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# Importance of the Soil Mechanics for the Civil Engineers

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Soil Mechanics for Civil Engineers

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### Importance of the Soil Mechanics for the Civil Engineers

1. 1. Importance of the soil mechanics for the Civil Engineers In soil mechanics we study about the various properties of the soil to be used for various engineering construction works. There are various reasons that as a civil Engineer one must study this new branch of the Engineering science. (1) Foundations: All the civil Engineering structures, ultimately rest on the soil. They transfer their whole load to the soil, so we have to construct the foundations to retain these structures. In case of the hard soil/ having sufficient strength we can provide the shallow foundations. If we know the strength of the soil then we can decide which type of foundation is to be used. If the soil is weak in strength then we have to provide the deep foundations like pile foundation, well foundation etc. It is important to know the method to calculate the method to know the strength of the soil. (2) Earthen Dams: There are so many earthen dams constructed to retain the water. The soil to be used for the construction of these earthen dams must be suitable enough to use it in its construction. Various properties of the soil, like it permeability, strength, and density are checked on regular basis to know if the soil compacted to required density or not. The earthen dams are costly structure and also they have a high risk of getting failed, so they must be constructed with great care, so it is very important to study the properties of the soil. (3) Embankments: There are embankments constructed to raise the levels of the highways on the plains because there are chances of the floods etc, and also it is required to keep the foundation of the pavement above the water table. The embankments are generally constructed of the soil, which is tested for its various properties. There is need to design a economical embankment which is only possible by studying the various soil properties. (4) Canals or other retaining and under ground structures: The canals also are formed by the soil which are to be constructed to be impermeable and of enough strength. The retaining structure like the retaining walls, are constructed to retain the earth. The earth properties are important to know about. The properties like the earth pressure, shear strength etc gives us the idea to design the retaining structure. The soil strata is constantly investigated by the geologist to give the idea of the type of construction to be carried further in case of the tunneling. (5) There are various techniques to be used to improve the soil properties, which can help with the economy of the construction works dealing with the soil. Factors affecting stability of slope (Geotechnical Engg.) Hi,  Stability of Slopes A slope is an inclined boundary surface between air and the body of the earthwork e.g. high ways cut or fill, railway cu or fill, earth dams, levees and river training works.  Stability of slopes can be affected by various reasons.Some of them may result in the increased stress and some may cause reduction in the strength.  Factors causing increased stress: 1. Increased unit weight of soil by wetting. 2. Added external load (moving loads, buildings etc.)
2. 2. 3. Steep end slopes either by excavation or by erosion. 4. Shock loads  Factors causing reduction in strength: 1. Vibrations and earthquakes 2. increase in moisture content 3. increase in pore water pressure. 4. loss of cementing material. Now, to analyse the stability of the slopes, there are few assumptions: 1. The problem is two dimensional. 2. Coulomb's theory can be used to compute shear strength and shear strength is assumed as uniform along the slip surface. 3. The flownet, in case of seepage, can be drawn and seepage force can be evaluated. Methods of Stability analysis of slope(Geotech) Hi here is a brief introduction to the stability analysis of the slopes. There are following methods of stability analysis:  (A)Slip circle method or Swedish circle method or Method of Slices: This method assumes the surface of sliding is an arc of a circle. Soil is either purely cohesive or it will be a cohesive and frictional soil. So, analysis can be done separately as below:  Analysis of purely cohesive soils: An arc of a circle is assumed as the failure plane and the weight of the circular arc wedge provides a disturbing moment. This disturbing moment is stabilized by the stabilizing moment developed due to the presence of the cohesion along the arc surface. These two moments are equated to get the limiting values. The ratio of the resisting moment to the stabilizing moment gives us the Factor of safety.  Cohesive and frictional soils: In such soil again a circular arc is assumed to be a failure wedge and the weight of the wedge is resolved into tangential and normal directions. The tangential weight will provide us with the disturbing moment about the center of rotation and the normal force will provide us the frictional resistance which along with the cohesion force will produce the stabilizing moment.  (B) Friction circle method This method is applicable to cohesive as well as frictional soils and assumes the failure surface as an arc of a circle. There is a small circle known as friction circle. Stability of Earth Dam (slope) Hello, Earth dams must be safe against slope and foundation failure for all operating conditions. There are three generally recognized critical stages based on pore pressure condition for which the stability of the embankment should be ascertained. These three situations are  End of construction  Steady state seepage  rapid state seepage. Usually construction pore pressure reach their maximum value when the embankment reaches maximum height. After the reservoir has been filled for a long time, pore pressures are determined by steady-stage seepage conditions and may be estimated by the construction of flow net. Rapid lowering of the reservoir produces the third critical situation, particularly for low permeable soils. Upstream slope stability may be critical for the construction or rapid draw-down condition. The downstream slope should be checked for the construction and steady-seepage condition. GATE, PSUs preparation- Soil Engineering ( notes)- Part 11 Hello there, Welcome to the part 11 of the one liner notes useful for the preparation of GATE and other similar examinations.