ion s


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SITE INVESTIGATION FOR PILING

After-construction effects of:

1. Ex pansive soil ( next semester )

2. Negative friction / downdrag

3. Slope instability


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PILESPILES - - designdesign1. Geotechnical

- strength and stiffness

serviceability

2. Pile structural strength

3. Pile material durability


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GEOTECHNICAL STRENGTH

Vertical compression loading:

ULTIM ATE GEOTE CHN IC AL S TRE NG TH

- or capacity, R ug

b bssug Af Af R !


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f s = average, fully mobilized, skin friction

(= INTERFACE friction and adhesion )f b = ultimate base bearing pressure

Dependent upon S OIL TYP ES OIL P ROFI LE

P ILE M ATERIA LINS TA LL ATIO N


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Low load Ultimate load

f s = X max

f s =X

max

f o r the

full

l e ng thf s


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Calculations

Circular pile, length, L:

R ug = 7 f s (T Dl) + f b (T Db 2 /4 )

where D b = diameter of base

Note 1: f s may vary down the sha f t

(add contributions)

Note 2: f b on ly at b ase


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Design geotechnical strength, R g*

R g* = g R ug > S*

(design action effect )


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Reduction factor gon Geotechnical Strength

How good are the soil / pile data?

Have piles been p r oo f l o aded ?

Is design based on site investigation?Static analysis

Is design based on driving instrumentedpiles? Dynamic pile testing

Is design based on driving records?

Dynamic analysis


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Reduction factor g

Pile load testing 0.7 to 0.9

Static analysis 0.4 to 0.65

Dynamic load testing 0.5 to 0.85

Dynamic analysis* 0.45 to 0.65

*caution on clay sites


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The equivalent factor of safety is usually

between 2 and 2.5 for static analysis

based on

good s oi l data

and s i te inv est ig at ion


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STRUCTURAL STRENGTH

Reduction factor, s

Concrete - f rom AS3600

Grout - f rom AS3600 and x reduction factor Steel - f rom AS4100

Timber - compression 0.85- tension 0.7- bending 1


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1. CLEAN SANDS - Jd only

The skin friction term

tan)(K f ovss d!

(L ATERA L S TRE SS ) x FRI CTIO N C OEFFI C IENT


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KULHAWY (1984 ) sand parameters

Pile Type K sKo

H

Jd

Bored piles 0.7 to 1 1

Displacementpiles

see below

- precast concrete 0.75 to 2 0.8 to 1

- smooth steel 0.75 to 2 0.5 to 0.7


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END BEARING, f b

qv b b N)(o

d!

Analogous to the surcharge term inbearing capacity analysis


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Nq for Piles in Sand

Nq = fn (density & method of construction )

Driven piling increases ID and Jd , locally

[Meyerhof 1959 ]

NOTE : minm. penetration into bearing stratum= 5 B


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Densification 5B Rule

J o = 30 r

J = 34 r

J = 47 r

Half pile

CL

Layer 2

B

Layer 1

J o = 30 r

5B


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Nq typical values, driven piles[AS2159 (1978 )]

SandConsistency

Density Index,ID

(%)

Nq

LOOSE 20-40% 60

MEDIUMDENSE

40-75% 100

DENSE 75-90% 180


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Limiting (maximum ) values of

f s and f b for sands

f s max = 110 kPa

f b max = 15 MPa

After Tomlinson 1995


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CLAYS, SILTS

The skin friction OR side shear term

- effective stresses and drained strength?

BUT the pwps are uncertain- Total stress analysis acceptable

Adhesion u pu ps ccF !!

since F = pile fle xibility factor and F = 1 for L/B


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E p(c u )

( W d vo )

1 < 0.35

0.5 > 0.8

Generally, E p = 1.0 for c u < 40 kPa

E p = 0.4 for c u > 150 kPa

Otherwise , Semple + Rigden (1984 ):


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Adhesion factors, E , for bored piles in clays:

Stiff clays E = 0.45

Stiff f issured clays f smax } 100kPa

T omlinson ( 1995 )

Other clays E = ( E p - 0.1 )

W eltman & Healy ( 1978 )


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End Bearing Term, f b

Total Stress Analysis of Saturated NC Clay

f b = 9c u Nc = 5.14

d cNc = 8.4 for infinitely deep footing

s cd cNc = 9 + for a circular or square,

deep footing


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PILE PARAMETERSfrom CPT (field test )

CPT = Cone Penetration Test

OR electronic friction cone

- designed specifically for interpreting

pile parameters

- 36 mm diameter cone (60 r) is pushed into

the soil at 2 cm/sec

1.2 m in a minute


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Sleevefriction, f sc

Tipresistance, q c

CPT provides acontinuous recordwith time (= de p th ) of q c and f sc


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PILE PARAMETERS from CPT

(A) f s f sc , directly from cone

Scale effect : small cone displaces less soil

conservative for sands!

CLAY SOILS ..f s = f sc

SANDS f s = 2f sc

(BUT f s = f sc for H-piles )


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PILE PARAMETERS from CPT

(B ) f b measured directly q c

Scale effect:De Beer

Consider a loose sand overlying a dense sanddeposit

Small cone senses layer over less depth than alarge diameter pile


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qc (MPa )

loose sand

dense sand

Depth (m )

yc


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RAMIFICATIONS

Interpretation of CPT for f b

Various formulations e xist, e.g.

CRAIG Av. q c 3B above

pile base level

AND B below

e.g. 0.4 m dia. pile founded at 10 m requires

average q c between 8.8 m and 10.4 m


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Energy IN = Energy OUT

Blow 1 Blow 3

Pile headdisplacement

c

S

S


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Pileresistance

Piledisplacement

c S


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Pile Driving AnalysisThe Wave equation / C APW AP

Based on differential eqn. for thetransmission of compression waves

Measure @ pile headStrain => driving forceAcceleration => velocity & displacement

Then adjust soil parameters to give best matchwith output


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The Wave Equation

Ram,W1

Pile

cap, W 2

Spring constant,K, for cap block

Pile

segments,W3 to W i

R 3

Shear resistance

Base resistance

R i


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ram

cap

pile 1

pile 2

t = 0

a

t = 2

a

a

a

Blocks, springs and dashpotst = 1

a

a


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BENEFITS

Dr iving stresses evaluated

Rat ion a l se l e c t ion of driving equipment& fall heights

Dr iving e ff ici e nc y f a c t o rs not required cf Hiley formula

Pile capacity may be evaluated a f ter

in sta ll at ion small hammer blow required


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PILE DRIVING

Ideally : Wh = 0.5 xWp to 2 xWp

To avoid overstressing pile head:

- use heavier hammers, less drop- for concrete piles, Broms suggested ( 197 3);

(m)h3(MPa) emax !


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Pile Capacity fromPile Driving Records

Saturated clays : pile capacity is underestimated

Why?

C a pa ci t y inc reases w i th t i me

Re-strike (to just move pile ) months later?


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% of longterm

capacity

t /d 2 (days /m 2)

50

100

01 10 100 1000

f o r

cH

= 40 m 2 /yr

P o ul o s


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EXAMPLE

For soil with the horiz. coefficient of consolidation c H from the previous slide,time taken for a 400 mm diameter drivenpile (d 2 = 0. 16 m 2) to reach 75% of thelong term capacity will take appro x.

T= 100d 2, or 16 days


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Remaining DesignConsiderations

Piles and downdrag

Group actionSettlement


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1. PILES IN CONSOLIDATING SOIL

Adhesion factor may be negative!

CRAIG - for NC clay undergoing consolidation

J d! tan'oss'os

0 .25$


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The Situation

Recent fill or

Consolidating soil

Stable Soil

sett l eme n t


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PILE GROUPS

Group efficiency

Group capacity not always = 7 (pile capacities )

RATIO of group to pile capacity = EFFICIENCY

- close spacings in loose sand are e ff ici e n t

- close spacings in clay are in e ff ici e n t

- Block Action may determine Group capacity


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Block Action

4x4 pile group, dia. d,spacing, s

Block base, (3s + d )2,perimeter, 4(3s + d )L

L


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Calculations

Adhesion rather than cohesion for sides

Base resistance is L/ B u 5?

For design , adopt the smaller of GroupCapacity and 7 (pile capacities )

NOTE: unlikely to need except f or close piles in saturated clays, s < 4d


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S ettlements

Are usually small:Slip should be includedPile elastic compression can dominate

Refer: Poulos for settlement calculations

Caution: Block action of groups may stress far deeper than any pile in the group greater settlements!


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Settlements of Blocks

L

C ompressible soil layer

Stressbowls


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PILES - SUMMARY

Pile capacity depends largely on installation

1. Single Piles (a ) STATIC ANALYSIS

Sands: f sma x and f bma xClays - adhesion factors, E p , E

- f b= 9cu

(b ) CPT DATA

- better parameter evaluation


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SUMMARY

2. Pile Groups Block Action may

diminish capacity, ANDincrease settlement

1. Single Piles (c ) Dynamic Analysisdriving data used(gives capacity at the

time of pile-driving )

deep foundations students 2006

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