Transcript No Slide Title

DESIGN OF DEEP
FOUNDATIONS
George Goble
Consulting Engineer
In this Lecture I Will Discuss the Deep
Foundations Design Process
Both Driven Piles and Cast-in-Place
Systems
Both Geotechnical and Some of the
Structural Aspects
MY BACKGROUND
Structural Engineer – Minor in Soil Mechanics
Experience in Construction and Several Years as
a Structural Designer
Designed Several Large Pile Foundations
Thirty Years as a College Professor Teaching
Structures and Mechanics, Emphasizing Design
Research on Optimum Structural Design
and on
the Dynamics of Pile Driving
Managed the Research that Developed Dynamic
Methods for Pile Capacity Prediction
Founded PDI and GRL
Now Have a Bridge Testing and Rating Business
WHY DO THIS?
• Driven Pile Design is Often Not Well Done
– Not dangerous but excessively conservative
– Design process not clearly understood
– Large cost savings possible
– Capabilities of modern hammers not
recognized
– Many job specs are poorly written
FUNDAMENTAL ADVANTAGES
OF THE DRIVEN PILE
• We know the material that we put in the
ground before we drive
• Because it is driven each pile
penetrates to the depth required to get
the capacity
• Capacity can be determined accurately
by driving observations
FOUNDATION DESIGN PROCESS
• Process is Quite Complex (Unique)
• Not Complete Until the Driving Criterion is
Established in the Field
• Structural Considerations can be Critical
– But Structural Properties Known in Advance of
Pile Installation
• Factor of Safety (Resistance Factor)
Dependent on Methods of Capacity
Determination and Installation Quality Control
I Will Discuss the Basis for the
Design.
Since early in the 19th Century a
Design Approach Called Allowable
Stress Design (ASD) Has Been
Used.
Will Discuss the Fundamental Basis
for ASD
GENERAL STRUCTURAL DESIGN
PROCESS
ASD HISTORICAL BACKGROUND
• Rational Analyses Appeared Early
1800’s
• Analysis Linear Elastic Based - Steel
• Well Developed by Late 1800
• Basic Concept – Do not Exceed Yield
Stress
• Produced an Orderly Basis for Design
ASD BASIS
STRESS
y
a
STRAIN
Define an ALLOWABLE STRESS
a = C y
For Steel Beams C = 0.4 to 0.66
ALLOWABLE STRESS DESIGN
• “Safe” Stress or Load Permitted in
Design
– Allowable Stress Determined by
Dividing the Yield Strength of the
Material by a Factor of Safety that is
More than One
– The Factor Provides Safety Margin
– Factor Selected by Experience
STRENGTH DESIGN
• Not All Structures Have Linear Load-Stress
(or Load-Strength) Relationship
• Example – Columns
• Behavior Understood by Late 1800’s
• Strength Non-Linear and Dependent on
Slenderness Ratio and Can Be Calculated
• Factor of Safety Introduced
• Universally Used in Geotechnical Design
• Still Called ASD
WHY LRFD?
• First Adopted by ACI Building Code – 1956 in
an Appendix
• Adopted 1963 as Equal to ASD
• Strength Design Necessary for Particularly for
Concrete Columns
• Desirable to Split Safety Margin on Both Loads
and Strength
• Adopted Different Factors on Different Load
Types
• Adopted in Practice in about Two Years
• All Factors Determined Heuristically
ASD
Qi = Rn/F.S.
LRFD
γij Qij = k Rnk
Gravity Loads
ASD - D + L
LRFD - ACI: 1.2D + 1.6L
LRFD - AASHTO: 1.25D + 1.75L
PROBABILITY RAISES
ITS UGLY HEAD
• Concept First Proposed in 1969 by Cornell in
ACI Journal Article
• Extensive Research Developed Rational
Load and Resistance Factors for Structural
Elements
• AISC Code Adopted LRFD mid-1980’s
• Ontario Bridge Code Adopted 1977
• AASHTO Bridge Code Adopted LFD 1977
• AASHTO Bridge Code Adopted LRFD after
Extensive Research Project, 1994
STRENGTH AND LOAD DISTRIBUTION
fR(R),fQ(Q)
Load Effect (Q)
Resistance (R)
A
a b
Q
Rn R
R,Q
STRENGTH MINUS LOAD DISTRIBUTION
UNDERSTAND THE
LIMITATIONS
• Load and Resistance Factors not Unique
– Several Factors Selected Based on One Condition
• Design Process Must Be Well-Understood by
Code Developers
• Strength Data May Be Dependent on
Undefined Variables
FROM THE HANDLING
OF THE LOADS ALONE
IT
IS A BIG IMPROVEMENT
OVER ASD
LOAD FACTORS FOR SELECTED CODES
Code
Dead Load
Live Load
AASHTO Bridge Code
1.25
1.75
ACI 318-02
1.20
1.60
AISC & ANSI 577
1.20
1.60
Ontario Bridge Code
1.20
1.40
Canadian Code
1.20
1.60
Euro Code
1.35
1.50
Danish Code
1.00
1.30
Australian Code
1.25
1.50
API Code
1.30
1.50
But
There Are Many Loads
And Load Combinations
For Instance,Two Important Ones
In AASHTO
Str I = 1.25D + 1.75 L + …
Str IV = 1.50 D
COMPARE F.S. WITH  FOR
DIFFERENT L/D RATIOS
γD QD + γL QL=  Rn ( QD + QL)F.S. = Rn
γD + γLQL/QD =  (1 + QL/QD)F.S.
(γD + γLQL/QD)/ (1 + QL/QD) =  (F.S.)
ACI 318-95
0.800
0.700
AASHTO
0.600
Australian Code
0.500

Eurocode
0.400
API Code
0.300
AISC, ANSI 577, and
Canadian Bridge
Code
Ontario Bridge
Code
0.200
0.100
0.000
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Danish Foundation
Code
(L/D)
Resistance Factors as Function of L/D
at F.S.=2.0 for Several Different Codes
AASHTO
Equivalent
Resistance
Factors for Given
F.S., Function of
L/D
Dead L.F. = 1.25
Live L.F. = 1.75
Phi As A Function of L/D for Various F.S.
Load Cases Str I and IV
1.2
1.1
F.S.=1.40
1
0.9
F.S.=1.60
Phi
0.8
F.S.=2.00
0.7
0.6
F.S.=2.50
0.5
F.S.=3.00
F.S.=3.75
0.4
F.S.=5.00
0.3
0.2
0
0.1
Str IV
0.2
Str I
0.3
0.4
0.5
0.6
0.7
0.8
L/D
Str I = 1.25 D + 1.75 L
Str IV = 1.50 D
0.9
1
SUMMARY
• LRFD Is an Improvement Based on
the Split Safety Margins Alone
– Both between Load Types and
Strength
• Load and Resistance Factors nonUnique
• Clearly Written, Unique Codes
Necessary
SUMMARY (Cont.)
• Probabilistic Load and Resistance
Factor Determination Attractive
– Probabilistic Factors Must Be Based on a
Clear Understanding of the Design
Process
– Must Have Good Data!!!!!!
• Designer Needn’t Know How to Obtain
Resistance Factors from Probability
FOUNDATION DESIGN PROCESS
• Combined effort of geotechnical,
structural and construction engineer
• Local contractor may provide input
• Large design capacity increases are
often possible for driven piles
• Both design and construction
practice need improvement
FOUNDATION DESIGN PROCESS
Establish requirements for structural
conditions and site characterization
Obtain general site geology
Collect foundation
experience from the area
Plan and execute subsurface
investigation
FOUNDATION DESIGN PROCESS
• Preliminary loads defined by structural
engineer
• Loads will probably be reduced as
design advances
• Improved (final) loads must be used in
final design
FOUNDATION DESIGN PROCESS
Plan and execute subsurface
investigation
Evaluate information and
select foundation system
Deep Foundation
Shallow Foundation
Foundation Design Process
Deep Foundation
Driven Pile
Drilled Shaft
Select Drilled Shaft
Foundation Design Process
Drilled Shaft
Select Shaft Type and
Factor of Safety or Resistance Factor
By Static Analysis, Estimate Unit
Shaft Friction and End Bearing
Select Cross Section and
Length for Required Capacity
(Structural Engineer?)
Foundation Design Process
Prepare Plans and Specifications
Select Contractor
Verify Shaft Constructability
and Capacity
Install and Inspect Production
Shafts
QUESTION
Where does the Geotechnical
Strength Variability come from?
Foundation Design Process
Deep Foundation
Driven Pile
Select Driven Pile
Drilled Shaft
FOUNDATION DESIGN PROCESS
Define Subsurface Conditions
Select Capacity Determination Method
Select Quality Control Procedures
Determine Safety Factor or Resistance Factor
Determine Working Loads and Loads Times Factor of Safety
Gives Required Ultimate or Nominal Resistance for ASD
For LRFD Determine Loads Times Load Factors
Get Factored Load - Divide by  Factor to
Get Required Nominal Resistance
Penetration Well Defined
Penetration Not Well Defined
DRIVEN PILE DESIGN
PROCESS
Penetration Well Defined
• Pile Depth is Defined by a
Dense Layer or Rock
• The Length is Easily Selected
Based on the Depth to the
Layer
FOUNDATION DESIGN PROCESS
Select Pile Type and Size
Determine Unit Shaft Friction and
End Bearing With Depth
Estimate Required Pile Length
Do a Preliminary Drivability Check
1
DRIVEN PILE DESIGN PROCESS
GENERAL
• Capacity Verification Method
– More Accurate Methods Justify a Smaller
Safety Factor (Larger Resistance Factor)
• Choices
–
–
–
–
Static load test
Dynamic test
Wave equation
Dynamic formula
DRIVEN PILE DESIGN PROCESS
GENERAL
• Q. C. Method
– As Q.C. is Improved, Factor of Safety
can decrease (Resistance Factor can
Increase)
• e.g., Better Capacity Determination Method
• Increased Percentage of Piles Statically or
Dynamically Tested
• Critical piles tested
DRIVEN PILE DESIGN PROCESS
GENERAL
• Make Pile Static Capacity Prediction
– Predict Unit Shaft Friction and End Bearing
with Depth
– Prediction Should Be Best Possible
• Do Not Adjust with Resistance Factor
– Note Any Minimum Depth Requirements
– Pile Size Determined With Knowledge of
Loads
DRIVEN PILE DESIGN PROCESS
GENERAL
• Pile Size Selection Should Consider Loads
• Structural Limit State Must Also Be Considered –
Lateral Loads
• Close Structural and Geotechnical Coordination
Necessary
• Maybe Pile Size Selection by Structural Engineer
– Foundation Engineer
• Length Will Be Obvious if Piles to Rock
DRIVEN PILE DESIGN PROCESS
• At this stage a proposed foundation
design is complete
• All other strength limit states must be
checked
• Drivability must be checked
• All serviceability limit states also
checked
DRIVEN PILE DESIGN PROCESS
Evaluate Drivability
Design
Satisfactory?
NO
YES
Prepare plans and specifications
Select Contractor
DRIVEN PILE DESIGN PROCESS
• Drivability usually evaluated by wave
equation
– Must satisfy driving stress requirement
– Blow count must be reasonable
– Hammer and driving system assumed
• If dynamic formula used it will determine
required blow count
– Dynamic formula will not detect excessive
driving stresses
DRIVEN PILE DESIGN PROCESS
Change
Driving
System
Select Contractor
Contractor Advises Proposed
Hammer and Driving System
Perform Drivability Analysis
Hammer
Satisfactory?
NO
DRIVEN PILE DESIGN PROCESS
• This is the same as above except the
driving system is now known (given by
Contractor)
DRIVEN PILE DESIGN PROCESS
Hammer
Satisfactory?
YES
Set driving criteria
Drive test pile to criteria
Verify test pile capacity
Capacity/stress
satisfactory?
NO
DRIVEN PILE DESIGN PROCESS
Capacity/stress
satisfactory?
NO
YES
Drive production piles
Undertake construction control
and monitor installation
Resolve pile installation problems
and construction procedures
QUESTION
Where does the Geotechnical
Strength Variability come from?
THE END