soil physics lecture 4[1]

SOIL PHYSICSSOLID PHASE

• The solid phase is the only permanent component of the soil.

• The solid phase is made up of:

Mineral particles: have various shapes and sizes

Organic matter: has no definite shape (amorphous)

• Particles size and mineral composition determine the nature and behavior of the soil:

Surface area Porosity Interactions with fluids Thermal regime

SOIL PHYSICSSOLID PHASE: Particle Sizes, Shapes and Specific Surface

• Soil particles can be classified according to

Size Shape or geometry Density Mineral composition

Size

• Based on size

50 μm < sand < 2,000 μm2 μm < silt < 50 μmclay < 2 μm

SOIL PHYSICSSOLID PHASE: Particle Sizes, Shapes and Specific

Surface

Soil Texture: Defined as the relative proportions of sand, silt & clay fractions.


Surface

• Soil Texture carries both quantitative and qualitative terms:

Qualitatively: refers to the feel (e.g. coarse, gritty, smooth)

Quantitatively: refers to the measured distribution of particle sizes and the proportions of the relative size ranges of particles comprising a soil.

• Soil texture is thus an intrinsic attribute of a soil and the most often used to characterize a soil.


Textural fractions

• Traditional method involved dividing the separates (sand, silt, and clay) based on particle diameter.

• The current procedure involves the use of a standard method called mechanical analysis.

• Unfortunately, there is still no universally accepted scheme of classifying particle sizes.

• The conventional definition of soil material is particles < 2 mm.


Surface


Surface

Soil Classes

Textural class is determined on the basis of the mass ratios of three fractions: sand, silt, and clay.

Soils with different proportions are assigned to different classes.

These classes are based on soil textural triangle.

We use mainly the USDA triangle which has 12 classes.


Soil Textural Triangle


Particle Size Distribution

• Since soil is a continuous array of particle sizes, division into discrete fractions is arbitrary.

• Although textural triangle is widely followed and useful in many ways, it is important to evaluate the complete array and distribution of particle sizes.•

• This can be achieved use of particle size distribution curves.

• These curves are a plot of the % of soil particles vs the diameters smaller than a denoted diameter.


Surface

Particle Size Distribution


Surface

Mechanical Analysis Mechanical analysis is the procedure to determine the

particle size distribution of soil samples.

• Dispersion of soil aggregates. Soil is often aggregated and must be separated into discrete particles.

• The main cementing agents are organic matter, CaCO3

and iron oxides.

• Deflocculation is carried out by chemical dispersants and mechanical methods.


Surface

Mechanical Analysis

Steps:

• Dispersion of soil aggregates:

Oxidative dispersion : by use of hydrogen peroxide (H2O2)

Chemical dispersion: using calgon (Na hexametaphosphate)

Mechanical dispersion: by use of a mechanical stirrer

• Settling in sedimentation cylinder based on Stoke’s law.

2

9

2 rg

t

Lv Fp )(

42

9 22 d

rtg

L

Fp

)(

d

t

A

21/


Surface

Mechanical Analysis

Steps:

• Measurement of particle distribution using:

pipette method

hydrometer method.


Surface

Mechanical Analysis

Question 7: Using Stoke’s law, calculate the time needed for all sand particles to settle out of a depth of 0.2 m in an aqueous suspension at 30°C.

How long would it take for all silt particles to settle out?


Mechanical Analysis

Question 7: Using Stoke’s law, calculate the time needed for all sand particles to settle out of a depth of 0.2 m in an aqueous suspension. How long would it take for all silt particles to settle out?

Given:Radius of sand particles > 25 μmDynamic viscosity = 1.1×10-3 kg m-1 s-1

Density of water = 9.81 kg m-3

Particle Density = 2650 kg m-3


Surface

Mechanical Analysis22

9

rg

Lt

Fp )(

smm kgs m.

m.smkg.tsand

)( ) ( 97

102598126508192

201011922632

113

hrmm kgs m.

m.smkg.tsilt

)( ) ( 17

10198126508192

201011922632

113


Surface

Shape

The property and behavior of the soil in bulk is affected by:

• Particle size

• Mineral composition

• Particle shape Regular shape Irregular shape

• Sand and silt particles are spherical or cubical shape.

• Clay are platelet-like or lath-like shaped.


Surface

Specific Surface

• The specific surface of a soil mineral is defined as the surface area As of particles per unit mass (am) or per unit volume (av) or per unit bulk volume of soil (ab).

where Ms is the mass of particles of volume Vs contained in a bulk volume Vt of soil.

• The units are traditionally expressed in m2 g-1 or m2 cm-3

• Convert to SI units: m2 g-1 = 103 m2 kg-1 and m2 cm-3= 106 m2 m-3

ssm MAa / ssv VAa / tsb VAa /


Specific Surface

• Specific surface is related to: Size Shape

• Many physical, chemical and biological reactions occur at surfaces.

• Determination of specific surface is based on:

Particle geometry Adsorption characteristics Heat of immersion


SurfaceParticle Geometry

a) Cube with size, L

b) Sphere with diameter, d

c) Regular polyhedron with diameter, d

LL

LS

ss 66

3

2

26 Larea 3 Lmass s

2 4 rarea srmass 3 3

4 dr

rS

ss 63

4

43

2

(s) nA)area (S kn 6

sk(s)dρnAmass d

S s6



d) lath and rod shaped clay particlesaδδ areasurface 42 2 a

δ

2 avolume s amass 2

21242

2

2

a

a

aS

ss

)(

aδ ss

S422

δ

aδ

11



e) plate-like clay particles)2 bδaδ(ab areasurface

b

aδ

abvolume s abmass

ab

ab

baabS

ss

1112)(2

baδ , s

S2

SOIL PHYSICSSOLID PHASE: Particle Sizes, Shapes and

Specific Surface

Question 8: Estimate the approximate specific surface of a soil composed of :

10% coarse sand: 1,000μm (av. diameter)

20% fine sand: 100μm (av. diameter)30% silt: 20μm (av. diameter) 20% kaolinite clay: 40 nm (av. platelet

thickness)10% illite clay: 5 nm (av. thickness)10% montmorillonite: 1 nm (av. thickness).

soil physics lecture 4[1]

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