Geotechnical Engineering
Geotechnical engineering is the branch of Civil Engineering concerned with the engineering behaviour of earth materials. Geotechnical engineering uses principles of Soil Mechanics and Rock Mechanics to Investigate subsurface conditions and materials.
According to Karl Terzaghi (1883-1963)
“Unfortunately, soils are made by nature and not by man, and the products of nature are always complex…As soon as we pass from steel and concrete to earth, the omnipotence of theory ceases to exist. Natural soil is never uniform. Its properties change from point to point while our knowledge of its properties are limited to those few spots at which the samples have been collected. In soil mechanics the accuracy of computed results never exceeds that of a crude estimate, and the principal function of theory consists in teaching us what and how to observe in the field.”
Rock: The source of soils
Most of the non organic materials that are identified as soil originated from rock as the parent material.
The rocks that form the earth’s surface are classified as to origin as:
Igneous Rocks
• Are those formed directly from the molten state of magma.
• The molten magma that cooled rapidly at or near earth surface are called extrusive or volcanic type rocks. Eg. Basalts, Rhyolites and Andesites.
• If the molten rock cools very slowly, the different materials segregate into large crystals forming a coarse-grained or granular structure . Intrusive or plutonic type ,Eg. Granite
Sedimentary Rocks
• Are formed from accumulated deposits of soil particles or remains of certain organisms that have become hardened by pressure or cemented by minerals.
• Cementing materials such as silica , Calcium Carbonate ,iron oxides are abundant
• For E.g., Limestones, *Dolomites, Sandstone, Shale, Conglomerate and Breccia
Metamorphic Rocks
Results when any type of existing rock is subject to metamorphism, the change brought about by combinations of heat, pressure and plastic flow so that the original rock structure and mineral composition are changed.
• Limestone >MARBLE; Shale>SLATE or PHYLLITE; Granite >GNEISS; Sandstone>QUARTZITE
Major soil types based on particle size:
• The major engineering categories of soil are gravel, sand, silt and clay
• Particles larger than gravel are called cobbles or boulders
• Gravel and sands are considered coarse-grained soils
• Silt(very tiny particles of disintegrated rock)and clay particles are considered fine-grained soils because of their small particle sizes.
Soils can be grouped into two broad categories:
• Residual –Formed from weathering of rock and remain at the location of their origin.
• Transported –those materials that have beenmoved from their place of origin by agencieslike, gravity, water, glaciers, or man-either singularly or in combination
Transported Soils are classified:
• Alluvial –transported in running water [rivers]
• Lacustrine –deposited in quiet lakes
• Marine –deposited in sea water
• Aeolin –transported by wind
• Glacial –by ice [Glaciation –massive. moving sheets of ice
• Colluvial –deposited through action of landslide
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