311 GEOTECHNICAL ENGINEERING

1. FCE 311 GEOTECHNICAL ENGINEERING
3. SOIL FORMATION
Department of Civil & Construction Engineering
University of Nairobi

2. 3.1 Definition of Soil Formation
• Soil formation is the process by which soil
is created. The formation of soil happens
over a very long period of time. Soil is
formed from the weathering of rocks and
minerals.

3. 3.2 Weathering
• Weathering is the process of breaking
down rocks.
• Weathering occurs in situ or “with no
movement”, and thus should not be
confused with erosion, which involves
the movement of rocks and minerals by
agents such as water, ice, wind, and
gravity.

4. Physical Weathering
Involves the breakdown of rocks and soils
through direct contact with atmospheric
conditions, such as
• heat,
• water,
• ice and
• pressure,
This is without any change in chemical
condition.

5. Physical Weathering - Cont’d
The soil formed due to physical weathering
will be cohesionless (sand and gravel).
Physical agencies causing mechanical
weathering of rocks are;
• Daily and seasonal temperature changes.
• Flowing water, glaciers and wind, which
produce impact and abrasive action on
rock.
• Splitting action of ice.
• Growth of roots of plants in rock fissures,

6. Chemical Weathering
Chemical weathering changes the
composition of rocks by decomposing the
parent minerals, transforming them into
new compounds such as clay silica
particles, carbonates and iron oxides.
The decomposition of rock is the result of the
following reactions;
• Oxidation
• Carbonation
• Hydration
• Leaching

7. 3.3 Residual and Alluvial Soils
Residual Soils
• Are those which have remained over the
parent rock from which they have been
formed. They are relatively shallow in
depth. They are characterized by a
gradual transition from soil through
partially weathered rocks, fractured and
fissured rock, to bedrock.

8. Alluvial Soils
• Are the soils which have been
transported and subsequently deposited
by flowing water.
• An alluvial fan is formed when the
velocity of a soil-laden stream suddenly
deceases due to abrupt decrease in
gradient.
• Flood-plains are formed on the sides of a
stream due to overflowing of flood water.
A delta is formed just before a stream
reaches the standing water of the sea.