chapter 4 site exploration part i
DESCRIPTION
Geotechnical engineeringTRANSCRIPT
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Site Exploration and Characterization
“Subsurface material properties cannot be specified; they must be deduced through exploration.”
Charles Dowding (1979)
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Objectives
Determine location and thickness of soil and rock strata (subsurface soil profile)
Determine location of groundwater tableRecover samples for laboratory testingConduct lab and/or field testingIdentify special problems and concerns
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Project AssessmentLiterature SearchField ReconnaissanceSubsurface Drilling and SamplingLaboratory Testing of Soil Samples
Site Exploration; General Strategy
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Type, location and approximate dimensions of the proposed development
Existing topography and any proposed grading
Any previous developments
Project Assessment
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Geologic Maps (e.g. USGS, Bureau of Economic Geology)
Soil Survey Reports (USDA Soil Conservation Service)
Geotechnical ReportsHistoric Groundwater Data
Literature Search
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Conventional Aerial Photographs Geologic features (landslides, faults),
topography, drainage patterns Site history
Infrared Aerial Photographs Springs, seepage zones Useful in slope stability investigations
Remote Sensing
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Any previous developments, grading etc.
Site topography, any signs of slope instability (landslides, soil creep)
Site drainage conditionsRock outcropsSite access
Field Reconnaissance
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Field Exploration
Site Boring LayoutTest Borings or Test Pits?
It depends on the type of materials, and what you want to know.
Number and Frequency of BoringsDepth, Sampling Methods and
Field Testing
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III. Subsurface Exploration/Sampling
Borehole Spacing Site conditions/uniformity type of structure (bridge, building,
landfill) typically one borehole/2500 ft2
Also see Table 4.1 (p.108)Borehole Depths
Magnitude of loading/soil conditions Also see Table 4.3 (p.109)
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How Many Borings?
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How Deep?
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Dial Before You Dig
One Call SystemUnderground Utility Locator System
In Texas:1-800-344-83771-800-DIG-TESS
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Look Up and Live!
Safety AwarenessRegular Emphasis
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Subsurface Drilling Auger Drilling
Solid Stem AugerHollow Stem Auger
Rotary Drilling Bucket Auger Percussion (or Cable Tool) Drilling
Drilling and Sampling of Soils
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Auger Drilling
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Hollow Stem Auger Casing with outer spiral Inner rod with plug/or pilot
assembly For sampling, remove pilot
assembly and insert sampler Typically 5ft sections, keyed,
box & pin connections Maximum depth 60-150ft
Auger Drilling
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Hollow-Stem Augers
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Bit at the end of drill rod rotated and advanced
Soil/rock cuttings removed by circulating drilling fluid
Common drilling fluid; bentonite in water with slurry density 68-72pcf
Air may be used as drilling fluid
Rotary Drilling
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Not common in US in geotechnical explorations
Heavy impact drilling tools lifted and dropped
Impact loosens soil and rock Cuttings removed with a
bailer Slow process; Used in
environmental explorations where drilling fluid is not permitted
Cable Tool Drilling
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Rock Drilling, Coring, Augering
http://www.globaldrilsup.com/cat-index.html
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Soil and Rock Sampling
Disturbed samples In-place structure is not preserved Okay for determination of soil index
properties
“…Estimating the nature of the formation from the cuttings is like identifying the cow from the hamburgers.”
G.F. Sowers
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Soil and Rock Sampling
Undisturbed samples Minimizes effects from potential
disturbance Needed for determination of in-situ
density, in-situ permeability, soil shear strength and compressibility
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Soil Samplers
Standard Split Spoon Samplers Shelby Tube SamplersPiston SamplersHeavy Wall Samplers
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Standard Split Spoon Samplers
Thick wall (0.25in) cylinderSampling tube is split along the
lengthHammered into the ground
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Standard Split Spoon Sampler
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Shelby Tube (Thin-wall) Samplers
Thin wall (1/16in) sampling tube
Sampler pushed into the ground hydraulically
Sample extruded from tube
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Piston Samplers
Minimizes sample disturbance caused by back-pressure
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Heavy-Wall Samplers
Thicker walls provide better strength & durability
However, it creates more disturbance
Sampler pounded into the ground
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Groundwater Monitoring
Groundwater level must be determined during geotechnical investigation
Can be accomplished by leaving selected soil borings open
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In-situ Testing
When it is difficult to obtain “undisturbed” samples
Cohesionless soils, Sensitive claysIn-situ Test Methods
Standard Penetration Test (SPT) Cone Penetration Test (CPT) Pressuremeter Test Flat Plate Dilatometer Test
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Standard Penetration Test (SPT)
140 lb (63.5 kg) Hammer 30in (76 cm) free fall Drive sampler over 18 inchesRecord no. of blows per each 6 inch
penetrationSPT blow count=blows for 2nd 6 inch
penetration + blows for 3rd 6inch penetration
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Standard Penetration Test (SPT)
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Types of SPT Hammers
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SPT
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SPT
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Corrections to SPT blow Counts
Factors affecting SPT blow count: Hammer Efficiency (See Table 4.3)Borehole diameter (See Table 4.4)Type of sampler (See Table 4.4)Rod length (See Table 4.4)
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SPT Correction Factors
60.0
NCCCEN RSBm
60
hammer efficiency (Em) …. Table 4.3 bore hole diameter (CB)…….Table 4.4. sampler correction (CS) ……Table 4.4 rod length (CR) ………Table 4.4
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SPT Overburden Correction
)(Customary /2000
)(2
60601z
ftlbNN
(SI) 100
)( 60601z
kPaNN
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Use of SPT Data
To Determine Relative Density, Dr
From AASHTO Chart From Eq. (4.3) p.122
To determine From Figure 4.11 (p.123)
To determine C From AASHTO Chart