MODULE 1 - KTU NOTES
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Transcript of MODULE 1 - KTU NOTES
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MODULE 1
GEOLOGY
INTRODUCTION TO GEOLOGY
The word ‘Geology’ is derived from the Greek words ‘GEO’ which means Earth
and ‘LOGOS’ which means science. GEOLOGY is the science that deals with the study
of the Earth as a planet. It deals with:
Origin, age and structure of Earth.
Evolution, modification and extinction of various surface and sub-surface
physical features like mountains, plateaus, plains, valleys, basin, caves,
etc.
Materials making up the earth
Nature and functioning of atmosphere
The study of all water bodies existing on surface or underground
Interaction of atmosphere, lithosphere and hydrosphere
Physical, dynamic and physicochemical processes operating on and within
the earth
Agents and forces involved and evolved in such processes
RELEVANCE OF GEOLOGY IN CIVIL ENGINEERING
Scope of Engineering Geology
Engineering geology is the application of geology in design, construction and
performance of civil engineering works. The objectives of engineering geology are:
It enables a civil engineer to understand engineering implications of
certain conditions related to the area of construction, which are geological
in nature.
It enables a geologist to understand the nature of geological information
that is essential for a safe design and construction of a civil engineering
project.
The principle objective of an engineering geologist is the protection of life
and property against damage caused by various geological conditions.
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The area covered by Engineering geology includes geological hazards, geotechnical data,
material properties, landslide and slope stability, erosion, flooding, dewatering, seismic
studies etc. The most important role of an engineering geologist is to interpret the
landforms and earth processes, to identify potential geologic and related man-made
hazards that may impact civil engineering structures and human development.
The importance of Engineering geology in development can hardly be exaggerated. It
helps to:
Identify areas susceptible to failure due to geological hazards
Establish design specifications
Select the best site for engineering purposes
Select best engineering materials for construction
Recognize potential difficult ground conditions prior to detailed design and
construction
Engineering geology is the branch of applied sciences which deals with the application of
geology for a safe, stable and economic design and construction of a civil engineering
project. The application of geological knowledge in planning designing and construction
of big civil engineering projects is an absolute essential. Most of the civil engineering
projects involve some excavation of soils and rocks, or involve loading the Earth by
building on it. In some cases, the excavated rocks may be used as constructional material,
and in others, rocks may form a major part of the finished product, such as a motorway
cutting or the site of a reservoir. The feasibility, planning and design, construction and
costing, and safety of a project may depend critically on the geological conditions where
the construction will take place. The scope of engineering geology is best studied with
reference to major activities of the profession of a civil engineer which are:
a) Construction
b) Water Resource Development
c) Town and Regional Planning
(a) Geology in Construction Jobs
Full geological information about the site of construction or excavation and about the
natural materials of construction is of great importance, in all types of heavy construction
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jobs such as buildings, towers, tanks, dams and reservoir, highway bridges, traffic and
hydropower tunnels etc.
The aspect of geology has full relevance in all the three aspects of each construction i.e.
planning, designing and execution.
i) Planning
Following geological information is greatly useful in planning an engineering project.
Topographical maps: Such maps give details of relief features and are essential
to understand relative merits and demerits of all the possible sites for the proposed
structure.
These maps are useful in determining:
a) Presence of nature of slopes, size, contours.
b) Depth of valleys and gorges
c) Rate of change of elevation in various direction
Topographic map
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Topographic map of two hills
Hydrological Maps: These maps give broad details about the distribution and
geometry of the surface water channels and also the occurrence and depth
contours of groundwater.
Hydrological Map: Subernarekha basin
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These maps play an important role in planning of engineering projects. The
stability and cost of engineering structures are greatly influenced by surface water
and groundwater.
Geological Maps: It gives an idea of availability of construction materials in a
particular area. Petrological characters and structural disposition of rock types as
developed in the proposed area are depicted in geological maps.
Geological Map
Fracture and displacement that the rocks might have undergone in the past can be
obtained from these maps. These maps act as a guide for locating and limiting
exploratory operation (test holes etc.).
ii) Design
In many cases the geological characters and conditions are the main criteria considered
for the engineering design of an engineering project. For example, if a choice has to be
made between an earthen dam, a gravity dam and an arch dam at a particular site, the
ultimate type of dam chosen and its design would be based mostly on the geological
conditions of the site. Dams as high as Bhakra Dam and as long as Hirakud Dam of
India cannot be built across every river valley of the world. The final size, shape and
design parameters of the dam are fixed based on the profile of gorge or the valley and the
strength of rocks at the base and on embankments.
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Some of the geological characters that have a direct or indirect influence on the design of
a proposed project are:
The existence of hard bed rocks and their depth from and inclination with the
surface.
The mechanical properties like compressive strength, shear and transverse
strength, modulus of elasticity, porosity and permeability, resistance to decay and
disintegration along and across the site of the proposed project.
Presence, nature and distribution pattern of planes of structural weakness like
joints, faults, folds etc.
Presence nature and distribution pattern of zones of weak materials like shear
zones, clay bands etc.
The position of ground water table including points of recharge and discharge and
variations during different periods of the year.
Seismic character of the area as studied from the seismic history and prediction
about future seismicity.
iii) Construction
The engineer responsible for the quality control of the construction materials will derive
enormous benefit from his geological background of the nature material such as sand,
gravel, crushed rocks and soils.
Similarly for construction in geologically sensitive areas as those of coastal area, seismic
zones and permafrost regions, knowledge of geological history of the area is of great
importance.
a) Coastal area: behavior of rocks towards waves, current and marine environment
must fully be understood in planning and execution stage. Special type of
construction may become essential in this area.
b) Seismic region: Construction should be well balanced and light weight. Hence
lightweight materials are used and architectural fancies are to be avoided.
c) Permafrost region (soil remains permanently frost up to certain depth): Problems
can be solved only by proper understanding of the ground below.
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d) Construction of underground projects like tunnels cannot be undertaken without a
thorough knowledge of the geological characters and setting of the rocks and their
relevance to the loads imposed on or relieved from them
Rocks, being anisotropic do not behave according to empirical thumb rules. The
stability of structures constructed on or through or with rocks, depends on understanding
the nature of rocks.
(b) Geology in Water Resource Development
Exploration and development of water resources have become very important areas of
activities for scientists, technologists and engineers in all parts of the world. The water
resource engineers have to understand the water cycle in all essential details.
Water cycle
The study of water cycle is an essential prerequisite for effective planning and execution
of major water resource development programmers on national and regional level.
Geological information is of fundamental importance in exploration and exploitation of
water resources of a region from surface and sub-surface reserves of water. The water
bearing properties of rock bodies and factors that influence storage, movement and yield
of water from aquifers are geological problems. A thorough geological knowledge about
the rock strata is essential is essential for designing a water supply project.
(c) Geology in Town and Regional Planning
A town planner is concerned essentially with utilization of land in a best and aesthetic
manner possible for developing cities and towns for meeting social needs in different
areas. The primary aim of a town planner is to derive maximum benefit from natural
environment with minimum disturbance. Hence, he must possess a broad perspective of
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the nature and properties of all elements making the environment of the area under study.
The roles played by the materials making the land like rocks, soils, vegetation, water
bodies etc. in the evolution of natural landscape must be understood.
The regional Town Planner is responsible for adopting an integrated approach in all such
cases of allocation of land for developmental projects. Thus a change induced in the
natural setup of an area due to a proposed new project is going to lead a series of changes
in the adjoining and even in distant places.
In nature, nothing works in isolation. As such all sound planning must be in tune with the
natural features and processes of a region. For example an industrial township located in
the banks of a perennial river may provide natural drainage for effluents from industry but
is harmful to flora, fauna and human population located downstream.
SUBDIVISIONS OF GEOLOGY
Geology is a very vast subject which necessitates it to study under different subdivisions.
Following are the important branches of geology:
(a) Physical Geology
It deals with the origin, development and ultimate fate of various surface features of the
Earth with its internal structure. It also deals with the role played by internal agents like
volcanism and earthquakes and external agents like wind, water and ice on the physical
features of the earth. The configuration of rock bodies, water bodies and huge moving
deposits of ice on the surface and their structures also form important subjects of physical
geology.
(b) Geomorphology
It is a part of physical geology. It deals with the study of surface features of the earth,
mainly study of the land surface like mountains, plains, plateaus, valleys and basins and
various other landforms associated with them. The structure and evolution of these
landforms through space and time are also included in this branch.
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(c) Mineralogy
Mineralogy is that branch of geology, which deals with formation, occurrence,
aggregation, properties and uses of minerals. Mineralogy is divided into sub-branches like
crystallography, optical mineralogy and descriptive mineralogy and so on.
Crystallography deals with internal structure and external manifestations of minerals
occurring in crystallized form in the natural process or made from synthetic processes.
(d) Petrology
Formation of various types of rocks, their mode of occurrence, composition, textures and
structures, geological and geographical distribution on the earth are all studied under
petrology. It is subdivided into three distinct branches: Igneous petrology, Sedimentary
petrology and metamorphic petrology.
(e) Historical Geology
The past history of the Earth can be obtained from the study of rocks and features
associated with them. The study of rocks gives an idea about their nature and time of
formation, composition, constitution, magnetism and structural disposition and fossils. It
reveals a lot about the events that have passed since their formation. The details of
climate, biological and environmental conditions, prevailing just before, during and after
the formation of these rocks can be accurately estimated from the study. Paleo-geography,
paleontology and stratigraphy are three distinct subdivisions of Historical Geology.
(f) Economic Geology
This branch deals with the study of those minerals and rocks and other materials like
fuels, ores of all the metals and non-metals, building stones, salt deposits and industrial
minerals. Economic Geology deals with the mode of occurrence of these materials,
principles involved in their formation and accumulation, their properties, structural and
other controls that help to make their extraction economical.
In addition to the above major branches of geology, there are a few other new branches of
science in which geology makes a very important and basic component. A few of them
are:
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a) Geo-chemistry deals with:
Chemical constitution of the Earth as a planet and its various parts
Distribution and abundance of different elements and their compounds
Trends of reorganization in the elements of earth during its geological
history
b) Geo-physics:
Water and oil bearing strata below the Earth’s surface are identified by
geophysical principles and processes.
Application of physics is an absolute essential in areas of geo-magnetism,
geo-thermometry, geo-electricity, geo-cosmology, and seismology.
Study of interior of earth was done by applying knowledge of seismic
waves.
c) Geo-hydrology
Deals with geological aspects of groundwater and surface water bodies
(occurrence and movement through different types of rock).
d) Mining geology
Deals with exploration and exploitation of economic mineral deposits, for
locating mines.
e) Rock mechanics
Deals with the study of behaviour of rocks under various type of loads
imposed on them.
f) Geo-mechanics
Deals with the study of natural force fields as acting on the Earth on global
and regional levels.
g) Meteorology
Deals with the study of physical, chemical and biological aspects of
atmosphere.
h) Oceanography
Youngest of geological sciences dealing with physical, structural, genetic
and other aspects of oceans.
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i) Engineering geology
A new field of applied sciences that has developed due to interaction
between the civil engineering practice and geological sciences.
Deals with the application of geology for a safe, stable and economic
design and construction of a civil engineering project.
WEATHERING
Weathering is the process of decay and disintegration of rocks under the influence of
certain physical and chemical agencies of the atmosphere. The most important aspect of
weathering is that the weathered product remains lying over and above or near to the
parent rock unless it is removed from there by some other agency of the nature.
Examples:
(a) Rocks exposed to frost action at higher altitude in cold climates disintegrate into small
fragments. These fragments remain strewn over the slopes itself.
(b) Rocks exposed to high temperatures in deserts gradually disintegrate into smaller
pieces that remain close to the parent rock.
CLASSIFICATION OF WEATHERING
Weathering is classified mainly into two classes: 1. Mechanical/physical weathering and
2. Chemical weathering
1. Mechanical/physical weathering
It is a natural process of in-situ disintegration of rocks into smaller fragments and
practices through essentially physical processes without a change in their composition. It
is one of the most common geological processes of slow natural rock disintegration in all
parts of the world. The variations in temperature and organic activity are the two
important factors that bring about this change under specific conditions.
Temperature variations cause extensive mechanical weathering of rocks exposed on the
surface. This occurs mainly by two ways viz. frost action in cold regions and thermal
effects (insolation) in hot arid regions.
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(a) Frost action
In areas of cold and humid climates, temperature often falls below the freezing point of
water repeatedly during winter. As a result of this, the water within the pores, cracks,
fractures and cavities of rocks will freeze, which increases the volume of water by about
10%.The original openings and cracks are widened in the first stage, thereby
accommodating more and more water which will freeze in subsequent cycles.
(a)Water-filled crack (b)Freezes to ice (c)Breaks the rock
Fig 1.3: Frost action
A freezing cycle is often followed by a thawing cycling (melting of ice within
cavities).The repetition of freezing and thawing cycles over many years, results in the
exertion of internal stresses within the cavities leading to gradual disintegration of rocks.
This is depicted in figure.
Exudation is a process seen in porous rocks near seashore. It is similar to frost action. The
disintegration takes place due to formation of crystals of sodium chloride etc.
(b) Thermal Effects (Insolation)
In arid, desert and semi-arid regions, summer and winter temperatures differ
considerably. Similarly difference between day and night temperatures is also
considerable in such regions. Thus rocks (especially top layers) in those areas are exposed
to repeated variations in temperatures. This develops tensile stresses due to alternate
expansion and contraction. Hence the body of rock gradually breaks into smaller pieces.
For instance, in some deserts like Kara Qum, rocks are exposed to high temperatures as
70-80℃ in summer and are cooled down to -10℃ in winter. This extreme difference in
temperatures results in disintegration of rocks due to Insolation.
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Exfoliation: This type of weathering is found in deserts and other areas of temperature
extremes. In a thick rock body or where the rock is layered, upper layers are mostly
affected due to temperature variations. As a result, the upper layers peel off from the
underlying rock mass.
Exfoliation: Castle rock track
In many cases, such a change is accompanied by chemical weathering, developing curved
surfaces. The process of peeling off of curved shells from rocks under the influence of
thermal effects in association with chemical weathering is called exfoliation. In the figure
it could be seen that a large disc-shaped flake has slipped off the upper surface of the
large boulder on to the boulders beneath.
(c)Unloading
This is a kind of mechanical weathering where fractures develop in confined rock
masses due to removal of the overlying rock cover due to prolonged erosional
work of other agencies.
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Confined rock mass under stress
These rock masses remain confined from sides and uncovered from above. Due to
pressure relief from above, they expand in the upward direction.
Stresses are relieved due to unloading
As a result of this joints develop in the rock mass parallel to the uncovered surface
dividing them into sheets.
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Weathering continues
This rupture itself is a mechanical breakdown of rocks and it paves way for further
weathering along the joint planes.
2. Chemical weathering
Chemical weathering is a process of alteration of rocks of the crust by chemical
decomposition by atmospheric gases and moisture. The processes of chemical reactions
between the surfaces of rocks and atmospheric gases take place till a chemical
equilibrium is established.
Chemical weathering disintegrates the rocks in a number of ways depending upon the
mineralogical composition of rocks and the nature of chemical environment surrounding
them. The main processes of chemical weathering are solution, hydration and hydrolysis,
oxidation and reduction, carbonation, base-exchange and formation of colloids.
I. Solution
Some rocks contain one or minerals that are soluble in water to some extent. Rock salt,
gypsum and calcite are some of the minerals soluble in water. Some minerals are not
soluble in water. The solvent action of water for many common minerals is enhanced
when carbonated. For example limestone which is not soluble in water is soluble in
carbonated water.
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II. Hydration and hydrolysis
These two processes indicate the direct attack of atmospheric moisture on individual
minerals of a rock that affect its structural make up. When the surfaces of many crystals
(having partially unsatisfied valences) come in contact with polarized water molecules,
any one of the following reactions can occur:
i. The ions tend to hold the polarized side of water molecule and form a hydrate.
This process of addition of the water molecule is called hydration. For example:
CaSO4 + 2H2O CaSO42H2O
Anhydrite gets slowly converted to gypsum by hydration as shown above.
ii. The process in which exchange of ions occur whereby water enters into the crystal
lattice of mineral, is called hydrolysis.
K + AlSi3O8 + H+ H Al Si3 O8 + K
+
Weathering of Orthoclase (K + AlSi3O8) occurs as shown above.
III. Oxidation and reduction
Iron bearing minerals and hence rocks are prone to oxidation and reduction. The effects
are observed from colour changes produced in iron bearing rocks. For example:
4Fe + 3O2 2Fe2O3 (brown colour)
Fe2O3+H2O Fe2O3.H2O
Ferrous iron (Fe++) is oxidized to ferric iron (Fe +++) when exposed to air rich in
moisture. Ferric iron is oxidized to stable ferric hydroxide.
Iron oxide in rocks and minerals reduces to elemental iron in presence of decaying
vegetation, which supplies carbonaceous content causing reduction.
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IV. Carbonation
It is the process of weathering of rocks under the combined action of atmospheric CO2
and moisture. The carbonic acid formed corrodes silicate bearing rocks. For example:
2KAlSi3O8 + 2H2O+ CO2 Al2Si2O5(OH)4 + K2CO3+ 4SiO2
Felspar orthoclase decomposes to form a clay mineral, a soluble bicarbonate and silica.
V. Colloid formation
The processes of hydration, hydrolysis, oxidation and reduction on rocks and minerals
often result in splitting of particles into smaller particles called colloids. These colloids
are characterized by atoms with only partially satisfied electrical charges. Weathering of
clay minerals, silica and iron oxides often results in colloid formation. These colloids are
easily precipitated as their charges are satisfied and form stable products.
SPHEROIDAL WEATHERING
As the name indicates, the breaking of original rock mass into spheroidal blocks is called
spheroidal weathering. It is caused by the combined action of mechanical and chemical
weathering.
Spheroidal weathering
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The original rock mass is split into small blocks by development of parallel joints due to
thermal effects. Simultaneously, chemical weathering corrodes the border and surfaces of
blocks causing their shapes roughly into spheroidal contours.
ROLE OF PLANTS AND ORGANISMS – Organic weathering
Hydrogen ions (H+) are released at the roots of plants during their growth and
metabolism. These ions replace K+, Ca
2+, Mg
2+ ions etc. from the rocks surrounding the
root system. Thus these rocks and minerals undergo decomposition. Root systems of big
plants and trees creep into the pre-existing cracks in the nearby rocks. Thus the cracks
widen and the rocks break into fragments. Action of rodents on rocks will also cause the
disintegration of rocks. Man has been breaking the rocks since very beginning for many
purposes. The decay and disintegration of rocks by living things is called organic
weathering.
FACTORS AFFECTING WEATHERING
Following are the factors which affect weathering:
(i)Nature of rock
Some rocks are easily affected by weathering, while some remain unaffected even under
the same conditions. Chemical composition of rocks is a major factor which determines
the stability of rock in a given environment. For example, sandstone is highly resistant to
weathering as it is made up of quartz (SiO2) which is a highly weathering resistant
mineral. Granites, on the other hand undergoes chemical decay to a large extend.
The rate of weathering of a rock mass which has a massive, compact and dense structure
will be slow as that compared to that having a fractured structure. It is because the gases
and moisture find easy pathways into the body of rocks through these fractures.
(ii)Climate
Same types of rocks exposed in three or more types of climates may show entirely
different trends of weathering. Cold and humid conditions favour both mechanical and
chemical weathering. Dry and cold climates, on the other hand will not favour any type of
weathering due to absence of moisture.
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Chemical weathering will be predominant in hot and humid climate, while mechanical
weathering is predominant in hot and arid areas (due to expansion and contraction of
rocks).
(iii)Physical environment
The rate of weathering is greatly affected by the topography of the area where rocks are
directly exposed. Rocks in slopes assist in removal of the weathering end product
comparatively faster and make fresh rock surface exposed to weathering. In case of rocks
in level lands, the weathered products accumulate over the parent rock and slow down
further disintegration.
PRODUCTS OF WEATHERING
The product of weathering include (a) Regolith (b) Mineral and rock formation
(a)Regolith: It includes all the weathered material, which covers the parent rock or is
lying close to it. These materials deposit on the surface of the parent rock in huge
thickness. In many cases, the weathering of rocks becomes slow after the formation of
weathered layers at the top.
Regolith
It is because the overlying cover acts as a barrier for the atmospheric agencies to further
act on the parent rock. The upper part of regolith is termed a soil. Regolith has been
broadly used to express:
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(i)Eluvium: It is the end product of weathering that happens to lie over and above the
parent rock. Eluvium forms a thin or thick layer on the parent rock, depending on the
duration for which weathering has been operative on it. Regolith is another term for
eluvium.
(ii)Deluvium: It is the end product of weathering that has been moved to some distance
after its formation due to weathering. The weathered products get deposited at the base of
the slope and form heaps of various thickness. Gravity and rain-wash are the major agents
which remove these weathered products to some distance.
Weathering results in the formation of a few minerals and rocks. These include:
(i)Clay minerals: Weathering of silicate rocks under humid climatic conditions, results in
the formation of Montmorrilonite, Kaolinite and Illite. Montmorrilonite is formed by the
hydration of volcanic dust in semi dry climates. Hydration and carbonation of igneous
rocks under humid climates form Kaolinite.
(ii)Ores of Aluminium: The weathering of clay rocks produces ores of aluminium like
bauxite (Al2O3.nH2O) and laterite.
ENGINEERING SIGNIFICANCE OF WEATHERING
Soil is the ultimate product of weathering. A clear knowledge of the genetic
background of soils is required for the better understanding of engineering
properties of soil. It helps in proper planning and design of Engineering projects
built on soil or rocks.
When foundations are carried down to the bed rock, knowledge of degree of
weathering, depth of weathered cover and the trend of weathering in that area is of
utmost importance for the ultimate safety of the project.
For a construction engineer it is necessary to find out:
a. The extent to which the area for the proposed project has deteriorated due
to weathering. It is necessary to remove loose weathered materials and
carry foundation to solid rock.
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b. The effect of weathering on construction materials to be used in the
project. This helps to select construction materials that are more durable to
weathering.
Chemical weathering breaks the bonds between the rocks that make the slope,
causing the instability of slope. The slope rocks lose shearing strength and will
finally fail. Therefore, slope stability must also ensure protection of slope rocks
from weathering.
The response of stones (like marbles, limestone and granite) to chemical
environment must carefully be studied by the civil engineers prior to
recommending them for major constructions. Disfiguring, pitting, honeycombing
and loss of surface appearance are common effects of chemical weathering on
stones.
LABORATORY TESTS USED IN CIVIL ENGINEERING FOR ASSESSING
INTENSITY OF WEATHERING
The process of weathering weakens the rocks. Hence it is of great concern. The actual
behaviour of a rock mass subjected to a change in stress is governed by the mechanical
properties of the rock material and the geologic discontinuities like faults, joints, fissures
etc.
1. Rock Quality Designation (RQD)
It is frequently used to indicate the quality of rock mass. RQD is defined as the sum total
of lengths of the cores of length 10cm and longer recovered from the drilling, expressed
as a percentage of the total length of the hole drilled (Deere et al, 1967). Thus,
RQD = (Total length of cores in pieces of 10cm and longer x 100)/Length of run
The above equation is for core sticks of diameter 57.2mm. It is a measure of degree of
fracturing and amount of weathering in the rock mass.
RQD Rock quality
100-90 Excellent
90-75 Good
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75-50 Fair
50-25 Poor
Table 1.1: Quality of rocks
2. Unconfined compression test
It is the commonly used strength test on rocks. Test specimen shall preferably be a right
circular cylinder with length to diameter ratio 2 to 3. If the ratio is less than 2, usual
correction shall be applied taking standard slenderness ratio as 2. The ends of the
specimen should be flat, smooth and parallel. The ends should be exactly perpendicular to
the axis of the cylinder. Cores obtained are trimmed for the purpose.
A suitable loading machine shall be used for applying and measuring the axial load to the
specimen. Discs made of steel having a hardness of not less than 30 HRC shall be placed
at specimen ends. With abrasive rocks, these discs surfaces tend to roughen after a
number of specimens have been tested and hence need to be resurfaced from time to time.
Unconfined compression apparatus (proving ring type)
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The specimen, the discs and the spherical seat shall be accurately centered with respect to
one another and the loading frame. The curvature center of the seat surface should
coincide with the center of the top surface of the specimen.
The specimen is subjected to compression between the cross-head and the plate of a
compression testing machine. The compressive strength is given by peak load divided by
initial area of cross-section of the specimen i.e. qu = P/A.
The apparatus consists of a small load frame fitted with a proving ring to measure the
vertical stress applied to the specimen. The deformation of the sample is measured using
a separate dial gauge.
a. The ability of spherical seat to rotate freely shall be checked before each test.
b. The surfaces of the two bearing discs and the test specimen shall be wiped clean.
c. The specimen shall be kept on the lower disc.
d. The axis of the specimen shall be carefully aligned with the center of the thrust of
the spherical seat.
e. As the load is gradually brought to bear on the specimen, the movable portion of
the spherically seated disc shall be adjusted to ensure uniform seating.
f. Load on the specimen shall be applied continuously at a constant stress rate such
that failure will take place in about 5 to 15 minutes of loading.
g. Alternatively, the stress rate shall be within the limits of 0.5 MPa/s to 1 MPa/s.
h. The maximum load on the specimen shall be recorded in N within 1 percent
accuracy.
i. The number of specimens to be tested should be determined from practical
considerations, but at least five are required to obtain a representative value.
j. The unconfined compressive strength of the specimen shall be calculated by
dividing the maximum load carried by the specimen during the test, by the
average original cross-sectional area.
k. The report shall give uniaxial compressive strength for each specimen in the
sample, expressed to three significant figures, together with the average result for
the sample.
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3. Water absorption test
The sample shall be selected from the quarried stone or taken from the natural rock. The
test pieces shall be crushed or broken, and the material passing 20-mm IS sieve and
retained on 10-mm IS Sieve shall be used for the test.
The apparatus consists of cylindrical measuring glass jars capacities of 1000 ml and 100
ml (shall have graduation mark of 1 ml ) capacity, glass vessel of about l.5 litre capacity
and two dry absorbent cloths of 0.5 m2 area each, a balance of capacity 3 kg with an
accuracy of 1 g, a desiccator and an oven.
a. The test piece weighing about 1 kg shall be washed to remove particles of dust
and immersed in distilled water in a glass vessel at room temperature 20 to 30°C
for 24 hours.
b. Soon after immersion and again at the end of soaking period, entrapped air shall
be removed by gentle agitation achieved by rapid clock-wise and anti-clock-wise
rotation of the vessel.
c. The vessel shall then be emptied and the test piece be allowed to drain.
d. The test piece shall then be placed on a dry cloth and gently surface dried with the
cloth.
e. It shall be transferred to a second dry cloth when the first one removes no further
moisture.
f. The sample shall then be weighed.
g. The sample shall then be carefully introduced in the 1000 ml capacity measuring
cylinder and distilled water shall be poured by means of I00 ml capacity
measuring cylinder in the larger cylinder while taking care to remove entrapped
air, until the level of water in the larger cylinder reaches 1000 ml mark.
h. The quantity of water thus added shall be recorded in ml or expressed in gram
weight
i. The water in the larger cylinder shall be drained and the sample shall be carefully
taken out and dried in an oven at 100 to 110°C for not less than 24 hours.
j. It shall then be cooled in a desiccator to room temperature and weighed.
k. The room temperature during the test shall be recorded.
The water absorption shall be calculated from the following formula:
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Water absorption =
where A = weight of oven-dry test piece in g and
B = weight of saturated surface-dry test piece in g.
The water absorption shall be expressed as percentage by weight of oven-dry sample and
shall be the average of three determinations.
4. Test for Abrasion of Coarse Aggregates by the Use of Los Angeles Machine
The principle of Los Angeles abrasion test is to find the percentage wear due to the
relative rubbing action between the aggregate and steel balls used as abrasive charge.
Pounding action of these balls also exists during the test and hence the resistance to wear
and impact is evaluated by this test.
The Los Angeles machine consists of a hollow cylinder closed at both ends, having inside
diameter 70 cm and length 50 cm and mounted so as to rotate about its horizontal axis.
The abrasive charge consists of cast iron spheres of approximate diameter 4.8 cm and
each of weight 390 to 445 g. The number of spheres to be used as abrasive charge and
their total weight have been specified based on grading of the aggregate sample. The test
has been standardized by the ISI.
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The specified weight of aggregate specimen (5 to l0 kg, depending on gradation) is placed
in the machine along with the abrasive charge. The machine is rotated at a speed of 30 to
33 rpm for the specified number of revolutions (500 to 1000 depending on the grading of
the specimen). The abraded aggregate is then sieved on 1.7 mm IS sieve and the weight of
powdered aggregate passing this sieve is found.
The result of the abrasion test expressed as the percentage wear or the percentage passing
1.7mm sieve expressed in terms of the original weight of the sample. The Los Angeles
abrasion value of good aggregates acceptable for cement concrete, bituminous concrete
and other high quality pavement materials should be less than 30 percent. Values up to 50
percent are allowed in base courses like water bound and bituminous macadam. This test
is more dependable than other abrasion tests as rubbing and pounding action in the test
simulate the field conditions better. Also correlation of Los Angeles abrasion value with
field performance and specifications of the test values have been established.
ENGINEERING CLASSIFICATION OF WEATHERED ROCK MASSES
Weathering of wall rock (rock constituting discontinuity surfaces) is classified in
accordance with the recommendations of The International Society for Rock Mechanics
(ISRM) Committee on rock classification (1981 b):
1. Unweathered/fresh: No visible signs of weathering can be observed. The rock is
fresh and crystals appear bright.
2. Slightly weathered rock: Discontinuities are stained or discoloured and may
contain a thin filling of altered material. Discolouration may extend into the rock
from the discontinuity surfaces to a distance of up to 20% of the discontinuity
spacing.
3. Moderately weathered rock: Slight discolouration extends from discontinuity
planes for greater than 20% of the discontinuity spacing. Discontinuities may
contain filling of altered material. Partial opening of grain boundaries may be
observed.
4. Highly weathered rock: Discolouration extends throughout the rock and the
rock material is partly friable (easily crumbled). The original texture of the rock
has mainly been preserved, but separation of the grains has occurred.
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5. Completely weathered rock: The rock is totally discoloured and decomposed
and in a friable condition. The external appearance is that of soil.
SOIL PROFILE
Soil is formed as a result of weathering of parent rock. Soil is a mixture of organic matter,
various minerals and water which can support plant life on the surface of earth. A vertical
section from the surface down to the bed rock reveals various layers of soil, which is
termed as soil profile. Pedologists have identified these layers of soils and have
designated them as different horizons.
During the development of soil from a parent material, the actual transformation proceeds
through certain well defined stages. In mature soils, these stages appear as a series of
horizons. Such horizons, when arranged in descending order, are collectively said to form
a Soil Profile for that particular area.
A typical Soil Profile, beginning from surface and proceeding downwards, is generally
made up of three main horizons (there may be many sub-horizons in each main horizon).
Fig 1.4: Soil profile
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(i) The A-Horizon: It is characterized by finely divided particles. It extends from a few
centimeters to as much as a meter or more. It contains loose leaves, incompletely
decomposed organic matter and good amount of humus in humid regions. In the basal
zone of A-horizon, leaching effects may be seen.
(ii) The B-Horizon: This zone lies immediately below the A-horizon. It is free from the
staining of particles by humus. In arid regions, it may contain nodules of calcium
carbonate or gypsum. Colloid accumulation is maximum in this zone. The lower region of
B-horizon becomes more pebbly and coarse indicating transition to the C-horizon. The A
and B horizon together form the true soil, called Solum.
(iii) The C-Horizon: It is more a zone of weathered rock. In texture, it is often coarse
grained and pebbly; in composition, it retains all the evidence of its parent rock.
(iv)The D-Horizon: In a true soil profile, a sample from this horizon is the parent rock
itself, unaltered as yet. In some cases, it may the solid rock mass on which the other zones
are resting.
GEOLOGICAL CLASSIFICATION OF SOIL
Based on the whether the soils are found over and above the rocks from which these are
derived or transported far away from the place of formation, soils may be classified into
two types: (a) Residual soil and (b) Transported soil.
a. Residual soil: Soils which get deposited at the site of their formation itself or has
suffered very little transport are called residual soils. These soils generally show
no stratification, but would show well-distinguished horizons of soil profile.
Thickness of such soils varies from place to place. Depending on climate,
lithology, topography and the extent of time for which the soil formation
processes have been operative in that region, the thickness of soil varies from
place to place.
The chemical composition of the residual soil depends on the nature of the parent
rock and trends of chemical activity are determined by climate of the area. The
residual soils in cold climate are rich in humus. The deep residual soils are highly
leached and are infertile. Examples: Red and black soils.
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b. Transported soil: These soils are deposited far away from their parent rock.
They drift from the place of origin to the site of deposition by means of various
geological agents like slopes of area (gravity), rivers, glaciers, wind, lake and
marine activities. These soils are classified on the basis of main the agent involved
in their transport and deposition and are grouped into:
(i)Colluvial soils: These soils are formed from the rock materials that accumulate
at or near the base of the steep mountains by the action of gravity. They are stony
in nature. Very few mountain plants can grow on it.
(ii)Alluvial soils: These soils are formed by the action of rivers and are confined
to river basins. These soils are very fertile. Indo-Gangetic alluvium plains belong
to this type. These are made up of fine material and are clearly stratified. These
soils are very fertile due to negligible leaching and heterogeneous nature.
(iii)Glacial soils: These soils are transported and deposited by glacial action.
Rock fragments, which are formed under the glacial action show angularity with
striations. These soils are not fertile.
(iv) Aeolian soils: Wind is a very active agent for transport of dust, silt and sand
grade particles. These soils are formed due to wind action. These consist mainly
of silt and clay. Some are fertile. Loess is an example.
(v)Lacustrine soils: These soils are formed from the accumulation of debris in
lakes and other bodies of standing water. It is rich in organic material and often
classed among the organic soils. The Karewas of Kashmir is famous for
supporting saffron crops that are partly lacustrine in nature.
INTERIOR OF THE EARTH
The real interior of earth is nowhere exposed to our direct observation. The most
important source of information for earth’s interior is the study of seismic waves
(released during earthquakes and nuclear shocks).
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Seismological evidence
Seismology deals with the study of elastic waves generated within the earth during an
earthquake. Some of the facts related to seismic waves which are relevant to the earth’s
interior are as follows:
i. The point of origin of earthquake below the surface of earth is called its focus. In
every earthquake, 3 types of elastic waves, namely P-waves, S-waves and L-
waves are generated. The P and S waves (body waves) travel through the body of
the earth and the L-waves (surface waves) are confined mostly to near the surface
of the earth.
ii. The P and S waves are considered to be important in the study of interior of earth.
These waves travel with characteristic velocities through different media. The
abrupt changes in velocities of the body waves during their travel from focus to
various seismographic stations on the earth are detected. These waves are
recorded on the surface of the earth after having passed through materials deep
within the earth. From the arrival times of these waves, many important
conclusions can be drawn regarding the nature of materials lying in their path.
iii. The velocity of any one type of these waves at any depth within the earth can be
calculated from the study of travel time curves of the waves from previous
earthquake records.
Fig 1.1: Velocity-depth curves
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iv. If sharp and prominent changes are repeatedly observed in the velocity depth
curves, from different records, it indicates major variations in the nature of the
medium at those respective depths below the surface. The extension of this
analysis from surface to center of the earth gives a generalized picture of the
earth’s interior.
The body waves reach earth’s surface after being reflected and refracted at various depths
below. If the nature of earth from the surface to the center were uniform, no change on
the velocity of seismic waves traveling through it would be recorded on the opposite end.
Conversely, a major change in seismic velocity indicates a change in nature of medium or
material at that particular depth. This major change in velocity of seismic waves is called
a seismic discontinuity. This is very important in interpreting the internal structure of
earth.
The two most significant seismic discontinuities are: The Mohorovicic discontinuity
and the Mantle-Core discontinuity which marks the three major zones in the
constitution of the Earth: the crust, the mantle and the core.
Fig 1.2: Interior of earth
The crust
The crust is the upper most shell of the Earth that extends to variable depths below the
mountains (75 km), continents (35 km) and oceans (5 km). The Mohorovicic
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discontinuity marks the lower boundary of the crust. The continental crust is further
divided into three layers: A, B and C. The A or upper layer is made up of sedimentary
rocks. The B or middle layer is made up of granites, gneisses and other related igneous
and metamorphic rocks. It is this layer that is exposed on the surface because the
overlying A layer has been removed due to prolonged erosion. The C layer or lowermost
layer is made of basic minerals which are rich in magnesium silicates and hence
sometimes named as SIMA (Si for silica and Ma for magnesium).
The mantle
It is the second concentric shell of the Earth that lies beneath the crust everywhere. From
the lower boundary of the crust it continues up to a depth of 2,900km.It is sub divided
into upper mantle and lower mantle. A part of upper mantle from 100km to 500km depth
is in a plastic state, and is named as asthenosphere. It is believed to be the source of
volcanic activities and other processes.
The core
It is the innermost concentric shell of the Earth, extending from 2900km to 6371km i.e. to
the center of the earth. It can be distinguished into two zones: the outer core and the inner
core. The outer core behaves like a liquid while the inner core behaves like a solid
metallic body made up of iron and nickel (hypothesis).
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