GEOTECH - Soil Formation

Soil Formation –Origin of soil and grain size

Soils

formed by weathering of rocks due to mechanical disintegration of chemical decomposition.

geologic cycle

it consists of erosion, transportation, deposition and upheaval of soil

residual soil or sedentary soil

If the soil stays at the place of its formation just above the parent rock

transported soil

When the soil has been deposited at a place away from the place of its origin,

Residual Soil

have a gradual transition from relatively fine material near the surface to large fragments of stones at greater depth.

Transported Soil

Deposits of transported soils are quite thick and are usually uniform.

Three types of rocks

• Igneous Rock

• Sedimentary Rock

• Metamorphic Rock

Transportation of Soil

(1) Water Transported Soils

(2) Wind Transported Soils

(3) Glacier-Deposited Soils

(4) Gravity-Deposited Soils

(5) Soils transported by combined action

Flowing water

one of the most important agents of transportation of soils

alluvial deposits

All type of soils carried and deposited by water are known as

lacustrine deposits.

Deposits made in lakes are called

Marine deposits

formed when the flowing water carries soils to ocean or sea.

aeolian deposits

Soils deposited by wind are known as

Large sand dunes

It occurs in arid regions and on the leeward side of sea with sandy beaches

Loess plateau

These deposits have low density, high compressibility, low bearing capacity, and high permeability

Glaciers

are large masses of ice formed by the compaction of snow.

Drift

a general term used for the deposits made by glaciers directly or indirectly

till

Deposits directly made by melting of glaciers are called

Colluvial soils

such as talus, have been deposited by the gravity

Talus

consists of irregular, coarse particles.

four constituent parts of Soil

• Gravels

• Sand

• Silts

• Clays

Gravels

pieces of rocks with occasional particles of quartz, feldspar, and other minerals

Sand

a particles made of mostly quartz and feldspar

Silts

are the microscopic soil fractions that consist of very fine quartz grains and some flake-shaped particles that are fragments of micaceous minerals

Clays

mostly flake-shaped microscopic and submicroscopic particles of mica, clay minerals, and other minerals.

organizations have developed particle-size classifications

1. Massachusetts Institute of Technology (MIT)

2. U.S. Department of Agriculture (USDA)

3. American association of State Highway and Transportation Officials (AASHTO)

4. Unified Soil Classification System (USCS)

Mechanical analysis

the determination of the size range of particles present in a soil, expressed as a percentage of the total dry weight.

two methods in determining the particle size distribution of soil

1.SieveAnalysis

2.Hydrometer analysis

Sieve Analysis

used for particle sizes larger than 0.075 mm in diameter

Hydrometer analysis

used for particle sizes smaller than 0.075 mm in diameter

Effective size (D10)

This parameter is the diameter in the particle-size distribution curve corresponding to 10% finer. The effective size of a granular soil is a good measure to estimate the hydraulic conductivity and drainage through soil

Uniformity coefficient (Cu)

This parameter is defined as ratio of the D60 to D10.

Coefficient of gradation (Cc):

This parameter is defined as ratio of the (D30)2 to the product of D60 andD10

Sorting coefficient (S0):

This parameter is another measure of uniformity and is generally encountered in geologic works

Grading of Soil

Is the distribution of particles of different sizes in a soil mass. The grading can be determined from the particle distribution curve.

Curve I

represents a type of soil in which most of the soil grains are the same size. This is called poorly graded soil .

Curve II

represents a soil in which the particle sizes are distributed over a wide range, termed well graded. A well-graded soil has a uniformity coefficient greater than about 4 for gravels and 6 for sands, and a coefficient of gradation between 1 and 3 (for gravels and sands). A flat S curve represents a soil which contains the particles of different sizes in good proportion

Curve III

represents a soil might have a combination of two or more uniformly graded fractions. This type of soil is termed gap graded. A curve with a hump in which some of the intermediate size particles are missing.

Uses of Particle Size Distribution Curve

1. It is used in the classification of coarse-grained soil.

2. The coefficient of permeability of a coarse-grained soil depends to a large extent on the size of the particles.

3. The particle size is used to know the susceptibility of a soil to frost action.

4. The curve is required for the design of drainage filters.

5. The particle size distribution provides an index to the shear strength of the soil. Well-graded has high shear strength.

6. The compressibility can be judged from its particle size distribution curve. A uniform soil is more compressible than a well-graded soil.

7. The particle size distribution curve is useful in soil stabilization and for the design of pavement.

8. The particle size distribution curve may indicate the mode of deposition of a soil.

9. The particle size distribution curve of a residual soil may indicate the age of the soil deposit.

three major categories of Particle Shape

1. Bulky

2. Flaky

3. Needle shaped

1. Bulky

are formed mostly by mechanical weathering of rock and minerals. Geologists use such terms as angular, sub angular, sub rounded, and rounded to describe the shapes of bulky particles.

2. Flaky

have very low sphericity—usually 0.01 or less. These particles are predominantly clay minerals.

3. Needle shaped

are much less common than the other two particle types. Examples of soils containing needle shaped particles are some coral deposits and attapulgite clays.