Soil Properties and classification
Soil classification AND DESCRIPTIONSub and
SuperstructureSubstructure main purposes :
distribute the load of the structure over a large area of soil
or to transmit the load to hard stratum of soil below the
structure.
stability of structure against tilting, overturning, due to wind
earthquake or extremely asymmetric loading etc.
to prevent lateral movement of loose soils from under the
building To act as a firm and level base for construction of
superstructureThree Soil types
1. Agricultural soil (e.g. topsoil, subsoil) not considered as
engineering soils
2. Soil is a material that can be worked without drilling or
blasting. Consist of soft, loose, uncemented deposits (e.g.
gravels, clays, etc). There are 6 principle soil types considering
size and nature of soil particles. Boulders & cobbles Gravels
Sands Silts Clays Peats3. Rock is the hard, rigid & cemented
deposits (e.g. granite and sandstone) Solid material called
bedrock. Rock can be classified into three types of rock Igneous
rocks are those that solidify from the magma through either
intrusive (Granite) or extrusive (Basalt or Obsidian) processes
Sedimentary rocks result from external forces on the Earths
crust and are formed of particles deposited by rivers, glaciers,
the wind, the sea or by chemical deposition from lakes or the sea
(Limestone, Conglomerates & Sandstone)
Metamorphic rocks are igneous or sedimentary that have been so
altered by heat and/or pressure that they have lost their original
character and have often been re-crystallized into new types of
rocks (Marble or Slate)
Soil Composition, Structure & Fabric
Soil is grouping of solid separate particles forming a porous
structure. The pores or voids contain air and/or water. The
individual particles are quite free to move. Therefore soil is
regarded as a particulate system
Air = unimportant from an engineering point of view voids should
be reduced as far as possible
Water = very important for the engineering properties &
behaviour
Solid matter (soil skeleton) = varies widely in particle shape,
size & mineral composition organic or inorganic
Groundwater Profile
Two types of Engineering Soils
1. Granular Soils
Boulder, cobbles & gravels = angular to rounded rock
fragments
Sands = granular
Silts = similar to sands but smaller grains with some plasticity
& cohesion
The above particles are formed by mechanical weathering shape
tends to be bulky & equi-dimensional
Structure arrangement = each individual particle supported by
points of contact with adjacent particles
2. Cohesive Soils
Clays = from rock weathering, mainly due to chemical
weathering.
Particles are plate shaped
Possess varying degrees of plasticity & cohesion
A small amount of clay will influence the soil behaviour
Natural clay deposits contain 70% of sand/silt (sandy clay,
silty clay)
Structure arrangement =
Flocculation from inter particle attraction (Van der Waals or
secondary bonding) these forces increases particles closer
together.
Dispersed structure repulsive forces from electrically ve nature
of particle surface holds particles apartSoil classificationIn
practice soils contain more than one type e.g. sandy gravel, silty
sand, clayey silt, sandy clay). The British Standard classification
uses symbols to group the soils in accordance with its particle
size distribution or plasticity indicesPrimary letterSecondary
letter
Course grained soilsGravelWell-graded
Poorly-graded
SandUniformly-graded
Gap-graded
Fine grained soilsFinesLow plasticity
Intermediate plasticity
SiltHigh plasticity
Very high plasticity
ClayExtremely high plasticity
Upper plasticity range
Organic soilsPeatOrganic (may be suffixed to any group)
For mixtures of sand silt and clay the triangular classification
chart may also be used.
Figure 1 - The hypothetical XYZ soil classification scheme
Calculation of Moisture Content.
Bulk & Dry Density
Masses: Density () and Unit Weight ()
Density = Mass / Volume
Mass = no gravity
Weight = when gravity is taken into account
Unit weight = density x gravity (9.81ms-2)
Bulk density () Mg/m3 x 9.81m/s2 = Bulk unit weight () kN/m3
Dry density (d) Mg/m3 x 9.81m/s2 = Dry unit weight (d) kN/m3
Density of water (1000kg/m3 or) 1Mg/m3 x 9.81m/s2 = Unit weight of
water (w) = 9.81kN/m3
Activity: A soil sample has a diameter 50mm and a length of
85mm, a mass of 325g and a moisture content of 16.8% - calculate
the bulk and dry densities in Mg/m3.
Particle density
The particle density (specific gravity) of a material is the
ratio of the mass or weight of volume of a material to the mass or
weight of an equal volume of water. In soil mechanics the particle
density of the soil grains is given the symbol s (specific gravity
= Gs).
For coarse-grained soils, a 500-1000 ml density bottle (gas jar)
may be used, but for fine- grained soils, a special conical topped
glass jar, called a pyknometer, should be used.
Should be noted that the range of particle density of common
soil particles is very narrow: between 2.60 and 2.70 Mg/m3.
Guessing a value within this range will produce an error of no
more than 3%.
In order to make the laboratory test worthwhile for standard
soils therefore, a high level of accuracy in the procedures and
weighing is necessary, i.e. better than 1%.
Particle Size Distribution by Sieve Analysis The analysis
involves passing soil through a series of sieves of decreasing mesh
size and recording the proportion of soil retained on each.
Grading The shape of the Particle Size Distribution curve
indicates the range of particle sizes within a soil. A well graded
soil has approximately equal proportions of particle sizes and the
curve is usually smooth. A poorly graded soil may contain a high
proportion of material within a narrow band.
Activity: A grading test undertaken on a sample of sand gave the
following results tabulated below. Draw the particle size
distribution curves for the sample, determine the effective size,
uniformity coefficient of the sample and state the type of
curve.
BS sieve size
(mm)Mass retained (grams)% retainedCumulative %
retainedCumulative %
passing
6.30-
2.3676
1.1870
0.6075
0.30200
0.1575
0.0754
500
Using the particle size distribution curve describe the soil
using the following classification tablesClassification tables
Consistency Limits (Atterberg Limits)
These stages describe the consistency of the soil
Which in turn relate to its engineering properties
The moisture content at which the soil becomes plastic is
defined as the Plastic Limit (P.L.)The moisture content at which
the soil changes from plastic to liquid is defined as the Liquid
Limit (L.L.).The range of moisture content over which the soil
remains in a plastic condition is defined as the Plasticity Index
(P.I.).
TESTS TO DETERMINE LIQUID AND PLASTIC LIMITS PLASTIC LIMITThe
moisture content is determined at which a thread of soil can be
rolled without breaking until it is only 3mm in diameter.LIQUID
LIMIT
By cone penetrometer.
The classification of fine soils is based on soil
plasticity.
Plasticity is a function of a fine soils capacity to absorb
water and remain in a cohesive state. If water is added to a coarse
soil, the water will fill the voids and saturate the sample, and
further water will simply drain off and not be absorbed by the
soil. If water is added to a fine soil, water will initially fill
the voids.
When the soil is saturated it will continue to absorb water due
to the properties of the clay minerals and an increase in volume of
the soil mass will occur.
At the same time the soil is progressively softened by the water
which increases the distance between the clay minerals and
decreases the attracting forces between them, thus decreasing the
cohesion. As the soil gets weaker it becomes pliable and when it is
sufficiently pliable to be rolled out into a thread, it is said to
be in a plastic state.
Eventually the soil loses all its strength and starts to flow
under its own weight, when it is said to be in a liquid state.Use
of the A Line Classification chart The classification chart for
fine soils divides the soil according to ranges of liquid limit,
& plasticity index.
Generally the higher the Liquid Limit the higher the plasticity
is said to be. Silts and organic soils have a low Plasticity Index
(i.e. a small range of moisture content over which they are
plastic) compared to their Liquid Limit. Clays have a high
Plasticity Index in relation to their Liquid Limit.
The clay minerals have the capacity to take in moisture and
still retain some cohesion.To classify a fine soil according to the
chart:i) Determine Liquid and Plastic Limits, and therefore
Plasticity Index.ii) Using values of PI and LL, plot the soil on
the chart.iii) Observe which segment the soil comes into (CL, CI,
ML etc)iv) Write down the soil name, CLAY for C soils, SILT for M
soils.v) Follow the name with the plasticity i.e. CI = CLAY of
intermediate plasticity.
Example: Use the A Line Classification chart to classify the
following soilCone Penetration Test
Agricultural soil (topsoil overlying subsoil)
Ground Level
?m
solid particles
water
air
Idealised Model of Soil Structure
Typical Soil Structure
Agricultural soil (topsoil overlying subsoil)
At any stage the water table could be encountered upper zone of
the soil which is fully saturated
Bedrock
?m
Bedrock
Engineering soil
?m
Engineering soil
Pyknometer
Boring made from ground level
The upper level of the water table is the level at which water
would stand
Excavation
Gas jar
Plastic
Liquid
WATER
mass
EMBED Equation.DSMT4
s =
SUPERSTRUCTURE
SUBSTRUCTURE
EMBED Equation.DSMT4
EMBED Equation.DSMT4
325g
where : w = moisture content (expressed as decimal)
w = moisture content (expressed as percentage)
EMBED Equation.DSMT4
EMBED Equation.DSMT4
V
SOLID
AIR
EMBED Equation.DSMT4
EMBED Equation.DSMT4
EMBED Equation.DSMT4
EMBED Equation.DSMT4
M
V
EMBED Equation.DSMT4
EMBED Equation.DSMT4
EMBED Equation.DSMT4
SAND
mass
EMBED Equation.DSMT4
WATER
w1= mass of container (g)
w2 = mass of container & wet soil (g)
w3 = mass of container & dry soil (g)
AIR
Activity: Calculate the moisture content for the following:
EMBED Excel.Sheet.8
Ms
EMBED Equation.DSMT4
SOLID
Flocculated structure
WATER
AIRR
SOLID
Dispersed structure
Mw
EMBED Equation.DSMT4
85mm
50mm
Shrinkage limit, SL
Liquid limit, LL
Plastic limit, PL
Boundaries between the states are known as the consistency
limits
EMBED Equation.DSMT4
Moisture content below which no further reduction in volume will
occur
Measured by slowly drying a sample out & periodically
measuring its volume & mass
Graph of moisture content vs volume
Empirical boundary defined as the moisture content at which a
3mm diameter thread of soil can be rolled by hand without breaking
up
Empirical boundary defined as the moisture content at which the
soil is assumed to flow under its own weight measured using a cone
penetrometer or Cassegrande cup
Plasticity index, PI is the range of moisture content PL &
LL
PI=LL-PL
Semi-Solid
Solid
Moisture content increases
Cohesive soil
0 10 20 30 40 50 60 70 80 90 100 110 120
70
60
50
40
30
20
10
6
0
Liquid limit (%)
ML
MV
MH
MI
ME
CE
CH
CV
CI
CL
Plasticity index (%)
Upper plasticity range (U)
CI
Extremely high plasticity (E)
Very high (V)
High (H)
Intermediate(I)
Low (L)
A-line
CI
0 10 20 30 40 50 60 70 80 90 100 110 120
SILT
CLAY
70
60
50
40
30
20
10
6
0
Liquid limit (%)
ML
EMBED Excel.Sheet.8
EMBED Excel.Sheet.8
To classify a fine soil according to the chart:
i) Determine Liquid and Plastic Limits, and therefore Plasticity
Index.
w1= mass of container (g)
w2 = mass of container & wet soil (g)
w3 = mass of container & dry soil (g)
EMBED Equation.DSMT4
MV
Calculate the moisture content
EMBED Equation.DSMT4
PI= 40 13.5 = 26.5%
= 13+14/2 = 13.5%
EMBED Excel.Sheet.8
Plastic Limit Test
PI=LL-PL
Plasticity index
Plastic limit = average of results
EMBED Equation.DSMT4
MH
MI
ME
CE
CH
CV
CI
CL
Plasticity index (%)
Upper plasticity range (U)
Extremely high plasticity (E)
Very high (V)
High (H)
Intermediate(I)
Low (L)
A-line
SILT
CLAY
CI = CLAY of intermediate plasticity.
PI= 26.5%
LL=40%
ii) Using values of PI and LL, plot the soil on the chart.
iii) Observe which segment the soil comes into (CL, CI, ML
etc)
iv) Write down the soil name, CLAY for C soils, SILT for M
soils.
v) Follow the name with the plasticity
_1263829410.unknown
_1263906760.unknown
_1263961112.xlsSheet1
Testw1w2w3Cone penetration (mm)moisture content %
110.320.11817.227
211.21815.821.248
3101915.723.858
412.221.817.530.181
Sheet1
0
0
0
0
moisture content %
Moisture content (%)
Cone Penetration (mm)
Sheet2
Testw1w2w3moisture content %
A10.226.324.413
B13.225.323.814
Sheet3
_1263961765.xlsSheet1
Testw1w2w3Cone penetration (mm)moisture content %
110.320.11817.227
211.21815.821.248
3101915.723.858
412.221.817.530.181
Sheet1
0
0
0
0
moisture content %
Moisture content (%)
Cone Penetration (mm)
Sheet2
Testw1w2w3moisture content %
A10.226.324.413
B13.225.323.814
Sheet3
_1263961766.unknown
_1263961114.unknown
_1263961764.unknown
_1263961111.xlsSheet1
Testw1w2w3Cone penetration (mm)moisture content %
110.320.118.017.227
211.218.015.821.248
310.019.015.723.858
412.221.817.530.181
Sheet1
0
0
0
0
moisture content %
Moisture content (%)
Cone Penetration (mm)
Sheet2
Testw1w2w3moisture content %
A10.226.324.413
B13.225.323.814
Sheet3
_1263961110.unknown
_1263830096.unknown
_1263830098.unknown
_1263829519.unknown
_1263829406.unknown
_1263829408.unknown
_1263829409.unknown
_1263829407.unknown
_1263829404.unknown
_1263829405.unknown
_1263829402.unknown
_1263829403.unknown
_1263829122.xlsSheet1
Testw1w2w3Cone penetration (mm)moisture content %
18.118.215.517.236
27.316.213.721.239
37.717.714.823.841
46.916.113.230.146
Sheet1
0
0
0
0
moisture content %
Moisture content (%)
Cone Penetration (mm)
Sheet2
Testw1w2w3moisture content %
A16.526.624.329
Sheet3
_1263829401.unknown
_1263829121.unknown