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Soil mechanics is an important discipline for many branches of engineering, such as Civil Engineering , Geotechnical Engineering and Engineering Geology . It is used in the Design of Foundations to support Structure s, Embankment s, Retaining Wall s, Earthworks and underground openings. The percolation of Water through soils is of importance in the construction of Tunnels and deep foundations, and obviously water-related structures like Bridge pilings and Dam s. BASIC CHARATERISTICS OF SOILS Soil is made up of 3 components: air, water, and solids. The solids are particles that range in size from clay particles the size of dust to giant boulders. The amount of air and water within a sample of soil affects its properties to carry a load. In addition, the types of particles that constitute the soil affect the properties as well. SEEPAGE Seepage is the flow of a fluid through the soil pores, in downward or upward direction. Seepage under Dam s and sheet Pile walls is often estimated using the simple graphical construction known as a Flownet . Seepage can cause Erosion of soil when the seepage velocity is great enough, and so it is an important consideration when any structure is designed which may experience a Head difference from one point to another. Erosion of the supporting soil can lead to failure of the structure, and this is a common cause of dam failure. Seepage in upward direction reduces the effective stress of soil particles. In case the hydraulic gradient is equal to or greater than the critical gradient, effective stress reduces to zero. The condition is termed as quicksand or boiling condition. EFFECTIVE STRESS ''σ ''' The concept of effective stress is central to understanding behaviour of water under different conditions. Effective stress is a measurement of the load borne by the soil skeleton. This pressure determines the ability of soil to resist Shear Stress . If the effective stress in a soil is reduced to zero, ''quick'' condition is said to occur (see Quicksand ). Effective stress (''σ ''' ) of a soil is calculated from two easily measured parameters, total stress (''σ'') and pore water pressure (''μ'') according to: σ' = σ - μ where all three terms have units of Pressure . Total Stress ''σ'' The total stress ''σ'' is equal to the Overburden Pressure , it is simply the weight of everything which rests on the soil, including the soil above. Total stress doesn't always increase with increasing depth. Pore water pressure ''μ'' The pore water pressure ''μ'' can be calculated as the Hydrostatic Pressure of water according to Fluid Statics if it is assumed that the flow of water through soil is slow. This assumption is valid under most conditions (quick condition being a notable exception). Pore water pressure can be estimated as zero above the Water Table and increases Linearly with increasing depth below the water table. SHEAR STRENGTH STRESSES AND DISPLACEMENTS CONSOLIDATION THEORY When water flows into or out of a soil mass without causing the volume to change, the flow is known as ''seepage''. If, on the other hand, the flow of water within a soil mass induces a volume change, then the flow is referred to as ''transient''. The process of volume change triggered by a transient flow is known as ''consolidation''. It is related to the change in effective stresses within the soil matrix due to a surface loading (or unloading) or variation in the water table. The ''excess porewater pressure'' (i.e. load-induced porewater pressure) generated in both cases causes the water to be either squeezed out of the soil mass (positive pore water pressure) or sucked into the soil matrix (negative porewater pressure). This movement of water continues at a ''changing rate'' until all excess pressure has dissipated, and the equilibrium of stresses has been restored according to the effective stress principle. If at some stage during its geological history the soil has been subjected to unloading, e.g. disappearance of an ice cover or a severe erosion, then the present pressure due to the overburden pressure (self weight) is smaller than that which existed before the onset of the unloading process, and the soil is known as ''overconsolidated''. If, on the other hand, the soil has not been subjected to any unloading during its entire geological history, then the present overburden pressure, constitutes the largest pressure that the soil has ever experienced, and the soil is referred as ''normally consolidated''. LATERAL EARTH PRESSURE Retaining structures are subjected, apart from their self weight, to lateral thrusts whose intensity and direction depend on the movement (or lack of it) of the structure itself. The type of thrust is examined using the coefficient of earth pressure defined as: # If the wall does not move at all then is referred as the ''coefficient of earth pressure at rest'' . Where, (Jaky's Solution, 1944). # If the wall is pushed into the soil then at failure, the coefficient reaches its maximum value known as the ''coefficient of passive earth pressure'' . # If the wall is moved away from the soil it supports, then at failure the ratio reaches its minimum value, known as the ''coefficient of active earth pressure'' . BEARING CAPACITY Ultimate bearing capacity of soil is the value of the average contact Pressure between the foundation and the soil which will produce shear failure in the soil. Safe or maximum bearing capacity is the maximum contact pressure to which the soil can be subjected to without risk of Shear Failure . Ultimate stress is divided by the factor of safety used. STABILITY OF SLOPES GROUND INVESTIGATION Ground investigation is the major means of obtaining information which will affect the planning, design and construction of a new project. It can be divided into two stages - primary and secondary. Primary investigation is usually carried out before construction and depends on the nature of the project. It may include a surface investigation (topographic survey, service placement, estimation of excavation volumes, surface grades needed for drainage), and a subsurface investigation (location of ground water, soil types, soil depth to required Bearing Capacity , soil properties). The secondary investigation is usually an ongoing process throughout construction and is concerned with site accessibility, conditions and safety. REFERENCES
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