Geomaterial Characterization Sub-topics

Geomaterial Characterization

Sub-topics

• Chemical characterization

pH, TDS, EC, BOD, COD

Sulphite and Chloride contents

Cation-Exchange Capacity

Pore-solution sampling

Corrosion potential

Sorption-Desorption

• Thermal Characterization

• Electrical Characterization

Pore Solution Extraction by Centrifugation

Laboratory technique

• Soil sample mixed with immiscible liquid (CCl4)

• Centrifuged in a tube at a particular rotational speed

• Pore solution is displaced by CCl4

Pore solution could be extracted even from dry soils

Quantity of pore solution extracted depends on soil type

Results obtained cannot be generalized

Importance of Lysimetric studies

Field studies No control of boundary conditions,

cost and time intensive

Laboratory studies Cannot simulate field conditions, Spatial variability cannot be taken into account

Lysimetric study Intermittent approach Simulates In-situ conditions with

better control on boundary conditions

Lysimeter Device which creates a control volume of soil

for studying various contaminant transport

mechanisms under in-situ conditions

Lysimeter identified as a potential tool for studying radioactive contaminant

Interaction and migration in Geoenvironment

R is the soil spiked

(with Cs, Co & Tritium)

Slurry of native soil

Vial for pore-solution collection

Details of the suction sampler

Stopper

Sample Collector

Flexible rubber tube

To the

vacuum

Pump

Ceramic thimble

Perspex tube

Soil slurry

Screw cap

Activities at a Glance

TDR studies

200

180

160

140

120

100

80

60

40

20

0

0.0 0.1 0.2 0.3 0.4 0.5

De

pth

(cm

)

15/06/05 20/06/05

05/07/05 14/07/05 18/07/05

26/7 flash floods

26/08/05 27/09/05GSL

Hanumantha Rao, B, Sridhar, V., Rakesh, R.R., Singh, D.N., Narayan, P.K. and

Wattal, P.K., “Application of In-situ Lysimetric Studies for Determining Soil Hydraulic

Conductivity”, Geotechnical and Geological Engineering, 2009, DOI 10.1007/

s10706-009-9260-5. Published Online: 13 May 2009.

Variation of 137Cs and 60Co activity concentration with

depth in dry soils after a period of 500 days

Pressure Membrane Extractor

S PG

PME

A

P PG

C R

RU

B

Air inlet Pressure gauge

Drain

Expelled water to the sampling bottle

Air pressure

Limitations

Expensive instrumentation

Cumbersome methodology

Intensive & rigorous sample preparation, time consuming

Complicated procedure for calibration and analysis

Requirement of skilled and trained personnel

Pore-solution extraction/Analysis (PME)

AAS

ICP-MS

Gas chromatography

Ion selective electrodes

Impedance spectroscopy

(Impedance analyzer)

Electrical resistivity methods

(Probes)

Electro-magnetic methods

(Time domain Reflectometry)

Dielectric constant

(Ground penetrating radar)

CHEMICAL CHARACTERIZATION

for

ASSESSING SOIL CONTAMINATION

Direct methods Indirect methods

Used for measuring soil suction and characterizing unsaturated soil

Soil Suction Matric(x) suction (soil matrix)

Osmotic suction (salts)

Total

Suction

Soil-water characteristic curve (SWCC)

w

AEV

wr

w : water content

: Soil suction

Exploring the possibility of WP4 (dewpoint potentiameter)

AN INDIRECT METHODOLOGY

FOR ASSESSING SOIL CONTAMINATION

Block chamber

Working principle of WP4

Measuring range- 0 to 80 MPa

Works on relative humidity principle

WP4 measures total suction of soil

Uncontaminated soil : Total suction = Matric(x) suction

Contaminated soil : Total suction = Matric(x) suction + Osmotic suction

SWCC of uncontaminated and contaminated soil of same type

would be different

The difference between SWCCs would indicate soil contamination

A Case study

Soil used: Marine soil designated as contaminated soil (CS)

Source: Collected from the coastal area of Mumbai, India

Soil property Value

Specific gravity 2.64

Particle size characteristics

Coarse sand (4.75-2.0 mm) 4

Medium sand (2.0-0.420 mm) 9

Fine sand (0.420-0.074mm) 11

Silt size (0.074-0.002 mm) 44

Clay size (< 0.002 mm) 32

Consistency limits

Liquid limit (%) 61

Plastic limit (%) 37

Plasticity index (%) 24

Soil Classification (USCS) MH

Oxide % by weight

SiO2 33

Al2O3 11

Fe2O3 12

TiO2 2

CaO 6

Chlorides (ppm) 9840

Sulphites (ppm) 40

CEC (meq/100g) 4.04

Physical properties Chemical properties

As such the soil is

contaminated

Soil subjected to washing to nullify contamination

No. of washings LS Chloride (ppm) Sulphite (ppm)

1 2 6750 15

2 4 1850 10

3 6 800 10

4 8 250 5

5 10 90 < 5

0 1 2 3 4 5 6 7

0

10

20

30

40

50

60

70

80

90

100

(

ms/c

m)

No. of washings

Washing nullifies contamination

100

101

102

103

104

105

106

107

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

(kPa)

w

Contaminated soil

Washed soil

Difference due to

contamination

For geotechnical engineers, it’s very important subject

Metal corrosion in undisturbed soils is generally very low regardless of

the soil composition (e.g. metal piles, reinforcement of foundation etc.)

Corrosion of metal (steel) in disturbed soils (e.g., buried pipelines that

are backfilled) is strongly affected by soil conditions & properties.

Soil changes its chemical and physical nature continuously over time

and seasonally.

Corrosion Potential of Soils

Pipeline damage

from pitting/corrosion

• Chloride content

• Moisture content

• Oxygen content/Redox potential

• Soil permeability/texture

• pH/Acidity

• Temperature

• Soil resistivity

• Drainage characteristics

• Sulfate/Sulfite ion concentrations

• Microbiological activity

• Stray currents (from cathodic protection, DC traction

system viz., train, metro)

• Spillage of corrosive substance/pollution

Soil Characteristics & Environmental Variables

Clay in the soil mass reduces movement of air (oxygen) and water, i.e.

low aeration, when wet, and hence increase in local (pitting) corrosion.

High plasticity of clay (swelling/shrinking soils) can pull off susceptible

coatings on the structures.

Clay is susceptible to cracking (during wetting-drying cycles) which

helps transport of air and moisture to the structures buried in it.

Sand promotes aeration and moisture distribution & hence, soluble

salts and gases (air/oxygen) are easily transported to structures,

causing greater general corrosion but less pitting.

Soil Classification/Texture

Bored Cast in-situ piles

Reinforcement in concrete pile exposed due to leaching of concrete

Chloride and Sulphate content of water found well within prescribed

limit & hence water not corrosive.

Ryzner index (RI) of water was found out to be 7.7 & hence water is

corrosive and unsaturated

pH scale for Soils

Langelier Saturation Index (LI)

Determines if calcium carbonate will precipitate or not

LI = pH – pHs

pH = actual pH value measured in the water

pHs = pH of the water in equilibrium with solid CaCO3

If LI > 0 calcium carbonate will precipitate

If LI < 0 calcium carbonate won’t precipitate

The CaCO3 layer deposited on surfaces acts as a protective coating.

Ryznar Index

Determines the degree of scale formation

RI = 2 pHs – pH

RI < 5.5 heavy scale will form

5.5 < RI < 6.2 scale will form

6.8 < RI < 8.5 water is corrosive

RI > 8.5 water is very corrosive

ASSESSMENT OF CORROSION POTENTIAL OF SOILS

Durability of underground structures is seriously affected by corrosion of

the concrete (IS: 456-2000)

Specifications for type of cement, minimum cement content, maximum

water-cement ratio, etc., to be adopted stringently, based on the exposure

of the concrete to different concentrations of sulphates in the soil or

ground water.

However, for assessment of corrosion potential of underground structures,

chemical properties of the soil need to be considered in details.

Corrosion is an electrochemical process

Certain conditions must exist for the corrosion to occur (corrosion cell)

Effects of soil characteristics on corrosion

By Victor Chaker, J. David Palmer

ASTM Committee G-1 on Corrosion of Metals

Soil (Electrolyte)

Metallic connection

Anode Cathode

Electric current

Electrochemical

reaction

Corrosion

The “Corrosion cell”

Soil Electrolyte Therefore properties of soils play a crucial role

in accelerating corrosion.

Properties of soils: Electrical resistivity

pH

moisture content

Porosity

sulphate and chlorides content

redox potential

presence of micro-organism

temperature

are important for evaluating the corrosion

potential of soils (DIN 50929-3).

For corrosion, the elements that are soluble in water are important:

– Base forming: Na, K, Ca, Mg (raise pH).

– Acid forming: Carbonate, Bicarbonate, Chloride ion, Nitrate, and Sulfate (lower pH).

Rating based on the soil fraction

Rating based on the electrical resistivity

Rating based on the pH

Rating Based on the ground water status

Rating based on the sulphite content

Rating based on the chloride content

Based on different soil characteristics, a certain rating (R1 to

R6) for the soils has been assigned and the sum of these

ratings is a measure of the overall soil corrosivity.

Rating based on the soil fraction

Soil fraction % by

weight R1

Clay & silt <10 +4

10 to 30 +2

30 to 50 0

50 to 80 -2

>80 -4

Organic matter, e.g.:

muddy or swampy

soils:

peat, mud, marsh

>5 -12

Severely polluted:

due to fuel ash, slag

coal, coke, refuse,

rubbish or waste water

- -12

Rating based on the electrical resistivity

Resistivity (.m) R2

>500 +4

200 to 500 +2

50 to 200 0

20 to 50 -2

10 to 20 -4

<10 -6

Rating based on the pH

PH R3

>9 +2

5.5 to 9 0

4.0 to 5.5 -1

<4 -3

Higher conductivity: high corrosion rate

(efficient electrolyte)

Rating Based on the ground water status

Ground water status R4

No groundwater 0

Groundwater -1

Groundwater at times -2

Rating based on the sulphite content

Sulphite content (g/l) R5

<0.15 0

0.15 to 1 -2

1 to 2 -4

>2 -6

Rating based on the chloride content

Chloride content (ppm) R6

<100 0

100-2000 -2

2000-10000 -4

>10000 -6

Total assessment of the corrosion potential

Summation of R1- R6

R

Corrosion

potential

0 Virtually not

corrosive

-1 to -4 Slightly

corrosive

-5 to -10 Corrosive

< -10 Highly

corrosive

Chloride ions: Cause pitting of steel

and decrease soil resistivity.

Dissolved Oxygen concentration in the soil moisture determines its RP(potential diff. between the electrodes), higher the oxygen content, higher would be the RP The difference in the RP may lead to the formation of the “corrosion cell” Low soil RP indicates conditions conducive to anaerobic microbiological activities. RP varies with time, moisture content variations, micro-organism activities etc. RP measurements may not be accurate assessment of corrosion potential of soils.

Redox Potential (mV) (Std. H Scale) Aeration Corrosivity

>400 strong aeration Noncorrosive

200 to 400 Aeration Weak

100 to 200 weak aeration Moderate

0-100 Non to weak Severe

Negative Not aerated Extremely sever

Soil Corrosivity based on Redox (Reduction-Oxidation) Potential

ORP (Oxidation Reduction Potential)

In well aerated soils, Fe3+ exhibits red, yellow, and brown colors.

In poorly aerated soils, the oxygen content is low & soils are gray in color due to

reduced state of the Fe.