Basic Characteristics of Soils Outline 1. The Nature of Soils - - PowerPoint PPT Presentation

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

• f 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~ 4m, 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+

• r 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

(Holtz and Kovacs, 1981)

• 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)

3.2 Grain Size Distribution (Cont.)

Log scale

(Holtz and Kovacs, 1981)

Effective size D10: 0.02 mm D30: D60:

3.2 Grain Size Distribution (Cont.)

• Describe the shape

Example: well graded

• Criteria
• Question

What is the Cu for a soil with

• nly one grain size?

2 ) 9 )( 02 . ( ) 6 . ( ) D )( D ( ) D ( C curvature

• f

t Coefficien 450 02 . 9 D D C uniformity

• f

t Coefficien

2 60 10 2 30 c 10 60 u

     

mm 9 D mm 6 . D ) size effective ( mm 02 . D

60 30 10

  

) sands for ( 6 C and 3 C 1 ) gravels for ( 4 C and 3 C 1 soil graded Well

u c u c

      

Answer

• Question

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

D Finer

1 D D C uniformity

• f

t Coefficien

10 60 u

 

Grain size distribution

3.2 Grain Size Distribution (Cont.)

• 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

(Holtz and Kovacs, 1981)

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

Increasing water content

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)

(Holtz and Kovacs, 1981)

• 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

4.2.1 Casagrande Method (Cont.)

 

. log ) ( / log ,

1 2 2 1

cont N I w value positive a choose N N w w I index Flow

F F

    

N w

• Multipoint Method

Das, 1998

4.2.1 Casagrande Method (Cont.)

• 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 . tan 25

tan

          



content moisture ing correspond w blows

• f

number N N w LL

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

(Holtz and Kovacs, 1981)

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. content water the is w PL LL PL w PI PL w LI     

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) mm 002 . : fraction clay ) weight ( fraction clay % PI A  

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