## Soil mechanics notes

Soil Mechanics: According to “terzagha” is an application of laws of mechanics and hydraulics to engineering problems dealing with sediments and other unconsolidated accumulations of solid particles produced by the mechanical and chemical disintegration of rocks regardless of whether or not they contain in admixture of organic constituents.
Voids: Spaces between grains of sand, gravel or soil, occupied by air or water or both.
Void Ratio: The ratio of the volume of voids to the volume of the solids in a sample of soil or aggregate e= Vv/Vs.
Plasticity: Plasticity is defined as that property of a soil, which allows to be deformed rapidly, without rupture, without clarity rebound and with out volume charge.
Porosity (n) The ratio of the volume of voids to the total volume of a soil sample n= Vv/V.
Degree of Saturation: The degree of saturation (Sr) is defined as the ratio of the volume of water present in a given soil mass to the total volume of voids in it. Thus Sr = Vw/Vv
The degree of saturation is usually expressed as a percentage and is also known as percent saturation For a fully saturated sample Vw = Vv and hence Sr = 1 for perfectly dry sample V10= Zero and hence Sr = 0.

Consistency: The relative ease with which soil can be deformed. This term is mostly used for fine-grained soil, for which the consistency is related to large extent to water content. Consistency denotes the degree of firmness of the soil, which may be termed as soft, firm , stiff or hard fine-grained soil may be mixed with water form a plastic paste, which can be molded into any form by pressure. The addition of water reduces the cohesion making the soil easier to mould. Further addition of water reduces the cohesion until the material no longer retain, its shape under its own weight, but flows as a liquid.

Liquid Limit: “Liquid limit is the minimum moisture content, expressed as moisture content percentage by dry weight of soils, where the soil changes from liquid state to plastic state”.
(OR) “The minimum moisture content percentage by dry weight at which the soil starts flowing under its own weight.”
Plastic Limit: “Plastic limit is the minimum moisture content, expressed as percent moisture content by weight of dry soil, where the soil change from plastic state to semi-solid state”.

Shrinkage Limit: Shrinkage limit is the maximum moisture content of soil, expressed as percent moisture content by weight of dry soil, at which the soil changes from semi-soild state tosolid state without any change in volume. Also it is the minimum moisture content at which the soil is completely saturated.

Soil Classification:
1. Textural Classification: “ Soil classification of composite soils exclusively based on the particle size distribution is known as “Textural Classification”. Soil occurring in nature is composed of different percentage of sand, silt and clay size particles. Therefore this classification is based on relative proportions of silt, sand and clay. It explain by the triangular classification of U.S PRA.
Permeability: The property of a porous material which permits the passage or seepage of
water through its interconnecting voids is called permeability. A material having continuous voids is called permeable material. Gravels are highly permeable while clay is the least permeable and hence clay may be termed impermeable for all practical purposes. (Laminar, Turbulent, Transitional flows)

Darcy’s Law: The rate of flow per unit area of cross-section of saturated soil is directly proportional to hydraulic gradient “i”. (OR) The velocity of Laminar flow in saturated soil is directly proportional to hydraulic gradient “i”.
Mathematically: Q/A i , Q/A = Ki, Q = KiA or V i, V =Ki bcoz V= Q/A
Where K= coefficient of permeability A = Cross-sectional Area of soil
Q = discharge, i = hydraulic gradient, and i = h1 – h2/L.

Factors affecting permeability of Soil.
1. Size and Shape of particles.
2. Properties of pores fluid.
3. Void Ratio of Soil
4. Structural arrangements of soil particles
5. Entrapped air and foreign-matter/Degree of saturation.
6. Absorbed water in clay soil.
Bearing Capacity: The bearing power of bearing capacity of a soil is the maximum load per unit area. (usually in terms of tons per square foot or tonnes per square meter.) which the ground will support without yielding or displacement..
Factors Affecting Bearing Capacity:
1. Type of soil: Generally coarse-grained soil has more bearing capacity.
2. Depth of foundation: Safe bearing capacity of soil increases with depth.
3. Amount of settlement the structure can bear: Higher values of the save bearing capacity can be adopted if the structure can tolerate higher settlement.
4. Location of ground water: Save bearing capacity decreases as the water table nears the foundations.
5. Type and size of foundation: Certain types of foundation, allow higher bearing capacity
6. Past history of Soil: Pre-consolidated and dense soil allows higher bearing capacity.