Deep Foundations Students 2006
Transcript of 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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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 2
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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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 3
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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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 4
DRIVEN
PILING
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Types of Deep Foundations
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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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 11
Types of Deep Foundations
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concrete slurry inserted through tip asstring withdrawn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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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 12
Reference http://www.keller-ge.co.uk/index.html
Cast in-situ piling
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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 13
Dry mi x concrete plugcan be usedin place of steel cap
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C asing may be withdrawn
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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 15
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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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 16
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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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 17
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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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 18
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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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 19
PILESPILES - - designdesign1. Geotechnical
- strength and stiffness
serviceability
2. Pile structural strength
3. Pile material durability
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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 20
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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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 21
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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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 22
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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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 23
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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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 24
Design geotechnical strength, R g*
R g* = g R ug > S*
(design action effect )
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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 25
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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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 26
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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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 27
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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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 28
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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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 32
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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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 43
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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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 47
Energy IN = Energy OUT
Blow 1 Blow 3
Pile headdisplacement
c
S
S
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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 48
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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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 53
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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DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 54
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 )