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notes1-4-1

# notes1-4-1 - Soil Mechanics-I(CENG-2202 Chapter 4...

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Soil Mechanics-I(CENG-2202) Chapter 4 : Compressibility of soils - 47 - Department of Civil Engineering, Faculty of Technology Addis Ababa University 4. COMPRESSIBILITY OF SOILS 4.1 INTRODUCTION Foundations of structures usually rest on soil. Due to the additional stress introduced by a foundation or due to other loadings, a soil mass will compress. This compression is a sum of different and distinct processes: a. Deformation of soil particles b. Relocation of soil particles, and c. Expulsion of water or air from the void spaces In general, soil settlement caused by different loadings can be categorized into three categories: i. Immediate settlement (or elastic settlement) Such a settlement is caused by the elastic deformation of dry soil and of moist and saturated soils without any change in the moisture content. Immediate settlement calculations are generally based on equations derived from the theory of elasticity. ii. Primary consolidation settlement The volume of saturated soils under pressure will decrease due to the expulsion of water from the void spaces. The associated settlement is known as primary consolidation settlement. It is usually the predominant type of settlement in fine grained soils, especially in clays. iii. Secondary consolidation settlement This type of settlement is observed in saturated cohesive soils and is the result of the plastic adjustment of soil fabrics. If follows primary consolidation settlement when soil is under a constant effective stress. The total settlement of a foundation can then be given by S T = S c + S s + S e where S T = total settlement S c = primary consolidation settlement S s = secondary consolidation settlement S e = immediate settlement The total settlement obtained in this fashion must be within acceptable limits for any structure. For most practical purposes, the magnitude of the elastic settlement can be ignored compared to the consolidation settlements. Discussion here shall hence be limited to consolidation settlement. But it must be noted that the volume change due to expulsion of air is treated in geotechnical engineering under the title compaction characteristics of soils. Prior to discussion of soil settlements, discussing the nature of stresses in soils must be presented. It is due to stresses that soils undergo settlements.

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Soil Mechanics-I(CENG-2202) Chapter 4 : Compressibility of soils - 48 - Department of Civil Engineering, Faculty of Technology Addis Ababa University 4.2 EFFECTIVE STRESS Stresses in a soil mass may be caused by the self weight of the soil, and also by the external loads which may be applied to the soil. The pattern of stresses caused by the external loads is usually a complicated one, and the same may be the case for stresses induced by self weight of soil. However, there is one common situation in which the self weight of the soil gives rise to a very simple pattern of stresses-that is when the ground surface is horizontal and the nature of soil does not vary significantly in the horizontal direction. The stress caused in such a situation at a point in a soil mass is called the geostatic stress.
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