ENVIRONMENTAL GEOMECHANICS CE-641 Lecture No. 18 Prof. D N Singh - PowerPoint PPT Presentation

ENVIRONMENTAL GEOMECHANICS CE-641 Lecture No. 18 Prof. D N Singh Department of Civil Engineering

23.10.2018 Lecture No. 18 Lecture Name: Geomaterial Characterization Sub-topics • Thermal Characterization • Importance • Methodologies • Thermal properties • Influence of Various soil specific Parameters • Centrifuge Modelling • Cracking Characteristics • Electrical Characterization • Magnetic Characterization

IMPORTANCE (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

THERMAL PROPERTIES THERMAL RESISTIVITY (inverse is Conductivity, k) R T (inverse is Conductivity, k) THERMAL DIFFUSIVITY (  ) SPECIFIC HEAT (C p ) C p =(R T .  .  ) -1  is the density of the media K CAN BE CORRELATED TO HYDRAULIC CONDUCTIVITY

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  can be used for geomaterial characterization

The Transient Method Thermocouple Power supply leads leads (T-type) 95 mm Thermocouple Thermocouple leads Nichrome wire 6mm dia copper tube Stainless steel tube of dia 1.2mm Thermal probe Insulated T-type Thermocouple Grounded junction T-type thermocouple

Thermal probes and thermocouples

A.C. Power Supply  Constant Power Supply Unit Set Off on 0 300 600 900 1200 1500 Switch     0000 000.0 big small S Current 0 Timer Temperature indicator 000.0 000.0 000.0 Temperatures Field Thermal Probe Fine tuning Coarse tuning

Various Devices used for Thermal Property Determination Laboratory thermal probe Field thermal probe THERMODET DDTHERM (software)

Transient Method r Governing Equation for Line Heat Source in an Infinite Medium       2 1           2   Initial and boundary conditions: t r r r  =  0 , for t = 0, r =   θ   π lim 2 . k. r Q   r r 0 Solution of the Differential Equation:         n n Q 1 u       2 θ θ γ   r ( ) lnu  0 π u 4 k n.n!    1 n α 4 t  is the Euler’s constant and is equal to 0.5772.

For r  0 and t  , the higher order terms of u can be neglected 100 (a) Q t 80   θ θ 2 ( ) ln 2 1 π 4 k t 60 1 s 40  1   Q  R s.   0 C) 20 0.1 1 10 100 T  4 π   ( 100 (b) 80 60 40 20 0 5 10 15 20 25 30 35 40 t (min)

Details of the thermal property detector (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

Variation of temperature with time for THERMODET (a) 70 60 50 40 30 20 0 C) 1 10 100  ( 70 (b) 60 50 40 30 20 0 10 20 30 40 50 60 70 80 90 100 t (min)

Percentage change in temperature versus time factor curves 0 20 40  (%)  = D 60 2 T 80 H=  t 50 100 H=2D 120 0.01 0.1 1 T where  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

Variation of thermal resistivity with dry-density 1200 WC SS BC WS (a) 1000 800 600 Variation of thermal resistivity 400 with moisture content 0 C-cm/W) 200 400 600 3 )  d (g/cm SG1 SG2 SG3 Soil (b) { WS 500 350 SS 1.3 BC 400 WC 0 C-cm/W) R T ( 300 FA-3 300 250 1.6 R T ( 200 FA-1 FA-2 FA-3 BFS SF 1400 100 1200 (c) 0 0 10 20 30 40 1000 w (%) 800 600 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 3 )  d (g/cm

Variation of thermal diffusivity with dry-density 40 FA-1 FA-2 FA-3 BFS SF 2 /s) 35 -8 m 30  (x10 25 20 15 Variation of thermal diffusivity 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 with moisture content 3 )  d (g/cm 70 Soil 65 WS SS BC WC FA-3 60 2 /s) 55 -8 m 50 45  (x10  d =1.6 g/cm 3 40 35 } 1.3 30 25 20 0 10 20 30 40 w (%)

Variation of specific heat with dry-density 3.0 FA-1 FA-2 FA-3 BFS SF 0 C.g) 2.5 2.0 C p (J/ 1.5 1.0 0.5 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0  d (g/cc) 3.0 WS SS BC WC FA-3 2.5 0 C.g) 2.0 Variation of specific heat with C p (J/ moisture content 1.5 1.0 0.5 0 5 10 15 20 25 30 35 40 w (%)

Generalized Relationships Generalized thermal resistivity relationships, termed as DDTHERM, have been proposed by Singh and Devid (2000). Dry (single-phase) soils 1/R T = [ a .10(0.6243  d - 3)] Moist (single-phase) soils Clays and silts 1/R T = [ b .10(0.6243  d - 3)] 1/R T = [1.07log(w)+ c ].[10.(0.6243  d -3)] where R T is the soil thermal resistivity (  C.cm/W), w is the moisture content (%) and  d is the dry-density of the soil (g/cm 3 ). a, b and c depend on the % fraction of the soil and its moisture content and determining these parameters is a big challenge

W Fraction b Fraction a (%) Clay 0.219 4>w  2 Clay 0.243 Silt Silt 0.254 Silty sand 0.385 5  w>4 Clay 0.276 Fine sand 0.340 Silt 0.302 Coarse sand 0.480 Fraction c w Gravel 0.21 (%) Clay -0.73 >5 Silt -0.54 Silty sand 0.12 Fine sand 0.70  1 Coarse sand 0.73 Gravel 0.8 For clay and silt phase: Weight = (phase %), when 5  w(%)  2. Weight = Minimum of the (Absolute c value or phase %), when w (%) >5 Silty-sand, fine-sand coarse-sand and gravel: Weight = (phase %  c of the phase)+ phase %, when w (%)>1 Weight = a of the phase, when w (%)<1 (dry soils)

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)

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.

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.

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 ?

Centrifuge Setup Data logger Rheostat Switch-on Micro switch Switch-off Axis of rotation Test setup Batteries Thermocouple leads Power supply leads Geomaterial

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 C p (J/g- 2 /s) -6 10  (m 2 R T ( 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