D CEE 120 Soil format and nature etc

D CEE 120 Soil format and nature etc - CEE 120- Principles...

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CEE 120- Principles of Soil Mechanics Introduction to soil material - soil formation, nature of soil constituents, soil structure, unique nature of soil and its behavior 1 Soil formation, nature of soil constituents and soil structure Structure of planet Earth and formation of Earth’s surface To understand the characteristics, structure and diversity of soils encountered in geotechnical engineering practice, one needs to know how soils are formed, and to know how soils are formed it is necessary to know the structure and dynamics of the upper layers of our planet and for that matter the structure of our planet itself. The internal structure of the Earth is presented in Figure 1. The earth’s crust is the uppermost layer of the Earth and it is made of solid rock. The crust ranges in thickness from as small as 5 km beneath the oceans, called the oceanic crust, to as large as 70 km under some mountain ranges, called the continental crust. A common thickness of the continental crust is otherwise around 40 km. Below the crust is mantle, a relatively solid rocky layer around 2900 km thick. Below the mantle it is the outer core, a molten metallic layer, and in the center of the earth is the solid iron-reach inner core. As indicated in Figure 2, the interface between the crust and mantle is called the Mohorovi č i ć discontinuity, or simply Moho (or sometimes indicated just as M-discontinuity). The mantle is divided in the upper mantle up to 700 km deep and the lower mantle. The crust and the upper layer of the upper mantle down to approximately 100 km are called the lithosphere, which means the “sphere of rock”. The lithosphere is considered cool and rigid. The zone of the upper mantle below the lithosphere is called the asthenosphere. The asthenosphere and lower mantle are hot and in semi-solid state -- they can slowly flow under constant stresses, but behave as a solid when subjected to rapidly applied stresses, such as those associated with seismic waves. The lithosphere, a relatively thin layer of rigid, brittle rock material that surrounds the earth like an egg-shell is composed of tectonic plates. Figure 3 shows that there are six large continental- sized and about 14 smaller subcontinental plates. The plates are in constant slow motion. The lithospheric tectonics plates are supported from below by material of the asthenosphere that is 1
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more plastic, and it has been suggested that the plates move due to the shear stresses imposed at their bottom by the convection currents in the viscous mantle of asthenosphere, such as shown in Figure 4. Although several different models of the circulation pattern of the mantle below the lithosphere have been proposed, they all agree that the circulation is caused by the unequal distribution of heat within the earth. The mantle closer to the earth’s surface is cooler and therefore denser than the lower mantle, which is close to the hot outer core. The material from the denser upper mantle therefore sinks into the lighter lower mantle, while the lower lighter material moves
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This note was uploaded on 11/08/2010 for the course CEE 120 taught by Professor Vucetic during the Fall '10 term at UCLA.

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D CEE 120 Soil format and nature etc - CEE 120- Principles...

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