LEARNING OUTCOMES1.Explain the soil characteristics related

to geo-environmental 2.Explain basic soil test

Soils are formed by the disintegration (or more precisely, evolution) of rock material of the earth’s relatively deeper crust, which itself is formed by the cooling of volcanic magma.

The stability of crystalline structure governs the rock formation.

As the temperature falls, new and often more stable minerals are formed. For instance, one of the most abundant minerals in soils known as quartz acquires a stable crystalline structure when the temperature drops below 573°C.

The intermediate and less stable minerals (from which quartz has evolved) lend themselves to easy disintegration during the formation of soils.

The disintegration process of rocks leading to the formation of soils is called weathering.

It is caused by natural agents; primarily wind and water (note that these are the same agents that aid the evolution and life in other kingdoms).

The specific processes responsible for weathering of rocks are:

i. Erosion by the forces of wind, water, or glaciers, and alternate freezing and thawing of the rock material.

ii. Chemical processes, often triggered by the presence of water. These include:  Hydrolysis (reaction between H- and OH- ions of water and the ions of the rock minerals),

Chelation (complexation and removal of metal ions),

Cation exchange between the rock mineral surface and the surrounding medium

Oxidation and reduction reactions, Carbonation of the mineral surface because of the

presence of atmospheric CO2.

iii. Biological processes which, through the presence of organic compounds, affect the weathering process either directly or indirectly.

Once the rock material is weathered, the resultant soil may either remain in place or may be transported by the natural agencies of water, air, and glaciers.

In the former case, the soils are called residual soils.

Depending on the natural agent involved, the transported soils are called alluvial or fluvial (water-laid), aeolian (wind-laid), or glacial (ice-transported) soils.

Several subdivisions are often made based on the transportation and deposition conditions.

Five independent variables may be viewed as governing soil formation:

Climate - Amount of moisture available, temperature, chemical reaction speed and rate of plant growth

Organisms present

- Organisms influence the soil's physical and chemical properties and furnish organic matter to soil

Topography - Angle: like Steep is poorly developed soils but flat to undulating surface is the best. Orientation (direction the slope is facing) - soil temperature and Moisture

The nature of the parent material

- Original mineral makeup and important in young soils. Residual soil–from bedrock. Transported soil–carried from elsewhere

Time - varies for soils in different climates, locations

Five mains groups of mineral composition in soil (regular structure elements and atomic elements) are :

i. CARBONATES

- calcite and dolomite usually use in cement

ii. OXIDES

iii.

HYDROUS OXIDES

– gibbsite and brucite minus OH’s sheet in clay minerals

iv.

PHOSPHATE – using for fertilizer

v. SILICATE – 90% of all soil

Silicates constitute well over 90% of the earth's crust.

The fundamental unit of all silicate structures is the SiO4 tetrahedron.

It consists of four O2- ions at the apices of a regular tetrahedron coordinated to one Si4+ at the center.

The individual tetrahedral are linked together by sharing O2- ions to form more complex structures.

Silica tetrahedron: The silica tetrahedron consists of four oxygen ions and one silica ion.

The molecular arrangement is such that the four oxygen ions are spaced at what would be the corners and tip of a three-dimensional, three-sided pyramid, with the silicon located within the pyramid.

Oxygen ions at the base are shared by adjacent tetrahedrons, thus combining and forming a sheet.

QUARTZQUARTZ

Commonly found in soil and the mineral composition SiO2.

The Quartz shape are in three dimensions and each of quartz cannot absorb in acid and cannot break easily.

There is no isomorphous substitution in quartz, and each silica tetrahedronis firmly and equally braced in all directions.

As a result, quartz has no planes of weakness and is very hard and highly resistant to mechanical and chemical weathering.

Quartz is not only the most common mineral in sand and silt-sized particles of soils, but quartz or amorphous silica is frequently present in colloidal (1 to 100 nm) and molecular (< 1 nm) dimensions.

some of the silicon atoms are replaced by aluminum. This results in a negative charge and in distortion of the crystal structure, because Al atoms are larger than Si atoms.

The negative charge is balanced by taking in cations such as K+, Na+, and Ca+ in orthoclase, albite, and anorthite feldspars, respectively. The distortion of the lattice and the inclusion of the cations cause cleavage planes that reduce the resistance of feldspars to mechanical and chemical weathering.

For these reasons, feldspars are not as common as quartz in the sand-, silt-, and claysized fractions of soils, even though feldspars are the most common constituent of the earth's crust.

Common micas such as muscovite and biotite are often present in the silt- and sand-sized fractions of soils.

In a unit sheet of mica, which is 1 nm thick, two tetrahedral layers are linked together with one octahedral layer.

In muscovite, only two of every three octahedral sites are occupied by aluminum cations, whereas in biotite all sites are occupied by magnesium.

In well-crystallized micas one fourth of the tetrahedral Si+4 are replaced by A1+3.

The resulting negative charge in common micas is balanced by intersheet potassiums. In a face-to-face stacking of sheets to form mica plates, the hexagonal holes on opposing tetrahedral surfaces are matched to enclose the intersheet potassiums.

The alumina octahedron consists of six-oxygen and one-aluminum.3 oxygen is in the top place of the octahedrons, and three are in the bottom plane. The aluminum is within the oxygen grouping. It is possible that the aluminum ion may be replaced with magnesium, iron, or other neutral ions. The aluminum sheet is 5 x 10-7mm thick. Oxygen from the tip of a silica tetrahedron can share an alumina sheet, thus layering sheets. Different sheet arrangements are then combined to form the different clay minerals. The composition and typical properties of the more commonly occurring clay minerals are Kaolinite, Illite and Montmorillonite

KAOLINITEKAOLINITE

is a common mineral in soils and is the most common member of this subgroup. A Kaolinite is the most prevalent clay mineral and is very stable, with little tendencies for volume change when exposed to water. Kaolinite layers are stack together to form relatively thick particle. Particles are plate shaped. The composition is one-silica, one alumina sheet that is very strongly bonded together. Kaolinites have very little isomorphous substitution in either the tetrahedral or octahedral sheets and most kaolinites are close to the ideal formula Al2Si2O5 (OH) 4.

Illite - has irregular plate shape, more plastic than kaolinites.

Its does not expand when exposed to water unless potassium deficiency exists. This clay is most prevalent in marine deposits.

The composition is an alumina sheet sandwiched between two silica sheets to form a layer. Potassium provides the bonds between the layers.

MONTMORILLONITEMONTMORILLONITE has irregular plate shapes or is fibrous because

of the weak bond between layers this clay readily absorbs water between layers.

This mineral has a great tendency for large volume change. The composition is an alumina sheet sandwiched between two silica sheets to form a layer.

Iron or magnesium may replace the alumina in the aluminum sheet.

The soil type or category is based on particle size, however, where the soil particle size is too small to be observed, an additional physical property, known as plasticity is utilized as a criterion for evaluation

Soil is all the material located above bedrock and can be grouped into four major categories or types including gravel, sand, clay and silt.

These four categories can be reduced to two groups termed coarse-grained soil and fine-grained soil.

Particle size and shape affects the mechanical behavior of soils, however, the effect of varies for coarse-grained and fine-grained soils.

The size and shape of the granular soil particles can increase or decrease the tendency of particles to fracture, crush and degrade.

The grading of gravels and sands may be qualified in the field as well graded (good representation of all particle sizes from largest to smallest).

Poorly graded materials may be further divided into uniformly graded (most particles about the same size) and gap graded (absence of one or more intermediate sizes).

Soil structure is the shape that the soil takes based on its physical and chemical properties; it is the geometric arrangement of soil particles with respect to one another.

The process of sedimentation or rock weathering creates the initial soil structure.

Among the many factors that effect soil structure is the shape, size, and mineral composition of the soil particles, and the nature and composition of soil water.

The basic terminology used to define the soil structure are single-grained, honeycombed, flocculated and dispersed with variations dependent upon the composition of the soil.

The particle arrangement of cohesionless soils (gravel, sand and silt) has been likened to arrangements attained by stacking marbles, or “single-grained”.

In single grained structures soil particles are in a stable position, with each particle in contact with the surrounding ones.

For similar sized particles large variations in the void ratio are related to the relative position of the particles.

The term cohesive is used for clay soils, which have an inherent strength, based on their particle structure which provides considerable strength in an unconfined state.

The cohesiveness of a clay is due to its’ mineralogy and is a controlling factor determining the shapes, sizes, and surface characteristics of a particle in a soil.

It determines interaction with fluids. Together, these factors determine plasticity,

swelling, compression, strength, and fluid conductivity behavior

Fine grained soils are identified on the basis of some simple tests for :

i. Dry strength

Dry strength is a qualitative measure of how hard it is to crush a dry mass of fine grained soil between the fingers. Clays have very high dry strength and silts have very low dry strength.

ii. Dilatancy Dilatancy is an indication of how quickly the moisture from a wet soil can be brought to the surface by vibration. In silty soils, within a few strikes water rises to the surface making it shine. In clays, it may require considerable effort to make the surface shiny. In other words, dilatancy is quick in silts and slow in clays.

iii. Toughness

Toughness is a qualitative measure of how tough the soil is near its plastic limit (where the soil crumbles when rolled to a 3 mm diameter thread). Toughness increases with plasticity. Silty soils are soft and friable (crumble easily) at Plastic Limits (PL), and clays are hard at PL. The fines can also be identified by feeling a moist pat; clays feel sticky and silts feel gritty. The stickiness is due to the cohesive properties of the fines, which is also associated with the plasticity, and therefore clays are called cohesive soils. Gravels, sands and silts are called granular soils.

The grain size distribution of a coarse grained soil is generally determined through sieve analysis, where the soil sample is passed through a stack of sieves and the percentages passing different sizes of sieves are noted.

The grain size distribution of the fines are determined through hydrometer analysis, where the fines are mixed with distilled water to make 1000 ml of suspension and a hydrometer is used to measure the density of the soil-water suspension at different times.

Limit Descriptioni. Shrinkage

limit which is the water content at which the soil passes from solid to semisolid state

ii. Plastic limit which is the water content at which transition from semisolid to plastic state takes place

iii. Liquid limit which indicates the water content required in order for the clayey soil to begin exhibiting flow characteristics like liquids

The BS 5930:1999 (Code of Practice for Site Investigations) summarizes the purposes of laboratory testing to be to describe and classify the samples, to investigate the fundamental behavior of the soils in order to determine the most appropriate method to be used in the analysis, and to obtain soil parameters relevant to the technical objectives of the investigation.

The laboratory tests for soils commonly carried out include:

• Moisture content, which read in conjunction with liquid and plastic limits gives an

• indication of undrained strength;

• Liquid and plastic limits to classify fine grained soil and the fine fraction of mixed

• soils;

Particle size distribution to give the relative proportions of gravel, sand, silt and clay;

Organic matter which may interfere with the hydration of Portland cement;

Mass loss of ignition which measures the organic content in soil, particularly peat;

Sulfate content which assesses the aggressiveness of the soil or groundwater to buried concrete;

pH value which is usually carried out in conjunction with sulfate contents tests;

California bearing ration (CBR) used for the design of flexible pavements;

Soil strength tests such as Triaxial compression, unconfined compression and vane shear;

Soil deformation tests; Soil permeability tests.