DEEP FOUNDATIONS

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DEEP FOUNDATIONS

• D. A. Cameron
• Rock and Soil Mechanics 2006
• NOTE all photos are from UC at Davis
• http//cgpr.ce.vt.edu/photo_album_for_geotech/GeoP
  hoto.html

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Why go deep?

• A Near surface soils inadequate
• weak relative to applied loads
• erodible
• watercourses, scour of soil

• B Load orientation
• lateral loading raked piles
• uplift loading - anchors
• C Settlement concerns

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Types of Deep Foundations
Deep foundations usually L/B gt 5 L pile
length, B dia. or breadth of pile

• Driven Piles
• MATERIALS
• - wood, precast concrete, steel
• SECTIONS
• - octagons, solid circles, rings, H-sections
• LIMITATIONS
• Vibrations due to driving? Head room?

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DRIVEN PILING
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Types of Deep Foundations

• 2. Bored Concrete Piles
• Large diameter?
• Increased base diameter?
• underreamed
• Excavation support?
• Bentonite slurry
• Limited 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
Concrete displaces fluid
Weak soil, high WT
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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 as string
  withdrawn
• Etc, etc,etc

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Cast in-situ piling
Reference http//www.keller-ge.co.uk/index.html
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Dry mix concrete plug can be used in place of
steel cap
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Casing may be withdrawn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 support
• Bentonite slurry used?
• slippery side contact (smeared)
• Stress relaxation expected for DISPLACEMENT
  PILES

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NON-DISPLACEMENT PILE

• Soil is removed
• The excavation may or may not be supported

DISPLACEMENT PILE

• Soil is displaced within the adjoining soil
  mass
• Displaced volume ? pile volume

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SITE INVESTIGATION FOR PILING

• Soil strength and stiffness
• Soil chemical analysis ? corrosion
• Possible obstructions to installation
• Potential for damage to adjoining structure due
  to ground heave
• Vibrations

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SITE INVESTIGATION FOR PILING

• After-construction effects of
• Expansive soil (next semester)
• Negative friction / downdrag
• Slope instability

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PILES - design

• Geotechnical
• - strength and stiffness ? serviceability
• Pile structural strength
• Pile material durability

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GEOTECHNICAL STRENGTH

• Vertical compression loading
• ULTIMATE GEOTECHNICAL STRENGTH
• or capacity, Rug

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fs average, fully mobilized, skin
friction ( INTERFACE friction and adhesion)fb
ultimate base bearing pressureDependent upon
SOIL TYPE SOIL PROFILE PILE MATERIAL
INSTALLATION
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Low load
fs ? max
fs ? max for the full length
fs ltlt ? max
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Calculations

• Circular pile, length, L
• Rug ?fs(?Dl) fb(?Db2/4)
• where Db diameter of base
• Note 1 fs may vary down the shaft
• (add contributions)
• Note 2 fb only at base

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Design geotechnical strength, RgRg ?g Rug gt
S (design action effect)
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Reduction factor ?gon Geotechnical Strength

• How good are the soil / pile data?
• Have piles been proof loaded?
• Is design based on site investigation?
• Static analysis
• Is design based on driving instrumented piles?
  Dynamic pile testing
• Is design based on driving records?
• Dynamic analysis

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Reduction factor ?g
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• The equivalent factor of safety is usually
  between 2 and 2.5 for static analysis
• based on
• good soil data
• and site investigation

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STRUCTURAL STRENGTH

• Reduction factor, ?s
• Concrete - from AS3600
• Grout - from AS3600 and x reduction factor
• Steel - from AS4100
• Timber - compression 0.85
• - tension 0.7
• - bending 1

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CLEAN SANDS - ?? only

• The skin friction term

(LATERAL STRESS) x FRICTION COEFFICIENT
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KULHAWY (1984) sand parameters
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END BEARING, fb
Analogous to the surcharge term in bearing
capacity analysis
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Nq for Piles in Sand

• Nq fn (density method of construction)
• Driven piling increases ID and ??, locally
• Meyerhof 1959
• NOTE minm. penetration into bearing stratum
  5B

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Densification 5B Rule
CL
?o 30?
Half pile
? 47?
? 34?
?o 30?
Layer 2
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Nq typical values, driven pilesAS2159 (1978)
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Limiting (maximum) values of fs and fb for sands

• fs max 110 kPa
• fb 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

since F pile flexibility factor and F 1
for L/Blt50
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Generally, ?p 1.0 for cu lt 40 kPa ?p 0.4
for cu gt 150 kPa Otherwise, Semple Rigden
(1984)
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Adhesion factors, ?, for bored piles in clays

• Stiff clays ? 0.45
• Stiff fissured clays fsmax ? 100kPa
• Tomlinson (1995)
• Other clays ? (?p - 0.1)
• Weltman Healy (1978)

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End Bearing Term, fb

• Total Stress Analysis of Saturated NC Clay
• fb 9cu
• Nc 5.14
• dcNc 8.4 for infinitely deep footing
• scdcNc 9 for a circular or square,
• deep footing

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PILE PARAMETERS from CPT (field test)

• CPT Cone Penetration Test
• OR electronic friction cone
• designed specifically for interpreting
• pile parameters
• 36 mm diameter cone (60?) is pushed into the soil
  at 2 cm/sec
• 1.2 m in a minute

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CPT provides a continuous record with time (
depth) of qc and fsc
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PILE PARAMETERS from CPT

• fs ? fsc , directly from cone
• Scale effect small cone displaces less soil
• ? conservative for sands!

CLAY SOILS..fs fsc SANDSfs
2fsc (BUT fs fsc for H-piles)
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PILE PARAMETERS from CPT

• (B) fb measured directly ? qc
• Scale effectDe Beer
• Consider a loose sand overlying a dense sand
  deposit
• Small cone senses layer over less depth than a
  large diameter pile

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qc (MPa)
loose sand
dense sand
Depth (m)
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RAMIFICATIONS

• Interpretation of CPT for fb
• Various formulations exist, e.g.
• CRAIG Av. qc 3B above
• pile base level
• AND B below
• e.g. 0.4 m dia. pile founded at 10 m requires
  average qc between 8.8 m and 10.4 m

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Pile Parameters from Driving Analysis Hiley
Formula

• R(Sc/2) ?Whh
• R pile resistance
• S pile set
• c temporary elastic compression
• ? efficiency factor
• Wh hammer weight
• h drop height

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Energy IN Energy OUT
Pile head displacement
S
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Pile resistance
Pile displacement
c
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The efficiency factor

• Wp pile weight
• e coefficient of restitution
• k output efficiency of the hammer

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The Hiley Formula

• Simple expression
• Requires driving efficiency of system
• Requires simple measurement of pile displacement
  near design depth, for regulated driving energy
• WARNING
• Good record in sands, not so good in clays

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Pile Driving Analysis The Wave equation /
CAPWAP

• Based on differential eqn. for the transmission
  of compression waves
• Measure _at_ pile head
• Strain gt driving force
• Acceleration gt velocity displacement
• Then adjust soil parameters to give best match
  with output

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The Wave Equation
Ram, W1
Spring constant, K, for cap block
Pile cap, W2
R3
Pile segments, W3 to Wi
Shear resistance
Ri
Base resistance
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Blocks, springs and dashpots
t 0
a
ram
cap
pile 1
pile 2
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BENEFITS

• Driving stresses evaluated
• Rational selection of driving equipment fall
  heights
• Driving efficiency factors not required
• cf Hiley formula
• Pile capacity may be evaluated after installation
• small hammer blow required

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PILE DRIVING

• Ideally Wh 0.5xWp to 2xWp
• To avoid overstressing pile head
• - use heavier hammers, less drop
• - for concrete piles, Broms suggested (1973)

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Pile Capacity from Pile Driving Records

• Saturated clays pile capacity is underestimated
• Why?

Capacity increases with time Re-strike (to
just move pile) months later?
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100
of long term capacity
for cH 40 m2/yr Poulos
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t /d2 (days /m2)
0
1
10
100
1000
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EXAMPLE

• For soil with the horiz. coefficient of
  consolidation cH from the previous slide, time
  taken for a 400 mm diameter driven pile (d2
  0.16 m2) to reach 75 of the long term capacity
  will take approx.
• T 100d2, or 16 days

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Remaining Design Considerations

• Piles and downdrag
• Group action
• Settlement

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1. PILES IN CONSOLIDATING SOIL

• Adhesion factor may be negative!
• CRAIG - for NC clay undergoing consolidation

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The Situation
Recent fill or Consolidating soil
Stable Soil
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PILE GROUPS

• Group efficiency
• Group capacity not always ?(pile capacities)
• RATIO of group to pile capacity EFFICIENCY
• close spacings in loose sand are efficient
• close spacings in clay are inefficient
• - Block Action may determine Group capacity

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Block Action
L
4x4 pile group, dia. d, spacing, s
Block base, (3s d)2, perimeter, 4(3s d)L
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Calculations

• Adhesion rather than cohesion for sides
• Base resistance is L/B ? 5?
• For design, adopt the smaller of Group Capacity
  and ?(pile capacities)

NOTE unlikely to need except for close piles in
saturated clays, s lt 4d
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Settlements

• Are usually small
• Slip should be included
• Pile 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
Stress bowls
Compressible soil layer
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PILES - SUMMARY

• Pile capacity depends largely on installation
• Single Piles (a) STATIC ANALYSIS
• Sands fsmax and fbmax Clays -
  adhesion factors, ?p, ?
• - fb 9cu
• (b) CPT DATA
• - better parameter evaluation

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SUMMARY
1. Single Piles (c) Dynamic
Analysis driving data used (gives
capacity at the time of pile-driving)

• 2. Pile Groups Block Action may diminish
  capacity, AND increase settlement