Geotechnical Investigation Report by ertwiw878272

VIEWS: 0 PAGES: 41

									Geotechnical Investigation
        Report

          July 2008

   Hadi J. Yap, PhD, PE, GE


                              1
                   Table of Contents
1.0   Introduction                             8.0   Recommendations
2.0   Scope of Services                              8.1    mat foundation
3.0   Field Investigation                            8.2    pile foundation
                                                     8.3    below-grade walls
4.0   Laboratory Testing
                                                     8.4    basement floors
5.0   Geology and Seismicity
                                                     8.5    seismic design
      5.1   regional geology
                                                     8.6    site preparation
      5.2   regional seismicity and faulting
                                                     8.7    excavation
      5.3   geologic hazards
                                                     8.8    dewatering
6.0   Site Conditions                                8.9    shoring
      6.1   site conditions
                                                     8.10   earthwork
      6.2   subsurface conditions
                                                     8.11   utilities
7.0   Discussions and Conclusions                    8.12   construction monitoring
      7.1   foundation support                       8.13   site drainage
      7.2   groundwater
                                               9.0  Additional Geotechnical
      7.3   excavation
                                                    Services
      7.4   dewatering
                                               10.0 Limitations
      7.5   shoring and underpinning
                                                                                      2
                 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.)



                                                    3
                    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

                                                                                                4
                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
       test borings
       Cone Penetration Tests (CPTs)


                                           5
United States Coast Survey Map - February 1852
                                                 6
7
            Laboratory Testing

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

   Corrosivity


                                           8
                Site Conditions
   Describe site history, if known
       reclamation history
       past development
       previous grading

   Describe existing conditions
       surface conditions
       existing site use
       known obstructions
                                      9
         Subsurface Conditions
   Describe soil encountered
       thickness
       density/strength
       compressibility


   Groundwater conditions



                                 10
    Typical Stratigraphic Layers
   Fill
   Dune Sand
   Bay Mud
   Colma Formation
   Old Bay Clay
   Franciscan Complex Bedrock

                                   11
12
    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




                                                     13
    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’




                                                      14
    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


                                                     15
    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
                                               16
    Typical Stratigraphic Layers
   Old Bay Clay
       consists of stiff to hard overconsolidated
        clay
       may contain sand and gravel lenses
       relatively strong
       moderately compressible



                                                     17
    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


                                                  18
        Geology and Seismicity
   Regional Geology
   Seismicity & Faulting
       distance to faults

   Geologic Hazards
       ground shaking
       liquefaction
       lateral spreading
       landsliding
       tsunami

                                 19
    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

                                                     20
                      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



                                                                21
              Groundwater


   Depth groundwater encountered

   Likely fluctuations

   Design




                                    22
        Ground Improvement


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

                             23
                    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                                24
          Shallow Foundations

   Footings
       isolated
       continuous
       grid (waffle)

   Mat



                                25
         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

                                                    26
            Deep Foundations


   Piers

   Piles




                               27
                      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


                                                      28
                  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



                                                       29
                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
                                                              30
                   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
                                                           31
                         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



                                                                  32
            Seismic Design
   Provide site class (A through F)

   Provide spectral acceleration values for
    Maximum Considered Earthquake
    (MCE) and Design Earthquake (DE)




                                               33
            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 (PI>75)
                more than 120 feet of soft/medium stiff clay (Bay Mud)




                                                                           34
                 Excavation

   Allowable slopes
   Shoring
   Dewatering




                              35
Site Preparation & Earthwork
   Site preparation
       remove pavements, organics
       abandon utilities
       abandon/remove old foundations
       overexcavation


   Earthwork
       compaction requirements
       acceptable fill materials
                                         36
                     Utilities

   Settlement
       hangers
       flexible connections
       sleeved connections




                                 37
                 Pavements

   Flexible
   Rigid
   Pavers
       pedestrian
       vehicular



                             38
               Site Drainage

   Keep water away from building

   Collect all downspouts and surface water

   All water should be directed to storm drain




                                               39
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

                                                  40
                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




                                            41

								
To top