IIT Bombay ENVIRONMENTAL GEOMECHANICS ENVIRONMENTAL GEOMECHANICS CE-641 CE-641 Department of Civil Engineering Department of Civil Engineering DR. D. N. SINGH DR. D. N. SINGH dns@civil.iitb.ac.in dns@civil.iitb.ac.in www.civil.iitb.ac.in/~dns www.civil.iitb.ac.in/~dns
IIT Bombay 7.11.2014 Lecture No. 12 Lecture Name: Geomaterial Characterization Sub-topics Thermal Characterization Importance Methodologies Thermal properties Influence of Various soil specific Parameters Centrifuge Modelling Environmental Geomechanics Lecture No. 12 D N Singh
IMPORTANCE IIT Bombay (in REAL LIFE SITUATIONS) HIGH LEVEL RADIOACTIVE WASTE DISPOSAL HIGH VOLTAGE UNDERGROUND POWER CABLES ROADS, PIPELINES, STRUCTURES IN COLD REGIONS AGRI- & AQUA-CULTURE FIELDS/ SOLAR PONDS GROUND IMPROVEMENT TECHNIQUES (SOIL HEATING & FREEZING) ENERGY CONSERVATION SCHEMES TRANSMISSION OF HOT FLUIDS (CHEMICALS/GAS) HEAT LOSS FROM THE BASEMENTS OF BUILDINGS Environmental Geomechanics Lecture No. 12 D N Singh
IIT Bombay THERMAL PROPERTIES THERMAL RESISTIVITY (inverse is Conductivity, k) R T (inverse is Conductivity, k) THERMAL DIFFUSIVITY ( α ) SPECIFIC HEAT (C p ) C p =(R T . ρ . α ) -1 ρ is the density of the media K CAN BE CORRELATED TO HYDRAULIC CONDUCTIVITY Environmental Geomechanics Lecture No. 12 D N Singh
IIT Bombay Factors Influencing Thermal properties of Geomaterials Type of Soil Moisture Content Distribution and Size of the Grains Density of the Soil Temperature and Pressure Presence of Contaminants Method of Measurements C p , R T , and α can be used for geomaterial characterization Environmental Geomechanics Lecture No. 12 D N Singh
IIT Bombay The Transient Method Thermocouple leads Stainless steel tube of dia 1.2mm Thermal probe Insulated T-type Thermocouple Grounded junction T-type thermocouple Environmental Geomechanics Lecture No. 12 D N Singh
IIT Bombay Thermal probes and thermocouples Environmental Geomechanics Lecture No. 12 D N Singh
IIT Bombay A.C. Power Supply • Constant Power Supply Unit Set Off on 0 300 600 900 1200 1500 Switch • • • • 0000 S 000.0 big small Current 0 Timer Temperature indicator 000.0 000.0 000.0 Temperatures Field Thermal Probe Fine tuning Coarse tuning Environmental Geomechanics Lecture No. 12 D N Singh
IIT Bombay Various Devices used for Thermal Property Determination Laboratory thermal probe Field thermal probe THERMODET DDTHERM (software) Environmental Geomechanics Lecture No. 12 D N Singh
Transient Method IIT Bombay Governing Equation for Line Heat Source in an Infinite Medium r Initial and boundary conditions: θ = θ 0 ,for t = 0, r = ∞ ∂ θ = − lim 2 π . k. r Q ∂ r → r 0 Solution of the Differential Equation: ( ) ( ) n n − ∞ Q 1 u ∑ 2 − = − − − r ( θ θ ) lnu γ = 0 u 4 π k n.n! = 1 n 4 α t γ is the Euler’s constant and is equal to 0.5772. Environmental Geomechanics Lecture No. 12 D N Singh
IIT Bombay For r → 0 and t →∞ , the higher order terms of u can be neglected 100 (a) Q t 80 − = 2 ( θ θ ) ln 2 1 4 π k t 60 s 1 40 − 1 Q = R s. 20 0 C) 0.1 1 10 100 T 4π 100 θ ( (b) 80 60 40 20 0 5 10 15 20 25 30 35 40 t (min) Environmental Geomechanics Lecture No. 12 D N Singh
Details of the thermal property detector IIT Bombay (THERMODET) Power leads Thermocouple leads Cap of the probe Rubber washer Top cap 25 mm thick Styrofoam 5 mm thick Perspex disk 220 mm long SS tube Compacted soil 140 mm Thermocouple Thermal probe 25 mm thick Perspex disk 20 mm thick Styrofoam Rubber washer Bottom cap 70 mm Environmental Geomechanics Lecture No. 12 D N Singh
IIT Bombay Variation of temperature with time for THERMODET (a) 70 60 50 40 30 20 0 C) 1 10 100 θ ( 70 (b) 60 50 40 30 20 0 10 20 30 40 50 60 70 80 90 100 t (min) Environmental Geomechanics Lecture No. 12 D N Singh
IIT Bombay Percentage change in temperature versus time factor curves 0 20 40 µ (%) α = 60 80 H= ∞ 100 H=2D 120 0.01 0.1 1 T where α is the thermal diffusivity D is the diameter of the soil sample T is the time factor corresponding to 50% change in temperature t 50 is the time corresponding to 50% change in temperature Environmental Geomechanics Lecture No. 12 D N Singh
IIT Bombay 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 fact that the type of the soil is Fly Ash 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. 12 D N Singh
IIT Bombay 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. 12 D N Singh
IIT Bombay 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. 12 D N Singh
IIT Bombay 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. 12 D N Singh
IIT Bombay 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. 12 D N Singh
IIT Bombay Environmental Geomechanics Lecture No. 12 D N Singh
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