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# [DAY-1] Gate 2021 | Civil Engineering Key Notes & Formulas- Soil Mechanics Part – 1

Sep 17, 2020

Soil mechanics: It is the branch of civil engineering which deals with the application of the principles of mechanics and hydraulics to engineering problems related to soils.

A. Origin of Soils: Soils are formed by the weathering of rocks due to mechanical disintegration or chemical decomposition.

• If the soil is located at the places of origin, it is called residual soil. If the soil located away from the origin, it is called transported soil.
• Soils deposited from suspension in running water are called “Alluvial deposits”.
• Soils formed in still water like lakes are called “Lacustrine deposits”.
• Soils formed in seawater are called “Marine deposits”.
• Soils transported by wind are called “Aeolian soils”.
• Soils transported by ice are called “glacial deposits”.

B. Three Phase System: The soil mass consists of solids, liquid, and gases. Liquid and gases together called voids.

Total volume, V = Vs + Vw + Vv

C. The following are the basic volume relations:

1. Void ratio (e) is the ratio of the volume of voids (Vv) to the volume of soil solids (Vs), and is expressed as a decimal.

2. Porosity (n) is the ratio of the volume of voids to the total volume of soil (V ), and is expressed as a percentage.

Void ratio and porosity are inter-related to each other as follows:
and

3. The volume of water (Vw) in a soil can vary between zero (i.e. a dry soil) and the volume of voids. This can be expressed as the degree of saturation (S) in percentage.

For a dry soil, S = 0%, and for a fully saturated soil, S = 100%.

4. Air content (ac) is the ratio of the volume of air (Va) to the volume of voids.

5. Percentage air voids (nais the ratio of the volume of air to the total volume.

D. Basic weight relations:

1. The ratio of the mass of water present to the mass of solid particles is called the water content (w), or sometimes the moisture content.

Its value is 0% for dry soil and its magnitude can exceed 100%.

2. The mass of solid particles is usually expressed in terms of their particle unit weight  or specific gravity (Gs) of the soil grain solids .

where = Unit weight of water

For most inorganic soils, the value of Gs lies between 2.60 and 2.80. The presence of organic material reduces the value of Gs.

3. Dry unit weight  is a measure of the amount of solid particles per unit volume.

4. Bulk unit weight is a measure of the amount of solid particles plus water per unit volume.

5. Saturated unit weight is equal to the bulk density when the total voids is filled up with water.

6. Buoyant unit weightor submerged unit weight is the effective mass per unit volume when the soil is submerged below standing water or below the ground water table.

E. The following are some inter-relations:

1.
2.
3.
4.

5.
6.
7.

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