chapter i basic characteristics of soils · • the atterberg limits are usually correlated with...

Chapter I

Basic Characteristics of Soils

Outline

1. The Nature of Soils (section 1.1 Craig)

2. Soil Texture (section 1.1 Craig)

3. Grain Size and Grain Size Distribution (section

1.2 Craig)

4. Particle Shape (part of section 1.4 Craig)

5. Atterberg Limits (section 1.3 Craig)

6. References

1. Soil Definitions

• Civil Engineers define soils as any un-cemented

accumulation of Mineral particles formed by the

weathering of rocks, the voids spaces between

the particles contain water and or air.

weathering of rocks

Physical weathering Sands, Gravel

Chemical Weathering Clay

1.1 Origin of Clay Minerals

“The contact of rocks and water produces clays, either at or near the surface

of the earth” (from Velde, 1995).

Rock +Water Clay

For example,

The CO2 gas can dissolve in water and form carbonic acid, which will

become hydrogen ions H+ and bicarbonate ions, and make water slightly

acidic.

CO2+H2O H2CO3 H+ +HCO3-

The acidic water will react with the rock surfaces and tend to dissolve the K

ion and silica from the feldspar (common Mineral on earth crest). Finally, the

feldspar is transformed into kaolinite.

Feldspar + hydrogen ions+water clay (kaolinite) + cations, dissolved silica

2KAlSi3O8+2H+ +H2O Al2Si2O5(OH)4 + 2K+ +4SiO2

•Note that the hydrogen ion displaces the cations.

1.2 Basic Unit-Silica Tetrahedra

Hexagonal

hole

1 Si

4 O

(Si2O10)-4

Replace four

Oxygen with

hydroxyls or

combine with

positive union

(Holtz and Kovacs, 1981)

Tetrahedron

Plural: Tetrahedra

1.3 Synthesis

Noncrystall

ine clay -

allophane

Mitchell, 1993

1.4 1:1 Minerals-Kaolinite

Basal spacing is 7.2 Å

• Si4Al4O10(OH)8. Platy shape

• The bonding between layers are van der

Waals forces and hydrogen bonds (strong

bonding).

• There is no interlayer swelling

• Width: 0.1~ 4m, Thickness: 0.05~2 m

layer

Trovey, 1971 ( from

Mitchell, 1993)

17 m

1.5 2:1 Minerals-Illite

potassium

• Si8(Al,Mg, Fe)4~6O20(OH)4·(K,H2O)2. Flaky shape.

• Some of the Si4+ in the tetrahedral sheet are replaced by the Al3+, and some of the Al3+ in the octahedral sheet are substituted by the Mg2+ or Fe3+. Those are the origins of charge deficiencies.

• The charge deficiency is balanced by the potassium ion between layers. Note that the potassium atom can exactly fit into the hexagonal hole in the tetrahedral sheet and form a strong interlayer bonding.

• The basal spacing is fixed at 10 Å in the presence of polar liquids (no interlayer swelling).

• Width: 0.1~ several m, Thickness: ~ 30 Å

7.5 m Trovey, 1971 ( from

Mitchell, 1993)

K

1.5 2:1 Minerals - Montmorillonite (Smectite)

n·H2O+cations

5 m

• Si8Al4O20(OH)4·nH2O (Theoretical unsubstituted). Film-like shape.

• There is extensive isomorphous substitution for silicon and aluminum by other cations, which results in charge deficiencies of clay particles.

• n·H2O and cations exist between unit layers, and the basal spacing is from 9.6 Å to (after swelling).

• The interlayer bonding is by van der Waals forces and by cations which balance charge deficiencies (weak bonding).

• There exists interlayer swelling, which is very important to engineering practice (expansive clay).

• Width: 1 or 2 m, Thickness: 10 Å~1/100 width (Holtz and Kovacs, 1981)

1.6 Elementary Particles Arrangement

2. Soil Texture

2.1 Soil Texture

The texture of a soil is its appearance or “feel” and it

depends on the relative sizes and shapes of the

particles as well as the range or distribution of those

sizes.

Coarse-grained soils:

Gravel Sand

Fine-grained soils:

Silt Clay

0.075 mm (USCS) -0.06 mm BS

Sieve analysis Hydrometer analysis

2.2 Characteristics


3. Grain Size and Grain Size

Distribution

3.1 Grain Size

4.75 USCS

BS

0.075

2.0 0.06 0.002

USCS: Unified Soil Classification

BS: British Standard

Note:

Clay-size particles

Clay minerals

For example:

Kaolinite, Illite, etc.

For example:

A small quartz particle may have the

similar size of clay minerals.

3.2 Grain Size Distribution

(Das, 1998) (Head, 1992)

•Sieve size

3.2 Grain Size Distribution (Cont.)

Coarse-grained soils:

Gravel Sand

Fine-grained soils:

Silt Clay

0.075 mm (USCS)

•Experiment

Sieve analysis Hydrometer analysis

(Head, 1992)


Log scale


Effective size D10: 0.02 mm

D30: D60:


• Describe the shape Example: well graded

•Criteria

•Question

What is the Cu for a soil with

only one grain size?

2)9)(02.0(

)6.0(

)D)(D(

)D(C

curvatureoftCoefficien

45002.0

9

D

DC

uniformityoftCoefficien

2

6010

2

30c

10

60u

mm9D

mm6.0D

)sizeeffective(mm02.0D

60

30

10

)sandsfor(

6Cand3C1

)gravelsfor(

4Cand3C1

soilgradedWell

uc

uc

Answer

•Question

What is the Cu for a soil with only one grain size?

D

Fin

er

1D

DC

uniformityoftCoefficien

10

60u

Grain size distribution


• Engineering applications

It will help us “feel” the soil texture (what the soil is) and it will

also be used for the soil classification (next topic).

It can be used to define the grading specification of a drainage

filter (clogging).

It can be a criterion for selecting fill materials of embankments

and earth dams, road sub-base materials, and concrete aggregates.

It can be used to estimate the results of grouting and chemical

injection, and dynamic compaction.

Effective Size, D10, can be correlated with the hydraulic

conductivity (describing the permeability of soils). (Hazen’s

Equation).(Note: controlled by small particles)

The grain size distribution is more important to coarse-grained soils.

4. Particle Shape

Important for granular soils

Angular soil particle higher friction

Round soil particle lower friction

Note that clay particles are sheet-like.

Rounded Subrounded

Subangular Angular


Coarse-

grained

soils

5. Atterberg Limits

and

Consistency Indices

4.1 Atterberg Limits

Liquid Limit, LL

Liquid State

Plastic Limit, PL

Plastic State

Shrinkage Limit, SL

Semisolid State

Solid State

Dry Soil

Fluid soil-water mixture

Incr

easi

ng w

ater

co

nte

nt

The presence of water in fine-grained soils can significantly affect

associated engineering behavior, so we need a reference index to

clarify the effects

4.2 Liquid Limit-LL

Cone Penetrometer Method

(BS 1377: Part 2: 1990:4.3)

•This method is developed by the

Transport and Road Research

Laboratory, UK.

•Multipoint test

•One-point test

Casagrande Method

(ASTM D4318-95a)

•Professor Casagrande standardized

the test and developed the liquid

limit device.

•Multipoint test

•One-point test

Particle sizes and water

•Passing No.40 Sieve (0.425 mm).

•Using deionized water.

The type and amount of cations can significantly affect the

measured results.

4.2.1 Casagrande Method

N=25 blows

Closing distance =

12.7mm (0.5 in)


•Device

The water content, in percentage, required to close a

distance of 0.5 in (12.7mm) along the bottom of the

groove after 25 blows is defined as the liquid limit

4.2.1 Casagrande Method (Cont.)

Liquid limit Test

Reference: Budhu: Soil Mechanics and Foundation


.log

)(/log

,12

21

contNIw

valuepositiveachooseNN

wwIindexFlow

F

F

N

w

•Multipoint Method

Das, 1998


•One-point Method

• Assume a constant slope of the

flow curve.

• The slope is a statistical result of

767 liquid limit tests.

Limitations:

• The is an empirical coefficient,

so it is not always 0.121.

• Good results can be obtained only

for the blow number around 20 to

30.

121.0tan

25

tan

contentmoistureingcorrespondw

blowsofnumberN

NwLL

n

n

4.3 Plastic Limit-PL

The plastic limit PL is defined as the water content at which

a soil thread with 3.2 mm diameter just crumbles.

ASTM D4318-95a, BS1377: Part 2:1990:5.3


4.3 Plastic Limit-PL (cont)

Plastic Limit-PL

4.4 Typical Values of Atterberg Limits

(Mitchell, 1993)

4.6 Indices

•Plasticity index PI

For describing the range of

water content over which a

soil was plastic

PI = LL – PL

•Liquidity index LI

For scaling the natural water

content of a soil sample to

the Limits.

contentwatertheisw

PLLL

PLw

PI

PLwLI

LI <0 (A), brittle fracture if sheared

0<LI<1 (B), plastic solid if sheared

LI >1 (C), viscous liquid if sheared

Liquid Limit,

LL

Liquid State

Plastic Limit, PL

Plastic State

Shrinkage Limit, SL

Semisolid State

Solid State

PI

A

B

C

4.6 Indices

•Activity A

(Skempton, 1953)

mm002.0:fractionclay

)weight(fractionclay%

PIA

Normal clays: 0.75<A<1.25

Inactive clays: A<0.75

Active clays: A> 1.25

High activity:

•large volume change when wetted

•Large shrinkage when dried

•Very reactive (chemically)

•Purpose

Both the type and amount of clay

in soils will affect the Atterberg

limits. This index is aimed to

separate them.

Mitchell, 1993

• Soil classification

(the next topic)

• The Atterberg limits are usually correlated with some engineering properties such as the permeability, compressibility, shear strength, and others.

In general, clays with high plasticity have lower permeability, and they are difficult to be compacted.

The values of SL can be used as a criterion to assess and prevent the excessive cracking of clay liners in the reservoir embankment or canal.

4.7 Engineering Applications

The Atterberg limit enable

clay soils to be classified.

5. References

Main References:

Craig’s Soil Mechanics 7th edition

Holtz, R.D. and Kovacs, W.D. (1981). An Introduction to GeotechnicalEngineering Prentice

Hall. (Chapter 1 and 2)

Others:

Head, K. H. (1992). Manual of Soil Laboratory Testing, Volume 1: Soil Classification and

Compaction Test, 2nd edition, John Wiley and Sons.

Lambe, T.W. (1991). Soil Testing for Engineers, BiTech Publishers Ltd.

Mitchell, J.K. (1993). Fundamentals of Soil Behavior, 2nd edition, John Wiley & Sons.

Das, B.M. (1998). Principles of Geotechnical Engineering, 4th edition, PWS Publishing

Company. (Chapter 2)

Budhu M. (2007)”Soil Mechanics and Foundations” Wiley, New York

chapter i basic characteristics of soils · • the atterberg limits are usually correlated with...

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