RELIABILITY OF SOIL POROSITY ESTIMATION FROM SEISMIC WAVE VELOCITIES

Transcription

1 5 th International Conference on Geotechnical and Geophysical Site Characterisation Technical Session 3-Characterisation in Rock and Residual Soil RELIABILITY OF SOIL POROSITY ESTIMATION FROM SEISMIC WAVE VELOCITIES Sebastiano Foti & Federico Passeri

2 Scientific Background I Laboratory and empirical analyses for porosity n: fundamental soil state parameter The initial reference Theory of Propagation of Elastic Waves in a Fluid-Saturated Porous Solid-Low frequencies range (Biot, 1956) A new possible in-situ measurement method Porosity of fluid-saturated porous media from measured seismic wave velocities (Foti et al., 2002) Initial fundamental hypotheses Binary continuum. Linear elastic and isotropic soil skeleton Undrained conditions and saturated soil by a perfect fluid Low dependency on the Poisson s Ratio of the evacuated soil skeleton Uncompressibility of soil grains (K S ) 2

3 Scientific Background II Further investigations were published aiming at validating the formula: Case studies of the Sicily s Bridge, Pisa Tower, Porto Empedocle, Canada, Florence, Gioia Tauro and Bocca di Chioggia (Foti and Lancellotta, 2004) (Jamiolkowski, 2012) (Lai and Crempien, 2012) Validation made comparing predicted values with laboratory results (Foti et al., 2002) (Jamiolkowski et al., 2009) Analysis of the well-posedness of the inverse problem and the existence of the solution (Lai and Crempien, 2012) POSITIVE RESULTS FROM THE VALIDATION PROCESS 3

4 Uncertainties sources Geophysical tests Degree of saturation Undrained conditions Poisson s Ratio ν SK Ancillary parameters Velocity of sound in water Porosity Estimation Soil type 4

5 Three major uncertainties factors Velocity of compressional waves in water Particularly dependent on the temperature (Lubbers and Graff, 1998) Choice of an initial typical range V w = 1464,8 ± 17,4 m/s Poisson s ratio of the evacuated soil skeleton In case of enough data above the watertable (for homogeneous soil deposits) ν sk = V P 2 2V S 2 Otherwise a larger uncertainty has to be accounted ν sk = 0.25 ± 0.1 Each test with advantages and disadvantages Difficulties in collecting a reliable statistical sample In situ geophysical tests 2 V P 2 V S 2 5

6 Errors propagation analysis (XH test) Relative error Derived quantities After a full derivation for each variable, it is possible to determine the influence of the single parameter on the final result. 6

7 Surface charts for XH tests I Relative errors colour legend Each material shows different realistic boundaries for the porosity due to: Reliable relative errors bars t p 3.5-5% In situ state of stress Mineralogy In situ density Dependency V S -n Relationship V P -V S Typical porosity values 7

8 Surface charts for XH tests d % V w 2.5-4% ν SK % 8

9 Case study #1: Zelazny Geotechnical surveys with repeated Cross-Hole Tests operated by the same company Only 8 sites provided the sufficient degree of saturation The distance between the boreholes is the most influent parameter A sufficient statistical sample was gathered (standard deviation of results) A full description of the operations is provided in: Jamiolkowski-Geotechnical characterization of copper tailings at Zelazny Most site (2015) 9

10 XIX 4 E-5 E (calculated ) Depth (m) XIX 4 E-5 E (standard ) Zelazny Most results Relative percentage error on porosity (%) 0 0 Relative percentage error on porosity (%) tp Vw d vsk tp Vw d vsk

11 Case study #2: Cross-hole, Down-hole, Suspension Logging and SDTM tests operated by different companies Intra-method and inter-method comparisons Limited number of samples (half maximum statistical dispersion) Availability of laboratory data A full description of the operations and the InterPACIFIC project is provided in Garofalo et al. (2016 a,b) 11

12 Cross-hole tests results Depth (m) SL tests results Depth (m) Mirandola results 0 n (-) n (-) Team 1 Team 2 Team 3 Team 4 Team 5 Team 6 Measured Team 1 Team 2 Measured

13 Conclusions An important advantage could be a water temperature measurement The Poisson s ratio of the evacuated soil skeleton is a relevant parameter Excellent results with the suspension logging test The formulation could become a tool aiming at assessing the reliability of each velocity profile A parametric analysis of uncertainties must be carried out taking into account only reliable parts of the charts In Poland only intra-method differences (XH), since in Mirandola the inter-method (different companies and operators) variability was assessed 13

14 Thank you for your attention A special acknowledgement to Prof. Michele Jamiolkowski for the Zelazny Most data 14