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
Importance of Electrical Properties of Geomaterials In Geotechnical Engineering 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….. 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 order of magnitude greater than that of the soils and geomaterials, dry soil= 3). For air, dielectric permittivity= 1.
Electrical Properties EPs of geomaterials are their response to the applied electric field Electrical Resistivity ( ) Dielectric constant (k) k = ε s / ε o METHODS • Low frequency resistivity methods (<100 Hz) • High frequency dielectric methods (10 4 -10 9 Hz) ADVANTAGES OVER OTHER METHODS: • Non destructive • Fast and easy • Incorporate response of the micro-structure (of the soil mass)
Parameters influencing Electrical Properties of 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)
Parameters Influencing Electrical conductivity 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.
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, NH 4 , 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.
Frequency 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 opposes 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.
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 of 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>10 7 Hz) are based on the • wave propagation concept. Methods based on low frequencies (f<10 6 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 Voltage Measurement SAMPLE Electrode Electrode
METHODS FOR LABORATORY MEASUREMENT OF SOIL RESISTIVITY B. Four-electrode method Power supply Voltage Measurement SAMPLE Electrode Electrode
Low Frequency Method 5 6 4 C V 7 3 V 2 8 C 1 1-2-3-4, 2-3-4-5, 3-4-5- 6, ……
Electrical Resistivity Box (100 mm cube) Plate electrode V A I L A R L Plate electrode = resistivity R= resistance Electrode point A = Area of electrodes L = spacing between the electrodes
ELECTRICAL RESISTIVITY BOX (TWO-ELECTRODE METHOD) 12 cm Point Electrodes @ 12 cm 3 cm It is difficult to determine A A R L 12 cm R . a a: shape factor for the electrode
ELECTRICAL RESISTIVITY PROBE Top nut 1 2 23 4 3 55 mm Lock nut 32 Ebonite ring Copper electrode 25 25 95 mm 25 20 SS pointed tip 16
Generalized relationship for Determining Soil Electrical Resistivity = A e (-(Sr-5)/B) Relationship between Electrical Resistivity and Thermal Resistivity Log ( ) = C R Log (R T ) C R = 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 2 nd 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 Developed Coulomb’s law Coulomb (1736-1806) Maxwell (1881) Electrical conductivity of a heterogeneous media Extended Maxwell’s equations for ellipsoidal particles Fricke (1924) Formation Factor= -m Archie (1942) (FF: electrical resistivity of saturated soil divided by the electrical resistivity of its pore fluid) Researcher AC Soil Property Determination of Water content Smith and Rose 100 kHz - 10 MHz (1933) Soil structure/Particle Arulanandan and 1 - 100 MHz orientation, electrolyte effect Smith (1973) Determination of water content Topp et al. (1980) 20 MHz - 1 GHz soil liquefaction, relative density Arulmoli et al. (1985) DC
State-of-the-art Researcher AC Soil Property Lovell (1985) 4 Hz porosity, permeability Loon et al. (1990) 0.1-1 GHz Conductivity of soil Arulanandan (1991) 50 MHz Porosity Thevanayagam (1993) All ranges porosity, pore fluid Knoll and Knight (1994) 0.1-10 MHz clay %, porosity, Shang et al. (1995) 60 Hz conductivity of clay Thevanayagam, (1995) 1 MHz - 1 GHz electrical dispersion in soils
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