Sub-topics Thermal Characterization Influence of Various Parameters - - PowerPoint PPT Presentation

Sub-topics

• Thermal Characterization
• Influence of Various Parameters
• Centrifuge Modelling
• Electrical Characterization

It is quite difficult to state the quantitative value of resistivity

• f any soil mainly due to the

fact that the type of the soil is not clearly defined in most of the practical situations. For instance, the word clay can cover a wide variety of soils.

Effect of the type of soil

1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 200 400 600 800 1000 1200 1400

Black Cotton Soil Silty Sand Fine Sand Coarse Sand Fly Ash

Thermal Resistivity (deg C-cm/watt) Dry density (g/cc)

Heat conduction through soil is largely electrolytic, the quantity of water present plays an important role. The amount of water present is dependent on a number of factors viz. weather, time of the year, nature of the sub-soil and the depth of permanent water table. Dry soils depict low conductivity. It is mainly due to the presence of air, a poor conductor (4000°C-cm/watt), separates the solid grains (4°C-cm/watt) of the soil. If the moisture content (Resistivity of water 165°C-cm/watt) of the soil increases, then conductivity also increases. Saturated soil has high conductivity as compared to the water. The moisture content, from where rate of decrease of resistivity is less, is known as critical moisture content for the soil.

Effect of moisture content

5 10 15 20 25 30 35 200 400 600 800 1000 1200 1400

Dry density 1.0g/cc 1.1g/cc 1.2g/cc 1.3g/cc 1.4g/cc Thermal Resistivity (deg C-cm/watt) Moisture Content ( % )

The soil has greatest potential for induced instability when moisture content is below its critical moisture content. A soil that is better able to retain its moisture, as well as is efficient to re-wet when dried, will have better thermal performance characteristics. This is best accomplished with a well-graded sand to fine gravel (sound mineral rock), with a small percentage of fines (silt and clay), that can be easily compacted to a high density. For maximum density the smaller grains efficiently fill the spaces between the larger particles, and the fines enhance the moisture

• retention. A sound mineral aggregate, without organics, and without

porous particles, ensures effective thermal condition.

The effect of moisture content on the thermal conductivity of some soils has been found to depend on whether the soil is in the process of drying or wetting. During the drying process thermal conductivity is higher. The amount of water present in the soil is a major factor in determining its resistivity The resistivity of water is governed by the amount of salts dissolved in it (which change electrical properties as well), as such the resistivity of water is governed by the amount of salts dissolved in it. Quite a small quantity of dissolved salts reduces the resistivity very considerably Different salts have different effects and this is probably the reason for the resistivity of similar soils but from different localities is different considerably.

Hysteresis effect Effect of dissolved salts in water

The grain size and its distribution strongly effect the manner in which the moisture is held. With large grains, the pore space available will be higher (due to the presence of air) resulting in higher resistivity or lower conductance. For a well-graded soil, higher soil density can be achieved by compaction (the space between the large grains gets occupied by the smaller ones and hence resistivity reduces). If the size and shape of grains are in such a way that they form a compact dense structure, then the resistivity of the soil decreases.

Effect of particle size, distribution and closeness

• f packing of the grains

Soil resistivity is a function of the pore fluid properties, as such the viscosity

• f the pore fluid effects the resistivity. As such, soil resistivity increases as the

temperature gets reduced. For subzero temperatures, the resistivity rises sharply. This may be attributed to the high resistivity values associated with ice. In the freezing process of soil, ice cementation occurs and the adhesive forces increase as the temperature decreases. This possibly leads to better interfacial heat transfer with a consequent increase in thermal conductivity of the frozen ground. Thermal conductivity of the frozen soil is greater than that of the unfrozen soil (because ice has a conductivity value about four times that of the water). For porosity=100% (i.e. zero solid volume) the conductivity of saturated frozen soil may be expected to approach the value for ice, while that of unfrozen saturated soils approaches the conductivity of water. At the other extreme, as the porosity decreases to zero, the conductivity should tend towards that of the solid particle.

Effect of temperature

Soil resistivity varies due to changes in its moisture content and temperature---seasonal variation. High resistivity occur during the periods when the moisture content is low and the ground temperature is high. Resistivity survey should be done throughout the year. Particularly during the driest ground conditions. Due to soil anisotropy (stratification), the resistivity may not be same in all directions. The resistivity parallel to the bedding surface is more than the resistivity perpendicular to it. Soil mass stratification can be described as the ratio of parallel resistivity to the normal resistivity.

Seasonal variations Anisotropy

Summary of scaling factors

PARAMETER SCALING FACTOR Length 1/N Void ratio 1 Acceleration N Force 1/N

2

Stress 1 Strain 1 Velocity N Thermal conductivity

?

Thermal l diffusivity

?

Specific heat

?

Heat flux ? Mass 1/N

3

Mass density 1

Time (diffusion)

1/N

2

Hydraulic Conductivity N

Data logger Batteries

Axis of rotation

Rheostat Micro switch Switch-on Switch-off Test setup Thermocouple leads Power supply leads Geomaterial

Centrifuge Setup

Thermal properties at Different Centrifugation Efforts

1 10 100 10 100 1000 10000

SS-D1 SS-D2 SS-D3 SS-D4 SS-SUB

RT (

0C-cm/W)

1 10 100 10

• 8

10

• 7

10

• 6

10

• 5

10

• 4

SS-D1 SS-D2 SS-D3 SS-D4 SS-SUB

 (m

2/s)

50 100 150 200 1 2 3 4 5 SS-D1 SS-D2 SS-D3 SS-D4 SS-SUB

Cp (J/g-

0C)

N

Krishnaiah, S. and Singh, D. N., “A Methodology to Determine Soil Moisture Movement Due to Thermal Gradients”, Experimental Thermal and Fluid Science, 27, 2003, 715-721. Krishnaiah, S. and Singh, D. N., "Centrifuge modelling of heat migration in soils," International Journal of Physical Modeling in Geotechnics.4(3), (2004), 39-47

Some results

5 10 15 20 25 20 30 40 50 60 70 80

125 100 50 N=1

5 10 15 20 25 r(cm) 0.3 1.5 2.0 3.0 4.0 5 10 15 20 25

 (

0C)

t (min)

5 10 15 20 25 1 2 3 4 5 20 30 40 50 60 70

 (

0C)

r (cm)

t (min) 5 10 15 20 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5

r z

10 20 30 40

 (%)

r (m)

35 days 25 days 15 days t=10 days

N 50 100 125

Time modeling

20 40 60 80 100

r=2.0 m r=1.5 m

R

2=0.9762

N 50 100 125 Best-fit for all data

 (%)

t (days)

0.9801 50 100 150 200 250 20 40 60 80 100 0.9625 50 100 150 200 250

SS-D2 SS-D1

0.9856

Modeling of models

However, such studies can not be conducted for rocks/concrete/stiff soils. Alternative???

Descritization

θ (°C) Results

Mathematical Modelling (Using ANSYS 6.0)

Perspex mold Thermocouple (TC) Thermal probe Soil sample r

Test setup

5 10 15 20 25 30 20 30 40 50 60 70 80

2 3

 (

0C)

4

r=1.5 cm

ANSYS 6.0 Experimental

t (min)

Validation of FEM (ANSYS) results with experimental results

 (C) Finite Element Model Centrifuge test x rp (cm) tp (min) rm (cm) tm (min) 32.05 75 12905 1.5 10 1.83 28.84 100 15485 2.0 10 1.88 26.80 150 17105 3.0 10 1.90

Scale factor for time

 

N log t t log x

10 m p 10

        

x m p

N t t 