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Note for Engineering Geology - EG By JNTU Heroes

  • Engineering Geology - EG
  • Note
  • Jawaharlal Nehru Technological University Anantapur (JNTU) College of Engineering (CEP), Pulivendula, Pulivendula, Andhra Pradesh, India - JNTUACEP
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 The knowledge about the nature of the rocks in tunneling and construction of roads.  The foundation problems of dams, bridges and buildings are directly related with geology of the area where they are to be built.  The knowledge of ground water is necessary in connection with excavation works, water supply, irrigation and many other purposes.  The knowledge of Erosion, Transportation and Deposition (ETD) by surface water helps in soil conservation, river control.  Geological maps and sections help considerably in planning many engineering projects.  If the geological features like faults, joints, beds, folds are found, they have to be suitably treated. Hence, the stability of the rock structures is important.  Pre-geological survey of the area concerned reduces the cost of planning work. Minerals, Rocks and soils constitute earth materials. evaluation and operations in civil engineering practice. They play a vital role in the site Whether it is tunneling, hydro-electric projects, ground water development, foundation for structures, study of slope stability etc.. a basic understanding of the earth materials is essential. Thus, study of minerals, rocks and soils forms the first step in civil engg point of view. Hence, a civil engineer should know the introduction of Geology and its branches and importance of a few branches such as Physical Geology, Petrology; Structural Geology and so on …… IMPORTANCE OF GEOLOGY PHYSICAL GEOLOGY, PETROLOGY & STRUCTURAL IMPORTANCE OF PHYSICAL GEOLOGY: It deals with the geological processes which bring about changes in the crust and upon the surface of the earth. It also deals with the surface features of the earth ( land forms ) or its topography. The earth is 3|Page

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concentrically divided into a number of spheres viz., Atmosphere ; (2) Hydrosphere and (3) Lithosphere . (1) The outermost sphere is Atmosphere which consists of several gases and vapours and envelopes the earth. Atmosphere is essentially a mixture of N2 and O2 with smaller quantities of vapour, CO2 etc… Geologically atmosphere is important as the medium of climate and weather. Hydrosphere includes the natural waters of the earth ie., oceans, seas, lakes, rivers, streams and underground water. Lithosphere is the outer part of the earth’s crust consisting of rocks and minerals. The geological processes include Denudation, Deposition, Earth movements, Igneous activity and metamorphism. Denudation: The sum of the processes which result in the general lowering of the land surfaces or when erosion takes place, fresh country rock surfaces will be exposed and this process is called DENUDATION. Denudation consists of weathering, transportation and erosion. Weathering is the process by which rocks are broken down and decomposed by the action of external agencies such as wind, rain, temperature changes. Weathering is the initial stage in the process of denudation. Transportation is the main agency by which materials are moved by means of Gravity, running water ( rivers, streams ); Ice ( glaciers ); Wind etc.. Erosion: Mechanical disintegration or chemical decomposition of rocks and their subsequent displacement is called as erosion or erosion is the destructive process due to the effect of the transporting agents. The chief agents of erosion are running water, wind etc.. Deposition : The material is transported mechanically and deposit (eg: sand ). Earth movements include the uplift and depressions of land areas & sea floors. Igneous activity includes emission of lavas, gases, other volcanic products etc Metamorphism: The process by which changes are brought about in rocks within the earth’s crust by the agencies of Heat, Pressure and Chemical fluids. Thermal metamorphism : heat alone acts Dynamic metamorphism : involves stress to break up the rocks Regional/Dynamothermal metamorphism: Both heat & pressure involves Retrograde metamorphism : produces lower grade metamorphic rocks Auto Metamorphism : chemical adjustment in newly solidified igneous rocks, brought about by a decrease in temperature . 4|Page

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Geological works of Rivers A river is one of the major geological agent which carries out its work. mainly divided into three stages, namely 1. River Erosion 2. River Transportation 3. River Deposition The work is River Erosion: Erosion means mechanical disintegration or chemical decomposition of rocks are transported from the site with the help of natural agencies like wind and running water (or) subsequent displacement. River is a powerful eroding agent and carries out its work in different ways such as hydraulic action, solution and abrasion / attrition etc.  Hydraulic action: The physical breakdown of rocks take place naturally and greater the movement greater will be the erosion. In the initial and youth stages, the rivers acquire more considerable kinetic energy. When such water dashes against rock forcefully, it will break and this will be more effective if 1. The rocks are already weathered. 2. They are porous and are not well cemented. 3. Those posses fractures, cracks etc.  Solution: This process, is a part of hydraulic action which involves only chemical decay of rocks. This is an invisible process and very effective under favourable conditions.  Attrition: This is a mechanical weathering process. When the rock fragments hit the rocks which are already exposed, abrasion take place. Thus the rock fragments during abrasion undergo wear and tear which is called attrition. During transportation, heavier and larger materials move slowly while finer and lighter material move fast.. When attrition take place the angular edges disappear and spherical, ellipsoidal stones etc are formed after a long journey. River Transportation: A river transports its material physically as well as in a solution form. The transport system is divided into three groups. 1. Bed load comprises heavier particles of sand, pebbles, gravels etc.. which are transported mainly by their rolling, skipping, along the bottom of stream. 2. Suspended load consists of silt, fine sands, clay etc.. and such load is carried by river in its body of water in suspension. As the river is moved, the load is also carried along with it. Thus load is transported continuously without break till conditions are favourable. This type of natural suspension and separation of sediments account to their size is called Sorting. 5|Page

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3. Dissolved load: Material is transported in a solution condition. The ability to transport the sediments is influenced by river velocity, density etc.. River Deposition is the last phase of geological work of a river. Among the different kinds of river deposits, a few are listed below: Alluvial cones and fans: River sediment is known as alluvium. If the deposit is spread over a small area but has a relatively steep slope, it is called an alluvial cone. On the other hand, if the deposit is spread over a large area and has a gentle slope, it is called an alluvial fan. Placer deposits: The placer deposits are characteristically composed of heavier metals such as Gold, Platinum, Chromite, magnetite, Rutile, Ilmenite, Monazite etc. which are commonly economic minerals. Eg: Rand placer deposit of South Africa is famous for gold. Delta deposits: Most of the rivers reach this stage just before they merge with the sea. Rivers Ganga and Brahmaputra have built up the best deltaic regions of the world. Deltas are very fertile and valuable for agriculture. Natural levees. During the time of floods, the river carries a very large scale of river dumps along its course on either side which are known as natural levees. Eg silt, clay . MEANDER DEVELOPMENT A meander in general is a bend in a (moving with smooth twists & turns) water coarse. A meander bend is formed when the moving water in a stream erodes the out banks and widens its valley. If the river encounters any obstacle, it shall not have the capacity to uproot it and therefore it takes a diversion and continues its downward coarse.. This is responsible for the formation of deposits known as placer deposits. By virtue of its relatively weak condition the river compulsorily undergoes a number of curves or bends which makes its path zig-zag. These bends are called meanders and the 6|Page

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