Chapter I: Introduction

Chapter 1
1.1 Soil Mechanics – is the branch of
science that deals with the study of the
physical properties of soil and the behavior
of soil masses subjected to various types
of forces.
Soil mechanics is the study of the
engineering behavior of soil when it is
used either as a construction material or
as a foundation material.
1.2 Geotechnical Engineering is the subdiscipline
of civil engineering that involves natural
materials found close to the surface of the
earth. It includes the application of the
principles of soil mechanics and rock
mechanics to the design of foundations,
retaining structures, and earth structures.
LEANING TOWER OF PISA
1.3 Foundation engineering applies the
knowledge of geology, soil mechanics, rock
mechanics, and structural engineering to the
design and construction of foundations for
buildings and other structures.

The most basic aspect of foundation engineering

deals with the selection of the type of
foundation, such as using a shallow or deep
foundation system.
Another important aspect of foundation
engineering involves the development of
design parameters, such as the bearing
capacity or estimated settlement of the
foundation.

Foundation engineering could also include the

actual foundation design, such as determining
the type and spacing of steel reinforcement in
concrete footings.
1.4 Soil means, the loose unconsolidated
inorganic material on the earth’s crust
produced by the disintegration of rocks,
overlying hard rock with or without organic
matter.

Soil – defined as an uncemented

aggregates of mineral grains and decayed
organic matter ( solid particles ) with liquid
and gas in the empty spaces between the
solid particles.
Formation of Soils
Most of the soils that cover the earth are
formed by the weathering of various rocks.

Two general types of weathering:

1. Mechanical Weathering
2. Chemical Weathering
1. Mechanical weathering is the process by
which rocks are broken into smaller and
smaller pieces by physical forces, including
running water, wind, ocean waves, glacier ice,
frost, and expansion and contraction caused
by the gain and loss of heat.
2. Chemical weathering is the process of
chemical decomposition of the original rock.

Example:
chemical weathering of feldstar can
produce clay minerals.

Most rock weathering is a combination of

mechanical and chemical weathering
Feldspar
Soil produced by the weathering of rocks can be
transported by physical processes to the other
places. The resulting soil deposits are called
transported soils.

Some soils stay where they were formed and

cover the rock surface from which they derive.
These soils are referred to as residual soils.
Transported soils can be subdivided into five
major categories based on the transporting
agent:
1. Gravity transported soil
2. Lacustrine(lake)deposits
3. Alluvial or fluvial soil deposited by
running water
4. Glacial deposited by glaciers
5. Aeolian deposited by the wind
In addition to transported and residual soils,
there are peats and organic soils, which derive
from the decomposition of organic materials.
General types of soils:
1. Residual Soils – are found in areas where the
rate of weathering is more than the rate at
which the weathered materials are carried away
by transporting agents. The rate of weathering is
higher in warm and humid regions compared to
cooler and drier regions.

The nature of a residual soil deposit will

generally depend on the parent rock. When hard
rocks, such as granite and gneiss, undergo
weathering, most of the materials are likely to
remain in place.
These soil deposits generally have a top layer
of clayey or silty clay material, below which
are silty or sandy soil layers. These layers in
turn, are generally underlain by a partially
weathered rock, and then sound bedrock.

Residual soils that derive from chemical rocks

do not possess a gradual transition zone to the
bedrock.Chemical rocks such as limestone,
that are chiefly made up of calcite ( CaCo3 )
mineral.
The residual soils derived from the weathering
of limestone-like rocks are mostly red in color.

The residual soils immediately above the

bedrock may be normally consolidated. Large
foundations with heavy loads may be
susceptible to large consolidation settlements
on these soils.

Residual soils on a steep natural slope can move

slowly downward, and this is usually referred
to as creep.
2. Transported Soils
2.1 Gravity Transported Soil
Mud flows are one type of gravity
transported soil. In this case, highly saturated,
loose sandy residual soils, on relatively flat
slopes, move downward like a viscous liquid
and come to rest in a more dense condition.
The soil deposits derived from past mud flows
are highly heterogeneous in composition.
2.2. Alluvial Deposits
Alluvial soil deposits derive from the action of
streams and rivers and can be divided into two
major categories:
a) braided-stream deposits
b) deposits caused by the meandering belt
of streams.
a) Deposits from Braided Streams
Braided streams are high-gradient, rapidly
flowing streams that are highly erosive and
carry large amounts of sediment. Because of
the high bed load, a minor change in the
velocity of flow will cause sediments to
deposits. By this process, theses streams may
build up a complex tangle of converging and
diverging channels, separated by sandbars
and islands.
The deposits formed from braided streams are
highly irregular in stratification and have a
wide range of grain sizes. These deposits share
several characteristics:

1. The grain sizes usually range from gravel

to silt. Clay-sized particles are generally not
found in deposits from braided streams.
 2. Although grain size varies widely, the
soil in a given pocket or lens is rather uniform.

3. At any given depth, the void ratio and

unit weight may vary over a wide range within
a lateral distance of only a few meters.
 b) Meander Belt Deposits
The valley floor in which a river
meanders is referred to as the meander belt.
In a meandering river, the soil from the bank is
continually eroded from the points where it is
concave in shape and is deposited at points
where the bank is convex in shape. These
deposits are called point bar deposits, and
they usually consist of sand and silt-sized
particles.
Sometimes, during the process of erosion and
deposition, the river abandons a meander and
cuts a shorter path. The abandoned meander,
when filled with water, is called an oxbow
lake.

During floods, rivers overflow low-lying areas.

The sand and silt-size particles carried by the
river are deposited along the banks to form
ridges known as natural levees.
Finer soil particles consisting of silts and clays
are carried by the water farther onto the
floodplains. These particles settle at different
rates to form backswamp deposits, often
highly plastic clays.

2.3. Lacustrine Deposits

Water from rivers and springs flows into
lakes. In arid regions, streams carry large
amounts of suspended solids. Where the
stream enters the lake, granular particles are
deposited in the area forming a delta.
Some coarser particles and the finer particles;
that is, silt and clay, that are carried into the
lake are deposited onto the lake bottom in
alternate layers of coarse-grained and fine-
grained particles. The deltas formed in humid
regions usually have finer grained soil deposits
compared to those in arid regions.
2.4. Glacial Deposits
The glaciers carried large amounts of sand,
silt, clay, gravel and boulders.
Drift is a general term usually applied to
the deposits laid down by glaciers.
Unstratified deposits laid down by melting
glaciers are referred to as till.
The landforms that developed from the
deposits of till are called moraines.
A terminal moraine is a ridge of till that
marks the maximum limit of a glacier’s
advance.
 Recessional moraines are ridges of till
developed behind the terminal moraine at
varying distances apart.
The till deposited by the glacier between
the moraines is referred to as ground moraine.
The sand, silt, and gravel that are carried
by the melting water from the front of a
glacier are called outwash.
The melted water deposits the outwash,
forming outwash plains, also called
glaciofluvial deposits
2.5. Aeolian Soil Deposits
Loess is an aeolian deposit consisting of silt
and silt-sized particles. The grain-size
distribution of loess is rather uniform.

The cohesion of loess is generally derived

from a clay coating over the silt-sized
particles, which contributes to a stable soil
structure in an unsaturated state.
The cohesion may also be the result of the
precipitation of chemicals leached by
rainwater. Loess is a collapsing soil, because
when the soil becomes saturated, it loses its
binding strength between particles.
3. Organic Soil

Organic soils are usually found in low-lying

areas where the water table is near or above
the ground surface. The presence of a high
water table helps in the growth of aquatic
plants that, when decomposed, form organic
soil.
This type of soil deposit is usually encountered
in coastal areas and in glaciated regions.
Organic soils show the following
characteristics:
1. Their natural moisture content may
range from 200 to 300%
2. They are highly compressible
3. Laboratory tests have shown that, under
loads, a large amount of settlement is derived
from secondary consolidation.