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Nordic field committee meeting - NGF meeting 2013 Geophysical methods in geotechnical investigations

Program • 14.00 – 14.20 Introduction – relevant geophysical investigations in geotechnical field

surveys, by Rolf Sandven, Multiconsult, Norway

• 14.20 – 14.50 Use of shallow seismic measurements – what information can analyses of surface waves (MASW) provide? by Mike Long, University College Dublin, Ireland

• 14.50 – 15.10 Break with refreshments

• 15.10 – 15.30 Experiences with 2D resistivity measurements (Electrical Resistivity Tomography) at the surface, by Inger Lise Solberg, National Geological Survey, Norway

• 15.30 – 16.10 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

• 16.10 – 16.30 News and experiences in geotechnical geophysics in Finland, by Pauli Saksa, Geosto OY, Finland

• 16.30 – 16.50 Will future soil investigations be carried out by airborne devices? Information about Airborne Electromagnetic Measurements (AEM), by Andreas Pfaffhuber, NGI, Norway

• 16.50 - 17.00 Final remarks, short discussion

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Nordic field committee meeting 2013 – NGF meeting October 15th 2013

Geophysical methods in geotechnical investigations– short introduction

Senior geotechnical advisor Rolf Sandven, Multiconsult

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Nordic field committee meeting - NGF meeting 2013 Geophysical methods in geotechnical investigations

Content – Introduction - background

–Geotechnical use of geophysical data

– Determination of shear wave velocity vs and shear modulus Gmax

• 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)

–Geophysical methods in geotechnical investigations

– Possibilities and limitations in practical use

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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?

Nordic field committee meeting - NGF meeting 2013 Geophysical methods in geotechnical investigations

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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?

Nordic field committee meeting - NGF meeting 2013 Geophysical methods in geotechnical investigations

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Geophysical principles – shear wave velocity

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• 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 vs

• The shear wave velocity gives Gmax = rvs

2, which can be used in e.g. earthquake analyses

Nordic field committee meeting - NGF meeting 2013 Geophysical methods in geotechnical investigations

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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 vs in an approximately vertical direction

Presently based on cable transfer of recorded data

Detail of percussive plate

Test principle Equipment for S-CPTU

Nordic field committee meeting - NGF meeting 2013 Geophysical methods in geotechnical investigations

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MASW: Multi-channel analysis of surface waves • Similar principles as used in conventional seismic investigations

Generation of waves in the soil layers, extension ~ 10-15 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

Nordic field committee meeting - NGF meeting 2013 Geophysical methods in geotechnical investigations

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Low frequency geophones (4.5Hz)

(1) Generation of surface wave

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MASW – Multi-channel analyses of surface waves

Apex Geoservices for Multiconsult, Seut, Fredikstad 2012

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Determination of shear wave velocity using bender elements

Used for determination of initial shear modulus Gmax 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 (Gmax)

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

Nordic field committee meeting - NGF meeting 2013 Geophysical methods in geotechnical investigations

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Bender element receiver in pedestal

Bender element transmitter in top cap

Vs,G

Bender element: Shear wave velocity measurements in conventional triaxial test

Screen display during measurements

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Nordic field committee meeting - NGF meeting 2013 Geophysical methods in geotechnical investigations

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Geophysical principles – resistivity measurements

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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

Nordic field committee meeting - NGF meeting 2013 Geophysical methods in geotechnical investigations

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• 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: 10-100 Ωm

Silt: 80 – 200 Ωm

Dry crusted clay, coarse materials: over 100 Ωm

Rock: several thousands Ωm

Nordic field committee meeting - NGF meeting 2013 Geophysical methods in geotechnical investigations

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• 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

Nordic field committee meeting - NGF meeting 2013 Geophysical methods in geotechnical investigations

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Geo-elektriske målinger – utstyr, gjennomføring og muligheter

Electrode

Electrode-configuration

2D resistivity measurements from the surface (Electrical Resistivity Tomography)

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Example of interpretation of an ERT resistivity profile– E6 Haga-Skjerdingstad

Quick clay from soundings/lab

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• 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

Nordic field committee meeting - NGF meeting 2013 Geophysical methods in geotechnical investigations

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Example of R-CPTU profile in quick clay

Nordic field committee meeting - NGF meeting 2013 Geophysical methods in geotechnical investigations

quick clay

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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

Nordic field committee meeting - NGF meeting 2013 Geophysical methods in geotechnical investigations

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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

Nordic field committee meeting - NGF meeting 2013 Geophysical methods in geotechnical investigations

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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

Nordic field committee meeting - NGF meeting 2013 Geophysical methods in geotechnical investigations