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Site Exploration and Characterization; Part II
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In-situ Testing When it is difficult to obtain “undisturbed”
samples
Cohesionless soils, Sensitive clays, Cohesive
Stiff to Hard Soils
In-situ Test Methods
Standard Penetration Test (SPT)
Cone Penetration Test (CPT)
Vane Shear Test (VST)
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Standard Penetration Test (SPT)
140 lb (63.5 kg) Hammer
30in (76 cm) free fall
Drive sampler over 18 inches
Record no. of blows per each 6 inch penetration
SPT blow count=blows for 2nd 6 inch penetration + blows for 3rd 6inch penetration
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Standard Split Spoon Sampler
Thick wall (0.25in) cylinder
Sampling tube is split along the length
Hammered into the ground
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Standard Split
Spoon
Sampler
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Standard Penetration Test (SPT)
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Types of SPT Hammers
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SPT:
Automatic
Trip Hammer
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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
60601
z
ftlbNN
(SI) 100
)( 60601
z
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
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Cone Penetration Test (CPT)
Originally Developed in Netherlands 1930s
Further developments in 1950s
“Dutch Cone”
ASTM D 3441
Types of CPT devices
mechanical cone
electric cone
piezocone
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Mechanical
Cone
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Electrical Cone
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Cone Penetrometer
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CPT Truck
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Crawler Type CPT Truck
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CPT Truck;
Interior
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Cone Penetration Test (CPT)
Measures:
Cone Resistance, qc
Sleeve Resistance, fsc
Typical CPT results
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Typical CPT
Data
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Use of
CPT
Data
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CPT Versus SPT
CPT: Advantages over SPT
provides much better resolution, reliability
versatility; pore water pressure, dynamic soil
properties
CPT: Disadvantages
Does not give a sample
Will not work with soil with gravel
Need to mobilize a special rig
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Vane Shear Test
Originally developed by Swedish Engineer,
John Olsson in 1920s
Now Standardized as ASTM D2573
Specially suited for soft, sensitive clays
Quick test, used to determine undrained
shear strength
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Vane Shear Test
Drill test hole
Insert vane
Rotate head
Measure torque
Relate
resistance to
soil shear
strength
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Vane Shear Test
Relationship between Su and applied Torque:
37
6
d
TS
f
u
37
6
d
TS
f
u
Relationship between Su and applied Torque (after correction factor):
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Drilling and Sampling
Texas Cone Penetrometer
Developed in 1949
Useful for wide range
of SOIL and ROCK
types and strengths
Design Charts related
TCP values to soil
bearing strength
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Drilling and Sampling
Texas Cone Penetrometer
3” diameter
hardened
steel cone
60 degree
point
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Drilling and Sampling
Texas Cone
Penetrometer
DRIVING FORCE
170 Pound hammer,
24” drop
6” penetration or 50
blows, and repeat
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Drilling and Sampling
Texas Cone Penetrometer
PROCEDURE
Seat penetrometer cone
Make reference marks
Drive cone 12 inches
into soft materials or
100 blows into hard
materials
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Pressuremeter
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Pressuremeter Test
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Flat Plate Dilatometer
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Flat Plate
Dilatometer
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Comparison of In-Situ
Test Methods
Table 3.5
Simplicity & ruggedness
Ease of Testing
Resolution
Basis for Interpretation
Types of Soils
Equipment Availability
Potential for Future Development
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Reliability & Validity of Field
Penetration Test Data
Do you KNOW
you have
reliable results?
Do you KNOW
you have ANY
results?
Correlations
with other test
methods
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Ex Situ vs. In Situ Testing
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Ex-Situ (Laboratory) Tests
ex-situ -- “out of its
original place”
Laboratory testing is
the most common
method for measuring
soil and rock
properties
Numerous examples...
Moisture content
Unit weight
Sieve analysis
Atterberg limits
Compaction
Hydraulic conductivity
Consolidation
Direct shear
Triaxial shear
Unconfined
compression
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Ex-Situ (Laboratory) Tests
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Ex-Situ (Laboratory) Tests
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Data Presentation
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Data Presentation
Scope of Information
Log of Boring
Soil Test Boring
Records
Test Pit Records
Data Included
Field
Laboratory
Software Based
Programs
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Log of Boring
Required Information
Drilling & Sampling Depths & Methods
Field Test Data
Drilling Notes
Soil appearance, stratification
“A complete record…”
Pass/Fail
“If it’s not written down, it didn’t happen...”
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Data Presentation
Cross Sections
Source is soil
boring logs
Yields a 2D or 3D
rendering of the
subsurface
Interpolation
Extrapolation
Guesswork
Helps visualize
the subsurface
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Philosophy of Exploration
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Philosophy of Exploration
The Blind Men and the Elephant
John Godfrey Saxe
(1816-1887)
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Uncertainty vs. Risk
More often than not, you develop your
exploration not to find out the subsurface
conditions of the site, but to validate and
refine your assumptions of what you
believe are the likely subsurface
conditions at the site.
The exploration becomes an exercisein reducing uncertainty / risk.
“Much, you do not know.”
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Economics
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Balancing Cost & Risk
“The [scope of a subsurface exploration] for
any particular site is a difficult problem
which is closely linked with the relative cost
of the investigation and the project for
which it is undertaken.”
VNS Murthy: Geotechnical Engineering: Principles and Practices of Soil
Mechanics and Foundation Engineering