Transcript Slide 1

Steps in Foundation Engineering
Understand project and site
Develop design criteria
Identify possible foundation alternatives
Conduct soil investigation
Characterize site
Engineering analysis to evaluate alternatives
Develop recommendations and write report
Monitor design, construction and performance
Project and Site
Project intent (from owner)
Assess general soil and site conditions
– Previous borings, maps, reports, aerial
photography
Previous experience in area
– Adjacent structures (How were they constructed?
how did they perform?)
Constraints
– Local building codes, neighboring facilities, `
access issues, economic limitations
Develop Design Criteria
Allowable settlement and tilt
Acceptable factors of safety
Identify constructability issues
Obtain design loads (provided by structural firm)
Building codes (UBC, IBC, ACI, AISC, etc)
Develop Design Criteria
Allowable Settlement?
Total settlement
Tilt
Differential Settlement
Allowable Settlements
Settlement issues
• Connections with existing structures and utility lines
• Surface drainage
• Access by vehicles and pedestrians
• Aesthetics
Tilting and Differential Settlement
• Non-uniform site conditions
• Ratio of actual-to-design load differs over structure
• Ratio of dead load to live load differs over structure
• As-built dimensions differ from plan dimensions
Tilting
Lack of stability
D/H<1/500 (text)
Differential Settlement
Stresses in structure – Cracking
δ/L<1/500 (C B C)
δ/L<1/150 (text)
FS=1.5-2
Factors of Safety
FS = capacity/demand
FS = 2-3 for foundations (typical)
FS = 1.5 for slopes and walls (typical)
Factors of Safety – why?
Required reliability
Uncertainties in soil properties and applied loads
Construction tolerances (design vs. as-built)
Cost-benefit ratio of additional conservatism in design
Factors of Safety
Foundation failures are typically more costly than failures
in the superstructure
Construction tolerances in foundations are wider than
those in the superstructure. Thus, as-built dimensions may
differ from the design.
Extra weight on the superstructure increases foundation
loads.
Design Loads
Types of structural loads
• Normal, P
• Shear, VX, VY
• Moment, MX, MY
• Torsion, T
Design Loads
Dead Loads, D
Live Loads, L
Wind Loads, W
Earthquake, E
Earth Pressure Loads, H
Snow, rain, fluid, impact, stream, etc.
Soil Investigation
In-situ testing:
Identify materials (geology)
Clays (CPT, DMT, vane)
Sands (CPT, DMT, SPT)
Borings (get samples – GSD)
Laboratory
Engineering properties from soil samples
Engineering properties (e.g. w, γ, LL, GSD)
Strength
Consolidation
Identify Possible Foundation Alternatives
Consider types of foundations
Assess benefits and potential problems
Site Characterization
Geology
Soil and effective stress profiles
Identify and estimate key soil properties
Monitor
Design, Construction, and Performance
Final Comments
Soil mechanics is the basis that allows for the selection, design
and construction of foundations.
Foundation design = assessment and optimization of options
through analysis.
Requires the ability to consider what might go wrong.
“Failure” is an unacceptable difference between expected and
observed behavior.
Key goal as foundation engineer: build economical foundation that
works.
Use rules of thumb when possible to check for reasonableness.
Your Job
Understand physics and soil behavior
Develop understanding
Develop engineering judgment
Lifelong Learning