M1 Soil Formation and Structure.pdf

CE2218 GEOTECHNICAL ENGINEERING - 1: SOIL MECHANICS M1 SOIL FORMATION & STRUCTURE UST Manila  Civil Engineering Department Rock Cycle Rock Cycle Igneous Rock Sedimentary Rock Metamorphic Rock  Weathering Process  the process of breaking down rocks by me...

CE2218 GEOTECHNICAL ENGINEERING - 1: SOIL MECHANICS M1 SOIL FORMATION & STRUCTURE UST Manila  Civil Engineering Department Rock Cycle Rock Cycle Igneous Rock Sedimentary Rock Metamorphic Rock  Weathering Process  the process of breaking down rocks by mechanical and chemical processes into smaller pieces Mechanical Weathering Chemical Weathering  Soil Profile  is a natural succession of zones or strata below the ground surface and represents the alterations in the original soil material which have been brought about by weathering processes. It may extend to different depths at different places and each stratum may have varying thickness  Transported Soils  soil transported by physical processes to other places Gravity transported soil Lacustrine soils Alluvial or fluvial soils Glacial soils Aeolian soils Marine soils Residual Soils  soils stayed where they were formed and cover the rock surface from which they derive Organic Soils  decomposed aquatic plants formed in coastal areas and in glaciated regions  Broad Classification of Soils Coarse-grained soils  with average grain-size greater than 0.075 mm, e.g., gravels and sands Fine-grained soils  with average grain-size less than 0.075 mm, e.g., silts and clays These exhibit different properties and behaviour but certain general conclusions are possible even with this categorisation. For example, fine-grained soils exhibit the property of ‘cohesion’—bonding caused by inter- molecular attraction while coarse-grained soils do not; thus, the former may be said to be cohesive and the latter non-cohesive or cohesionless.  Some Commonly Used Soil Designations  Bentonite. Decomposed volcanic ash containing a high percentage of clay mineral— montmorillonite. It exhibits high degree of shrinkage and swelling.  Black cotton soil. Black soil containing a high percentage of montmorillonite and colloidal material; exhibits high degree of shrinkage and swelling. The name is derived from the fact that cotton grows well in the black soil.  Boulder clay. Glacial clay containing all sizes of rock fragments from boulders down to finely pulverised clay materials. It is also known as ‘Glacial till’.  Some Commonly Used Soil Designations  Loam. Mixture of sand, silt and clay size particles approximately in equal proportions; sometimes contains organic matter.  Loess. Uniform wind-blown yellowish brown silt or silty clay; exhibits cohesion in the dry condition, which is lost on wetting. Near vertical cuts can be made in the dry condition.  Marl. Mixtures of calcareous sands or clays or loam; clay content not more than 75% and lime content not less than 15%.  Soil Structure  The ‘structure’ of a soil may be defined as the manner of arrangement and state of aggregation of soil grains. The following types of structure are commonly studied: (a) Single-grained structure (b) Honey-comb structure (c) Flocculent structure  Single-grained Single-grained structure is characteristic of coarse-grained soils, with a particle size greater than 0.02 mm. Gravitational forces predominate the surface forces and hence grain to grain contact results. The deposition may occur in a loose state, with large voids or in a sense state, with less of voids.  Honey-comb This structure can occur only in fine-grained soils, especially in silt and rock flour. Due to the relatively smaller size of grains, besides gravitational forces, inter-particle surface forces also play an important role in the process of settling down.  Flocculent This structure is characteristic of fine-grained soils such as clays. Inter-particle forces play a predominant role in the deposition. Mutual repulsion of the particles may be eliminated by means of an appropriate chemical; this will result in grains coming closer together to form a ‘floc’. Formation of flocs is ‘flocculation’. But the flocs tend to settle in a honeycomb structure, in which in place of each grain, a floc occurs  Soil Texture The term ‘Texture’ refers to the appearance of the surface of a material, such as a fabric. It is used in a similar sense with regard to soils. Texture of a soil is reflected largely by the particle size, shape, and gradation.