**Topic: Phase relation of soils**

**Total volume, V**= Vs + Vw + Vv- Soils can be
**partially saturated**(with both air and water present), or be fully saturated (no air content) or be perfectly dry (no water content). - In a saturated soil or a dry soil, the
**three-phase system**thus reduces to two phases only. **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.**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.**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%.**

**Air content (ac)**is the ratio of the volume of air (Va) to the volume of voids.**Percentage air voids (na)**is the ratio of the volume of air to the total volume.- 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. - The mass of solid particles is usually expressed in terms of their particle unit weight or
**specific gravity (Gs)**of the soil grain solids. - 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. **Dry unit weight**is a measure of the amount of solid particles per unit volume.**Bulk unit weight**is a measure of the amount of solid particles plus water per unit volume.**Saturated unit weight**is equal to the bulk density when the total voids is filled up with water.**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.

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