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/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 failuresin the superstructure

Construction tolerances in foundations are wider thanthose 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)

LaboratoryEngineering properties from soil samplesEngineering properties (e.g. w, γ, LL, GSD)StrengthConsolidation

Identify Possible Foundation Alternatives

Consider types of foundations

Assess benefits and potential problems

Site Characterization

GeologySoil and effective stress profilesIdentify 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