soil investigation
TRANSCRIPT
FHWA - NHI Subsurface
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Chapter 5
In-Situ Geotechnical Tests
NHI Course on Subsurface Investigations
Lesson 7
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"In-Situ"
Latin: In its original position
Why perform in-situ testing?
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In-Situ Testing - Objectives
Select in-situ tests for augmenting, supplementing, and even replacing borings.
Realize the applicability of various in-situ methods to different soil conditions.
Recognize the complementary nature of in-situ direct push methods with conventional rotary drilling & sampling methods.
Recognize values for utilizing these methods and quality implications for their underuse.
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Outline of Geotechnical Site Characterization Methods
Drilling & Sampling In-Situ Tests
o Standard Penetration Test (SPT)o Cone Penetration Test (CPT + CPTu)o Flat Plate Dilatometer (DMT)o Pressuremeter (PMT)o Vane Shear (VST)
Geophysical Methodso Mechanical Waves (P-, S-, R-waves)o Electromagnetic (radar, resistivitity)
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In-Situ Geotechnical Tests for Soils
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Truck-Mounted Drill Rigs
Layne Drilling
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All-Terrain Drill Rigs
McLean, VA GT Campus, Atlanta, GA
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Track-Mounted Drill Rigs
Steele, Missouri
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Standard Penetration Test (SPT)
Split-Barrel Samplers
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Standard Penetration Test (SPT) Very common test worldwide
1902 - Colonel Gow of Raymond Pile Co.
Split-barrel sample driven in borehole
Conducted on 5-ft depth intervals (1.5-m).
ASTM D 1586 guidelines
Drop Hammer (140-lbs falling 30 inches) (63.5-kg hammer falling 0.76 meters)
Three-increments of 150-mm each; Sum last two increments = "N-value" (blows/ft)
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Standard Penetration Test (SPT)
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Standard Penetration Test
Advantages
Disadvantages Obtain Sample +
Number Simple & rugged
device at low cost Suitable in many
soil types Can perform in
weak rocks Available
throughout the U.S. (worldwide)
Obtain Sample + Number
Disturbed sample (index tests only)
Crude number for analysis
Not applicable in soft clays and silts
High variability and uncertainty
Corrections to SPT N-value
Nmeasured = Raw SPT Resistance (ASTM D 1586).
N60 = (ER/60) Nmeasured = Energy-Corrected N
Value where ER = energy ratio (ASTM D 4633). Note: 30% < ER < 100% with average ER = 60% in the U.S.
N60 CE CB CS CR Nmeas = Estimated corrected N
For Clean Sands: (N1)60 = CN N60 = Energy-
corrected SPT N-value normalized to an effective overburden stress level of one atmosphere:
(N1)60 = (N60)/(vo’)0.5 with stress given in atm. (Note:
1 atm = 1 bar = 100 kPa = 1 tsf).
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Standard Penetration Test (SPT)
4
6
8
10
12
14
16
0 10 20 30 40 50
Measured N-values
Dep
th (
met
ers)
Donut
Safety
Sequence
4
6
8
10
12
14
16
0 10 20 30 40 50
Corrected N60
Dep
th (
met
ers)
Donut
Safety
Trend
ER = 34 (energy ratio)
45
40
41
41
39
47
56
55
60
56
63
63
63
64
69
Data from Robertson, et al. (1983)
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ADSC Load Test Site at Georgia Tech Campus
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SPT Results at GT Campus
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SPT Results at GT Campus
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Cone Penetrometers
Cone Penetration Test (CPT)
Electronic Steel Probes with 60° Apex Tip ASTM D 5778 Procedures Hydraulic Push at 20 mm/s No Boring, No Samples, No Cuttings, No Spoil Continuous readings of stress, friction, pressure
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Cone Penetration Testing (ASTM D 5778)
Cone Penetration Vehicles
Mobile 25-tonne rigs with hydraulic pushing systems. Enclosed cabins to allow testing for all weather conditions
Cone Trucks
Cone Penetration Vehicles
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Electric Friction Cone Penetrometer
0
2
4
6
8
10
12
14
16
18
20
0 2 4 6 8
qT (MPa)
De
pth
(m
)
0
2
4
6
8
10
12
14
16
18
20
0 100 200 300
fS (kPa)
De
pth
(m
)
Georgia Tech Test Site
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Piezocone Penetrometers
Porewater Pressures Measured at Apex McClelland Penetrometer Design
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Cone Penetrometer Types
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Cone Penetration Test
Advantages
Disadvantages Fast and continuous
profiling of strata Economical and
productive Results not operator-
dependent Strong theoretical
basis for interpretation
Particularly suited to soft soils
High capital investment
Requires skilled operator for field use
Electronics must be calibrated & protected
No soil samples Unsuited to gravelly
soils and cobbles.
Corrections to CPT
*Need Type 2Piezo-Element
at Shoulderfor qc qt
Procedures for CPTu
Porous Element Materials•Sintered Metals•Ceramics•Plastics (disposable)
Saturation of Porous Elements:•Water•Glycerine•Silicone
Procedures:•Vacuum for 24-hours•Pre-saturated elements•Prophylactic to maintain fluids
Grease-Filled Slots - (no element)
CPTu Classification
Approximate Rules of Thumb:
Clean Quartz Sands
o tip stress qc ~ qt > 5 MPa (50
tsf)
o porewater pressures: u2 = ub ~
uo (near hydrostatic)
Soft to Firm to Stiff Intact Clays
o qt < 5 MPa (50 tsf)
o porewater pressures u2 different
than uo (usually greater)
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Geostratigraphy by Piezocone Tests, Blytheville, AR
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Geostratigraphy by Piezocone Tests, Blytheville, AR
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Geostratigraphy by Piezocone Tests, Blytheville, AR
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Geostratigraphy by CPTu at Univ. Mass-Amherst
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Vane Shear Test (VST)
Field Vane (FV) per ASTM D 2573 Performed at bottom of boring or by direct push placement of device Four-sided blade pushed into clays and silts to measure following:
suv (peak) = Peak Undrained Strength
suv (remolded) = Remolded Strength (after 10
revolutions)
Sensitivity, St = suv(peak)/suv (remolded)
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Vane Shear Test (VST)
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Vane Shear Devices
Scandinavian Vanes McClelland Offshore Vane
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Vane Shear Devices
Dutch Vane Equipment, Holland VST in Upstate NY
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Vane Shear Test
Advantages
Disadvantages Assessment of
undrained shear strength of clays
Simple test and equipment
Measure inplace sensitivity
Long history of use in practice, particularly embankments, foundations, & cuts
Limited to soft to stiff clays & silts with suv < 200 kPa
Slow & time-consuming
Raw suv needs empirical correction
Can be affected by sand seams and lenses
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Interpretation of Undrained Shear
Strength (suv) from
Vane Shear Test
HiDiDD
Ts
BTuv 6)cos/()cos/(
122
Height H
Width D
iT
iB
T = measured torque
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Interpretation of suv from Vanes with H/D =2
Geometries
33273.0
7
6
D
T
D
Tsuv
3265.0
D
Tsuv
3257.0
D
Tsuv
Rectangular
Nilcon
Geonor
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Results from Vane Shear TestsSan Francisco Bay Mud, MUNI Metro Station
0
5
10
15
20
25
30
0 10 20 30 40 50 60 70 80
Vane Strength, suv (kPa)
De
pth
(m
ete
rs)
Peak
Remolded
0
5
10
15
20
25
30
0 1 2 3 4 5
Sensitivity, St
De
pth
(m
ete
rs)
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Mobilized Strength: mob = Rsuv Correction Factor (Chandler, 1988)
0.5
0.6
0.7
0.8
0.9
1
0 20 40 60 80 100 120
Plasticity Index, PI (%)
Van
e C
orr
ecti
on
Fac
tor,
R tf = time to failure (minutes)
10
100
103
104
Correction for
Embankments Under
Normal Rates of
Construction
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Field Vane Equipment
Nilcon (mechanical) Geonor (mechanical) A.P. vanden Berg Geotech AB (electrovane) Envi (memovane)
Lab Vane EquipmentASTM D 4648
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Flat Plate Dilatometer Test
Direct push of stainless steel plate at 20-cm intervals; No borings; no cuttings.
Introduced by Marchetti (1980).
18o angled blade
Pneumatic inflation of flexible steel membrane using nitrogen gas
Two pressure readings taken (A and B) within about 1 minute
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Flat Plate Dilatometer Test (DMT)
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Dilatometer Test (DMT)
Advantages
Disadvantages Simple and Robust
Equipment
Repeatable and Operator-Independent
Quick and Economical
Theoretical Derivations for elastic modulus, strength, stress history
Difficult to push in dense and hard materials
Primarily established on correlative relationships
Needs calibration for local geologies
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Flat Plate Dilatometer
Marchetti Device (ASCE JGE, March 1980;ASTM Geot. Testing J., June 1986)
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Flat Dilatometer Test Calibrations: A, B (positive values)
Readings: contact pressure "A" and expansion pressure "B" with depth
Corrections for membrane stiffness in air: p0 A + A p1 = B -B
DMT INDICES:
ID = material index = (p1-po)/(po-uo)
ED = dilatometer modulus = 34.7(p1-po)
KD = horizontal stress index =(po-uo)/vo’
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DMT in Piedmont Residuum, Charlotte, NC
0
2
4
6
8
10
12
14
16
0 200 400 600 800
Modulus ED (atm)
0
2
4
6
8
10
12
14
16
0 500 1000 1500
Pressure (kPa)
De
pth
(m
ete
rs)
PoP1
0
2
4
6
8
10
12
14
16
0 1 10
Material Index ID
Clay Silt Sand
0
2
4
6
8
10
12
14
16
0 5 10 15
Horiz. Index KD
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Computerized DMT System
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Pressuremeter Test (PMT)
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Pressuremeter Test (PMT)
0
1
2
3
4
5
0 100 200 300 400 500 600
Volume Change (cc)
Pre
ssu
re (
tsf)
0
1
2
3
4
5
0 10 20 30 40 50
Creep (cc/min)
Pre
ssu
re (
tsf)
Prebored PMT data from Utah DOT project
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Pre-Bored Pressuremeter
Menard Pressure Panel Texam Monocell Probe
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Self-Boring Pressuremeter
Professor Jean Benoit, UNH
Cambridge-Type Probe
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CPTu in Piedmont Weathered Schist
23
34
71
34
56
67
50/6"
50/2"
50/3"
SPT-N (bpf)
Fugro Sounding at MARTA site, North Atlanta
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Dual-Element Piezocone
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Cone Penetrometers
Triple-Element Piezocone (Norwegian Institute of Technology)
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Cone Penetrometers
Quad-Element Piezocone (Oxford University)
Memocone (cableless system)
Memory chip in penetrometer. Sychronizewith depth wheel & data logger at surface
Audio- or Acoustic-cone (cableless system)
Uses audio signal to send CPTu data upthe inside of the rods. Decoder at surface
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Piezocone Penetrometers
Various Penetrometers Used at Georgia Tech
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SmallPortableTrack
CPT Rig
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GT Geostar Anchored Cone Rig
Mud Island, Mississippi River, Memphis, TNMud Island, Mississippi River, Memphis, TN
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CPT Track Truck
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Geophysical Methods Mechanical Wave Measurements
Crosshole Tests (CHT) Downhole Tests (DHT) Spectral Analysis of Surface Waves Seismic Refraction Suspension Logging
Electromagnetic Wave Techniques Ground Penetrating Radar (GPR) Electromagnetic Conductivity (EM) Surface Resistivity (SR) Magnetometer Surveys (MT)