Geotechnical Engineering

Soil Phase Relations & Index Properties

Void ratio, porosity, water content, unit weights and Atterberg limits — the descriptive vocabulary of soil.

PART 1

Topic Breakdown & Traps

Soil is a three-phase system (solids, water, air). The void ratio

e

(voids ÷ solids) and porosity

n

(voids ÷ total) describe packing; the basic identity is

S e = w G_{s}

. Atterberg limits mark the water contents separating the solid, plastic and liquid states of fine soils.

Void ratio / porosity:

e = \frac{n}{1 - n}, n = \frac{e}{1 + e}

Basic relation:

S e = w G_{s}

Dry unit weight:

γ_{d} = \frac{γ}{1 + w} = \frac{G _{s} γ _{w}}{1 + e}

Plasticity index:

P I = LL - P L

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** $e$ vs $n$ .** Void ratio is voids/solids (can exceed 1); porosity is voids/total (always $< 1$ ).
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Water content can exceed 100% for soft clays — it is mass of water per mass of solids.
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Bulk vs dry unit weight. Divide by $(1 + w)$ to convert bulk to dry.

PART 2

Q1BASIC1 Mark · NAT

A soil has porosity

40%

. Its void ratio is _____.

Q2MEDIUM2 Marks · NAT

A soil has bulk unit weight

20 kN/m^{3}

and water content

25%

. Its dry unit weight is _____ kN/m³.

Q3HARD2 Marks · NAT

A soil has

w = 20%

G_{s} = 2.70

and void ratio

0.6

. Its degree of saturation is _____ %.