SOIL MECHANICS

BY:
ENGR. ZOHAIB KHALIQ

DEPARTMENT OF CIVIL ENGINEERING

INSTITUTE OF SOUTHERN PUNJAB
 CHAPTER NO. 1
“INTRODUCTION”

2
Soil Mechanics
 CHAPTER NO. 1 “INTRODUCTION”

Soil:
Soil is an unconsolidated (loose) agglomerate of minerals
with or without organic matter (plants and animals residue at
different stages of decomposition) found at or near the
surface of earth crust with which and upon which civil
engineers build their structures.
Soil Mechanics:
Soil mechanics is the branch of civil engineering technology
concerned with the study of soil and its behaviour under
different types of loads (external forces, temperature
changes, moisture variations etc.) .

3
Soil Mechanics
 CHAPTER NO. 1 “INTRODUCTION”

Geotechnical Engineering:
Geotechnical engineering is the branch of civil engineering
which deals with the analysis, design and construction of
foundations , slopes, retaining structures, embankments,
tunnels roads and other systems that are made of or
supported by soil or rock.

4
Soil Mechanics
 CHAPTER NO. 1 “INTRODUCTION”

Soil Formation:
In general, soils are formed by weathering of rocks. The
physical properties of soil are dictated primarily by the
minerals that constitute the soil particles and, hence, the
rock from which it is derived.

5
Soil Mechanics
 CHAPTER NO. 1 “INTRODUCTION”

Weathering:
Weathering is the process of breaking down rocks by
mechanical and chemical processes into smaller pieces.
(1) Mechanical weathering
Mechanical weathering may be caused by the expansion and
contraction of rocks from the continuous gain and loss of
heat, which results in ultimate disintegration. Frequently,
water seeps into the pores and existing cracks in rocks. As
the temperature drops, the water freezes and expands. The
pressure exerted by ice because of volume expansion is
strong enough to break down even large rocks. Other
physical agents that help disintegrate rocks are glacier ice,
wind, the running water of streams and rivers, and ocean
waves. It is important to realize that in mechanical
weathering, large rocks are broken down into smaller pieces
without any change in the chemical composition.

6
Soil Mechanics
 CHAPTER NO. 1 “INTRODUCTION”

(2) Chemical weathering

In chemical weathering, the original rock minerals are
transformed into new minerals by chemical reaction. Water
and carbon dioxide from the atmosphere form carbonic acid,
which reacts with the existing rock minerals to form new
minerals and soluble salts. Soluble salts present in the
groundwater and organic acids formed from decayed organic
matter also cause chemical weathering.
Chemical decomposition alters chemical composition may be
due to any one of following processes:
• Oxidation
It occurs in rocks containing iron. The oxygen in the air
reacts with ferrous matter and decomposes them. It is similar
to the rusting of steel.

7
Soil Mechanics
 CHAPTER NO. 1 “INTRODUCTION”

• Carbonation
The mineral containing iron, calcium, magnesium, sodium,
potassium etc. can be decomposed by carbonic acid which is
formed by carbon dioxide with water. Thus practically all
igneous rocks decompose in this manner.
• Hydration
Decay of rock caused by water combined with some rock
minerals is called hydration. This process is more intensive
in humid then in the arid climates.
• Leaching
Leaching is the process whereby water soluble salts are
dissolved and washed out from the soil by rainfall,
percolating water, surface run off or other water.
• Desilication
Desilication consists in leaching out dissolved silica freed in
the case of other chemical processes.

8
Soil Mechanics
 CHAPTER NO. 1 “INTRODUCTION”

Types of soil deposits:

(1) Residual Soils

The soils formed by the weathering at their place of origin are
called residual soils.
An important characteristic of residual soil is the gradation of
particle size. Fine-grained soil is found at the surface, and
the grain size increases with depth. At greater depths,
angular rock fragments may also be found.

9
Soil Mechanics
 CHAPTER NO. 1 “INTRODUCTION”

(2) Transported Soils

The transported soils may be classified into several groups,
depending on their mode of transportation and deposition:
• Glacial soils—formed by transportation and deposition of
glaciers
• Alluvial soils—transported by running water and
deposited along streams
• Lacustrine soils—formed by deposition in quiet lakes
• Marine soils—formed by deposition in the seas
• Aeolian soils—transported and deposited by wind
• Colluvial soils—formed by movement of soil from its
original place by gravity, such as during landslides
• Pyroclastic soils—Materials ejected from volcanoes and
transported by gravity, wind and air.

10
Soil Mechanics
 CHAPTER NO. 1 “INTRODUCTION”

Soil types w.r.t grain size:

Soil Type Description Average grain size

Coarse: 80 mm to 20 mm
Gravel Rounded and/or angular bulky hard rock
Fine: 20 mm to 4.75 mm

Coarse: 4.75 mm to 2 mm
Sand Rounded and/or angular bulky hard rock Medium:2 mm to 0.425 mm
Fine:0.425 mm to 0.075 mm

Particles smaller than 0.075mm, exhibit little or no

Silt 0.075 mm to 0.002 mm
strength when dried

Particles smaller than 0.002mm, exhibit significant

Clay < 0.002 mm
strength when dried: water reduces strength

11
Soil Mechanics
 CHAPTER NO. 1 “INTRODUCTION”

Soil Texture:
Texture of soil is its appearance or feel and it depends on the
relative sizes and shapes of the particles as well as the range
or distribution of those sizes.
Soils with respect to texture can be divided into two groups:
(1) Coarse textured soil
• Includes gravels, sands and their mixtures
• Classified on the basis of grain sizes
(2) Fine textured soil
• Includes clay and silt
• The presence of water affects the engineering properties

12
Soil Mechanics
 THE END

13
Soil Mechanics