ENVIRONMENTAL GEOMECHANICS CE-641 Lecture No. 20 Prof. D N Singh - - PowerPoint PPT Presentation

ENVIRONMENTAL GEOMECHANICS

CE-641 Lecture No. 20

• Prof. D N Singh

Department of Civil Engineering

30.10.2019 Lecture No. 20 Lecture Name: Geomaterial Characterization

Sub-topics Electrical Characterization

• Importance
• Electrical Properties (Resistivity & Dielectric constant)
• Influence of Various Parameters
• Methods of Measurement
• Generalized Relationships
• Relationship between Thermal and Electrical Resistivities
• Laboratory & Field Investigations
• State-of-the-art
• Electrical Properties Ohmic Conduction in Geomaterials
• Electrical Impedance
• Determination of Electrical Properties
• Flow of AC in Geomaterials: Basic Models

Magnetic Characterization

Becoming essential for predicting/determining: Water content & Saturation Degree of compaction Porosity Hydraulic conductivity Liquefaction potential of the soil mass Detecting and locating geomembrane failures To estimate corrosive effects of soil on buried steel/concrete To investigate effects of soil freezing on buried structures Estimating soil salinity for agricultural activities.

Importance of Electrical Properties of Geomaterials In Geotechnical Engineering

Change in water content leads to change in the dielectric permittivity of the water-geomaterial system or vice versa. This fact leads to determination of water content of the geomaterial if its dielectric constant is known. Many sensing techniques have been developed, over years and are still being developed for measuring soil moisture and some of these techniques are: Capacitance probe/FD Sensor Time domain reflectometry, TDR, probe Useful for rapid determination of in-situ moisture content that too under non-invasive and non-destructive manner. Measure volumetric moisture content. Water has a high dielectric permittivity (=81, which is more than an

• rder of magnitude greater than that of the soils and geomaterials,

dry soil= 3). For air, dielectric permittivity= 1.

Importance…..

METHODS

• Low frequency resistivity methods (<100 Hz)
• High frequency dielectric methods (104 -109 Hz)

Electrical Resistivity () Dielectric constant (k)

ADVANTAGES OVER OTHER METHODS:

• Non destructive
• Fast and easy
• Incorporate response of the micro-structure (of the soil mass)

EPs of geomaterials are their response to the applied electric field

Electrical Properties

k = εs/εo

Parameters influencing Electrical Properties

• f Geomaterials
• Porosity and the pore structure
• Water content
• Salinity level
• Cation exchange capacity of the soil
• Temperature
• Type and Frequency of the current

Parameters influencing Liquefaction of soils

• Grain shape and size
• Porosity & Relative Density
• Variation of Water table
• External Forces/Disturbances---shearing
• Resistivity = f (void ratio)=f (density)
• Change in resistivity= f (change in the void ratio)

= f (change in the density)

The conduction of electricity in geomaterials takes place through the moisture-filled pores. Therefore, EC of the soil is influenced by the interactions between the following soil parameters: Pore continuity: Water-filled pores that are connected directly with the neighbouring pores tend to conduct electricity more readily. Soils with high clay content have numerous, small water-filled pores that are quite continuous and usually conduct electricity better than sandy soils. Compaction normally increases the pore continuity and hence the soil EC. Water content: Dry soil conductivity is less than the moist soils.

Parameters Influencing Electrical conductivity

Salinity level: Increase in concentration of electrolytes (salts) in pore solution will increase the EC appreciably. Cation exchange capacity (CEC): Mineral soils containing high levels of organic matter (humus) and/or minerals such as Montmorillonite, Illite or Vermiculite have a much higher ability to retain positively charged ions (such as Ca, Mg, K, Na, NH4, H) than soils lacking these constituents. The presence of these ions in the pores enhances EC in the same way that salinity does. Temperature: As temperature decreases, towards the freezing point of water, EC decreases slightly. Below freezing point, pores become increasingly insulated from each other and overall EC declines rapidly.

Capacitance, Inductance and Resistance are strongly dependent on the frequency of the current input. Capacitance is the property of an electric circuit that opposes any change in voltage and is dependent on the frequency. Water (due to its dipole nature) in the pores is largely responsible for the residual high-frequency capacitance. It varies from a high value, at low frequency, to a low value, at high

• frequency. Capacitance values at high frequency correspond to the

background capacitance of the water in the medium. An inductor is a electronic component that stores energy in the form of a magnetic field. Inductance is the property of an electric circuit that

• pposes current. However, in most of the geomaterials (unless they

contain Iron) this component is not significant. Resistance is the opposition to the flow of current in an electric circuit and it decreases rapidly with the increase of frequency.

Frequency

The type of current used plays an important role: DC and low frequency AC (<100Hz) are employed for determination of soil resistivity. For frequency >100 Hz, the conductivity is noticed to increase with the applied frequency. On the other hand, high frequency (>1MHz) dielectric response

• f geomaterials can be employed to characterize the soil fabric

structure such as: particle shape, Size, orientation and porosity These studies highlight the presence of water (dielectric constant 81) in increasing the dielectric constant of the wet soil as compared to its dry state (dielectric constant 5). The dielectric constant is noticed to remain constant only if the applied frequency is >50 MHz. TDR and capacitive devices are employed for finding the dielectric constant of the geomaterials based on which its characterization can be done.

Laboratory & Field Investigations

• Two-electrode or four-electrode methods
• Application of :

Surface Network Analyzer (SNA) Impedance analyzer LCR meter

• Methods based on high frequencies (f>107 Hz) are based on the

wave propagation concept.

• Methods based on low frequencies (f<106 Hz) are based on

equivalent elements (as the wavelength is much larger than the size of the measurement device).

METHODS FOR LABORATORY MEASUREMENT OF SOIL RESISTIVITY

• A. Two-electrode method

Power supply SAMPLE Electrode Electrode Voltage Measurement

METHODS FOR LABORATORY MEASUREMENT OF SOIL RESISTIVITY

• B. Four-electrode method

Power supply SAMPLE Electrode Electrode Voltage Measurement

2 3 4 5 6 7 8 1 C V V C

1-2-3-4, 2-3-4-5, 3-4-5-6, ……

Low Frequency Method

Electrical Resistivity Box (100 mm cube)

Plate electrode Plate electrode

Electrode point

L A I V    L A R  

A= Area of electrodes L= spacing between the electrodes = resistivity R= resistance

ELECTRICAL RESISTIVITY BOX (TWO-ELECTRODE METHOD)

12 cm @ 3 cm 12 cm 12 cm Point Electrodes

It is difficult to determine A

L A R   a . R  

a: shape factor for the electrode

1 2 3 4 16 Ebonite ring 55 mm 25 25 25 95 mm Lock nut Top nut SS pointed tip Copper electrode 32 23 20

ELECTRICAL RESISTIVITY PROBE

Generalized relationship for Determining Soil Electrical Resistivity  = Ae(-(Sr-5)/B) Relationship between Electrical Resistivity and Thermal Resistivity

Log () = CRLog (RT)

CR = A+B.e (-SrC) A, B and C = f (Fine content)

Sr : Degree of saturation

Field Investigations

Ground Penetrating Radar (GPR) Time Domain Reflectometry (TDR) Capacitance sensor Portable dielectric probe (PDP) Electrical conductivity probe (ECP) Monitoring Slope deformation & Movement

2nd International Symposium and Workshop on Time Domain Reflectometry for Innovative Geotechnical Applications (TDR 2001). www.iti.northwestern.edu/tdr/tdr2001/proceedings/

State-of-the-art

Researcher Contribution Coulomb (1736-1806) Maxwell (1881) Fricke (1924) Archie (1942) Developed Coulomb’s law Electrical conductivity of a heterogeneous media Extended Maxwell’s equations for ellipsoidal particles Formation Factor= -m (FF: electrical resistivity of saturated soil divided by the electrical resistivity of its pore fluid) Researcher AC Soil Property Smith and Rose (1933) Arulanandan and Smith (1973) Topp et al. (1980) Arulmoli et al. (1985) 100 kHz - 10 MHz 1 - 100 MHz 20 MHz - 1 GHz DC Determination of Water content Soil structure/Particle

• rientation, electrolyte effect

Determination of water content soil liquefaction, relative density

Researcher AC Soil Property Lovell (1985) Loon et al. (1990) Arulanandan (1991) Thevanayagam (1993) Knoll and Knight (1994) Shang et al. (1995) Thevanayagam, (1995) 4 Hz 0.1-1 GHz 50 MHz All ranges 0.1-10 MHz 60 Hz 1 MHz - 1 GHz porosity, permeability Conductivity of soil Porosity porosity, pore fluid clay %, porosity, conductivity of clay electrical dispersion in soils

State-of-the-art

Electrical Properties of Geomaterials

Electrical properties (conductivity, , and dielectric constant, k) can be used for geomaterial characterization. Electrical conductivity is a measure of charge mobility in response to an electric field. Dielectric constant is a measure of the capacity of a material to reduce the strength of an electric energy field and to behave like an insulator. Variation in electrical properties with the frequency of AC

Electrical Properties of Geomaterials

• Electrical conduction in moist geomaterials occurs as a result of the

movement of ions

• These materials are dielectric material (characterized by polarization)
• However, they behave neither as a conducting material nor as a

perfectly dielectric material, and hence they can be modeled as a ‘lossy dielectric material’.

• A frequency-dependent complex permittivity, k, is used to capture both

amplitude and phase information.

    A d C k

For the parallel plate capacitor

Dielectric Constant k k=ε/εo where, ε = material permittivity εo = permittivity of free space = 8.85410-12 (F/m) k =(k-j·k) k= real part of k (depends on polarizability) k= imaginary part of k (losses due to the conduction and polarization)

Ohmic Conduction in Geomaterials: Basics

• Conduction of current in due to ionic movement
• I = .V : Resistivity
• Factors affecting electrical conduction in case of coarse-grained soils:
• void ratio
• degree of saturation
• Grain size & shape & orientation
• Pore structure
• the nature of the pore fluid and its conductivity
• Electrical conduction in fine-grained soils:

Complex phenomenon, due to development of double layers around the grains Negligible surface charge of grains

Electrical Impedance

Z=V(t)/I(t) =Vcost/Icos(t-) =R-jX where, R is resistance, which is the real part of Z (= Z), X is the imaginary part of Z (=Z)

• Resistivity term is applicable to DC
• Impedance – Resistance offered by soil mass to AC
• Impedance captures both frequency and amplitude information

Impedance is frequency (of AC) dependent

Basic Model R C

Element Impedance Admittance Resistor (R) Inductor (L) Capacitor (C) Z = R+j0 Z = 0+jωL Z = 0-j(ωC)-1 Y = 1/R+j0 Y = 0-j(ωL)-1 Y = 0+jωC Elements in series : Elements in parallel :





i i equiv

Z Z





i i equiv

Y Y

Determination of Electrical properties of Soils

140 mm Connector Perspex sheet 30 mm Scale Sample SS electrode 100 mm 30 mm 10 mm Base plate

Perspex box

Specimen

Plate electrode

Impedance cells

Impedance cells

Analysis of experimentally

• btained impedance data can be

done by: Cole-Cole plot Nyquist plot----widely employed Bode plot

Equivalent circuit

CE CS CE RE RS RE

• Z ''

Z ' (2RE+RS) Rs

Perspex box

Specimen

Plate electrode

Details of a typical Impedance Cell Nyquist plot

20 40 60 80 100 120 20 40 60 80 100 120

• Z



Z



SS1 SS2 SS3

R

Electrode polarization

Nyquist Impedance plot

SS1: Grade-1 sand (Coarse) SS2: Grade-2 sand (Medium) SS3: Grade-3 sand (Fine)

• Z''

Z'

ω=

Development of Equivalent Circuits Fitting Circuits to Impedance Data Using Z-view software (Johnson, 2003)

1 2 3 4 5 EXP CKT1

• Z'' (104)

Z' (104)

Development of Equivalent Circuits

1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 EXP CKT1 EXP CKT2 X EXP CKT4 EXP CKT3 EXP CKT5

• Z'' (104)

EXP CKT6

Z' (104)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 2 4 6 8 10 12 14 16 18 20

R (10

3



Rgb Rg

Grain resistance Rg Grain boundary resistance Rgb

The soil can be characterized as a granular material, if Rgb is negligible or very low. For these soils, the order of magnitude

• f the Rg would be very high.

The soil can be characterized as a fine- grained soil if both Rgb and Rg are present in the equivalent circuit. However, values of these resistances should be quite low as compared to the granular soils/materials.

c c a b b a

: Conduction path : Soil grains : Electrodes

: Air

Basic Models to Depict Flow Paths of AC in Dry Geomaterials

AC flow through a dry soil may occur due to: (i) a-a (the surface of the soil grains, which is mainly due to the presence of surface charge carriers/ions) (ii) b-b (the soil cluster, wherein soil grains are in contact with each other and current may flow through the interconnected grains) (iii) c-c (partly through the soil grains and partly through the air present in the voids, which is a least likely path due to its very high resistance, unless the air is contaminated with fumes of water or chemicals

d b d c c a a b

: Conduction path : Water filled voids : Soil grains : Electrodes

: Air

Basic Models to Depict Flow Paths of AC in Partially Saturated Geomaterials

AC flow through a partially-saturated soil may occur through: (i) a-a (interconnected pores filled with pore- solution, which offers least resistance to the flow

• f current)

(ii) b-b (interconnected soil grains) (iii) c-c (partly through the connected soil grains and partly through interconnected pores) (iv) d-d (partly through soil grains and partly through the voids, which contain air and pore- solution.

c b c b a a

: Conduction path : Water filled voids : Soil grains : Electrodes

Basic Models to Depict Flow Paths of AC in Saturated Geomaterials

As the air is not present in the voids, the AC can flow through; (i) a-a (continuous pore-fluid) (ii) b-b (interconnected soil grains) (iii) c-c (partly through interconnected soil grains and partly through the pore-fluid).