Geotechnical Investigation Report

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Geotechnical InvestigationReport

• July 2008
• Hadi J. Yap, PhD, PE, GE

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Table of Contents

• 1.0 Introduction
• 2.0 Scope of Services
• 3.0 Field Investigation
• 4.0 Laboratory Testing
• 5.0 Geology and Seismicity
• 5.1 regional geology
• 5.2 regional seismicity and faulting
• 5.3 geologic hazards
• 6.0 Site Conditions
• 6.1 site conditions
• 6.2 subsurface conditions
• 7.0 Discussions and Conclusions
• 7.1 foundation support
• 7.2 groundwater
• 7.3 excavation
• 7.4 dewatering
• 7.5 shoring and underpinning

• 8.0 Recommendations
• 8.1 mat foundation
• 8.2 pile foundation
• 8.3 below-grade walls
• 8.4 basement floors
• 8.5 seismic design
• 8.6 site preparation
• 8.7 excavation
• 8.8 dewatering
• 8.9 shoring
• 8.10 earthwork
• 8.11 utilities
• 8.12 construction monitoring
• 8.13 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 analysis
• Develop 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
• subgrade preparation
• criteria for fill, quality, placement and
  compaction
• pavement design
• construction considerations

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Field Investigation

• Evaluate existing data
• TR database
• city records
• geologic maps
• historic maps
• Perform site reconnaissance
• Develop field investigation program
• test pits
• dynamic cone penetrometer tests
• test borings
• Cone Penetration Tests (CPTs)

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United States Coast Survey Map - February 1852
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Laboratory Testing

• Geotechnical parameters
• index testing for classification
• shear strength
• compressibility
• R-value (for pavement design)
• Corrosivity

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Site Conditions

• Describe site history, if known
• reclamation history
• past development
• previous grading
• Describe existing conditions
• surface conditions
• existing site use
• known obstructions

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Subsurface Conditions

• Describe soil encountered
• thickness
• density/strength
• compressibility
• Groundwater conditions

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Typical Stratigraphic Layers

• Fill
• Dune Sand
• Bay Mud
• Colma Formation
• Old Bay Clay
• Franciscan Complex Bedrock

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Typical Stratigraphic Layers

• Fill
• Heterogeneous soil consisting of sands, clays,
  silts, gravels, construction debris
• 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
• clean fine-grained sand, wind-blown deposit
• covers the majority of San Francisco
• Typically loose in upper 10
• Typically medium dense, 10 to 30
• Typically dense below 30

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Typical Stratigraphic Layers

• Bay Mud
• consists of clay and silt with occasional sand
  lenses and organic material
• relatively low strength material
• relatively compressible material
• If underlain by new fill, it could settle,
  causing downdrag on piles

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Typical Stratigraphic Layers

• Colma Formation
• consists predominantly of sands with occasional
  clay lenses
• typically contains between 0 to 20 silt/clay
• 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
• may contain sand and gravel lenses
• relatively strong
• moderately compressible

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Typical Stratigraphic Layers

• Franciscan Complex bedrock
• deformed bedding planes and shear zones due to
  seismic activity
• highly variable hardness strength
• moderately to highly weathered
• Relatively strong and incompressible
• excellent foundation support

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Geology and Seismicity

• Regional Geology
• Seismicity Faulting
• distance to faults
• Geologic Hazards
• ground shaking
• liquefaction
• lateral spreading
• landsliding
• tsunami

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Discussion, Conclusions Recommendations

• Discussion Conclusions
• 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

• Consolidation
• A slow process of squeezing water out in soft
  clay, resulting in denser packing of soil
  particles, when overlain by new fill
• Liquefaction
• Temporary loss of shear strength in loose sand
  due to a rise in excess pore water pressure
  generated by strong seismic shaking
• Seismic Densification
• Densification of loose sand above the water table
  due to ground shaking
• Foundation Settlement

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Groundwater

• Depth groundwater encountered
• Likely fluctuations
• Design

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Ground Improvement

• Stone columns
• Compaction grouting
• Chemical grouting
• Jet grouting
• Dynamic compaction

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Foundations

• The most appropriate foundation type depends on
• Subsurface conditions
• Building type and size
• loading conditions
• basement levels
• Site constraints
• noise
• vibrations
• proximity to existing improvements
• proximity to bay, channel
• Economics

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Shallow Foundations

• Footings
• 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
  non-support
• 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

• Piers
• Piles

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Piers

• Efficient can use one large diameter pier in
  lieu of several piles
• Lengths can be adjusted in the field reduce
  waste
• 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
• Use where soft soil or un-documented fill is
  present
• Can be used at sites with high groundwater table
  or thick Bay Mud
• Fabricated at yard (concrete) good quality
  control
• Moderately high capacity

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Retaining Walls

• Restrained vs. Unrestrained Walls
• Design wall pressure is higher for restrained
  walls
• Drained vs. Undrained Walls
• undrained walls are designed to resist
  hydrostatic pressure
• Drainage
• drainage panels
• gravel drain
• weep holes/pipes
• Waterproofing
• where moisture transmission is unacceptable

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Floor Slabs

• Slab-on-grade
• supported on ground
• less expensive
• will lose support and settle/crack if settlement
  occurs beneath slab
• Structural slab
• supported on foundations
• more expensive
• can span areas of non-support due to liquefaction
  and/or settlement

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Floor Slabs

• Vapor Barrier
• typically included beneath floor slabs above the
  water table
• consists of
• 4 crushed rock
• 10 mil visqueen
• 2 sand
• alternatives include Griffolyn T65-G
• 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)
• Provide spectral acceleration values for Maximum
  Considered Earthquake (MCE) and Design Earthquake
  (DE)

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Seismic Design

• Site Class
• A hard rock
• B rock
• C very dense soil and soft rock
• D stiff soil profile
• E soft soil profile
• F
• liquefiable
• more than 10 feet of peat
• more than 25 feet of very high plasticity clays
  (PIgt75)
• more than 120 feet of soft/medium stiff clay (Bay
  Mud)

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Excavation

• Allowable slopes
• Shoring
• Dewatering

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Site Preparation Earthwork

• Site preparation
• remove pavements, organics
• abandon utilities
• abandon/remove old foundations
• overexcavation
• Earthwork
• compaction requirements
• acceptable fill materials

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Utilities

• Settlement
• hangers
• flexible connections
• sleeved connections

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Pavements

• Flexible
• Rigid
• Pavers
• pedestrian
• vehicular

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Site Drainage

• Keep water away from building
• Collect all downspouts and surface water
• All water should be directed to storm drain

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Additional Geotechnical Services

• Review geotechnical aspects of plans and
  specifications
• Observation geotechnical aspects of construction
• confirm subsurface conditions are as expected
• contractor performs work in accordance with plans
  and specifications

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Limitations

• 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