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. \ ? Geocongress 2014 23rd February 2014 ? ? ? 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 Geocongress 2014 23rd February 2014 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) Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO Geophysical methods for geotechnical site characterization Applicability of in situ tests (Mayne et al, 2002) Geocongress 2014 23rd February 2014 SEBASTIANO FOTI 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 SEBASTIANO FOTI 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 Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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] Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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) Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO Geophysical methods for geotechnical site characterization Pollutants and waste detection Buried waste disposal Courtesy of Valentina Socco Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 Saturated vs unsaturated(for coarse materials) Identification and monitoring of plumes SEBASTIANO FOTI POLITECNICO DI TORINO 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). Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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. Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO Geophysical methods for geotechnical site characterization Typical strain ranges for geotechnical problems (Atkinson, 2000) Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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) Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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) SEBASTIANO FOTI POLITECNICO DI TORINO 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). Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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 SEBASTIANO FOTI POLITECNICO DI TORINO 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) Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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. Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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 SEBASTIANO FOTI POLITECNICO DI TORINO 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 SEBASTIANO FOTI POLITECNICO DI TORINO 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 SEBASTIANO FOTI POLITECNICO DI TORINO Geophysical methods for geotechnical site characterization Degree of saturation Also very limited desaturation has a strong effect on the VP Valle-Molina (2006) Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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) Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO Geophysical methods for geotechnical site characterization Example: Zelasny Most tailing dam West dam Geocongress 2014 23rd February 2014 Jamiolkowski, 2012 SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 SEBASTIANO FOTI S: saturation POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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) SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 Large volumes are investigated Complex interpretation (indirect measurements based on inversion procedures or heavy processing) Accuracy and resolution at depth SEBASTIANO FOTI POLITECNICO DI TORINO Geophysical methods for geotechnical site characterization In-hole vs surface methods \ VS1 VS2 VS3 Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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) Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 15 15 SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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) Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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 SEBASTIANO FOTI POLITECNICO DI TORINO 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) SEBASTIANO FOTI POLITECNICO DI TORINO 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 SEBASTIANO FOTI log (VP2 ) , ...., log (VPn+1 ) )] POLITECNICO DI TORINO Geophysical methods for geotechnical site characterization Example on synthetic data (Piatti et al., 2012b) Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO Geophysical methods for geotechnical site characterization Example on synthetic data (Piatti et al., 2012b) Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO Geophysical methods for geotechnical site characterization Experimental data Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO Geophysical methods for geotechnical site characterization The PO river LENGTH: 650 km DISCHARGE ave.= 1450 m3/s max.= (nov 2000): 13000m3/s Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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? Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 Z SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 Z SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 25 30 50 100 150 200 250 300 ro [Ohm.m] SEBASTIANO FOTI -18 100 200 300 400 500 Vs [m/s] POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 SEBASTIANO FOTI dobs reference model Geometric regularization Structural inversion POLITECNICO DI TORINO 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: Geocongress 2014 23rd February 2014 thickness (h) density (ρ) S-wave velocity (VS) P-wave velocity (VP) Resistivity (Rho) SEBASTIANO FOTI Poisson’s ratio POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 thickness (h) density (ρ) S-wave velocity (VS) P-wave velocity (VP) Resistivity (Rho) SEBASTIANO FOTI POLITECNICO DI TORINO 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) Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO Geophysical methods for geotechnical site characterization Field example: Results (Garofalo, 2014) ν = 0.13 Bergamo and Socco (2013) Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO Geophysical methods for geotechnical site characterization Flowslides of 1998 in Campania Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO Geophysical methods for geotechnical site characterization (Cosentini et al., 2012) Mapping resistivity into degree of saturation Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO 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 Geocongress 2014 23rd February 2014 SEBASTIANO FOTI POLITECNICO DI TORINO
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