**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 = V_{s} + V_{w} + V_{v}

**C. The following are the basic volume relations:**

**1.** **Void ratio (e)** is the ratio of the volume of voids (V_{v}) to the volume of soil solids (V_{s}), 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 (V_{w}) 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 (a _{c})** is the ratio of the volume of air (V

_{a}) to the volume of voids.

**5.** **Percentage air voids (n _{a}) **is 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** (**G _{s}**) of the soil grain solids .

where = Unit weight of water

For most inorganic soils, the value of G_{s} lies between 2.60 and 2.80. The presence of organic material reduces the value of G_{s}.

**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 weight**or **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.**