8 soil description and classification

FCE 311 GEOTECHNICAL ENGINEERING
8. SOIL DESCRIPTION AND
CLASSIFICATION
Department of Civil & Construction Engineering
University of Nairobi

8.1 Introduction
Soil description
• details of both material and mass
characteristics.
Soil classification
• Is allocation of a soil to a limited number
of groups on the basis of material
characteristics only, viz: - particle size
distribution and plasticity.

8.1 Introduction
Soil classification
• Independent of the in situ mass condition
of the soil mass.
• It is unlikely that any two soils will have
the same description, while they could
have the same classification.

8.1 Introduction
Areas where soil description and
classification are useful.
• Foundations
• Seepage.
Cases where classification is by the
far the most important.
• Embankment construction where
classification of material is key factor in
choice and construction process.

8.1 Introduction
• It is essential that a standard language
be used in description and classification
of soils.
• A comprehensive description should
include the characteristics of both soil
material and in situ soil mass.

8.1 Introduction
Material characteristics
• Determined from disturbed samples of
soil
• The principal material characteristics are
particle size distribution (or grading) and
plasticity.
• Secondary material characteristics are
colour of the soil, shape, texture and
composition of the particles.

8.1 Introduction
Mass characteristics
• Determined in the field and laboratory
when undisturbed samples are available.
• Includes firmness, strength, details of
any bedding, discontinuities and
weathering.
• Arrangement of minor geological details
should be carefully described as this can
influence the engineering behaviour of in
situ soil considerably. holes etc.

8.1 Introduction
Mass characteristics
• Such minor geological details (macro-
fabric features) are thin layers of fine
sand and silt in a clay strata, silt filled
fissures in clay, small lenses of clay in
sand, organic intrusions and root holes
etc.

8.2 Soil Description
• A detailed description of the method of
describing soils is contained in BS 5930.
• The basic soils are
– boulders,
– cobbles,
– gravels,
– sand,
– silt and
– clay.

8.2 Soil Description – cont’d
BS 5930
• A soil is of basic type sand or gravel
(coarse soil), if after removal of boulders
and cobbles, over 65% of the material is
in the sand and gravel range.

8.2 Soil Description – cont’d
BS 5930
• A soil is of basic type silt or clay (fine
grained soil) when over 35% of the soil is
in the silt and clay range.
• Mixtures containing over 50% boulders
and cobbles are referred to as very
coarse soils.
• The descriptions may be of the form
COBBLES with finer material or gravelly
SAND with occasional cobbly
BOULDERS.

8.2 Soil Description –
Terminologies
Table 8.1: Soil description terminology
Table

8.2 Soil Description –
Field identification tests
Table 8.2: Field identification tests

8.3 Soil Classification System
Purpose of Soil Classification
• A soil classification system is an
arrangement of different soils into soil
groups having similar properties.
• The purpose of soil classification is to
make possible the estimation of soil
properties by association with soils of the
same class whose properties are known.

Purpose of Soil Classification
Thus with consistent soil classification it
is possible to;
• Use data of others in predicting
foundation performance.
• Build the geotechnical engineers data
base for application of design.
• Maintain a permanent record which can
be easily understood by others should
problems develop later.

There are several methods of soil
classification.
Two such methods in use by engineers
in Kenya are;
A. BRITISH SOIL CLASSIFICATION
B. UNIFIED SOIL CLASSIFICATION

• Based on the particle size distribution
and the plasticity plotted on a plasticity
chart.
• Plasticity chart is a plot of the soil PI
against LL.

• Cobbles and boulders retained on
63mm BS Sieve Size are removed from
the soil before the classification.
• Percentage of this very coarse portion
is determined and mentioned in the
report.
• Soil groups in the classification are
noted by the group symbols composed
of main and qualifying descriptive
letters.

Main terms Qualifying terms
GRAVEL
SAND
G
S
Well graded
Poorly graded
Uniform
Gap graded
W
P
Pu
Pg
FINE SOIL,
FINE SILT (M
SOIL)
CLAY
F
M
C
Of low plasticity (LL<35)
Of intermediate plasticity (LL 35-50)
Of high plasticity (LL 50-70)
Of very high plasticity (LL 70-90)
Of extremely high plasticity
Of upper plasticity range (LL<35)
Organic (may be a suffix to any group
L
I
H
V
E
U
O
Peat Pt

• The letter describing the dominant
group is placed first in the group
symbol.
• If the group has significant organic
content the suffix O is added as the last
letter.

Examples:
• SW – well graded SAND
• SCL – very clayey SAND (the clay in
the sample is of low plasticity)
• CIS – sandy CLAY of intermediate
plasticity
• MHSO – organic sandy SILT of high
plasticity

• Fine grained soils represented by a
point on the plasticity chart.
• Plasticity chart divided into:
– low plasticity (LL<35) and upper (U)
plasticity zones (LL>35).
– Upper plasticity zones are subdivided
into Intermediate plasticity (LL 35-50),
High plasticity (LL 50-70), very high
plasticity (LL 70-90) and extremely
high plasticity (LL >90).

Plasticity Chart (BS 5930: 1981)
0
10
20
30
40
50
60
70
0 10 20 30 40 50 60 70 80 90 100 110 120
PlasticityIndex(%)
Liquid limit (%)
CL
CI
CH
CV
CE
ML MI
MH
MV
ME
A line - PI = 0.73(LL-20)

• The recommended standard for soil
classification is the British Soil
Classification System and this is
detailed in BS 5930 Site Investigation.

• Based on the particle size distribution
and the plasticity as plotted on a
plasticity chart.
Differences with British Standard
• Detail in the unified classification is
reduced.
• Classification is simplified.

Differences with British Standard
• Separation of the coarse and the fine
grained soils is basically determined on
the 50% percentage fraction instead of
the 35 and 65% used in the British
classification system.
• In addition the division of the plasticity
chart is limited to the 50% value for the
lower and the higher plasticity for the
purposes of classifying the fine grained
soils.

In this system soils are broadly divided into
three divisions;
• Course-grained soils – if more than
50% by weight is retained on No. 200
ASTM sieves (American Society for
Testing Materials)
• Fine-grained soils – if more than 50%
by weight passes through No. 200
ASTM sieve
• Organic soils

Group symbols as indicated below;
Course-grained soils:
• Gravel: G
• Sand: S
Fine grained soils:
• Silt: M
• Clay: C
• Organic soil: O

• No. 200 sieve is of aperture size 0.074
mm
• No. 4 sieve is of aperture size 4.76 mm

Unified classification system plasticity
chart
CH
MH or OH
MLor OL
CL
CL-ML
ML
A line - PI = 0.73(LL-20)

SHORTCOMINGS OF
CLASSIFICATION SYSTEM
• The classification systems are based on
the properties of the grains and their
remolded properties.
• They do not consider the properties of
the materials in situ.

SHORTCOMINGS OF
• Yet it is the intact nature of soils in the
field that determine the behaviour of the
soil during and after construction.
• The foundation engineer should
therefore take recognition of the
description of soil described above

SHORTCOMINGS OF
• It is also important to have basic
understanding of the geology of the soil
to have a complete description and
understanding of the soil under study.

8 soil description and classification

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