Soil mechanics1

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Chapter 1: Basic characteristics of soils
- The nature of soils.
- Particle size analysis.
- Plasticity of fine soil.
- Soil description and classification.
- Phase relationships.
- Soil compaction.
- Field compaction

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The nature of soils
Soil is any uncemented or weakly cemented
accumulation of mineral particles formed by
the weathering of rocks, the void space
between the particles containing water and/or
air.

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Residual soil
The products of weathering remain at their
original location.
Transported soil
If the products are transported and deposited in
a different location. The agents of
transportation being gravity, wind, water and
glaciers.
During transportation the size and shape of
particles can undergo change and the particles
can be sorted into size ranges.

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Soil Formations and Deposits
The resultant soil particles retain the same
composition as that of the parent rock.
Soil Formations from
rock
Physical Chemical

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 Unloading
– e.g. uplift, erosion, or change in fluid pressure.
 Thermal expansion and contraction
 Alternate wetting and drying
 Crystal growth, including frost action
 Organic activity
–e.g. the growth of plant roots.
(Mitchell, 1993)
Physical processes

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The final products due to weathering

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 Hydrolysis
– is the reaction with water
–will not continue in the static water.
–involves solubility of silica and alumina
Chelation
–Involves the removal of metal ions.
Cation exchange
– is important to the formation of clay minerals
Oxidation and reduction.
Carbonation
–is the combination of carbonate ions such as the
reaction with CO2
Chemical Process

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Chemical weathering results in the formation
of groups of crystalline particles of colloidal
size (<0:002 mm) known as clay minerals.
The basic structural units of most clay minerals
are a silicon–oxygen tetrahedron and an
aluminium–hydroxyl octahedron

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Clay mineral particles consist of stacks of
these layers, with different forms of bonding
between the layers. The structures of the
principal clay minerals are represented in:
(a) Kaolinite
(b) Illite
(c) Montmorillonite

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Kaolinite
- structure based :
Single sheet of silica combined with
a single sheet of gibbsite.
- The bond
Hydrogen bonding relatively strong
- A kaolinite particle may consist of
over 100 stacks.

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Illite
- structure based :
single sheet of gibbsite combined
with two sheet of silica.
- The bond
non-exchangeable potassium ions
Hydrogen bonding relatively weak.
- In the silica sheet there is partial
substitution of silicon by
aluminium.

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Montmorillonite
- structure based :
single sheet of gibbsite combined
with two sheet of silica.
- The bond
non-exchangeable potassium ions
Hydrogen bonding relatively weak.
- In the gibbsite sheet there is partial substitution of
aluminium by magnesium and iron, and in the silica
sheet there is again partial substitution of silicon by
aluminium.

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Forces of repulsion and attraction
The surfaces of clay mineral particles carry
residual negative charges.
The negative charges result in cations present
in the water in the void space being attracted to
the particles.
Forces of repulsion and attraction act between
adjacent clay mineral particles. Repulsion
occurs between the like charges of the double
layers, the force of repulsion depending on the
characteristics of the layers.

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Dispersed structure
If there is net repulsion the
particles tend to assume a face-
to-face orientation.
Flocculated structure
If there is net attraction the
orientation of the particles
tends to be edge-to-face or
edge-to-edge.

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In turn these elementary aggregations combine to form
larger assemblages, the structure of which is
influenced by the depositional environment.
Two possible forms of particle assemblage, known as
- Bookhouse structures
-turbostratic structures

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Assemblages can also occur in the form of
connectors or a matrix between larger particles. An
example of the structure of a natural clay, in
diagrammatical form

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PARTICLE SIZE ANALYSIS
The particle size analysis of a soil sample
involves determining the percentage by mass of
particles within the different size ranges.
The particle size distribution of a coarse soil
can be determined by the method of sieving.
The mass of soil retained in each sieve is
determined and the cumulative percentage by
mass passing each sieve is calculated.

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Particle sizes in soils can vary from over
100mm to less than 0.001 mm. In British
Standards the size ranges detailed

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A coarse soil is described as well graded if there is
no excess of particles in any size range and if no
intermediate sizes are lacking. In general,
represented by a smooth, concave distribution
curve.
Well-graded soil:

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Uniform soil:
A coarse soil is described as poorly graded soil if a
high proportion of the particles have sizes within
narrow limits.
Gap-graded or step-graded soil:
large and small sizes are present but with a
relatively low proportion of particles of
intermediate size.
Poorly graded soil:

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The effective size D10
The size such that 10% of the particles are
smaller than that size.
Other sizes such as D30 and D60 can be defined
in a similar way.

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The general slope and shape of the distribution
curve can be described by means of the
coefficient of uniformity (CU) and the
coefficient of curvature (CZ), defined as
follows:
The higher the value of the coefficient of
uniformity the larger the range of particle sizes in
the soil.

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Plasticity of fine soil
Plasticity is an important characteristic in the case of
fine soils.
The term plasticity describing the ability of a soil to
undergo unrecoverable deformation without cracking
or crumbling.
In general, depending on its water content (defined as
the ratio of the mass of water in the soil to the mass of
solid particles)

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The liquid limit (LL, wL):
The upper limits of the range of water content over
which the soil exhibits plastic behavior. Or the change
of consistency from plastic to liquid state.
Also, the minimum moisture content at which the soil
will flow under its own weight. The moisture content
(in %) required to close a distance of 12.7mm along the
bottom of the groove after 25 blows.

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The plastic limit (PL, wP):
The lower limits of the range of water content over
which the soil exhibits plastic behavior. Or the change
of consistency from brittle/crumbly to plastic state.
Also, the moisture content (in %) at which the soil
when rolled into threads of 3.2mm in diameter,
crumbles. PL is the lower limit of the plastic stage of
the soil. The test is simple and performed by repeated
rolling of ellipsoidal size soil mass by hand on a
ground glass plate.

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Shrinkage Limit (SL):
The moisture content (in %) at which the volume
change of the soil mass ceases.
SL = [(m1-m2)/m2] x100 x [(Vi-Vf)ρw/m2]x100

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The plasticity index (PI, IP)
The water content range itself:
The liquidity index (IL):
The natural water content (w) of a soil relative to the
liquid and plastic limits can be represented.

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Soil description and classification
Soil description
Includes details of both:
- Material characteristics:
The color of the soil and the shape, texture and
composition of the particles.
- Mass characteristics:
compactive state (coarse soils) or stiffness (fine
soils) and details of any bedding, discontinuities and
weathering.

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Soil classification
Soil is allocated to one of a limited number of groups
on the basis of material characteristics using Standered
System.

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SW – well-graded SAND
SCL – very clayey SAND (clay of low plasticity)
CIS – sandy CLAY of intermediate plasticity
MHSO – organic sandy SILT of high plasticity.

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Soil A consists of 100% coarse material (76% gravel size;
24% sand size) and is classified as GW: well-graded, very
sandy GRAVEL.
Soil B consists of 97% coarse material (95% sand size; 2%
gravel size) and 3% fines. It is classified as SPu: uniform,
slightly silty, medium SAND.

43. Soil C comprises 66% coarse material (41% gravel size; 25%
sand size) and 34% fines (wL 26.25, IP 9.25, plotting in the CL
zone on the plasticity chart).
The classification is GCL: very clayey GRAVEL (clay of low
plasticity). This is a till, a glacial deposit having a large range
of particle sizes.
Soil D contains 95% fine material: the liquid limit is 42 and
the plasticity index is 18, plotting just above the A-line in the
CI zone on the plasticity chart.
The classification is thus CI: CLAY of intermediate plasticity.