Steps in Foundation Engineering

1
Steps in Foundation Engineering Understand
project and site Develop design
criteria Identify possible foundation
alternatives Conduct soil investigation Characte
rize site Engineering analysis to evaluate
alternatives Develop recommendations and write
report Monitor design, construction and
performance
2
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
3
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)
4
Develop Design Criteria Allowable
Settlement? Total settlement Tilt Differen
tial Settlement
5
Allowable Settlements
6
Settlement issues Connections with existing
structures and utility lines Surface
drainage Access by vehicles and pedestrians
Aesthetics
7
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
8
Tilting Lack of stability D/Hlt1/500
(text) Differential Settlement Stresses in
structure Cracking d/Llt1/500 (C B C) d/Llt1/150
(text) FS1.5-2
9
Factors of Safety FS capacity/demand FS
2-3 for foundations (typical) FS 1.5 for
slopes and walls (typical)
10
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
11
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.
12
Design Loads Types of structural loads
Normal, P Shear, VX, VY Moment, MX, MY
Torsion, T
13
Design Loads Dead Loads, D Live Loads, L Wind
Loads, W Earthquake, E Earth Pressure Loads,
H Snow, rain, fluid, impact, stream, etc.
14
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, ?, LL, GSD) Strength Conso
lidation
15
Identify Possible Foundation Alternatives Conside
r types of foundations Assess benefits and
potential problems
16
Site Characterization Geology Soil and effective
stress profiles Identify and estimate key soil
properties
Monitor Design, Construction, and Performance
17
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.
18
Your Job Understand physics and soil
behavior Develop understanding Develop
engineering judgment Lifelong Learning