Effective Stress Concepts In Soil

EFFECTIVE STRESS CONCEPTS IN SOIL

At any plane in a soil mass, the total stress or unit pressure ? is the total load per unit area. This pressure may be due to i) self weight of soil ii) over burden on the soil.

The total pressure consists of two distinct components: inter granular pressure or effective pressure and the neutral pressure or pore pressure. Effective pressure ?' is the pressure transmitted from particle through their point of contact through the soil mass above the plane.

Such a pressure, also termed as inter granular pressure, is effective in decreasing the voids ratio of the soil mass and in mobilizing its shear strength. The neutral pressure or the pore water pressure or pore pressure is the pressure transmitted through the pore fluid.

Therefore, this pressure is also called neutral pressure (u). Since the total vertical pressure at any plane is equal to the sum of the effective pressure and pore water pressure we have,

s = s ' + u

1 Submerged soil mass:

2 Soil mass with surcharge:

When the water level in the reservoir is corresponding to the flood level (H.F.L), the portion to the u/s of the dam will be saturated. The water level in the u/s pervious shell will be practically the same as the H.F>L. Due to capillarity, water will rise through a height hc. If the top of the core is situated at a height y< hc above the H.F.L, , the capillary forces ill pull the water in descending part of the earth dam, and will slowly empty it. This process is known as capillary siphoning.

4 Formation of meniscus:

When a solid or hollow tube, wet with water is partly inserted vertically in water, the molecules, due to attraction between the molecules of water and the material, climb the solid surface forming a curved meniscus adjacent to the walls of the tube or rod.

5 Saturated soil with capillary fringe:

6 Soil Shrinkage Characteristics in Swelling Soils

Objectives

Understand soil swelling and shrinkage mechanisms, and the development of desiccation cracks;

Distinguish between soils having different magnitude of swelling, as well as the consequences on soil structural behaviour; Know methods to characterize soil swell/shrink potential;

Construct soil shrinkage curves, and derive shrinkage indices, as well to apply them to assess soil management effects.

7 Bulking Of Sand:

As the moisture content of a fixed weight of sand increases, the volume also increases--up to a point. This is known as "bulking".

Bulking of loose, moist sand in the increase in its volume as compared to dry sand. Bulking is a well known phenomenon particularly in the trade of aggregate for proportioning of concrete. This phenomenon has been known since 1892 when it is was investigated by Feret at French school of Bridges and Roads.

This bulking phenomenon of sand is explained by moisture hulls or films which surround the sand particles. The contact moisture films, adsorbed to the sand particles by moisture surface  tension forces, tend to cause the sand particles to occupy a larger volume as compared to their dry state. Generally bulking of sand increases as the particle size of sand decreases. This is because of the increase in the specific surface area of the sand. Upon further subsequent increase in moisture content in sand, when a maximum increase in bulking volume is attained, bulking in its turn decreases, and upon the inundation of the sand the surface tension forces are neutralized, and most of the bulking, in such a case vanishes. As a consequence, the sand particles now rearrange themselves into a denser packing.

Effect of bulking on sand

Bulking of sand in a loose state of packing decreases the bearing capacity of sand considerably. In compacting sandy soils, low densities are usually achieved because of bulking.