Those properties which help to access the engineering behaviour of soil and which assist in determining its classification accurately are termed as index properties. Index properties include indices which help in determining the engineering behaviour such as
- Strength
- Load Bearing Capacity
- Swelling and Shrinkage
- Settlement etc.
The properties may be related to individual soil grain or aggregate soil mass.
The properties of individual particles can be determined from a remoulded disturbed sample. These properties depend upon the individual grains, their mineralogical composition, size and shape of grains and are independent of soil formation.
The soil aggregate properties depend upon the mode of soil formation, soil history and soil structure. These properties must be determined from the undisturbed sample or in the field directly.
Grain Shape
Grain shape is mainly useful in case of the soil of large size particles i.e. coarse-grained soil. Generally, soil grains in case of sand and gravel are called bulky. The sub-microscopic crystals of clay minerals are called flaky grains. Kaolinite, which is a clay mineral said to have needle shape grains.
The grains in gravel and sand are classified as angular, subangular, subrounded, rounded, well rounded.
Bulky grain classification is done on the basis of sphericity.
Grain Size Analysis
Grain size analysis is a method for the separation of the soil mass into different fractions based on the particle size. It is also termed as mechanical analysis or particle size analysis.
In this analysis, we usually express the proportion by mass of various size particles quantitatively.
Particles whose size is greater than 300 mm are called as Boulder whereas particles whose size lies in the range from 300mm to 80 mm are termed as cobbles.
Note: Cobbles and Gravel are not counted in the soil.
The properties of coarse-grained soil largely depend upon the particle size distribution however that of fine-grained soil depends upon structure, shape of grain and geological origin etc. Interparticle forces are dominant in case of fine-grained soil.
With the help of grain size analysis, we can determine the gradation and uniformity of soil. This gradation and uniformity help in the construction of embankments, dams etc however during the design of foundation, compressibility, bearing strength, shearing resistance etc. are more important.
The grain size analysis can be done through either sieving (for coarse-grained soil) or by sedimentation analysis using hydrometer (for fine-grained soil). In sieving, a known weight of soil mass is sieved through sieves of different pore size usually varying from 10mm to 150 microns. As per the IS :1498, the sieves are designated by the size of square opening either in mm or microns. (1000 micron = 1 mm)
