Geology and Soil Mechanics (Soil Classification) : - Titiksha Negi B.Tech (Ce) 4 SEM 1016568

GEOLOGY AND
SOIL MECHANICS
(SOIL
CLASSIFICATION)
-TITIKSHA NEGI
B.TECH (CE)
4th SEM
1016568
INTRODUCTION
The term "soil" can have different meanings, depending upon the field in which it is
considered.
To a geologist, it is the material in the relative thin zone of the Earth's surface within
which roots occur, and which are formed as the products of past surface processes.
The rest of the crust is grouped under the term "rock".
To a pedologist, it is the substance existing on the surface, which supports plant life.
To an engineer, it is a material that can be:
built on: foundations of buildings, bridges
built in: basements, tunnels
built with: embankments, roads, dams
supported: retaining walls
SOIL MECHANICS
 Soil Mechanics is a discipline of Civil Engineering involving the study
of soil, its behaviour and application as an engineering material.
 Soil Mechanics is the application of laws of mechanics and hydraulics
to engineering problems dealing with sediments and other
unconsolidated accumulations of solid particles, which are produced
by the mechanical and chemical disintegration of rocks, regardless of
whether or not they contain an admixture of organic constituents.
 Soil consists of a multiphase aggregation of solid particles, water, and
air. This fundamental composition gives rise to unique engineering
properties, and the description of its mechanical behavior requires
some of the most classic principles of engineering mechanics.
 Engineers are concerned with soil's mechanical properties. These
depend primarily on the nature of the soil grains, the current stress, the
water content and unit weight
FORMATION OF SOIL
 In the Earth's surface, rocks extend upto as much as 20 km depth. The major
rock types are categorized as igneous, sedimentary, and metamorphic.
 Igneous rocks: formed from crystalline bodies of cooled magma.
 Sedimentary rocks: formed from layers of cemented sediments.
 Metamorphic rocks: formed by the alteration of existing rocks due to heat

from igneous intrusions or pressure due to crustal movement.
 Soils are formed from materials that have resulted from the disintegration of
rocks by various processes of physical and chemical weathering. The nature
and structure of a given soil depends on the processes and conditions that
formed it:
 Breakdown of parent rock: weathering, decomposition, erosion.
 Transportation to site of final deposition: gravity, flowing water, ice, wind.
 Environment of final deposition: flood plain, river terrace, glacial ,marine.
 Subsequent conditions of loading and drainage: little or no surcharge, heavy

surcharge due to ice or overlying deposits, change from saline to freshwater,
leaching, contamination.
 All soils originate, directly or indirectly, from different rock types.
SOIL TYPES
 Soils as they are found in different regions can be classified into two broad
categories:
(1) Residual soils
(2) Transported soils
 Residual Soils
Residual soils are found at the same location where they have been formed.
Generally, the depth of residual soils varies from 5 to 20 m.
 Chemical weathering rate is greater in warm, humid regions than in cold, dry regions
causing a faster breakdown of rocks. Accumulation of residual soils takes place as
the rate of rock decomposition exceeds the rate of erosion or transportation of the
weathered material. In humid regions, the presence of surface vegetation reduces the
possibility of soil transportation.
 As leaching action due to percolating surface water decreases with depth, there is a
corresponding decrease in the degree of chemical weathering from the ground
surface downwards. This results in a gradual reduction of residual soil formation with
depth, until unaltered rock is found.
 Residual soils comprise of a wide range of particle sizes, shapes and composition.
SOIL CLASSIFICATION
 It is necessary to adopt a formal system of soil description and classification
in order to describe the various materials found in ground investigation. Such
a system must be meaningful and concise in an engineering context, so that
engineers will be able to understand and interpret.
 It is important to distinguish between description and classification:
 Description of soil is a statement that describes the physical nature and state
of the soil. It can be a description of a soil sample. It is arrived at by using
visual examination, simple tests, observation of site conditions, geological
history, etc.
 Classification of soil is the separation of soil into classes or groups each
having similar characteristics and potentially similar behaviour. A
classification for engineering purposes should be based mainly on mechanical
properties. The class to which a soil belongs can be used in its description.
 The aim of a classification system is to establish a set of conditions which
will allow useful comparisons to be made between different soils. The system
must be simple. The relevant criteria for classifying soils are the size
distributionof particles and the plasticity of the soil.
PARTICLE SIZE DISTRIBUTION
 For measuring the distribution of particle sizes in a soil sample,

it is necessary to conduct different particle-size tests.
 Wet sieving is carried out for separating fine grains from coarse
grains by washing the soil specimen on a 75 micron sieve mesh.
 Dry sieve analysis is carried out on particles coarser than 75
micron. Samples (with fines removed) are dried and shaken
through a set of sieves of descending size. The weight retained
in each sieve is measured. The cumulative percentage quantities
finer than the sieve sizes (passing each given sieve size) are then
determined.
 The resulting data is presented as a distribution curve with grain
size along x-axis (log scale) and percentage passing along y-
axis (arithmetic scale).
 Sedimentation analysis is used only for the soil fraction finer
than 75 microns. Soil particles are allowed to settle from a
suspension. The decreasing density of the suspension is
measured at various time intervals.
 In this method, the soil is placed as a suspension in a jar filled
with distilled water . The soil particles are then allowed to settle
down. The concentration of particles remaining in the
suspension at a particular level can be determined by using a
hydrometer. Specific gravity readings of the solution at that
same level at different time intervals provide information about
the size of particles that have settled down and the mass of soil
remaining in solution.
 The results are then plotted between % finer (passing) and log
size.
GRAIN SIZE DISTRIBUTION
 The size distribution curves, as obtained from coarse and fine

grained portions, can be combined to form one
complete grain-size distribution curve (also known
as grading curve). A typical grading curve is shown.
 From the complete grain-size distribution curve, useful
information can be obtained such as:
 1. Grading characteristics, which indicate the
uniformity and range in grain-size distribution.
2. Percentages (or fractions) of gravel, sand, silt and

clay-size.
GRADING CHARACTERISTIC
 A grading curve is a useful aid to soil description. The

geometric properties of a grading curve are
called grading characteristics.
CONSISTENCY OF SOIL
 The consistency of a fine-grained soil refers to its firmness, and it varies
with the water content of the soil.
 A gradual increase in water content causes the soil to change
from solid to semi-solid to plastic to liquid states. The water contents at
which the consistency changes from one state to the other are
called consistency limits (orAtterberg limits).
 The three limits are known as the shrinkage limit (WS), plastic limit
(WP), and liquid limit (WL) as shown. The values of these limits can be
obtained from laboratory tests.
 Two of these are utilised in the classification of fine
soils:
Liquid limit (WL) - change of consistency from plastic

to liquid state
Plastic limit (WP) - change of consistency from
brittle/crumbly to plastic state
 The difference between the liquid limit and the plastic
limit is known as the plasticity index (IP), and it is in
this range of water content that the soil has a plastic
consistency. The consistency of most soils in the field
will be plastic or semi-solid.
DETERMINATION OF LIQUID
LIMIT:-
DETERMINATION OF PLASTIC
LIMIT:-
INDIAN STANDARD SOIL
CLASSIFICATION SYSTEM
 Classification Based on Grain Size
The range of particle sizes encountered in soils is very large:
from boulders with dimension of over 300 mm down to clay
particles that are less than 0.002 mm. Some clays contain
particles less than 0.001 mm in size which behave as colloids, i.e.
do not settle in water.
 In the Indian Standard Soil Classification System (ISSCS), soils
are classified into groups according to size, and the groups are
further divided into coarse, medium and fine sub-groups.
The grain-size range is used as the basis for grouping soil particles
into boulder, cobble, gravel, sand, silt or clay.
Gravel, sand, silt, and clay are represented by group symbols G,

S, M, and C respectively.
Physical weathering produces very coarse and coarse soils.
Chemical weathering produce generally fine soils.
NEED AND SCOPE OF ENGINEERING
GEOLOGY AND SOIL MECHANICS
 The basic objectives of this course are-
 1. It enables a civil engineer to understand engineering
implication of certain condition related to area of
construction, which are essentially geological in nature.
 2. It enables a geologist to understand the nature of
geological information that is absolutely essential for a
safe design and construction of a civil engineering
projects.
TH A NK
…. . !
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GEOLOGY AND

 Sedimentary rocks: formed from layers of cemented sediments.

 Metamorphic rocks: formed by the alteration of existing rocks due to heat

 Transportation to site of final deposition: gravity, flowing water, ice, wind.

 Environment of final deposition: flood plain, river terrace, glacial ,marine.

 Subsequent conditions of loading and drainage: little or no surcharge, heavy

 For measuring the distribution of particle sizes in a soil sample,

 The size distribution curves, as obtained from coarse and fine

2. Percentages (or fractions) of gravel, sand, silt and

 A grading curve is a useful aid to soil description. The

Liquid limit (WL) - change of consistency from plastic

Gravel, sand, silt, and clay are represented by group symbols G,

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