17-08-2012, 03:54 PM
Soil description and classification
Soil description and classification.pdf (Size: 383.99 KB / Downloads: 94)
It is necessary to adopt a formal system of soil description and classification in order to describe the various materials found in ground investigation. Such a system must be comprehensive (covering all but the rarest of deposits), meaningful in an engineering context (so that engineers will be able to understand and interpret) and yet relatively concise. It is important to distinguish between description and classification: Description of soil is a statement describing the physical nature and state of the soil. It can be a description of a sample, or a soil in situ. It is arrived at using visual examination, simple tests, observation of site conditions, geological history, etc. Soil classification is the separation of soil into classes or groups each having similar characteristics and potentially similar behaviour. A classification for engineering purposes should be based mainly on mechanical properties, e.g. permeability, stiffness, strength. The class to which a soil belongs can be used in its description.
Description and classification
Basic characteristics of soils
Soil as an engineering material
Size range of grains
Shape of grains
Composition of grains
Structure or fabric
Soils consist
Soil as an engineering material
The term "soil" means different things to different people: To a geologist it represents the products of past surface processes. To a pedologist it represents currently occurring physical and chemical processes. To an engineer it is a material that can be:
Shape characteristics of SAND grains
SAND and larger-sized grains are rotund. Coarse soil grains (silt-sized, sand-sized and larger) have different shape characteristics and surface roughness depending on the amount of wear during transportation (by water, wind or ice), or after crushing in manufactured aggregates. They have a relatively low specific surface (surface area).
Shape characteristics of CLAY grains
CLAY particles are flaky. Their thickness is very small relative to their length & breadth, in some cases as thin as 1/100th of the length. They therefore have high to very high specific surface values. These surfaces carry a small negative electrical charge, that will attract the positive end of water molecules. This charge depends on the soil mineral and may be affected by an electrolite in the pore water. This causes some additional forces between the soil grains which are proportional to the specific surface. Thus a lot of water may be held asadsorbed water within a clay mass.