Geophysical Methods for Geotechnical Site Characterization

Short Course – 23rd of February 2014
Introduction to Geophysical Tests
VS (m/s)
Sebastiano Foti
(ITALY)
Email: [email protected]
www.soilmech.polito.it/people/foti_sebastiano
Geophysical methods for geotechnical site characterization
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
Geocongress 2014 23rd February 2014
SEBASTIANO FOTI
POLITECNICO DI TORINO
Geophysical methods for geotechnical site characterization
Geophysical Methods
Geophysical methods are indirect surveying techniques based on
measurements carried out on the ground surface or in holes. They
allow the distribution of physical properties of the subsurface to be
estimated and correlated with engineering information.
They are based on the excitation of an object with an energy field (artificial
or natural) and on the measurement of the object response.
The interpretation of the object response allows the object to be
characterised.
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SEBASTIANO FOTI
POLITECNICO DI TORINO
Geophysical methods for geotechnical site characterization
Geophysical parameters
•
•
•
•
•
•
Density
Electrical Conductivity (or Resistivity)
Electrical Permittivity
Magnetic Suscettibility
Chargeability
Seismic velocities
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SEBASTIANO FOTI
POLITECNICO DI TORINO
Geophysical methods for geotechnical site characterization
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 characterization
Applicability of in situ tests
(Mayne et al, 2002)
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POLITECNICO DI TORINO
Geophysical methods for geotechnical site characterization
Geotechnical and geoenvironmental site characterization
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.
Geocongress 2014 23rd February 2014
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POLITECNICO DI TORINO
Geophysical methods for geotechnical site characterization
Identification of stratigraphic sequence
Pugin et al., 2009
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 characterization
Lateral variations (shallow faults)
e.g. seismic methods: surface wave tests
Geological model
(expected)
2D VS model from
surface wave
analysis
Updated geological
model
[Ivanov et al., 2006]
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Geophysical methods for geotechnical site characterization
Cavity detection
Example 1: void detection in a minerary area in
canada with pseudo-2D VS sections from surface
wave analysis
Example 2: (ERT) Electrical Resistivity
Tomography and (GPR) Ground Penetrating Radar
surveys reveal a sinkhole beneath a house
ERT
VS
Xu et al., 2008
GPR
Dobecki and Upchurch, 2006
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Geophysical methods for geotechnical site characterization
Identification of stratigraphic sequence / local litography
e.g. electrical methods to identify clays below sands
Turesson and Lind, 2005
Non-seismic methods:
Powerful tools to investigate lateral variations at the site
(e.g. for assessing the potential for differential settlements)
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Geophysical methods for geotechnical site characterization
Hydro - geophysics
Sea coast-line
S1
Depth [ m ]
0
100
Land
Distance [ m ]
200
300
400
500
600
700
800
900
1000
1100
1200
-20
-40
-60
-80
-100
Salt water
intrusion
Macchiareddu - Cagliari (Italy)
50 150 250 350 450 550 650 750 850
[Ohm m]
Apparent resistivity pseudosection
Profile: n.3 Cardiga
2D rendering of time domain EM vertical 1D profiles for salt water
intrusion in coastal aquifer.
Courtesy of Alberto Godio
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Geophysical methods for geotechnical site characterization
Pollutants and waste detection
Buried waste disposal
Courtesy of Valentina Socco
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Geophysical methods for geotechnical site characterization
Hydrogeological / environmental applications
Electrical Resistivity Tomography (ERT)
(Martìnez-Pagàn et al., 2009)
Resistivity is sensitive to:
• pore fluid content
• pore fluid conductivity
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Saturated vs unsaturated(for coarse materials)
Identification and monitoring of plumes
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Geophysical methods for geotechnical site characterization
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 characterization
site characterization
for engineering
purposes
Geotechnical
and geoenvironmental
site characterization
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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Indagini geofisiche per la caratterizzazione geotecnica
Bulk waves
Longitudinal wave
(Primary wave – P)
VP =
λ + 2µ
=
ρ
M
ρ
Shear wave
(Secondary wave – S)
VS =
µ
=
ρ
G
ρ
(Animation courtesy of prof.Braile)
CISM Udine 7 Febbraio 2014
Sebastiano Foti
POLITECNICO DI TORINO
Geophysical methods for geotechnical site characterization
Seismic methods
In a linear elastic medium
Shear wave propagation
G = ρ V S2
In soils
G 0 = ρ V S2
Animation courtesy of
Dr. L. Braile, Purdue University
Gsec
G0
τ
G0
1.0
Gsec
G0
Gsec
γc
γ
γc
γ
Strain range of
geophysical test
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Geophysical methods for geotechnical site characterization
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 characterization
Typical strain ranges for geotechnical problems
(Atkinson, 2000)
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Geophysical methods for geotechnical site characterization
Site vs Lab (Pisa)
G0 lab (MPa)
120
90
60
30
0
0
30
60
90
120
G0 sito (MPa)
G 0 = ρ V S2
(Cross-Hole Test)
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Geophysical methods for geotechnical site characterization
G0 site vs lab
G
0
150
(m/s)
300
600
0.25
/G
0, sito
0.50
0.80 1.00
1.50
2.00
valori progetto
ROSRINE
campioni
ricostituiti
da sabbie
cementate
XX
X
X
X
X
campioni indisturbati
di sabbie cementate
V
S, sito
450
0.10
0,lab
750
900
Pisa
1050
0.0
valori usuali per
campioni di roccia
0.5
1.0
V
S,lab
G 0 = ρ V S2
Geocongress 2014 23rd February 2014
/V
1.5
S,sito
(Stokoe e Santamarina, 2000)
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Geophysical methods for geotechnical site characterization
Assessment of sample quality
The ratio
VS(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 characterization
Site characterization for seismic projects
120
30
100
G (MPa)
80
25
G ( γ ) lab
20
60
15
40
10
20
5
0
0
0.0001
0.001
0.01
0.1
D (%) ∆ u/σ '0 (%)
G 0(sito ) = ρ ⋅ VS 2
1
Shear
strain γ (%)
deformazione
tangenziale,
• G0 from in situ testing (geophysics)
• G/G0(γ) e D(γ) from laboratory tests
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Geophysical methods for geotechnical site characterization
Site characterization for seismic projects
120
30
 G(γ) 
G ( γ ) = (G 0 ) sito ⋅

 G 0  lab
25
80
20
60
15
40
10
20
5
0
0
0.0001
0.001
0.01
0.1
D (%) ∆ u/σ '0 (%)
G (MPa)
100
1
deformazione
tangenziale,
Shear
strain γ (%)
In situ tests investigate a large volume of soil
whereas laboratory testing concerns small samples
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Geophysical methods for geotechnical site characterization
Body Waves
Compressional wave
VP =
M
ρ
(after Bolt, 1976)
Shear wave
VS =
G
ρ
Vertically polarized SV
or
Horizontally polarized SH
Geocongress 2014 23rd February 2014
(after Bolt, 1976)
Direction of Propagation
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Geophysical methods for geotechnical site characterization
In a linear elastic isotropic homogeneous medium
VP =
M
ρ
VS: shear wave velocity
VP: dilational wave velocity
ρ: density
G: shear modulus
VS =
G
ρ
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 characterization
Biot Theory
Macroscopic approach: the medium is modeled as a binary continuum
arising 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.
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 characterization
Biot solution
Under the hypothesis of grain incompressibility, the velocity of propagation of body
waves in porous media can be written as:
VP =
VS =
(K
SK
F
K
+ 43 ⋅ G ) +
n
ρ
G
ρ
where
ρ = (1 − n) ⋅ ρ S + n ⋅ ρ F
Geocongress 2014 23rd February 2014
ρS grain density
ρF water density
KF
KSK
G
n
water bulk modulus
soil skeleton bulk modulus
shear modulus
porosity
νSK Poisson ratio of the (evacuated) soil skeleton
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Geophysical methods for geotechnical site characterization
Biot solution
Under the hypothesis of grain incompressibility, the velocity of propagation of body
waves in porous media can be written as:
VP =
VS =
(K
SK
F
K
+ 43 ⋅ G ) +
n
ρ
G
ρ
ρ = (1 − n) ⋅ ρ S + n ⋅ ρ F
where
ρS grain density
ρF water density
KF
KSK
G
n
water bulk modulus
soil skeleton bulk modulus
shear modulus
porosity
νSK Poisson ratio of the (evacuated) soil skeleton
4 ⋅ (ρ S − ρ F ) ⋅ K F
ρ − (ρ ) −
 1 −ν SK  2
2
 ⋅VS
VP − 2 ⋅ 
SK 
 1 − 2ν 
n=
2 ⋅ (ρ S − ρ F )
S
S 2
Geocongress 2014 23rd February 2014
ρS,ρF,KF: standard values
VP & VS: measured
νSK : range 0.1÷0.4
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Geophysical methods for geotechnical site characterization
Soil porosity from seismic velocities
Leaning Tower of Pisa site
Velocity of Propagation [m/s]
0
500
0
1000
1500
2000
2500
Porosity
0
0.2
0.4
0.6
0.8
1
10
Depth [m]
20
30
40
50
60
70
80
LAB (Laval)
LAB (Osterberg)
Vp
Vs
Cross-Hole
test
(Foti et al., 2002)
Geocongress 2014 23rd February 2014
4 ⋅ (ρ S − ρ F ) ⋅ K F
ρ − (ρ ) −
 1 −ν SK  2
2
 ⋅VS
VP − 2 ⋅ 
SK 
ν
1
−
2


n=
2 ⋅ (ρ S − ρ F )
S
S 2
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Geophysical methods for geotechnical site characterization
Degree of saturation
Also very limited desaturation has a strong effect on the VP
Valle-Molina (2006)
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Geophysical methods for geotechnical site characterization
Influence of degree of salutarion on liquefaction resistance
saturation degree strongly affect liquefaction resistance
VP can be used to monitor saturation and esclude liquefaction
Tsukamoto et al (2006)
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Geophysical methods for geotechnical site characterization
Example: Zelasny Most tailing dam
West dam
Geocongress 2014 23rd February 2014
Jamiolkowski, 2012
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Geophysical methods for geotechnical site characterization
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
σw : pore fluid conductivity
Archie
σt = σw nm Srp
n: porosity
Bruggeman
σt = σw n3/2
m = 3/2 : theoretical
Waxman & Smits
σt = X (σw + σs)
σs : clay surface conductivity
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S: saturation
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Geophysical methods for geotechnical site characterization
Example at Lab scale
Polito – 2D ERT (Borsic et al., 2005)
Identification of zones with different compaction
levels in sand
Coarse Matrix
n ≈ 0.48
Dense Inclusion
n ≈ 0.43
1
mS/cm
2
Estimated values with
Bruggeman equation
Matrix n ≈ 0.46
3
Inclusion n ≈ 0.42
3. Tomographic reconstruction
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Geophysical methods for geotechnical site characterization
In-hole vs surface methods
(Invasive vs Non-invasive methods)
2
1
D
Cross-Hole Test (CHT)
Down-Hole Test (DHT)
Seismic Cone (SCPT)
Seismic Dilatometer (SDMT)
P-S Suspension Logging
Vertical Seismic Profiling (VSP)
Geocongress 2014 23rd February 2014
X
3
n
X
Surface Waves Methods SWM
(SASW, MASW, microtremors)
Seismic Refraction
(P-waves or SH-waves)
Seismic Reflection
(P-waves or SH-waves)
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Geophysical methods for geotechnical site characterization
In-hole vs surface methods
Invasive Tests
Direct measurements: simple
and accurate interpretation
Costs and flexibility (in time and
space)
Good resolution also at great
depth
Non-intrusive (e.g. important for
waste landfills)
Easier standardization
Average properties (dynamic
behaviour of the whole soil
deposit)
Additional information from
borehole logging or the
penetration of the cone
Disadvantages
Non-Invasive Tests
Costs and necessity of planning
well in advance
Local measurement
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Large volumes are investigated
Complex interpretation (indirect
measurements based on inversion
procedures or heavy processing)
Accuracy and resolution at depth
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Geophysical methods for geotechnical site characterization
In-hole vs surface methods
\
VS1
VS2
VS3
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Geophysical methods for geotechnical site characterization
Combined use of geophysical methods
Synergies between different techniques can be
exploited at different level of integration:
• 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)
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Geophysical methods for geotechnical site characterization
Level 1: Comparison In-Hole methods vs SASW
Vs (m/s)
0
400
800
0
0
400
5
Saluggia
5
Depth (m)
Depth (m)
Cross Hole
25
Cross Hole
25
SASW-fk
SASW-fk
30
30
Vs (m/s)
400
Vs (m/s)
Vs (m/s)
800
0
400
800
0
1200
400
800
1200
1600
0
0
0
Pontremoli
Pontremoli
Pontremoli
site 1
site 2
site 3
Depth (m)
5
10
5
Depth (m)
0
Pisa
5
20
Down Hole
15
300
15
20
SASW-fk
200
10
10
10
100
0
15
Depth (m)
0
800
0
Castelnuovo
Depth (m)
Vs (m/s)
Vs (m/s)
10
5
10
Down Hole
Down Hole
Down Hole
SASW-fk
SASW-fk
SASW-fk
15
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15
15
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Geophysical methods for geotechnical site characterization
Level 2: Data integration and data fusion
Electrical resistivity tomography
Pugin et al., 2009
SH-wave seismic reflection
resistivity
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Geophysical methods for geotechnical site characterization
Combined use
•
•
•
•
Level 1: comparison for validation
Level 2: data fusion
Example: synergies of
Level 3: a priori info
seismic refraction and
Level 4: joint inversions
surface wave analysis
(SWM)
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Geophysical methods for geotechnical site characterization
Example of synergy: SW + VP refraction
Same testing setup and
equipment
P-waves
VP1
Receivers (geophones)
VP2
VP3 ≈ VP2
Rayleigh waves
Experimental data contain both
surface waves and
direct/refracted P waves
VS1
VS2
VS3
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Geophysical methods for geotechnical site characterization
P-WAVE REFRACTION
70
travel time [ms]
60
50
VP1
40
VP2
30
VP3 ≈ VP2
20
10
0
10
20
30
40
50
spacing [m]
60
KF
( K + ⋅ G) +
n
VP =
S
(1 − n) ⋅ ρ + n ⋅ ρ F
SK
4
3
Shallow water table masks
variation of the mechanical
properties of the solid skeleton
(influence of the pore fluid)
Geocongress 2014 23rd February 2014
70 0
0
80
200
400
600
Vp [m/s]
800
1000
1200
1400
1600
Water table
1
Depth [m]
0
2
3
4
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Geophysical methods for geotechnical site characterization
Experimental Data
Shear Wave Velocity (m/s)
Depth (m)
5
0
200
400
600
800 1000
starting profile
inversion #1
inversion #2
inversion #3
cross-hole test
10
15
20
25
Hp#1 Water table from P-wave refraction
Hp#2 No water table
Hp#3 Water table deeper than Hp #1
700
phase velocity, m/s
0
experimental
inversion #1
inversion #2
inversion #3
600
500
400
300
200
100
0
10
20
30
40
50
60
70
frequency, Hz
30
Geocongress 2014 23rd February 2014
(Foti and Strobbia, 2002)
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Geophysical methods for geotechnical site characterization
Level 4: joint inversion
(Piatti et al., 2012b)
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
T -1
1


+
L=
d
-g
m
C
d
-g
m
(
)
(
)
(
)
(
)
obs
obs
obs

 N + M + A 
dobs = ( log (VR1 ) , log (VR2 ) , ...., log (VRN ' ) ) ( log ( t1 ) , log ( t2 ) , ...., log ( tN '' ) ) 
gSW ( m) 
g ( m) = 

gPR ( m) 
m = [( log ( h1 ) , log ( h2 ) , ...., log ( hn ) ) ( log (VS1 ) , log (VS 2 ) , ...., log (VS n+1 ) )
Geocongress 2014 23rd February 2014
( log (V ) ,
P1
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log (VP2 ) , ...., log (VPn+1 ) )]
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Geophysical methods for geotechnical site characterization
Example on synthetic data
(Piatti et al., 2012b)
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Geophysical methods for geotechnical site characterization
Example on synthetic data
(Piatti et al., 2012b)
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Geophysical methods for geotechnical site characterization
Experimental data
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Geophysical methods for geotechnical site characterization
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
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Geophysical methods for geotechnical site characterization
The PO river
LENGTH: 650 km
DISCHARGE
ave.= 1450 m3/s
max.= (nov 2000): 13000m3/s
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Geophysical methods for geotechnical site characterization
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
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Geophysical methods for geotechnical site characterization
Seepage potential
Geology: alluvial deposits: recent sands, gravel, clay
TARGET: clayey layer: continuity, thickness
Water level can reach 10 m
above the ground surface
Anthropic soil
?
Thickness of low permeability layer?
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Geophysical methods for geotechnical site characterization
Geophysical investigation
large extension of the areas
Interest in fast geophysical tests from the surface
At a test site several
methods have been
tested and compared
VES ERT
HEP
SWM
Prefr SHrefr
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Geophysical methods for geotechnical site characterization
Combinations MASW + VES
ρ
app
Processing
VR
Apparent
resistivity
AB
dispersion
curve
ω
Inversion
VS
ρ 0
G0
VS profile
Z
resistivity
profile
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Z
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Geophysical methods for geotechnical site characterization
Joint inversion VES + MASW
Physical parameters: shear velocity and resistivity
Assumed parameter distribution: stack of homogeneous isotropic layers
VS, ρ
MODEL PARAMETERS:
n
ρ
n
VS
n-1 H
VS, ρ
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
From 4n-2 to 3n-1 unkowns
with the same experimental information
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Geophysical methods for geotechnical site characterization
Joint inversion VES + MASW
ρ
Processing
app
AB
VR
Joint Inversion
ρ
0
VS
dispersion
curve
ω
G0
VS profile
Z
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Z
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Geophysical methods for geotechnical site characterization
Field test results
(Comina et al., 2004)
Resistivity curves
3
10
Electric stratigraphy
Seismic stratigraphy
0
0
joint
joint
2
10
1
10 0
10
10 1
AB/2 [m]
10
2
-2
-2
-4
-4
-6
-6
-8
-8
Best estimate
single
of the -10
clay layer
thickness
-12
z [m]
joint
-10
Dispersion curves
350
Vr [m/s]
single
-12
300
single
250
joint
200
z [m]
ro [Ohm.m]
single
-14
-14
-16
-16
150
-18
1000
5
10
15
freq [Hertz]
20
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25
30
50 100 150 200 250 300
ro [Ohm.m]
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-18
100 200 300 400 500
Vs [m/s]
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Geophysical methods for geotechnical site characterization
The joint inversion algorithm
(Garofalo, 2014)
Surface-wave
propagation
Body-wave
propagation
Electrical
current flow
Dispersion
curves
P-wave
travel-time
Apparent
resistivity
Joint
Inversion
Physical Laws
Physical
inversion
Thickness h
S-wave velocity VS
m
P-wave velocity Vp
Resistivity Rho
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dobs
reference model
Geometric
regularization
Structural
inversion
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Geophysical methods for geotechnical site characterization
The model and the physical links
Unsaturated – skeleton
h1
VS,1
VP,1
Rho1
Saturated – mixture
of fluid and grains
h2
VS,2
VP,2
Rho2
VS,3
VP,3
Rho3
Half Space – clay
Model Parameters m:
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thickness (h)
density (ρ)
S-wave velocity (VS)
P-wave velocity (VP)
Resistivity (Rho)
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Poisson’s ratio
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Geophysical methods for geotechnical site characterization
The model and the physical links
Unsaturated – skeleton
h1
VS,1
VP,1
Rho1
Saturated – mixture
of fluid and grains
h2
VS,2
VP,2
Rho2
VS,3
VP,3
Rho3
Half Space – clay
Model Parameters m:
Seismic wave Velocities
Porosity
Resistivity
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thickness (h)
density (ρ)
S-wave velocity (VS)
P-wave velocity (VP)
Resistivity (Rho)
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Geophysical methods for geotechnical site characterization
Field example: Results
(Garofalo, 2014)
Saturated layer
4 ⋅ ( ρ S − ρ F )⋅ K F
ρ − (ρ ) −
 1− ν SK  2
2
⋅VS
VP − 2 ⋅ 
SK 
 1− 2ν 
φ=
2 ⋅ (ρ S − ρ F )
S
S 2
φ = 0.42 – 0.52
Bellotti and Selleri (1969)
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Geophysical methods for geotechnical site characterization
Field example: Results
(Garofalo, 2014)
ν = 0.13
Bergamo and Socco
(2013)
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Geophysical methods for geotechnical site characterization
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
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Geophysical methods for geotechnical site characterization
Flowslides of 1998 in Campania
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Geophysical methods for geotechnical site characterization
Sarno
Air-fall pyroclastic deposits
(Cascini et al., 2008)
flowslides occurred in May 1998
(Cascini et al., 2008)
Cover soils formed by volcanic ashes from the Vesuvio
(few meters thick) over a carbonatic bedrock
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Geophysical methods for geotechnical site characterization
Site characterization
Objectives
• 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
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
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Geophysical methods for geotechnical site characterization
Combination of different geophysical approaches
Surface wave method (MASW)
(Cosentini et al., 2012)
Seismic tomography (VP)
Electrical resistivity tomography
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Geophysical methods for geotechnical site characterization
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 characterization
(Cosentini et al., 2012)
Laboratory calibration of Archie’s law for unsat materials
σt = σw nm Srp
n: porosity
S: saturation
σw : pore fluid conductivity
The two exponet m and p are found by fitting laboratory data
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Geophysical methods for geotechnical site characterization
(Cosentini et al., 2012)
Mapping resistivity into degree of saturation
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Geophysical methods for geotechnical site characterization
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
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Geophysical methods for geotechnical site characterization
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 characterization
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)
Additional material available at
www.soilmech.polito.it/download
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