Geophysical methods in geotechnical investigations

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Leonard Norman
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1 Program Introduction relevant geophysical investigations in geotechnical field surveys, by Rolf Sandven, Multiconsult, Norway Use of shallow seismic measurements what information can analyses of surface waves (MASW) provide? by Mike Long, University College Dublin, Ireland Break with refreshments Experiences with 2D resistivity measurements (Electrical Resistivity Tomography) at the surface, by Inger Lise Solberg, National Geological Survey, Norway CPTU with resistivity measurements (R-CPTU) a combination of geophysical and geotechnical measurements - Short introduction of the method - Swedish experiences from R-CPTU and ERT investigations in the Gøta elv valley, by Hjørdis Løfroth, SGI, Sweden News and experiences in geotechnical geophysics in Finland, by Pauli Saksa, Geosto OY, Finland Will future soil investigations be carried out by airborne devices? Information about Airborne Electromagnetic Measurements (AEM), by Andreas Pfaffhuber, NGI, Norway Final remarks, short discussion 1

2 Nordic field committee meeting 2013 NGF meeting October 15th 2013 Geophysical methods in geotechnical investigations short introduction Senior geotechnical advisor Rolf Sandven, Multiconsult

3 Content Introduction - background Geotechnical use of geophysical data Determination of shear wave velocity v s and shear modulus G max Seismic CPTU (S-CPTU) Seismic analysis of shallow waves (MASW), (S-)CPTU) Bender element measurements (Lab) Determination of resistivity/electric conductivity Surface resistivity measurements (2D ERT) CPTU Down-the-hole resistivity (R-CPTU) Airborne Electromagnetic Measurements (AEM) Possibilities and limitations in practical use 3

4 Use of geophysical data Some remarks from S. Foti & T. Butcher (ISC 2004): «Capabilities and limitations of geophysical tests are not always well understood. This is not helped by lack of relationships between geophysically measured properties and the geotechnical characteristics of the ground needed for design» «Ground investigations should be problem driven. Geophysical measurements should focus on problems that they can help to solve» This was ten years ago, where are we now? 4

5 Use of geophysical data present trends Geophysical investigation methods have been quite common in geological surveys for many years More recently, geophysical investigation methods are also increasingly used in geotechnical investigations, due to e.g.: - More rational processing of test data due to growth of computational power - Increasing needs to detect special geological phenomena, such as e.g. mapping of quick clay areas - New requirements in present codes, e.g. better knowledge of shear wave velocity for earthquake design (Eurocode 8) More interaction between geophysicists and geotechnical engineers in an increasing number of projects may stimulate further use? 5

6 Geophysical principles shear wave velocity 6

7 Seismic methods Geotechnical field methods, e.g. S-CPTU (seismic CPTU) Deep seismic investigations, e.g. refraction-/reflection seismic Analyses of surface waves shallow seismic, e.g. MASW Time-distance diagrams give seismic velocities Used for determination of stratification and depth to bedrock Processing of measured data gives a shear velocity profile v s The shear wave velocity gives G max = rv s2, which can be used in e.g. earthquake analyses 7

8 Seismic CPTU (S-CPTU) Sonic wave triggered by blow on a percussive plate on the surface One (or preferably more) receiving unit(s) on the probe Provides the average velocity of v s in an approximately vertical direction Presently based on cable transfer of recorded data Equipment for S-CPTU Detail of percussive plate Test principle 8

9 MASW: Multi-channel analysis of surface waves Similar principles as used in conventional seismic investigations Generation of waves in the soil layers, extension ~ m Focus: Detection and measurement of surface waves (R- and S-waves) A large number of geophones on the surface allows elimination of background noise One energy release sufficient, e.g. use of a sledgehammer or similar The method can be used to detect velocity inversions (dense layer over soft layer) The processing of results provide a profile of shear wave velocity/shear modulus versus depth 9

10 (1) Generation of surface wave Low frequency geophones (4.5Hz)

11 MASW Multi-channel analyses of surface waves Apex Geoservices for Multiconsult, Seut, Fredikstad

12 Determination of shear wave velocity using bender elements Used for determination of initial shear modulus G max in the laboratory Important parameter for e.g. earthquake analyses In a triaxial test, the shear wave velocity is determined: before consolidation of the sample after consolidation, at start of shearing (G max ) at selected strain levels in the shearing phase (G) Test time per measurement : 1-3 milliseconds Somewhat more cumbersome preparation of test sample compared to an ordinary triaxial test 12

13 Bender element transmitter in top cap V s, G Screen display during measurements Bender element receiver in pedestal Bender element: Shear wave velocity measurements in conventional triaxial test

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15 Geophysical principles resistivity measurements 15

16 Resistivity measurements Test principle Utilize variations in the specific resistivity in the soil layers (electric resistance) An electric current (I) is applied to the soil between two electrodes, the electric voltage (V) is measured between two or more neighbouring electrodes. Different electrode configurations may be applied. Both airborne (AEM), surface (2D, ERT) and down-the-hole measurements (1D, R-CPTU) can be carried out made, but does not necessarily yield the same resistivity at the same point in the ground 16

17 Classification of specific resistivity Based on previous studies and experiences in Norway, Sweden and Canada Gradual transition and overlap between some soil types Specific resistivity influenced by local variations in mineralogy, salinity, density, pore water chemistry, degree of saturation etc. Influence on soil resistivity from various soil properties, soil constituents and geochemical properties not properly known? Salt, marine clay has the lowest resistivity: 1-10 Ωm Leached clay/possible quick clay: Ωm Silt: Ωm Dry crusted clay, coarse materials: over 100 Ωm Rock: several thousands Ωm 17

18 Airborne electromagnetic measurements (AEM) AEM systems transmits an electromagnetic signal from an airborne platform («bird») The signals induces secondary currents that are picked up by receiver coils units on the «bird» The signals are sensitive to the subsurface electrical resistivity The method may be used for various purposes, including mapping of quick clay areas 18

19 2D resistivity measurements from the surface (Electrical Resistivity Tomography) Geo-elektriske målinger utstyr, gjennomføring og muligheter Electrode Electrode-configuration

20 Example of interpretation of an ERT resistivity profile E6 Haga-Skjerdingstad Quick clay from soundings/lab 20

21 Use of R-CPTU (1D resistivity) Special module mounted behind a conventional CPTU-probe Continuous resistivity measurements in one boring (down-the-hole) Small influence volume and more local measurement compared to 2D ERT 1D and 2D resistivity measurements may be combined, but: Relatively good agreement for homogenous ground conditions Larger discrepancies when 3D-effects and inhomogenous conditions occur Input from R-CPTU may improve interpretation of ERT results 21

22 quick clay Example of R-CPTU profile in quick clay 22

23 Use of geophysical data - Possibilities Geophysical investigations may provide important information in the early investigation stages Quick mobilisation and cost-effective mapping of large areas (e.g. road projects, area mapping) Results from geophysical investigations provides continuous information between boreholes Geophysical methods are mostly non-intrusive, i.e. no disturbance of the soil during measurements Results may be used for optimal planning of traditional geotechnical investigations, particularly attractive in larger projects 23

24 Use of geophysical data Limitations Limited knowledge of geophysical methods among most geotechnical engineers Limited availability of test facilities, few companies perform geophysical measurements Some methods are sensitive to infrastructure in the ground, e.g. pipelines, cables etc. Interpretation and processing of results usually require special software and know-how Quality of interpretation may be somewhat influenced by boundary effects and topography of soil and rock strata Results need to be «calibrated» and compared to results from ordinary geotechnical methods 24

25 Use of geophysical data other features Important with good planning and interaction between geophysical and geotechnical expertise The quality of the interpretations is influenced by the basic knowledge of the investigated area At present, long waiting time for geotechnical investigations, probably less on geophysical. Information can be obtained earlier and may be used in temporary evaluations Geophysical investigations may be carried out at locations with limited access for drillrigs 25

ON THE INTERPRETATION OF SEISMIC CONE PENETRATION TEST (SCPT) RESULTS

Studia Geotechnica et Mechanica, Vol. XXXV, No. 4, 213 DOI: 1.2478/sgem-213-33 ON THE INTERPRETATION OF SEISMIC CONE PENETRATION TEST (SCPT) RESULTS IRENA BAGIŃSKA Wrocław University of Technology, Faculty