This document provides an introduction to geotechnical engineering and soil structure. It discusses how geotechnical engineering is involved in foundations, retaining structures, slope stability, and pavement design. Soil is described as a complex and unpredictable material compared to other structural materials like steel and concrete. The major types of soil deposits in India are outlined as alluvial, black cotton, laterite, desert, and marine soils. Field identification methods for cohesion and particle sizes are also summarized.

1. INTRODUCTION SOIL STRUCTURE

2. INTRODUCTION TO GEOTECHNICAL
ENGINEERING:
 Geotechnical Engineering is a term which includes Soil Mechanics,
Rock Mechanics and Geology.
 Soil Mechanics deals with action of forces on soil and with flow of
water on soil.
 Rock Mechanics deals with mechanics of rock.
 Geology deals with the study of Earth, the rocks of which it is
composed, and the processes by which they change.
 Out of the above 3, we mainly deal with Soil Mechanics for this
course.

3. APPLICATIONS TO CIVIL ENGINEERING:
A) Foundations:
 Every civil engineering structure – building or bridge or dam, it is
founded on or below the surface of earth.
 Foundations are required to transmit the load of the structure safely to
the soil.
 Foundation Engineering is an important branch in Geotechnical
Engineering.

5. B) Retaining Structures:
 When sufficient space is not available for a mass of soil to spread and
form a safe slope, a structure is required to retain the soil.
 Geotechnical Engineering gives the theories of Earth Pressure on
retaining structures.

6. C) Slope Stability:
 Whether natural slope or man nade slope – to prevent landslides,
slope stability is important.
 Geotechnical Engineering provides methods to check stability of
slopes.

7. D) Pavement Design:
 A pavement is a hard crust placed on soil for the purpose of providing
a smooth and strong surface on which the vehicles can move.
 Pavement consists of subgrade, sub base course, base course and
surface course.
 Behaviour of subgrade on various loading and environmental
conditions are studied in Geotechnical Engineering.
PAVEMENT DETAILS

8. COMPLEXITY OF SOIL STRUCTURE:
 Soil is a highly complex material.
 It differs from conventional structural materials such as steel and
concrete.
 Properties of steel can be accurately controlled and properties of
concrete can be controlled to some extent during its preparation.
 Soil is a material which has been subjected to changes of nature and
we cannot control that.
 Thus, soil is a heterogeneous and unpredictable material.
 Properties of soil also change with change in environmental, loading
and drainage conditions.
 Main engineering properties of steel and concrete are Elastic Modulus
(stress/strain), Tensile strength and Compressive Strength.
 Most of the design work can be done if these above properties are
known.

9.  However, engineering properties of soils depend on a lot of factors and
it is not possible to characterize them by 2 or 3 parameters.
 Detailed testing is required to determine the characteristics of soil and
only then design can be done.
 Steel and concrete can be inspected before use, soils for foundations
are at great depth and not open to inspection.
MAJOR SOIL DEPOSITS OF INDIA:
Soil deposits of India can be classified into 5 major groups.
A) Alluvial Soil:
 A large part of North India is covered with Alluvial soils.
 The distinct characteristics of Alluvial deposits is the existence of
alternating layers of sand, silt and clay.
 Thickness of each layer depends on the local terrain and nature of
floods in the rivers causing deposition.

10. B) Black Cotton Soil:
 A large portion of central India and a portion of South India are
covered with Black Cotton Soils.
 These soils are residual deposits formed from basalt rocks.
 These soils are clays of high plasticity.
 They have high shrinkage and swelling properties.
 Shear strength, Bearing capacity of this soil are extremely low.
C) Laterite Soil:
 Laterite soils are formed by decomposition of rock, removal of bases
and silica and accumulation of Iron oxide and Aluminium oxide.
 Presence of Iron oxide gives these soils the characteristic red or pink
colour.
 These are residual soils formed from basalt.
(Basalt: Most common type of solidified lava; a dense dark grey fine-
grained igneous rock)

11.  These soils exist in Central, Southern and Eastern India.
 These are soft and can be cut with a chisel when wet but they become
hard as time passes.
D) Desert Soil:
 A large part of Rajasthan and adjoining states is covered with sand
dunes.
 In this area, arid conditions exist with little rainfall. (Arid: No water)
 Size of the particles is in the range of fine sands.
 They are uniform in gradation.
 (Gradation): Proportion of material of each grain size present in a soil.
E) Marine Soil:
 They contain a large amount of organic matter.
 They are present along the coast.
 They have very low shear strength, highly plastic,highly compressible.

12. FIELD IDENTIFICATION OF SOILS:
Cohesion:
It is the bonding force or attractive fore between the particles of fine
grained soils that creates shear strength.
Sizes of soil:
Gravel – Between 80 mm and 4.75 mm.
Sand – Between 4.75 mm and 75 microns.
Silt – Between 75 microns and 0.002 mm.
Clay – Finer than 0.002 mm.

13. Thank You