Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site...
Transcript of Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site...
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Introduction to Geophysical Tests
Sebastiano Foti
VS (m/s)
(ITALY)(ITALY)(ITALY)(ITALY)Email: [email protected]
www.soilmech.polito.it/people/foti_sebastiano
Short Course – 23rd of February 2014
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Outline
• Geophysical methods– Scope and potential for geotechnical and
geoenvironmental characterization– Use of seismic velocities– Significance of other geophysical parameters– In-hole vs surface methods
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Geophysical Methods
Geophysical methods are indirect surveying techniques based onmeasurements carried out on the ground surface or in holes . Theyallow the distribution of physical properties of the subsurface to beestimated and correlated with engineering information.
They are based on the excitation of an object with an energy field (artificialor natural) and on the measurement of the object response.
The interpretation of the object response allows the object to becharacterised.
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Geophysical parameters
• Density• Electrical Conductivity (or Resistivity)• Electrical Permittivity• Magnetic Suscettibility• Chargeability• Seismic velocities
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Geophysical parameters
• Density• Electrical Conductivity (or Resistivity)• Electrical Permittivity• Magnetic Suscettibility• Chargeability• Seismic velocities
Direct relationship to mechanical parameters of the medium
(Elastic Moduli)
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Applicability of in situ tests
(Mayne et al, 2002)
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Geotechnical and geoenvironmental site characterizat ion
In the context of site characterization for engineering purposes, the role of geophysical methods is twofold:
• evaluation of geometrical boundaries to model subsoil conditions (e.g. stratigraphy but also physical inclusions or hydrogeological features);
• evaluation of physical/mechanical parameters of direct use for geotechnical modeling.
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Identification of stratigraphic sequence
Pug
in e
t al.,
200
9
Seismic methods: e.g. seismic reflection to identify an acquifer
In combination with conventional investigation :e.g. boreholes logs allow calibration / identification of litography
geophysical surveys allow for 2D/3D extension
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Lateral variations (shallow faults)
[Ivanov et al., 2006]
Geological model (expected)
2D VS model from surface wave analysis
Updated geological model
e.g. seismic methods: surface wave tests
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Cavity detection
Example 1: void detection in a minerary area in canada with pseudo-2D V S sections from surface wave analysis
Xu et al., 2008
VS
Example 2: (ERT) Electrical Resistivity Tomography and (GPR) Ground Penetrating Radar surveys reveal a sinkhole beneath a house
GPR
ERT
Dobecki and Upchurch, 2006
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
e.g. electrical methods to identify clays below sands
Ture
sson
and
Lin
d, 2
005
Powerful tools to investigate lateral variations at the site(e.g. for assessing the potential for differential settlements)
Identification of stratigraphic sequence / local li tography
Non-seismic methods:
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
100 200 300 400 500 600 700 800 900 1000 1100 1200
Distance [ m ]
-100
-80
-60
-40
-20
0
Dep
th [
m ]
50 150 250 350 450 550 650 750 850
[Ohm m]
Sea coast-line LandS1
Macchiareddu - Cagliari (Italy)
Apparent resistivity pseudosection Profile: n.3 Cardiga
Salt waterintrusion
2D rendering of time domain EM vertical 1D profiles for salt water intrusion in coastal aquifer.
Courtesy of Alberto Godio
Hydro - geophysics
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Pollutants and waste detection
Courtesy of Valentina Socco
Buried waste disposal
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Hydrogeological / environmental applications
Electrical Resistivity Tomography (ERT)
Resistivity is sensitive to:• pore fluid content • pore fluid conductivity Identification and monitoring of plumes
Saturated vs unsaturated(for coarse materials)
(Martìnez-Pagàn et al., 2009)
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Monitoring in environmental applications
Example:3D resistivity tomography on lab soil samples for diffusion of conductive plume monitoring. (Comina et al., 2011).
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
site characterization for engineering purposes
In the context of site characterization for engineering purposes, the role of geophysical methods is twofold:
• evaluation of geometrical boundaries to model subsoil conditions (e.g. stratigraphy but also physical inclusions or hydrogeological features);
• evaluation of physical/mechanical parameters of direct use for geotechnical modeling.
Geotechnical and geoenvironmental site characterizat ion
![Page 17: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/17.jpg)
POLITECNICO DI TORINOCISM Udine 7 Febbraio 2014 Sebastiano Foti
Indagini geofisiche per la caratterizzazione geotecnica
Bulk waves
(Animation courtesy of prof.Braile)
ρρµλ M
V P =+= 2
ρρµ G
V S ==ρρ
µ GV S ==
Longitudinal wave(Primary wave – P)
Shear wave(Secondary wave – S)
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Seismic methods
2SVG ρ=
20 SVG ρ=
In a linear elastic medium
In soils
G0
Gsec
G0
Gsec
1.0
γc γγc γ
τ
G0
Gsec
Strain range of geophysical test
Shear wave propagation
Animation courtesy of Dr. L. Braile, Purdue University
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Role of G0 in geotechnical engineering
• Evaluation of seismic site response
• Foundation vibrations
• Dynamic soil structure interaction
• Vibrations (e.g. railroads, industrial activities, …)
• Liquefaction suscettivity assessment
• Monitoring of ground improvement projects
• Correlation to operative values of G at medium strains
• Numerical simulations with advanced constitutive laws
• Evaluation of disturbance of soil samples
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Typical strain ranges for geotechnical problems
(Atkinson, 2000)
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Site vs Lab (Pisa)
0
30
60
90
120
0 30 60 90 120
G0 sito (MPa)
G0
lab
(MP
a)2
0 SVG ρ=(Cross-Hole Test)
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
0
150
300
450
600
750
900
10500.0 0.5 1.0 1.5
0.25 1.000.800.500.10 1.50 2.00
V
(m/s
)
V / V
S, s
ito
S,sitoS,lab
G / G0, sito0,lab
valori progetto ROSRINE
campioni ricostituitida sabbie cementate
X XX
XX X
campioni indisturbatidi sabbie cementate
valori usuali percampioni di roccia
Pisa
G0 site vs lab
20 SVG ρ= (Stokoe e Santamarina, 2000)
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Assessment of sample quality
The ratioVS(lab) / VS(field)Gives an indication of sample quality
it can be used also for coarse grained soils
DeGroot et al (2011).
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
0
20
40
60
80
100
120
0.0001 0.001 0.01 0.1 1
deformazione tangenziale, γγγγ (%)
G (
MP
a)
0
5
10
15
20
25
30
D (
%)
∆∆ ∆∆u/
σσ σσ' 0
(%
)
2S)sito(0 VG ⋅ρ=
lab)(G γ
� � �� ��� �Shear strain
Site characterization for seismic projects
• G0 from in situ testing (geophysics)• G/G0(γ) e D(γ) from laboratory tests
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
0
20
40
60
80
100
120
0.0001 0.001 0.01 0.1 1
deformazione tangenziale, γγγγ (%)
G (
MP
a)
0
5
10
15
20
25
30
D (
%)
∆∆ ∆∆u/
σσ σσ' 0
(%
) lab0
sito0 G
)(G)G()(G
γ⋅=γ
Shear strain
Site characterization for seismic projects
In situ tests investigate a large volume of soil whereas laboratory testing concerns small samples
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Body Waves
ρG
V S =
ρM
VP =
Vertically polarized SVor
Horizontally polarized SH
Compressional wave
Shear wave
(after Bolt, 1976)
Direction of Propagation
(after Bolt, 1976)
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
In a linear elastic isotropic homogeneous medium
ρM
VP =
ρG
V S =
VS: shear wave velocity
VP: dilational wave velocity
ρ: density
G: shear modulus
M: laterally constrained modulus
(oedometric conditions)
Note: In saturated soils VP is strongly influenced by the compressibility of the pore fluid (water)
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Macroscopic approach: the medium is modeled as a binary continuumarising from the superposition of a fluid and a solid phase occupying simultaneously the same regions of space. The porosity is the link between the two.
Biot Theory
Hypothesis:- isotropic, linear elastic soil skeleton- a non-dissipative compressible fluid saturates all voids- no relative motion between the solid and the fluid phases
(valid for low frequency range)
Writing the equations of motion for the porous media and applying the Helmholtz decomposition, it is possible to show the existence of two different compressional waves and of a unique shear waves. The fastest compressional wave is called of the first kind or P-wave, the slowest is called of the second kind or Biot wave.
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Biot solution
Under the hypothesis of grain incompressibility, t he velocity of propagation of body waves in porous media can be written as:
ρS grain density
ρF water density
KF water bulk modulus
KSK soil skeleton bulk modulus
G shear modulus
n porosity
νSK Poisson ratio of the (evacuated) soil skeleton
FS
S
FSK
P
nnwhere
GV
n
KGK
V
ρρρρ
ρ
⋅+⋅−=
=
+⋅+=
)1(
)( 34
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Biot solution
Under the hypothesis of grain incompressibility, t he velocity of propagation of body waves in porous media can be written as:
ρS grain density
ρF water density
KF water bulk modulus
KSK soil skeleton bulk modulus
G shear modulus
n porosity
νSK Poisson ratio of the (evacuated) soil skeleton
FS
S
FSK
P
nnwhere
GV
n
KGK
V
ρρρρ
ρ
⋅+⋅−=
=
+⋅+=
)1(
)( 34
)(2
211
2
)(4)(
22
2
FS
SSK
SK
P
FFSSS
VV
K
nρρ
νν
ρρρρ
−⋅
⋅
−−⋅−
⋅−⋅−−
=
ρρρρS,ρρρρF,KF: standard values
VP & VS: measured
ννννSK : range 0.1÷0.4
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Soil porosity from seismic velocities
0 0.2 0.4 0.6 0.8 1Porosity
LAB (Laval)LAB (Osterberg)
0
10
20
30
40
50
60
70
80
0 500 1000 1500 2000 2500
Dep
th [m
]
Velocity of Propagation [m/s]
VpVsCross-Hole
test
)(2
211
2
)(4)(
22
2
FS
SSK
SK
P
FFSSS
VV
K
nρρ
νν
ρρρρ
−⋅
⋅
−−⋅−
⋅−⋅−−
=(Foti et al., 2002)
Leaning Tower of Pisa site
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Degree of saturation
Valle-Molina (2006)
Also very limited desaturation has a strong effect on the VP
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Influence of degree of salutarion on liquefaction r esistance
Tsukamoto et al (2006)
saturation degree strongly affect liquefaction resistance� VP can be used to monitor saturation and esclude liquefaction
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Example: Zelasny Most tailing dam
Jamiolkowski, 2012West dam
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Non seismic methods
Quantitative use of geophysical parameters other than seismic velocities is less straightforward and typically require
the use of empirical correlations with geotechnical parameters
Example: electrical conductivity of soils
Trasport parameter related to:
- fluid properties (solubility of ionic species, concentration);
- mineralogy and specific surface of the solid grains;
- porosity and fabric
Archie σt = σw nm Srp n: porosity S: saturation
Bruggeman σt = σw n3/2m = 3/2 : theoretical
Waxman & Smits σt = X (σw + σs) σs : clay surface conductivity
σw : pore fluid conductivity
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
mS/
cm
3
3. Tomographic reconstruction
1
2
Coarse Matrix n ≈ 0.48
Dense Inclusion n ≈ 0.43
Matrix n ≈ 0.46
Inclusion n ≈ 0.42
Estimated values with Bruggeman equation
Polito – 2D ERT (Borsic et al., 2005) Example at Lab scaleIdentification of zones with different compaction
levels in sand
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
In-hole vs surface methods(Invasive vs Non-invasive methods)
XD
1 n32
X
Cross-Hole Test (CHT)Down-Hole Test (DHT)Seismic Cone (SCPT)Seismic Dilatometer (SDMT)P-S Suspension LoggingVertical Seismic Profiling (VSP)
Surface Waves Methods SWM(SASW, MASW, microtremors)
Seismic Refraction (P-waves or SH-waves)
Seismic Reflection (P-waves or SH-waves)
![Page 38: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/38.jpg)
Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Invasive Tests Non-Invasive TestsDirect measurements: simple and accurate interpretation
Good resolution also at great depth
Easier standardization
Additional information from borehole logging or the penetration of the cone
Costs and flexibility (in time and space)
Non-intrusive (e.g. important for waste landfills)
Average properties (dynamic behaviour of the whole soil deposit)
Large volumes are investigated
Costs and necessity of planning well in advance
Local measurement
Complex interpretation (indirect measurements based on inversion procedures or heavy processing)
Accuracy and resolution at depth
Dis
adva
ntag
esIn-hole vs surface methods
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
In-hole vs surface methods
\VS1
VS2
VS3
![Page 40: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/40.jpg)
Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Combined use of geophysical methods
• Level 1: comparison for validation / calibration• Level 2: data integration and data fusion (combining
different information on the same medium)• Level 3: a priori info (one method help the other)• Level 4: joint inversion (simultaneous interpretation of
different dataset)
Synergies between different techniques can be exploited at different level of integration:
![Page 41: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/41.jpg)
Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
0
5
10
15
20
25
30
0 100 200 300Vs (m/s)
Dep
th (m
)
Cross Hole
SASW-fk
Level 1: Comparison In-Hole methods vs SASW
0
5
10
15
0 400 800Vs (m/s)
Dep
th (m
)
Down Hole
SASW-fk
0
5
10
15
0 400 800 1200Vs (m/s)
Dep
th (m
)
Down Hole
SASW-fk
0
5
10
15
0 400 800 1200 1600Vs (m/s)
Dep
th (m
)
Down Hole
SASW-fk
0
5
10
15
0 400 800Vs (m/s)
Dep
th (m
)
Down Hole
SASW-fk
Pontremoli
site 1
Pontremoli
site 2
Pontremoli
site 3
Castelnuovo
0
5
10
15
20
25
30
0 400 800Vs (m/s)
Dep
th (m
)
Cross Hole
SASW-fk
Saluggia Pisa
![Page 42: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/42.jpg)
Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Level 2: Data integration and data fusion
Pug
in e
t al.,
200
9
SH-wave seismic reflection
Electrical resistivity tomography
resistivity
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Combined use
• Level 1: comparison for validation• Level 2: data fusion• Level 3: a priori info• Level 4: joint inversions
Example: synergies of seismic refraction and surface wave analysis
(SWM)
![Page 44: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/44.jpg)
Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Example of synergy: SW + V P refraction
Same testing setup and equipment
Rayleigh waves
VS1
VS2
VS3
Experimental data contain both surface waves and
direct/refracted P waves
Receivers (geophones)
VP1
VP2
VP3 ≈ VP2
P-waves
![Page 45: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/45.jpg)
Geophysical methods for geotechnical site character ization
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0
10
20
30
40
50
60
70
0 10 20 30 40 50 60 70 80spacing [m]
travel time [ms]
P-WAVE REFRACTION
Shallow water table masks variation of the mechanical properties of the solid skeleton (influence of the pore fluid)
0
1
2
3
4
0 200 400 600 800 1000 1200 1400 1600Vp [m/s]
Depth [m]
Water table
VP1
VP2
VP3 ≈ VP2
FS
FSK
P nnn
KGK
Vρρ ⋅+⋅−
+⋅+=
)1(
)( 34
![Page 46: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/46.jpg)
Geophysical methods for geotechnical site character ization
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Experimental Data
0 10 20 30 40 50 60 70100
200
300
400
500
600
700
frequency, Hz
phas
e ve
loci
ty,
m/s
experimentalinversion #1inversion #2inversion #3
Hp#1 Water table from P-wave refractionHp#2 No water tableHp#3 Water table deeper than Hp #1
0 200 400 600 800 10000
5
10
15
20
25
30
Dep
th (
m)
Shear Wave Velocity (m/s)
starting profileinversion #1inversion #2inversion #3cross-hole test
(Foti and Strobbia, 2002)
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Level 4: joint inversion
( )( ) ( )( )1 TL
N M A = + + +
-1obs obs obsd -g m C d -g m
( ) ( ) ( )( ) ( ) ( ) ( )( )1 2 ' 1 2 ''log , log , ...., log log , log , ...., logobs R R RN NV V V t t t d =
( ) ( )( )
SW
PR
=
g mg m
g m
( ) ( ) ( )( ) ( ) ( ) ( )( )1 2 1 2 1[ log , log , ...., log log , log , ...., logn S S Snh h h V V V +m =
( ) ( ) ( )( )1 2 1log , log , ...., log ]P P PnV V V +
A single inversion problems is solved considering all the available
experimental information: the best fit parameters for both VP and VS
models are obtained
A single misfit parameter include misfit on Rayleigh wave dispersion
curve and P-wave travel times
(Piatti et al., 2012b)
![Page 48: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/48.jpg)
Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Example on synthetic data
(Piatti et al., 2012b)
![Page 49: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/49.jpg)
Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Example on synthetic data
(Piatti et al., 2012b)
![Page 50: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/50.jpg)
Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Experimental data
![Page 51: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/51.jpg)
Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Case History #1
Combination of seismic and electrical methods for the assessment of site conditions for seepage analysis along an embankment
• Combination of several methods for reliable evaluation of cover thickness
• Joint inversion to improve accuracy
![Page 52: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/52.jpg)
Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
The PO river
LENGTH: 650 kmDISCHARGE
ave.= 1450 m3/smax.= (nov 2000): 13000m3/s
![Page 53: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/53.jpg)
Geophysical methods for geotechnical site character ization
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Seepage potential
Floods very often start with localized seepage that can degenerate causing inundations
10 extreme events each 100 years
Levees for a total length over 2400 km
![Page 54: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/54.jpg)
Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Water level can reach 10 m
above the ground surface
Anthropic soil
Thickness of low permeability layer?
?
Seepage potential
Geology: alluvial deposits: recent sands, gravel, clay
TARGET: clayey layer: continuity, thickness
![Page 55: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/55.jpg)
Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Geophysical investigationlarge extension of the areas
Interest in fast geophysical tests from the surface
VES ERT
HEP
SWM
Prefr SHrefr
At a test site several methods have been tested and compared
![Page 56: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/56.jpg)
Geophysical methods for geotechnical site character ization
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Z
VS profile
0VS G
ρρρρapp
AB
Combinations MASW + VES
VR
ωωωω
dispersion curve
Processing
Inversion
Z
0ρρρρ
Apparent resistivity
resistivity profile
![Page 57: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/57.jpg)
Geophysical methods for geotechnical site character ization
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Physical parameters: shear velocity and resistivity
Assumed parameter distribution: stack of homogeneous isotropic layers
MODEL PARAMETERS:
n ρn VS
n-1 H
LINK BETWEEN THE TWO MODELS:
geometry, thickness of the layers
(same position of interfaces: independent
variations of the two parameters, a variation
of resistivity does not imply a variation of
seismic shear velocity )
VS, ρ
VS, ρ
VS, ρ
VS, ρ
Joint inversion VES + MASW
From 4n-2 to 3n-1 unkowns with the same experimental information
![Page 58: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/58.jpg)
Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
ρρρρapp
AB
VR
ωωωω
Joint inversion VES + MASW
dispersion curve
Processing
Z
VS profile
Joint Inversion0VS G
Z
0ρρρρ
![Page 59: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/59.jpg)
Geophysical methods for geotechnical site character ization
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50 100 150 200 250 300-18
-16
-14
-12
-10
-8
-6
-4
-2
0
Electric stratigraphy
ro [Ohm.m]
z [m
]
joint
single
100 200 300 400 500-18
-16
-14
-12
-10
-8
-6
-4
-2
0
Seismic stratigraphy
Vs [m/s]
z [m
]
joint
single
0 5 10 15 20 25 30100
150
200
250
300
350
Dispersion curves
freq [Hertz]
Vr [m
/s]
single
joint
100 101
101
102
103
102
Resistivity curves
AB/2 [m]
ro [Ohm.m
]
single
joint
Field test results
Best estimate of the clay layer
thickness
(Comina et al., 2004)
![Page 60: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/60.jpg)
Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Joint Inversion
Dispersion curves
reference modelPhysical Laws
P-wavetravel-time
The joint inversion algorithmSurface-wave propagation
Body-wave propagation
Apparent resistivity
Electrical current flow
Geometric regularization
dobs
Thickness hS-wave velocity VSP-wave velocity Vp
Resistivity Rho
m Structuralinversion
Physicalinversion
(Garofalo, 2014)
![Page 61: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/61.jpg)
Geophysical methods for geotechnical site character ization
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The model and the physical links
Model Parameters m :
� thickness (h)� density (ρ) � S-wave velocity (VS)� P-wave velocity (VP)� Resistivity (Rho)
Poisson’s ratio
h1
h2
VS,1 Rho1 VP,1
VS,2 Rho2 VP,2
VS,3 Rho3 VP,3
Unsaturated – skeleton
Saturated – mixture of fluid and grains
Half Space – clay
� � �� �� �� � � � � �
![Page 62: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/62.jpg)
Geophysical methods for geotechnical site character ization
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Model Parameters m :
� thickness (h)� density (ρ)� S-wave velocity (VS)� P-wave velocity (VP)� Resistivity (Rho)
Porosity
� Seismic wave Velocities
� Resistivity
h1
h2
VS,1 Rho1 VP,1
VS,2 Rho2 VP,2
VS,3 Rho3 VP,3
Unsaturated – skeleton
Saturated – mixture of fluid and grains
Half Space – clay
� � �� �� �� � � � � �The model and the physical links
![Page 63: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/63.jpg)
Geophysical methods for geotechnical site character ization
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� Saturated layer
φ = 0.42 – 0.52
Bellotti and Selleri (1969)
Field example: Results (Garofalo, 2014)
φ =
ρS − (ρS)2 − 4⋅ (ρS − ρF ) ⋅ K F
VP2 − 2⋅ 1−ν SK
1− 2ν SK
⋅VS
2
2 ⋅ (ρS − ρF )
![Page 64: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/64.jpg)
Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Field example: Results
ν = 0.13
Bergamo and Socco(2013)
(Garofalo, 2014)
![Page 65: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/65.jpg)
Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Case history #2
Investigation of volcanoclastic slopes
• Combination of several in situ geophysical tests to increase the reliability of the results
• Combination of laboratory and in situ testing for the assessment of saturation conditions
![Page 66: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/66.jpg)
Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Flowslides of 1998 in Campania
![Page 67: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/67.jpg)
Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Sarno
Cover soils formed by volcanic ashes from the Vesuvio (few meters thick) over a carbonatic bedrock
Air-fall pyroclastic deposits flowslides occurred in May 1998
(Cascini et al., 2008) (Cascini et al., 2008)
![Page 68: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/68.jpg)
Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Site characterization
• Quantification of potential volume of the flow (for the design of mitigation infrastructures): thickness of the soil cover
• Prevision of onset of the flowslide: assessment and monitoring of saturation condition of the soil cover
Objectives
Critical issues
• Very difficult site logistics with steep and vegetated slopes poses strong limitations in the use of conventional site tests (boreholes and penetration testing)
• Necessity of investigating large areas
![Page 69: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/69.jpg)
Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Combination of different geophysical approachesSurface wave method (MASW)
Electrical resistivity tomography
Seismic tomography (VP)
(Cos
entin
i et a
l., 2
012)
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Comments
• Electical and seismic (VP) tomography show that the assumption of a layered medium in MASW is reasonable
• Inversion of MASW shows the relevance of higher modes at this site: surface wave analysis is not a simple and straightforward task
• The estimated thickness of the cover material is comparable with different methods
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Laboratory calibration of Archie’s law for unsat ma terials
σt = σw nm Srp
n: porosity
S: saturation
σw : pore fluid conductivity
The two exponet m and p are found by fitting laboratory data
(Cos
entin
i et a
l., 2
012)
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Mapping resistivity into degree of saturation
(Cos
entin
i et a
l., 2
012)
![Page 73: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/73.jpg)
Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Closing Remarks
• Geophysical test provide useful tools for geotechnical site characterization – evaluation of geometrical boundaries to model subsoil
conditions (e.g. stratigraphy but also physical inclusions or hydrogeological features);
– evaluation of physical/mechanical parameters of direct use for geotechnical modeling.
• VS � G0; sample quality
• VP � saturation; porosity (+M0 � ν for dry soils)
• Surface wave methods are cost and time effective but their interpretation is not simple
![Page 74: Introduction to Geophysical Tests - Soil Mechanics · Geophysical methods for geotechnical site characterization Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI](https://reader036.fdocuments.in/reader036/viewer/2022062402/5f051af17e708231d4114a13/html5/thumbnails/74.jpg)
Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Closing remarks
• Importance of choosing the right technique for the specific application
• Integration of different techniques reduces uncertainties
• Laboratory experimental can provide a framework and calibration for quantitative interpretation of field tests
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Geophysical methods for geotechnical site character ization
POLITECNICO DI TORINOGeocongress 2014 23rd February 2014 SEBASTIANO FOTI
Thank you for your attention
Acknowledgments
Prof. Laura Valentina Socco (DIATI - Politecnico di Torino)
Dr Cesare Comina (University of Torino)
Prof. Guido Musso (DISEG – Politecnico di Torino)
Dr Renato Cosentini (Politecnico di Torino)
Ms Flora Garofalo (PhD student at Politecnico di Torino)
Dr Paolo Bergamo (now at Queen’s University, Belfast – UK)
Dr Margherita Maraschini (now at Fugro - UK)
Dr Daniele Boiero (now at Western-Gico - UK)
Dr Claudio Piatti (now at D’Apollonia - Italy)
Dr Claudio Strobbia (now at Western-Gico - UK)
www.soilmech.polito.it/downloadAdditional material available at