SOIL MECHANICS

Prior to the understanding of the vertical fibre drain, its functional properties and its application in the required consolidation of soft soil before any superstructure construction, we should dwell into the properties of the earth crust.  The earth crust is rock and soil. The soil can be divided into boulders, gravel, sand, silt, clay, and fine-grained organic soil and peat soil.
Sand particles having particles larger than 60 micron is considered course and very course material. Between 2-60 micron is termed fine-grained and falls into the category silt. Between 0.1 - 2 micron is termed as very fine-grained and falls into the category clay. The very fine and fine-grained soil has the following physical properties. 

1. Plasticity - the soil can be rolled into thin thread.
2. Cohesion - the capacity of the soil to resist shearing stresses.
3. Thixotropy - thixotropy is a phenomenon of increase and decrease of the cohesion due to surface evaporation.
4. Consolidation- due to the application of pressure, the water contents of the soil decreases and therefore consolidates. This increases the sheer stress of soils.
5. Swelling- if the pressure is released while the soil is in contact with water, the volume of the soil is increased and also an increase in the water content.

Physical properties of soil - The above mentioned physical properties of soil including the study of hydraulic and mechanical properties of soil such as permeability, compressibility, shear characteristics, and stress and strain are factors that are useful in the design of the V.F.D for the improvement of the sub soil.

PERMEABILITY of soil depends upon the % of voids in the soil. The water held in the voids exert pressure on the porous materials through which it percolates. 

COMPRESSIBILITY of soil is a factor that depends upon the size of the soil particles, the porosity voids in the soil. Depending upon the applied load on the soil, the compression takes place until most of the voids are filled with the soil particles resulting in the resistance against ground water flow.

SHEAR CHARACTERISTICS OR SHEAR RESISTANCE OF SOIL

The soil fails when the shearing stress exceeds the shear resistance for the soil. The shear resistance of the soil is depended basically on the type of soil and the forces that will be exerted on the surface at ground level.

The above understanding of the properties of soil should lead us to design materials that will eventually keep a good sub soil drainage and moisture control systems so as to ensure stability of a road or superstructure by keeping the soil foundation in comparatively dry condition.