EFFECTS OF SEASONAL WEATHER CHANGES AND CONSISTENCY OF THE SOILS.

Soil study is necessary before loading and the main function of soil study is to furnish some general ideas and required principles to suppliment and guide the practical experience and a free vision and judgement of the Engineer in charge although it can not give exact solution however it is necessary to verify the bearing capacity of soil and the angle of repose at least below four feet of the Ground level where any structural elements are proposed by the construction agencies.Movements are caused by way of shrinking of the clay shrinking beneath the shallow foundation in dry summer seasons and subsequent swelling during the rains .The ground will dry most where it receives the greatest radiation from the sun .The movements associated with these seasonal changes are greatest at the surface and decrease with the depth but may go up to even three metres below the surface.Foundations on such clays should be placed at a depth at least four feet below the ground level as the shrinkage movements beyond that depth are likely to be small . Adjoining deeper excavations may dry out clays and cause settlement.Ground which is shrinkable will exhibit large cracks in the surface in dry weather and become very sticky during the rains .The soils are classified according to their consistency,plastic or friable depending upon the cohesion between soil particles.Friable soils are cohesionless and it is too necessary to judge the consistency as related to properties of stickiness,friability and plasticity.Cohesion is the internal molecular attraction which resists the rupture or shear of a material and is derived in fine gained soils from the water films which bind together the individual particles in the soil mass and is the characteristic of the fine materials with particles size below about 0.002 mm (clay) Cohesion of the soil decreases as the moisture content increases .Cohesion is greater in the well compacted clays than in badly -compacted soils and is independent of the external loads applied .Internal friction is due to the resistance of grains to sliding over each other and is the characteristics of the coarse material of particles size larger than about 0.002 mm .The magnitude of the internal friction of a granular mass depends upon the grading, shape and surface texture of the particles,the degree of compaction and the moisture content of the mass ,and the load to which it is subjected.Frictional resistance is highest with angular grains having a rough surface and of varied size and shape and the strength of the non cohesive soil depends entirely on internal friction.The true angle of internal friction of clay is seldom zero and may be as much as 26degree .The angles of internal friction for granular soils may be verified under the "Shear Test" .