Soil Classification

2

Soil Texture

3

Soil TextureThe 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) (Hong Kong)

Sieve analysis Hydrometer analysis

4

1.2 Characteristics(Holtz and Kovacs, 1981)

5

Grain Size and Grain Size Distribution

Take note!!!

The sizes of particles that make up soil varyover a wide range. Soils are generally called gravel, sand, silt, or clay, depending on the predominant size of particles within the soil. To describe soils by their particle size, several organizations have developed particle-size classifications.

Particle Size Distribution

boulders > 60mm 60mm > gravel > 2mm 2mm > sand > 60 m 60 m > silt > 2 m 2 m > clay

Each class may is sub-divided into coarse, medium and fine.

for sand:

2mm > coarse sand > 600 m 600 m > medium sand > 200 m 200 m > fine sand > 60 m

Classification boundaries either begin with a '2' or a '6'.

Soil-Particle Size Classification

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Grain Size

Gravel

Sand

Silt

Clay

4.75

Unit: mm (Holtz and Kovacs, 1981)

USCS

BS

0.075

2.0 0.06 0.002

USCS: Unified Soil Classification

BS: British Standard

• Data often presented as Particle Size Distribution Curves with logarithmic scale on X-axis

Particle Size Distribution (continued)

• S - shaped - but some conventions of curves going left to right, others, the opposite way around

sand

siltclay

A Problem • clay is used both as a classifier of size as above, and also to define

particular types of material. • clays exhibit a property known as cohesion (the "stickiness" associated with clays).

General Properties• Gravels ----- permeability is of the order of mm s-1. • Clays ----- it is 10-7 mm/s or less. • Compressibility of the soil increases as the particle size decreases.• Permeability of the soil decreases as the particle size decreases.

Particle Size Distribution (continued)

12

Sieve Sizes

(Das, 1998) (Head, 1992)

13

Grain Size Distribution (Cont.)

Coarse-grained soils:

Gravel Sand

Fine-grained soils:

Silt Clay

0.075 mm (USCS)

0.06 mm (BS) (Hong Kong)

•Experiment

Sieve analysis Hydrometer analysis

(Head, 1992)

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Grain Size Distribution (Cont.)

Log scale

(Holtz and Kovacs, 1981)

Fine

r

Effective size D10: 0.02 mm

D30: D60:

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Grain Size Distribution (Cont.)• Describe the shapeExample: well graded

•Criteria

•QuestionWhat 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

230

c

10

60u

mm9D

mm6.0D

)sizeeffective(mm02.0D

60

30

10

)sandsfor(

6Cand3C1

)gravelsfor(

4Cand3C1

soilgradedWell

uc

uc

16

Answer

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

D

Fine

r

1D

DC

uniformityoftCoefficien

10

60u

Grain size distribution

SAMPLE PROBLEM

1. For a soil with D60 =0.42mm, and D30 =0.21mm, and D10 =0.16, calculate and the coefficient of gradation.

Sol’n; CU =D60 /D10 =0.42mm/0.16mm=2.625 CC = (D30 )2 /(D10 )(D60) =(0.21)2 / (0.16)(0.42)=0.66

2. The following are the results of a sieve analysis:

US Sieve No.

Mass of Soil Retained on Each Sieve (g)

Opening Commulative Mass

% Passing

410204060

100200PAN

018.553.290.581.892.258.526.5

4.752.0

.850

.425

.250

.150

.075

018.571.5

162.2244

336.2394.1421.2

10095.60882.97761.49142.07020.1806.434

0

a.) Determine the percent finer than each sieve size and plot a grain-size distribution curve.b.) Determine D10 , D30 and D60 from the grain-size distribution curve.c.) Calculate the uniformity coefficient, Cu.d.) Calculate the coefficient of graduation, Cc.

Grain-size distribution curve

0.01 0.1 1 100

20

40

60

80

100

120

Solution

c.) Cu = D60 /D10 =0.4/0.12= 3.33

d.) CC = (D)2/(D60)(D10) =1.01

3.The particle-size characteristics of a soil are given in the table.

Sieve no. Opening % Passing

410204080

200

4.752.00

0.8500.4250.1800.075

1009064381813

Calculate the uniformity coefficient ( Cu) and coefficient of gradation ( Cc).

Sol’n:

Cu= D60/ D10= 0.73/0.019= 38.421

Cc = (0.29)2 / (0.73)(.019)=0.063

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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.

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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

Thank you!!!