IIT Bombay ENVIRONMENTAL GEOMECHANICS CE-641 Department of Civil Engineering DR. D. N. SINGH dns@civil.iitb.ac.in www.civil.iitb.ac.in/~dns
IIT Bombay Slide 1 16.10.2009 Lecture No. 18 Lecture Name: Geomaterial Characterization Sub-topics • Thermal Characterization • Influence of Various Parameters • Centrifuge Modelling • Electrical Characterization Environmental Geomechanics Lecture No. 18 D N Singh
IIT Bombay Slide 2 Effect of the type of soil It is quite difficult to state the Black Cotton Soil 1400 Silty Sand quantitative value of resistivity Fine Sand of any soil mainly due to the Coarse Sand 1200 Fly Ash fact that the type of the soil is Thermal Resistivity (deg C-cm/watt) not clearly defined in most of 1000 the practical situations. 800 600 For instance, the word clay 400 can cover a wide variety of soils. 200 0 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Dry density (g/cc) Environmental Geomechanics Lecture No. 18 D N Singh
IIT Bombay Slide 3 Effect of moisture content Heat conduction through soil is largely 1400 electrolytic, the quantity of water present plays Dry density an important role. 1.0g/cc 1200 1.1g/cc 1.2g/cc The amount of water present is dependent on 1.3g/cc a number of factors viz. weather, time of the Thermal Resistivity (deg C-cm/watt) 1000 1.4g/cc year, nature of the sub-soil and the depth of permanent water table. 800 Dry soils depict low conductivity. It is mainly due to the presence of air, a poor conductor 600 (4000 ° C-cm/watt), separates the solid grains (4 ° C-cm/watt) of the soil. If the moisture 400 content (Resistivity of water 165 ° C-cm/watt) of the soil increases, then conductivity also increases. 200 Saturated soil has high conductivity as 0 compared to the water. The moisture content, 0 5 10 15 20 25 30 35 from where rate of decrease of resistivity is Moisture Content ( % ) less, is known as critical moisture content for the soil. Environmental Geomechanics Lecture No. 18 D N Singh
IIT Bombay Slide 4 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. Environmental Geomechanics Lecture No. 18 D N Singh
IIT Bombay Slide 5 Hysteresis effect 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. Effect of dissolved salts in water 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. Environmental Geomechanics Lecture No. 18 D N Singh
IIT Bombay Slide 6 Effect of particle size, distribution and closeness of packing of the grains 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. Environmental Geomechanics Lecture No. 18 D N Singh
IIT Bombay Slide 7 Effect of temperature Soil resistivity is a function of the pore fluid properties, as such the viscosity of 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. Environmental Geomechanics Lecture No. 18 D N Singh
IIT Bombay Slide 8 Seasonal variations 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. Anisotropy 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. 9 Environmental Geomechanics Lecture No. 18 D N Singh
IIT Bombay Slide 9 Determination of Thermal Properties in a Geotechnical Centrifuge Though, several analytical and numerical models are available to model heat migration in geomaterials they lack simulation of the prototype conditions in terms of in-situ stresses. To overcome this, field tests, which are relatively costly, time consuming and difficult to perform, are found to be of immense help. Under these circumstances, a geotechnical centrifuge should be used for studying heat migration in geomaterials. Environmental Geomechanics Lecture No. 18 D N Singh
IIT Bombay Slide 10 Summary of scaling factors PARAMETER SCALING FACTOR Length 1/N Void ratio 1 Acceleration N 1/N 2 Force Stress 1 Strain 1 Velocity N 1/N 3 Mass Mass density 1 1/N 2 Time (diffusion) Hydraulic Conductivity N Thermal conductivity ? Thermall diffusivity ? Specific heat ? Heat flux ? Environmental Geomechanics Lecture No. 18 D N Singh
IIT Bombay Slide 11 Centrifuge Setup Data logger Rheostat Switch-on Micro switch Switch-off Axis of rotation Test setup Batteries Thermocouple leads Power supply leads Geomaterial Environmental Geomechanics Lecture No. 18 D N Singh
IIT Bombay Slide 12 Environmental Geomechanics Lecture No. 18 D N Singh
IIT Bombay Slide 13 Variation of voltage with time 2.0 N 1 50 100 125 1.5 Voltage (V) 1.0 0.5 0.0 0 5 10 15 20 25 t (min) Environmental Geomechanics Lecture No. 18 D N Singh
IIT Bombay Slide 14 Thermal properties at Different Centrifugation Efforts -4 5 10000 10 SS-D1 SS-D1 SS-D1 SS-D2 SS-D3 SS-D2 SS-D3 SS-D2 SS-D3 SS-D4 SS-SUB SS-D4 SS-SUB 4 SS-D4 SS-SUB -5 10 1000 0 C-cm/W) 0 C) 3 p (J/g- 2 /s) -6 10 α (m 2 C T ( R 100 -7 10 1 0 50 100 150 200 -8 10 10 1 10 100 1 10 100 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 Environmental Geomechanics Lecture No. 18 D N Singh
IIT Bombay Slide 15 Some results r(cm ) 0.3 1.5 2.0 3.0 4.0 r 50 N =1 100 125 80 70 60 0 C) 50 θ ( 40 30 20 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 t (m in) z t (m in) 5 10 15 20 70 60 0 C) 50 θ ( 40 30 20 0 1 2 3 4 5 0 1 2 3 4 5 0 1 2 3 4 5 0 1 2 3 4 5 r (cm ) Environmental Geomechanics Lecture No. 18 D N Singh
IIT Bombay Slide 16 Time modeling N 50 100 125 35 days 15 days 25 days t=10 days 40 30 20 10 0 µ (%) r (m) Environmental Geomechanics Lecture No. 18 D N Singh
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