Partical size and shape of soil.

                             PARTICAL SIZE AND SHAPE OF SOIL

The size of soil partical range from gravels to the finest possible clay size. Their characteristics also vary with the size. The soil partical coarse than 75 microns (0.075mm) are visible to the nake eye and can be examined by hand lens. Soil partical finer than 75 micron are not visible to the naked eye and can be observe only under a microscope. 

However, shape of partical smaller than 1 micron size can be determined only by means of an electron microscopy. The molecular structure of partical can be investigate by means of x-ray diffraction analysis.

The coarse fraction of soil consist of gravel and sand. The individual grains of gravels and sand may be angular, sub- angular, sub rounded etc.

Silt and clay constitute the finer fraction of soil. The partical may be angular, flaky os sometimes needle shaped.

Soil partical coarser then 0.075mm are generally termed as cohesionless and partical finer than this size are known as cohesive soils. From the engineering point of view, these two type of soil have distinctive characteristics. In case of cohesionless soils, gravitational force predominant in case of cohesive soils. Coarse fraction of soil consisting or angular grains is capable of supporting heavy loads can be compacted to a dense mass by vibration. 

The influence of shape of partical on the compressibility characteristics of soils are:

I) The shape of the partical is responsible for reduction in the volume of soil mass upon the application of pressure.
ii) Addition of small mixture of mica to sand results in large increase in its compressibility.

The effects of particle size and shape distribution on the constitutive behavior of the composite soils with a wide range of particle size were investigated.

 Two comparable sets of specimens were prepared: 

(1) Mixtures of fines silt and clay and an ideal coarse fraction beads and glass sand .

 (2) Mixtures of  natural coarse and fines fraction ( crushed granite gravels and river sand). Direct shear box testing was undertaken on thrty four samples and the structure of the shear surfaces, change in  water  content  and volume and the particle shape coefficient of the sheared specimens were examined.

The results indicate that the dilation and contraction a specimen exhibits is restrained with in the shear zone while the outer zones remain unchanged during shearing. Increased coarse fraction leads to an increase in the constant volume shear strength.
In  addition decreasing  or increasing  elongation convexity of the coarse fraction increases the constant volume friction angle.

The overall rough-ness of the shear surface at the constant volume state is negatively related to the smoothness  particle  positively and  convexity related to the area of the shear surface occupied by the particles with particular shapes. Two equations are proposed for the estimation of constant volume friction angle based on the proportion and shape coefficient and proportion  of the coarse fraction. 

It is hoped this will assist in considering the shear strength of mixed soils when  size of the coarse fraction makes laboratory testing difficult.