ENVIRONMENTAL GEOMECHANICS CE-641 Department of Civil Engineering - - PowerPoint PPT Presentation

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

5.9.2009 Lecture No. 11 Lecture Name: Geomaterial Characterization

Sub-topics

• Specific Surface Area determination
• Chemical characterization

Pore-solution sampling Corrosion potential Sorption-Desorption

• Thermal Characterization
• Electrical Characterization

IIT Bombay Slide 1 Environmental Geomechanics Lecture No. 11 D N Singh

Specific-surface Area (SSA)

Environmental Geomechanics Lecture No. 11 D N Singh IIT Bombay Slide 2

Soil-water-contaminant interaction is strongly dependent on SSA SSA is indicative of activity (reactivity) of fine-grained soils A classification scheme based on SSA would help to establish:

• Swelling and shrinkage characteristics
• Frost heave
• Collapse and compressibility
• Cation-exchange capacity
• Water retention characteristics
• Sorption and desorption characteristics

These characteristics mainly depend on the grain-size distribution of the soil (i.e., the clay-size fraction) and its mineralogical composition. SSA can capture the combined effect of these factors and hence, can be used for predicting engineering behavior of fine-grained soils.

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Determination of SSA of fine-grained soils

A. gas or vapor adsorption techniques BET nitrogen adsorption water-vapor adsorption

• B. absorption of the polar liquids and dyes on the soil surface

Ethylene glycol (EG) method Ethylene Glycol Monoethyle Ether (EGME) method p-Nitrophenol method Methylene blue (MB) dye method

• C. application of the state-of-the-art instruments

Mercury intrusion porosimetry (MIP) Internal reflectance spectroscopy X-ray diffraction Gas pycnometer

Environmental Geomechanics Lecture No. 11 D N Singh Slide 3

Specific-surface Area (SSA) Determination

Environmental Geomechanics Lecture No. 11 D N Singh IIT Bombay Slide 4

(a) Methylene blue (MB) absorption technique

0.5 g of the soil mixed with 50 ml of MB dye of different initial concentrations (C=5 to 104 mg/l), (L/S= 100) The mixture is agitated for 24 h on a mechanical shaker. The solution is filtered using the Whatman filter paper. The filtrate is transferred to microfuge tubes and centrifuged at 104 rpm for 30 min. This process helps in separating soil particles and the solution. The clear solution is decanted, collected in polypropylene tubes and stored in a refrigerator (@10 ºC). This process minimizes the precipitation of MB dye and evaporation of the solution. Later, the solution is suitably diluted and analyzed for determining the concentration

• f MB (i.e., Ce with the help of a UV-Spectrophotometer)

In order to establish the optimum UV-wavelength, at which the MB dye yields maximum absorbance a, dyes of different concentrations should be tested over a wide range of wavelength (λ=400 to 800 nm). Develop a calibration curve (i.e., a relationship between C and a, at optimal λ value). This calibration curve can be employed for determining concentration of the MB dye present in a solution.

MB Dye Absorption Method

200 400 600 800 1000 1 2 3

a

λ (nm)

665

C (mg/l) 4 6 8 10

Relationship between the absorbance and concentration of MB dye solution

Environmental Geomechanics Lecture No. 11 D N Singh IIT Bombay Slide 5

Absorbance versus wavelength response of MB dye solutions

UV-Spectrophotometer results

C=5.4·a

MB Dye Absorption Method

MB MB v

• pt

MB

A MW A C S ⋅ ⋅ =

Environmental Geomechanics Lecture No. 11 D N Singh IIT Bombay Slide 6

Ce : amount of MB dye in the solution Csi : amount of MB dye initially added to the soil sample Cse: that gets adsorbed on the soil particles after 24 h MWMB : Molecular weight

• f the MB (=319.87 g/mol)

AMB : the area covered by one MB molecule (=13 10-19 m2/molecule) Av : Avagadro’s number (=6.02 1023/mol)

Csi = Ci ⋅(L/S) Cse = (Ci-Ce)⋅(L/S)

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(b) Nitrogen gas adsorption technique

A surface-area analyzer (Autosorb, Quantachrome, USA) with N2 as an adsorbate is used. This instrument works on BET adsorption theory

Brunauer, S., Emmett, P. H., and Teller, E., “Adsorption of Gases in Multi-

molecular Layers,” Journal of the American Chemical Society, Vol. 60, 1938, pp. 309-319.

The instrument is calibrated with the help of standard reference material (Alumina) supplied by the manufacturer. 1 g of the air-dried soil is poured into a glass-cell and degassed under vacuum at 100 C, for a period of 24 h. This process helps in minimizing errors incurred due to rise of vapor pressure while adsorption of N2 takes place. Later, the sample is exposed to N2 corresponding to different relative pressures P/P0 . This process ensures optimal adsorption of N2 . At the end of the test the sample is weighed on a balance of accuracy 0.0001 g. The volume of N2 adsorbed Va on the sample (at pressure P) is recorded and adsorption isotherms are developed. Further, the SSA of the soil is determined by employing a single-point BET analyzer (Smartsorb-91). For this purpose, the degassed sample is filled in the sample-holder and is exposed to N2. SSBET is obtained with the help of the built-in software.

Environmental Geomechanics Lecture No. 11 D N Singh Slide 7

Nitrogen Gas Adsorption Technique

mol mol MBET Lm MBET LM

A V V

• r

V S

• r

S ⋅ =

Environmental Geomechanics Lecture No. 11 D N Singh IIT Bombay Slide 8

P: applied pressure P0 : saturation vapor pressure of N2 (= 1 atm at 77 °K) Va is the volume of N2 adsorbed at P, VLm is the volume of N2 required for mono-layer formation as per Langmuir isotherm, b is the parameter related to the maximum amount of N2 getting adsorbed on the sample, VMBET is the volume of N2 required to form mono-layer as per multi- point BET isotherm CMBET is a constant, which is proportional to heat of adsorption in first and subsequent adsorbed layers. Amol is the area covered by each N2 molecule (=16.2 10-20 m2).

Nitrogen Gas Adsorption Technique

0.0 0.1 0.2 0.3 0.4 10 20 30 40 50 0.0 0.1 0.2 0.3 0.4 15 30 45 60 75

(P/P0)/Va (g/cc)

Langmuir

P/(Va(P0-P)) (g/cc) P/P0

Multi-point BET

Langmuir and multi-point BET adsorption isotherms

Lm Lm a

V P P P V b 1 V P P ⋅ + ⋅ ⋅ = ⋅

MBET MBET MBET MBET MBET a

P P C V 1 C C V 1 P) (P V P ⋅ ⋅ − + ⋅ = −

Langmuir Isotherm Multi-point BET isotherm

Environmental Geomechanics Lecture No. 11 D N Singh IIT Bombay Slide 9

Mercury Intrusion Porosimetry

10

1

10

2

10

3

10

4

10

5

0.0 0.1 0.2 0.3 0.4 0.5

V (cc/g) p (psi)

∫

⋅ ⋅ =

Vmax Hg MIP

δ cos T ΔV S p

Environmental Geomechanics Lecture No. 11 D N Singh IIT Bombay Slide 10

δ =130 to 140˚ THg = 480 N/m = 0.48 dyne/cm

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(c) Ethylene Glycol Monoethyl Ether (EGME) absorption method

2 g air-dried soil is spread uniformly on the bottom of a glass dish (40 mm in internal diameter and 20 mm in height) and covered with a perforated watch- glass. These dishes are placed in a vacuum desiccator containing 250 g of P2O5, which helps in maintaining a constant vapor pressure inside the desiccator. The sample is evacuated by applying vacuum for 2 h and is weighed until it attains almost a constant weight. Later, 6 ml of EGME solution is added to the sample to form a slurry. The slurry is placed in the desiccator over a desiccant (mixture of 100 g CaCl2 and 20 ml EGME) for 12 h. This helps in maintaining a constant vapor pressure and minimizing the loss of EGME from the monolayer, which forms on the sample and the interlayer spacing of the soil minerals. Initial weight of the slurry along with the glass dish is measured, using the precision balance, and the dish is re-placed in the desiccator for evacuation under

• vacuum. The glass-dish is taken out of the desiccator, weighed and re-placed in it

several times, till a constant weight is attained.

Environmental Geomechanics Lecture No. 11 D N Singh Slide 11

Ethylene Glycol Monoethyl Ether (EGME) Absorption Method

600 1200 1800 2400 3000 0.0 0.7 1.4 2.1 2.8

W

EGME (g/g)

t (min.)

Air-dried sample Degassed sample (at 600 °C)

W

c

Environmental Geomechanics Lecture No. 11 D N Singh IIT Bombay Slide 12

wEGME is the amount of EGME required to form monolayer on a square meter of Bentonite (=2.86 10-4 g). External surface area, Sext, of the soil is obtained by suppressing its interlayer. For this purpose, the sample is degassed at 600 ºC for 5 to 6 h under vacuum, prior commencing the EGME test .

EGME c ext total

w W S

• r

S =

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(e) Air-adsorption method

A Ultra-pycnometer (Quntachrome, USA) is employed. The basic principle of this methodology is that SSA of the sample is proportional to its air-adsorption capacity Vair. Vair can be determined by measuring its density using Helium ρHe and Nitrogen ρN2 gases. The instrument is calibrated using the standard reference material (stainless-steel balls of different diameters) provided by the manufacturer. Later, the air-dried soil sample is degassed at 100 ºC, under application

• f vacuum till it attains a constant weight.

This procedure ensures complete removal (i.e., desorption) of the adsorbed air from the surface of the sample.

Slide 13 Environmental Geomechanics Lecture No. 11 D N Singh

Air-adsorption Method

        − =

2

N He

ρ 1 ρ 1 W Vair

.β W V S

air air

      =

10

• 2

10

• 1

10 10

1

10

2

10

• 1

10 10

1

10

2

10

3

S (m2/g) Vair/W (×10

• 8 m

3/g)

Estimation of β using MBET and EGME results Environmental Geomechanics Lecture No. 11 D N Singh IIT Bombay Slide 14

β is a constant parameter that defines the shape, least dimension and type of the soil particle W is the weight of the soil sample. Method yields only external surface area

10 100 1000 1 2 3 4

Activity SSA (m2/g)

Shah and Singh (2005) and CS & WC Cerato and Lutenegger (2004)

200 400 600 800 1000 40 80 120 160 Shah and Singh (2005), and CS & WC Low (1980) Cerato and Lutenegger (2004) Farrar and Coleman (1967)

• - - 95% confidence limit

CEC (meq./100g) SSA (m2/g)

1 10 100 1000 150 300 450 600

LL (%) SSA (m

2/g)

Shah and Singh (2005), and CS & WC Cerato and Lutenegger (2004) Dolinar and Trauner (2004)

Some Relationships

Arnepalli, D.N., Shanthakumar, S., Rao, H.B. and Singh, D.N., “Comparison of Methods for Determining Specific Surface Area of Fine-grained Soils", Geotechnical and Geological Engineering, 2008, 26(2), 121-137.

Environmental Geomechanics Lecture No. 11 D N Singh IIT Bombay Slide 15

IIT Bombay 200 400 600 2 4 6 8 10 12 20 40 60 80

Hedley et al.(2000) Banin and Amiel (1969) Dirksen and Dasberg (1993) Combine data (CD)

SSA (m

2/g)

CEC (meq/100 g) w

hygroscopic (%)

SSA=0.0012·(σh/σdry )+16.6 SSA=1.88kdiff +9.4

SSA (in m2/g) Shah, Paresh H. and Singh, D. N., "Methodology for Determination of Hygroscopic Moisture Content of Soils”, Journal of ASTM International. 3(2), (2006), 14 Pages.

σh, σdry : Hygroscopic and dry soil electrical conductivity, respectively kdiff (=kh-kdry)

Slide 16 Environmental Geomechanics Lecture No. 11 D N Singh