Transcript No Slide Title

Geotechnical Investigation
Report
July 2008
Hadi J. Yap, PhD, PE, GE
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Table of Contents
1.0
2.0
3.0
4.0
5.0
Introduction
Scope of Services
Field Investigation
Laboratory Testing
Geology and Seismicity
5.1
5.2
5.3
6.0
regional geology
regional seismicity and faulting
geologic hazards
site conditions
subsurface conditions
Discussions and Conclusions
7.1
7.2
7.3
7.4
7.5
foundation support
groundwater
excavation
dewatering
shoring and underpinning
Recommendations
8.1
8.2
8.3
8.4
8.5
8.6
8.7
8.8
8.9
8.10
8.11
8.12
8.13
Site Conditions
6.1
6.2
7.0
8.0
mat foundation
pile foundation
below-grade walls
basement floors
seismic design
site preparation
excavation
dewatering
shoring
earthwork
utilities
construction monitoring
site drainage
9.0
Additional Geotechnical
Services
10.0 Limitations
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Introduction
Present our understanding of the project:

Site: location, size, conditions

building type, number of stories and
basements, column loads

Site grading/fill to be placed

additional elements of the project (retaining
walls, parking areas, etc.)
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Scope of Services
Field
exploration
Laboratory
testing
Engineering
Develop
analysis
conclusions and recommendations regarding:

soil and groundwater conditions at the site

the most appropriate foundation type(s) for the structure

estimates of foundation settlement

lateral earth pressures for the design of permanent and temporary below-grade walls

site seismicity and seismic hazards, including ground rupture, liquefaction, lateral
spreading and differential compaction

San Francisco Building Code seismic design parameters
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subgrade preparation

criteria for fill, quality, placement and compaction

pavement design

construction considerations
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Field Investigation

Evaluate existing data

T&R database

city records

geologic maps

historic maps

Perform site reconnaissance

Develop field investigation program:

test pits

dynamic cone penetrometer tests
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test borings
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Cone Penetration Tests (CPTs)
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United States Coast Survey Map - February 1852
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Laboratory Testing

Geotechnical parameters
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

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index testing for classification
shear strength
compressibility
R-value (for pavement design)
Corrosivity
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Site Conditions


Describe site history, if known
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reclamation history
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past development

previous grading
Describe existing conditions

surface conditions

existing site use

known obstructions
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Subsurface Conditions

Describe soil encountered

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
thickness
density/strength
compressibility
Groundwater conditions
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Typical Stratigraphic Layers

Fill

Dune Sand
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Bay Mud
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Colma Formation
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Old Bay Clay

Franciscan Complex Bedrock
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Typical Stratigraphic Layers

Fill

Heterogeneous soil consisting of: sands,
clays, silts, gravels, construction debris
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Engineered fill or not?

If not-engineered fill, can not be relied
upon for foundation support
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Typical Stratigraphic Layers

Dune Sand
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clean fine-grained sand, wind-blown deposit
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covers the majority of San Francisco
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Typically loose in upper 10’
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Typically medium dense, 10’ to 30’
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Typically dense below 30’
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Typical Stratigraphic Layers

Bay Mud
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consists of clay and silt with occasional
sand lenses and organic material
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relatively low strength material
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relatively compressible material

If underlain by new fill, it could settle,
causing downdrag on piles
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Typical Stratigraphic Layers
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Colma Formation
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consists predominantly of sands with
occasional clay lenses
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typically contains between 0 to 20%
silt/clay
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relatively strong material

relatively incompressible material

excellent foundation support
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Typical Stratigraphic Layers

Old Bay Clay

consists of stiff to hard overconsolidated
clay
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may contain sand and gravel lenses
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relatively strong

moderately compressible
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Typical Stratigraphic Layers

Franciscan Complex bedrock
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deformed bedding planes and shear zones
due to seismic activity
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highly variable hardness & strength
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moderately to highly weathered
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Relatively strong and incompressible
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excellent foundation support
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Geology and Seismicity
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Regional Geology
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Seismicity & Faulting
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distance to faults
Geologic Hazards

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ground shaking
liquefaction
lateral spreading
landsliding
tsunami
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Discussion, Conclusions &
Recommendations

Discussion & Conclusions

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
discuss issues, alternatives, implications
conclude foundations type and settlement,
shoring, soil improvement
Recommendations

provide recommendations regarding the
geotechnical aspects of the project
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Settlement
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Consolidation
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Liquefaction
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Temporary loss of shear strength in loose sand due to
a rise in excess pore water pressure generated by
strong seismic shaking
Seismic Densification

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A slow process of squeezing water out in soft clay,
resulting in denser packing of soil particles, when
overlain by new fill
Densification of loose sand above the water table due
to ground shaking
Foundation Settlement
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Groundwater

Depth groundwater encountered
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Likely fluctuations
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Design
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Ground Improvement

Stone columns
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Compaction grouting
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Chemical grouting
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Jet grouting
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Dynamic compaction
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Foundations
The most appropriate foundation type depends on:

Subsurface conditions

Building type and size
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Site constraints
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loading conditions
basement levels
noise
vibrations
proximity to existing improvements
proximity to bay, channel
Economics
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Shallow Foundations

Footings



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isolated
continuous
grid (waffle)
Mat
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Shallow Foundations

suitable where competent material is
encountered at subgrade elevation

can be used in fill where it is improved and
building is small and light

mat can be used to span localized areas of nonsupport

mat can be used on softer soil in excavation
where weight of structure is equal to or less
than weight of soil removed
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Deep Foundations
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Piers
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Piles
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Piers
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Efficient – can use one large diameter pier in
lieu of several piles
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Lengths can be adjusted in the field – reduce
waste
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Derive capacity mainly from friction

Use casing and/or drilling fluid if groundwater
and/or loose soil is present
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Driven Pile

Concrete piles are economical in Bay Area
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Use where soft soil or un-documented fill is
present
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Can be used at sites with high groundwater table
or thick Bay Mud
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Fabricated at yard (concrete) – good quality
control

Moderately high capacity
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Retaining Walls

Restrained vs. Unrestrained Walls
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Drained vs. Undrained Walls

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undrained walls are designed to resist hydrostatic
pressure
Drainage
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Design wall pressure is higher for restrained walls
drainage panels
gravel drain
weep holes/pipes
Waterproofing
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where moisture transmission is unacceptable
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Floor Slabs
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Slab-on-grade
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supported on ground
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less expensive
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will lose support and settle/crack if settlement
occurs beneath slab
Structural slab
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
supported on foundations
more expensive
can span areas of non-support due to
liquefaction and/or settlement
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Floor Slabs
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Vapor Barrier
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typically included beneath floor slabs above the water
table
consists of:
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alternatives include Griffolyn T65-G
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4” crushed rock
10+ mil visqueen
2” sand
easier to install
takes less space
more expensive
perform moisture emission testing prior to placement of
floor covering
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Seismic Design

Provide site class (A through F)
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Provide spectral acceleration values for
Maximum Considered Earthquake
(MCE) and Design Earthquake (DE)
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Seismic Design
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Site Class
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A
B
C
D
E
F
hard rock
rock
very dense soil and soft rock
stiff soil profile
soft soil profile

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liquefiable
more than 10 feet of peat
more than 25 feet of very high plasticity clays (PI>75)
more than 120 feet of soft/medium stiff clay (Bay Mud)
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Excavation

Allowable slopes
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Shoring
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Dewatering
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Site Preparation & Earthwork

Site preparation
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
remove pavements, organics
abandon utilities
abandon/remove old foundations
overexcavation
Earthwork


compaction requirements
acceptable fill materials
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Utilities

Settlement

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
hangers
flexible connections
sleeved connections
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Pavements

Flexible
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Rigid
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Pavers
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
pedestrian
vehicular
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Site Drainage

Keep water away from building
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Collect all downspouts and surface water
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All water should be directed to storm drain
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Additional Geotechnical Services

Review geotechnical aspects of plans
and specifications
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Observation geotechnical aspects of
construction
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confirm subsurface conditions are as
expected
contractor performs work in accordance
with plans and specifications
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Limitations

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Conclusions and recommendations are
based on limited subsurface exploration
We should provide observation services to
check work is completed per plans, specs,
and our recommendations
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