Soil Classification

ENGINEERING DESIGN AND CONSTRUCTION

The Purpose

• To distinguish between the main soil types

• To assess the strength and structure of the soil

sand

clay

silt

Classification systems

• Unified Soil Classification Systems (USCS)

• BS Classification Systems (in BS 5930)• AASHTO/ASTM Classification Systems

Further reading:

1) Carter,M. Geotechnical Engineering Handbook. London: Pentech Press.

2) Other geotechnical references.

Soil Classification Tests

• Particle Size Analysis

• Atterberglimits

• Specific gravity

Particle size distribution

The size of particles within a soil mass can vary from less than 0.001 to over 100mm. The BS size ranges is shown below:

Sieve test (coarse grain)

• Oven dried sample of soil is weighted and passed through a series of sieves.

Particle size distribution form

Well graded

0

20

40

60

80

100

0.01 0.1 1 10 100

Grain size (mm)

Fine grain analysis

• Pipette method• Hydrometer method

Particle size distribution curve• Soil can be described according to the shape of

the curve and where it fits on to the chart.• Uniform soil – all particle are approximately the

same size, will have almost vertical curve.• Well graded soil – containing wide range of

particle size, will have a curve spread evenly across the chart.

• Poorly graded (or gap graded) – stretch across but shown deficiency of some intermediate sizes. Grading curve exhibit flat section or plateau.

Particle size distribution curve

• Effective size – the maximum particle size of the smallest 10% and called

D10

• Uniformity coefficient, CU – the ratio of the maximum particle size of the smallest 60% to effective size:

10

60

DD

CU =

Cu < 4.0 – uniformly graded

Cu > 4.0 – well graded or gap graded.

PSD curve

• Uniform soil will have Uniformity Coefficient approaching down to 1

• Well graded soil will have a high Uniformity Coefficient.

Example 1:PSD

7.40.06325.40.1845.30.35530.50.51.11.43.5212.92.85.75.68.46.32.3101.712.5020

Weight retained (g)Retained on sieve size (mm)

Total wt of sample = 147.2 g

2.03144.27.40.0637.110.4136.825.40.18

24.335.8111.445.30.35555.181.166.130.50.575.8111.635.61.11.476.6112.734.53.5278.9116.23112.92.887.7129.118.15.75.691.6134.812.48.46.397.3143.242.31098.8145.51.71.712.5100147.20020

% PassingWt. passing

Cumulative wt retained

Wt retainedSieve Sizes

Example 1: curve

0102030405060708090

100

0.01 0.1 1 10 100

Size (mm)

Perc

enta

ge p

assi

ng

D10 = 0.23

D60 = 0.7

U = D60/D10

= 0.7/0.23

=3.04

Atterberg limits (plasticity)• Quantitatively

describe the effect of varying water content on the consistency of fine-grained soil.

• The increasing water content cause the changes of the soil from solid to semi-solid to plastic and to liquid states.

Volume of Sample

Moisture content

SL PL

LLsolid

Semisolid

Plastic

liquid

Graphical representation….

solid Semi-solid plastic liquid

Increasing water content

SL LLPL

P.I.

The range of soil over which a soil is plastic.

Plasticity Index (PI)

The maximum moisture content at which further loss of moisture does not cause a decrease in the volume of the soil.

Shrinkage Limit (SL)

The minimum moisture content at which the soil can be rolled into a thread 3mm dia. w/out breaking up.

Plastic Limit (PL)

The minimum moisture content at which the soil will flow under its own weight.

Liquid Limit (LL)

PI

• Represent the relative amount of clay particles in soil

• Higher PI = greater amount of clay particles present = more plastic a soil.

• The more plastic soil will:– Be more compressible– Higher shrink swell potential– Less permeable

Liquid limit test

Casagrande Apparatus Cone penetrometer

Exercise

Sieve Sizes

Wt retained

50 037.5 15.5

20 1714 1010 116.3 33

3.35 114.51.18 63.30.6 18.2

0.15 170.063 10.5

The total mass of the sample was 311 g, plot the particle size distribution curve and from the curve, determine the effective size and uniformity coefficient. Classify the soil.

Answer: D10 = 0.7mm

D60 = 5.2 mm

Cu = 7.4

70 % gravel, 30 percent sand

Well graded Sandy Gravel.