SOIL MECHANICS A CIVL3401


Geotechnical Engineering is that part of engineering which is concerned with the
behaviour of soil and rock. Soil Mechanics is the part concerned solely with soils.
From an engineering perspective soils generally refer to sedimentary materials that have
not been cemented and have not been subjected to high compressive stresses.

As the name Soil Mechanics implies the subject is concerned with the deformation and
strength of bodies of soil. It deals with the mechanical properties of the soil materials
and with the application of the knowledge of these properties to engineering problems.
In particular it is concerned with the interaction of structures with their foundation
material. This includes both conventional structures and also structures such as earth
dams, embankments and roads which are themselves made of soil.

As for other branches of engineering the major issues are stability and serviceability.
When a structure is built it will apply a load to the underlying soil; if the load is too great
the strength of the soil will be exceeded and failure may ensue. It is important to realise
that not only buildings are of concern, the failure of an earth dam can have catastrophic
consequences, as can failures of natural and man made slopes and excavations.
Buildings or earth structures may be rendered unserviceable by excessive deformation of
the ground, although it is usually differential settlement, where one side of a building
settles more than the other, that is most damaging. Criteria for allowable settlement vary
from case to case; for example the settlement allowed in a factory that contains sensitive
equipment is likely to be far more stringent than that for a warehouse. Another
important aspect to be considered during design is the effect of any construction on
adjacent structures, for example the excavation of a basement and then the construction
of a large building will cause deformations in the surrounding ground and may have a
detrimental effect on adjacent buildings or other structures such as railway tunnels.

Many of the problems arising in Geotechnical Engineering stem from the interaction of
soil and water. For example, when a basement is excavated water will tend to flow into
the excavation. The question of how much water flows in needs to be answered so that
suitable pumps can be obtained to keep the excavation dry. The flow of water can have
detrimental effects on the stability of the excavation, and is often the initiator of
landslides in natural and man made slopes. Some of the effects associated with the
interaction of soil and water are quite subtle, for example if an earthquake occurs, then a
loose soil deposit will tend to compress causing the water pressures to rise. If the water
pressures should increase so that they become greater than the stress due to the weight of
the overlying soil then a quicksand condition will develop and buildings founded on this
soil may fail.

Soil mechanics differs from other branches of engineering in that generally there is little
control over the material properties, we have to make do with the soil at the site and this
is often highly variable. By taking samples at a few scattered locations we have to
determine the soil properties and their variability. At this stage in a project knowledge

of the site geology and geological processes is essential to successful geotechnical

Soil mechanics is a relatively new branch of engineering science, the first major
conference occurred in 1936 and the mechanical properties of soils are still incompletely
understood. The first complete mechanical model for soil was published as recently as
1968. Over the last 40 years there has been rapid development in our understanding of
soil behaviour and the application of this knowledge in engineering practice. The
subject has now reached a phase of development similar to that of structural mechanics a
century ago and the words of William Anderson in 1893 about structural engineers are
relevant today for geotechnical engineering, "There is a tendency among the young and
inexperienced to put blind faith in formulas (computer programs), forgetting that most
of them are based upon premises which are not accurately reproduced in practice, and
which, in any case, are frequently unable to take into account collateral disturbances
which only observation and experience can foresee, and common sense provide


1.1 Introduction

Soil mechanics is concerned with particulate materials (soils) found in the ground that
are not cemented and not greatly compressed. These soils usually have a sedimentary
origin, however, they can also occur as the result of rock weathering without any
transport of the particles. The soil particles can have varying sizes, shapes and
mineralogies, although these properties are usually interrelated. For instance the larger
sized particles are generally composed of quartz and feldspars, minerals that have high
strengths and the particles are fairly round. The smaller sized particles are generally
composed of the clay minerals kaolin, illite and montmorillonite, minerals that have
low strengths and form plate like particles. One of the most important aspects of
particulate materials is that there are gaps or voids between the particles. The amount
of voids is also influenced by the size, shape and mineralogy of the particles.

Because of the wide range of particle sizes, shapes and mineralogies in a typical soil a
detailed classification of each soil would be very expensive and inappropriate for most
geotechnical engineering purposes. However, some form of simple classification
system giving information about the engineering properties is required on all sites.
Why is this necessary?

 Usually the soil on site has to be used. Soils differ from other engineering materials
  in that one has very little, if any, control over their properties.

 The extent and properties of the soil at the site have to be determined.

 Cheap and simple tests are required to give an indication of the engineering
  properties such as stiffness and strength for preliminary design.

To achieve this continuous samples are recovered from boreholes, drilled to a depth
that will depend on the scale of the project. Observation of the core enables the
different soil layers to be determined and then classification tests are performed for
these different strata. The extent of the different soil layers can be determined by
correlating the results from different boreholes and this information is used to build a
picture of the sub-surface profile.

An indication of the engineering properties is determined on the basis of particle size.
This crude approach is used because the engineering behaviour of soils with very
small particles, usually containing clay minerals, is significantly different from the
behaviour of soils with larger particles. Clays can cause problems because they are
relatively compressible, drain poorly, have low strengths and can swell in the presence
of water.

1.2 Particle Size Definitions

The precise boundaries between different soil types are somewhat arbitrary, but the
following scale is now in use worldwide.

Gravel                 Sand               Silt           Clay
C      M        F      C      M     F     C    M   F     C    M    F
60 20       6        2    0.6   0.2   0.06 0.02 .006 .002 .0006 .0002

where C, M, F stand for coarse, medium and fine respectively, and the particle sizes
are in millimetres.

 the logarithmic scale. Most soils contain mixtures of sand, silt and clay particles, so
  the range of particle sizes can be very large.
 not all particles less than 2 m are comprised of clay minerals, and some clay
  mineral particles can be greater than 2 m. (A micron, m, is 10-6m).

1.3 Broad Classification

1.3.1 Coarse-grained soils

These include sands, gravels and larger particles. For these soils the grains are well
defined and may be seen by the naked eye. The individual particles may vary from
perfectly round to highly angular reflecting their geological origins.

1.3.2 Fine-grained soils

These include the silts and clays and have particles smaller than 60 m.

 Silts         These can be visually differentiated from clays because they exhibit
                the property of dilatancy. If a moist sample is shaken in the hand
                water will appear on the surface. If the sample is then squeezed in the
                fingers the water will disappear. Their gritty feel can also identify silts.

 Clays         Clays exhibit plasticity, they may be readily remoulded when moist,
                and if left to dry can attain high strengths

 Organic       These may be of either clay or silt sized particles. They contain
                significant amounts of vegetable matter. The soils as a result are
                usually dark grey or black and have a noticeable odour from decaying
                matter. Generally only a surface phenonomen but layers of peat may
                be found at depth. These are very poor soils for most engineering

1.4 Procedure for grain size determination

Different procedures are required for fine and coarse-grained material. Detailed
procedures are described in the Australian Standard AS 1289.A1, Methods of testing
soil for engineering purposes. These will be demonstrated in a laboratory session.

 Coarse      Sieve analysis is used to determine the distribution of the larger grain
              sizes. The soil is passed through a series of sieves with the mesh size
              reducing progressively, and the proportions by weight of the soil
              retained on each sieve are measured. There are a range of sieve sizes
              that can be used, and the finest is usually a 75 m sieve. Sieving can be
              performed either wet or dry. Because of the tendency for fine particles
              to clump together, wet sieving is often required with fine-grained soils.

 Fine        To determine the grain size distribution of material passing the 75m
              sieve the hydrometer method is commonly used. The soil is mixed with
              water and a dispersing agent, stirred vigorously, and allowed to settle
              to the bottom of a measuring cylinder. As the soil particles settle out of
              suspension the specific gravity of the mixture reduces. An hydrometer
              is used to record the variation of specific gravity with time. By making
              use of Stoke’s Law, which relates the velocity of a free falling sphere
              to its diameter, the test data is reduced to provide particle diameters
              and the % by weight of the sample finer than a particular particle size.

                Figure 1 A schematic view of the hydrometer test

1.5 Grading curves

The results from the particle size determination tests are plotted as grading curves.
These show the particle size plotted against the percentage of the sample by weight
that is finer than that size. The results are presented on a semi-logarithmic plot as

  shown in Figure 2 below. The shape and position of the grading curve are used to
  identify some characteristics of the soil.


% F i ne r




              0 .0 0 0 1   0 .0 0 1        0 .0 1      0 .1        1       10          100
                                           P a rtic le s ize (m m )
                                      Figure 2 Typical grading curves

  Some typical grading curves are shown on the figure. The following descriptions are
  applied to these curves

  W             Well graded material
  U             Uniform material
  P             Poorly graded material
  C             Well graded with some clay
  F             Well graded with an excess of fines

  The use of names to describe typical grading curve shapes and positions has
  developed as the suitability of different gradings for different purposes has become
  apparent. For example, well graded sands and gravels can be easily compacted to
  relatively high densities which result in higher strengths and stiffnesses. For this
  reason soils of this type are preferred for road bases. The suitability of different
  gradings is discussed in some detail by Terzaghi and Peck (1967).

  From the typical grading curves it can be seen that soils are rarely all sand or all clay,
  and in general will contain particles with a wide range of sizes. Many organisations
  have produced charts to classify soils giving names for the various combinations of
  particle sizes. One such example is given in Figure 3 below.

                                            40                  Clay






                                                                                                     e s (%

                        Sa n

                                            Sandy Clay                 Silty Clay                 40
                        80              Clay-Sand                        Clay-Silt
                                            Silty Sand                  Sandy Silt                               10
             100         Sand
                0        10       20        30        40 50 60                70        80        90        100
                                                      Silt Sizes (%)
                             LOWER MISSISSIPPI VALLEY DIVISION,
                                   U. S. ENGINEER DEPT.

                                       Figure 3 Classification Chart

Important observations from figure 3 are that any soil containing more than 50% of
clay sized particles would be classified as a clay, whereas sand and silt require 80% of
the particles to be in that size range. Also any soil having more than 20% clay would
have some clay like properties.

The hydrometer test is usually terminated when the percentage of clay sized particles
has been determined. However, there are significant differences between the
behaviour of the different clay minerals. To provide additional information on the soil
behaviour further classification tests are performed. One such set of tests, the
Atterberg Limit Tests, involve measuring the moisture contents of the soil at which
changes in the soil properties occur.

1.6 Atterberg Limits

These tests are only used for the fine-grained, silt and clay, fraction of a soil (actually
the % passing a 425 m sieve). If we take a very soft (high moisture content) clay
specimen and allow it to dry we would obtain a relation similar to that shown in
Figure 4.

As the soil dries its strength and stiffness will increase. Three limits are indicated, the
definitions of which are given below. The liquid and plastic limits appear to be fairly
arbitrary, but recent research has suggested they are related to the strength of the soil.


                            SL        PL                    LL
                                        Moisture Content (%)

                 Figure 4. Moisture content versus volume relation

 (SL) The Shrinkage Limit - This is the moisture content the soil would have had if
  it were fully saturated at the point at which no further shrinkage occurs on drying.
                                     weight of water      w
               moisture content                        w                          (1)
                                     weight of solids      ws

In the shrinkage test the soil is left to dry and the soil is therefore not saturated when
the shrinkage limit is reached. To estimate SL it is necessary to measure the total
volume, V, and the weight of the solids, ws. Then

                                             wV       1
                               SL  m                                              (2)
                                             ws        Gs

where w is the unit weight of water, and
      Gs is the specific gravity

 (PL) The Plastic Limit - This is the minimum water content at which the soil will
  deform plastically

 (LL) The Liquid Limit - This is the minimum water content at which the soil will
  flow under a small disturbing force

 (PI or Ip) The Plasticity Index. This is derived simply from the LL and PL

                               IP = LL - PL                                          (3)

 (LI) The Liquidity Index - This is defined as

                               m  PL    m  PL
                        LI                                                         (4)
                               LL  PL     Ip

The Atterberg Limits and relationships derived from them are simple measures of the
water absorbing ability of soils containing clay minerals. For example, if a clay has a
very high LI and LL it is capable of absorbing large amounts of water, and for instance
would be unsuitable for the base of a pavement. The LL and PL are also related to the
soil strength.

Remember that only the fraction finer than 425 m is tested in the Atterberg Tests. If
this fraction is only small (that is, the soil contains significant amounts of sand or
gravel) it might be expected that the soil would have better properties. While this is
true to some extent it is important to realise that the soil behaviour is controlled by the
finest 10 - 25 % of the particles

1.7 Classification Systems for Soils

Several systems are used for classifying soil. This is because these systems have two
main purposes

1. To determine the suitability of different soils for various purposes (see p8 Data

2. To develop correlations with useful soil properties, for example, compressibility
   and strength

The reason for the large number of such systems is the use of particular systems for
certain types of construction, and the development of localised systems.

1.7.1 PRA (AASHO) system

An example is the PRA system of AASHO (American Association of State Highway
Officials), which ranks soils from 1 to 8 to indicate their suitability as a subgrade for
pavements. The detailed classification is given in the Data Sheets p9.

1.     Well graded gravel or sand; may include fines
2.     Sands and Gravels with excess fines
3.     Fine sands
4.     Low compressibility silts
5.     High compressibility silts
6.     Low to medium compressibility clays
7.     High compressibility clays
8.     Peat, organic soils

1.7.2 Unified Soil Classification

The standard system used worldwide for most major construction projects is known as
the Unified Soil Classification System (USCS). This is based on an original system
devised by Cassagrande. Soils are identified by symbols determined from sieve
analysis and Atterberg Limit tests.

 Coarse Grained Materials

If more than half of the material is coarser than the 75 m sieve, the soil is classified
as coarse. The following steps are then followed to determine the appropriate 2 letter

1. Determine the prefix (1st letter of the symbol)

If more than half of the coarse fraction is sand then use prefix S

If more than half of the coarse fraction is gravel then use prefix G

2. Determine the suffix (2nd letter of symbol)

This depends on the uniformity coefficient Cu and the coefficient of curvature Cc
obtained from the grading curve, on the percentage of fines, and the type of fines.

First determine the percentage of fines, that is the % of material passing the 75 m

Then if % fines is     < 5% use W or P as suffix
                       > 12% use M or C as suffix
                       between 5% and 12% use dual symbols. Use the prefix from
                       above with first one of W or P and then with one of M or C.

If W or P are required for the suffix then Cu and Cc must be evaluated

                                       Cu 
                                  Cc 
                                       ( D60  D10 )

If prefix is G then suffix is W       if Cu > 4 and Cc is between 1 and 3
               otherwise use P

If prefix is S then suffix is W       if Cu > 6 and Cc is between 1 and 3
               otherwise use P

If M or C are required they have to be determined from the procedure used for fine
grained materials discussed below. Note that M stands for Silt and C for Clay. This is
determined from whether the soil lies above or below the A-line in the plasticity chart
shown in Figure 5.

For a coarse grained soil which is predominantly sand the following symbols are


 Fine grained materials

These are classified solely according to the results from the Atterberg Limit Tests.
Values of the Plasticity Index and Liquid Limit are used to determine a point in the
plasticity chart shown in Figure 5. The classification symbol is determined from the
region of the chart in which the point lies.

Examples                     CH        High plasticity clay
                             CL        Low plasticity clay
                             MH        High plasticity silt
                             ML        Low plasticity silt
                             OH        High plasticity organic soil (Rare)
                             Pt        Peat

                                      Comparing soils at equal liquid limit
                             50       Toughness and dry strength increase                 e
                                      with increasing plasticity index                "l
          Plasticity index


                             20                                               OH
                                                     CL                       or
                             10           CL                   OL
                                                          or                  MH
                              0          ML
                                  0    10 20       30      40 50 60           70      80      90   100
                                                            Liquid limit
                                            Plasticity chart
    Figure 5 Plasticity chart for laboratory classification of fine grained soils
                         for laboratory classification of fine grained soils
The final stage of the classification is to give a description of the soil to go with the 2-
symbol class. For a coarse grained soil this should include:

   the percentages of sand and gravel
   maximum particle size
   angularity
   surface condition
   hardness of the coarse grains
   local or geological name
   any other relevant information

If the soil is undisturbed mention is also required of

 stratification

                                                                                                                                                                            degree of compactness
                                                                                                                                                                            cementation
                                                                                                                                                                            moisture conditions
                                                                                                                                                                            drainage characteristics

                                                                                                                                                                    The information required, along with all the details of the Unified Classification
                                                                                                                                                                    Procedure is given in Figure 6. Note that slightly different information is required for
                                                                                                                                                                    fine-grained soils.

                                                                                                                                                                                                             Unified soil classification (including identification and description)

                                                                                                                  Field identification procedures                                                                          Group                                               Information required for                                                                                                                                                                        Laboratory classification
                                                                                                   (Excluding particles larger than 75mm and basing fractions on                                                          symbols           Typical names
                                                                                                                                                                                                                             1                                                     describing soils                                                                                                                                                                                    criteria
                                                                                                                         estimated weights)

                                                                                                                                                                                                                                                                                                                                                                                                           Depending on percentages of fines (fraction smaller than .075mm

                                                                                                                                                                                                                                                                                                                                                                                                                               Bordeline case requiring use of dual symbols
                                                                                                                                                                           Wide range of grain size and substantial        GW        Well graded gravels, gravel-
                                                                                                                                                          Clean gravels

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         C U = --- 60 Greater than 4

                                                                                                                                                                                                                                                                                                                                                                                                           Determine percentages of gravel and sand from grain size curve
                                                                                                                                                                                                                                                                           Give typical names: indicate ap-
                                                                                                                                                           (little or no

                                                                                                                                                                           amounts of all intermediate particle                      sand mixtures, little or no           proximate percentages of sand                                                                                                                                                                                       D 10

                                                                                                                                                                           sizes                                                     fines                                                                                                                                                                                                                                                                  2
                                                                                                                                                                                                                                                                           and gravel: maximum size:                                                                                                                                                                                             (D30 )
                                                                                                           More than half of coarse

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 -- -- -- --
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         C c = ------------- Between 1 and 3
                                                                                                            fraction is larger than

                                                                                                                                                                           Predominantly one size or a range of            GP        Poorly graded gravels, gravel-        angularity, surface condition,                                                                                                                                                                                       D10 x D60

                                                                                                                                                                                                                                                                                                                                                                                                           sieve size) coarse grained soils are classified as follows
                                                                                                                                                                           sizes with some intermediate sizes                        sand mixtures, little or no           and hardness of the coarse
                                                               More than half of material is larger than

                                                                                                                                                                           missing                                                   fines                                 grains: local or geological name                                                                                                                                                                      Not meeting all gradation requirements for GW

                                                                                                                                                                                                                                                                           and other pertinent descriptive
                                                                                                                                         amount of fines)

                                                                                                                                                                                                                                                                                                                   Use grain size curve in identifying the fractions as given under field identification
                                                                                                                                          Gravels with

                                                                                                                                                                           Non-plastic fines (for identification           GM        Silty gravels, poorly                 information and symbol in

                                                                                                                                                                           procedures see ML below)                                  graded gravel-sand-silt mixtures      parentheses.                                                                                                                                                                                          Atterberg limits below      Above "A" line with
                   The .075mm sieve size is about the smallest particle visible to the naked eye
                                                                          Coarse grained soils

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 "A" line or PI less than 4  PI between 4 and 7
                                                                            .075mm sieve size

                                                                                                                                                                           Plastic fines (for identification pro-          GC        Clayey gravels, poorly graded         For undisturbed soils add infor-                                                                                                                                                                                                  are borderline cases
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 Atterberg limits above "A" requiring use of dual

                                                                                                                                                                                                                                                                                                                                                                                                           More than 12% GM, GC, SM, SC
                                                                                                                                                                           cedures see CL below)                                     gravel-sand-clay mixtures             mation on stratification, degree

                                                                                                                                                                                                                                                                                                                                                                                                                               GW, GP, SW, SP
                                                                                                                                                                                                                                                                           of compactness, cementation,                                                                                                                                                                          line with PI greater than 7 symbols
                                                                                                                                                                           Wide range in grain sizes and sub-                                                              moisture conditions and drain-                                                                                                                                                                                     D
                                                                                                                                                                                                                           SW        Well graded sands, gravelly
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         C U =-- 60 Greater than 6
                                                                                                                                               Clean sands
                                                                                                                                               (little or no

                                                                                                                                                                           stantial amounts of all intermediate                                                            age characteristics.                                                                                                                                                                                                 -
                                                                                                                                                                           particle sizes                                            sands, little or no fines                                                                                                                                                                                                                                D 10
                                                                                                           More than half of coarse

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 (D )2
                                                                                                           fraction is smaller than

                                                                                                                                                                           Predominantely one size or a range of           SP        Poorly graded sands, gravelly                                                                                                                                                                                                                       C c = --- --30--
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 ------------ Between 1 and 3
                                                                                                                                                                                                                                                                           Silty sand, gravelly; about 20%                                                                                                                                                                                     D 10 x D60
                                                                                                                                                                           sizes with some intermediate sizes missing                sands, little or no fines             hard angular gravel particles

                                                                                                                                                                                                                                                                           12.5mm maximum size; rounded                                                                                                                                                                          Not meeting all gradation requirements for SW
                                                                                                                                         amount of fines)

                                                                                                                                                                           Non-plastic fines (for identification pro-      SM        Silty sands, poorly graded            and subangular sand grains

                                                                                                                                                                                                                                                                                                                                                                                                           Less than 5%
                                                                                                                                           Sands with

                                                                                                                                                                           cedures, see ML below)                                    sand-silt mixtures                    coarse to fine, about 15% non-                                                                                                                                                                        Atterberg limits below      Above "A" line with

                                                                                                                                                                                                                                                                                                                                                                                                           5% to 12%

                                                                                                                                                                                                                                                                           plastic lines with low dry                                                                                                                                                                            "A" line or PI less than 4  PI between 4 and 7
                                                                                                                                                                           Plastic fines (for identification pro-          SC        Clayey sands, poorly graded           strength; well compacted and                                                                                                                                                                                                      are borderline cases
                                                                                                                                                                           cedures, see CL below)                                    sand-clay mixtures                    moist in places; alluvial sand;                                                                                                                                                                       Atterberg limits above "A" requiring use of dual
                                                                                                                                                                                                                                                                           (SM)                                                                                                                                                                                                  line with PI greater than 7 symbols
                                                                                                                       Identification procedure on fraction smaller than .425mm
                                                                                                                                               sieve size
                                                                                                                                            Dry strength      Dilatency     Toughness
More than half of material is smaller than

                                                                                                                             Silts and clays

                                                                                                                                              crushing                     (consistency
                                                                                                                              less than 50

                                                                                                                               liquid limit

                                                                                                                                             character-      to shaking) near plastic
                                                                                                                                                istics                        limit)
                                                                                                                                                                                                                                    Inorganic silts and very fine sands,   Give typical name; indicate degree
                                                                                                                                                                             None to         Quick to                               rock flour, silty or clayey
                                                                                                                                                                                                               None        ML
           Fine grained soils

                                                                                                                                                                                                                                                                           and character of plasticity,
           .075mm sieve size

                                                                                                                                                                              slight          slow                                  fine sands with slight plasticity                                                                                                                                                                                60
                                                                                                                                                                                                                                                                           amount and maximum size of                                                                                                                                                               Comparing soils at equal liquid limit
                                                                                                                                                                           Medium to       None to very                             Inorganic clays of low to medium       coarse grains: colour in wet con-
                                                                                                                                                                                                             Medium                 plasticity, gravelly clays, sandy                                                                                                                                                                                50             Toughness and dry strength increase                             e
                                                                                                                                                                             high             slow                        CL,CI                                            dition, odour if any, local or                                                                                                                                                                                                                      in
                                                                                                                                                                                                                                    clays, silty clays, lean clays                                                                                                                                                                                                  with increasing plasticity index                         "l
                                                                                                                                                                                                                                                                           geological name, and other pert-                                                                                                                                                                                                             "A
                                                                                                                                                                                                                                                                                                                                                                                                                                  Plasticity index

                                                                                                                                                                            Slight to          Slow            Slight               Organic silts and organic silt-        inent descriptive information, and
                                                                                                                                                                            medium                                         OL       clays of low plasticity                symbol in parentheses                                                                                                                                                                                                              CH
                                                                                                                                                                                                                                                                                                                                                                                                                                                     30                                             CI
                                                                                                                                                                                                                                    inorganic silts, micaceous or
                                                                                                                          Silts and clays

                                                                                                                                                                            Slight to         Slow to        Slight to                                                     For undisturbed soils add infor-
                                                                                                                           greater than
                                                                                                                            liquid limit

                                                                                                                                                                                                                           MH       dictomaceous fine sandy or
                                                                                                                                                                            medium             none          medium                 silty soils, elastic silts             mation on structure, stratif-                                                                                                                                             20                                                            OH
                                                                                                                                                                                                                                                                           ication, consistency and undis-

                                                                                                                                                                           High to very                                             Inorganic clays of high                                                                                                                                                                                                                                                        or
                                                                                                                                                                                               None            High                                                        turbed and remoulded states,                                                                                                                                                                             CL              OL
                                                                                                                                                                              high                                         CH       plasticity, fat clays                  moisture and drainage conditions                                                                                                                                          10
                                                                                                                                                                                                                                                                                                                                                                                                                                                                              CL-ML            or                  MH
                                                                                                                                                                           Medium to       None to very      Slight to              Organic clays of medium to             Example                                                                                                                                                                    0
                                                                                                                                                                             high             high           medium        OH       high plasticity                        Clayey silt, brown: slightly plastic:                                                                                                                                          0              10       20     30    40 50 60            70         80        90   100
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                Liquid limit
                                                                                                                                                                               Readily identified by colour, odour                                                         small percentage of fine sand:
                                                                                              Highly organic soils                                                             spongy feel and frequently by fibrous       Pt       Peat and other highly organic soils    numerous vertical root holes: firm                                                                                                                                                                                   Plasticity chart
                                                                                                                                                                                                                                                                           and dry in places; loess; (ML)                                                                                                                                                                     for laboratory classification of fine grained soils

                                                                                                                                                                                                                        Figure 6 Unified Soil Classification Chart

Example - Classification using USCS

Classification tests have been performed on a soil sample and the following grading
curve and Atterberg limits obtained. Determine the USCS classification.


% Finer




           0 .0 0 0 1   0 .0 0 1    0 .0 1      0 .1        1           10     100
                                      P a rtic le s ize (mm)

Atterberg limits:            Liquid limit LL = 32, Plastic Limit, PL =26

Step 1: Determine the % fines from the grading curve

            %fines (% finer than 75 m) = 11% - Coarse grained, Dual symbols required

Step 2: Determine % of different particle size fractions (to determine G or S), and D10,
        D30, D60 from grading curve (to determine W or P)

            D10 = 0.06 mm, D30 = 0.25 mm, D60 = 0.75 mm

            Cu = 12.5, Cc = 1.38, and hence Suffix1 = W

            Particle size fractions: Gravel        17%
                                     Sand          73%
                                     Silt and Clay 10%

            Of the coarse fraction about 80% is sand, hence Prefix is S

Step 3: From the Atterberg Test results determine its Plasticity chart location

            LL = 32, PL = 26. Hence Plasticity Index Ip = 32 - 26 = 6

            From Plasticity Chart point lies below A-line, and hence Suffix2 = M

Step 4: Dual Symbols are SW-SM

Step 5: Complete classification by including a description of the soil

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