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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 soilThe products of weathering remain at their

original location.

Transported soilIf 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 samecomposition 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.

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.