Soil Composition

Soil Composition

Soil is a complex particulate material consisting of three distinct phases: Solids (soil particles), Water (liquid), and Air (gas). The interactions and relative proportions of these phases determine the engineering properties of the soil, such as strength, compressibility, and permeability.

The Three-Phase System

To simplify calculations, soil is often represented as a Phase Diagram (or Block Diagram) that separates the three components by volume and weight.

Components of the Phase Diagram

Solids ( $V_s$ , $W_s$ ): The mineral particles forming the soil skeleton.
Water ( $V_w$ , $W_w$ ): The fluid filling the void spaces.
Air ( $V_a$ , $W_a \approx 0$ ): The gas filling the remaining voids.

Fundamental Definitions:

Total Volume ( $V_t$ ) = $V_s + V_w + V_a$
Volume of Voids ( $V_v$ ) = $V_w + V_a$
Total Weight ( $W_t$ ) = $W_s + W_w$ (Weight of air is negligible)

Interactive Phase Diagram

Adjust the volumes below to see how they affect the Void Ratio, Porosity, and Degree of Saturation.

Soil Phase Diagram (Three-Phase System)

Input Parameters

Volume of Solids (Vs)1.00

Volume of Water (Vw)0.50

Volume of Air (Va)0.20

Specific Gravity (Gs)2.65

Adjusting volumes changes the Void Ratio ( $e$ ) and Porosity ( $n$ ). Adjusting water changes Saturation ( $S$ ) and Water Content ( $w$ ).

Calculated Ratios

Void Ratio (e)Vv / Vs

0.700

Porosity (n)Vv / Vt

41.2%

Degree of Saturation (S)Vw / Vv

71.4%

Water Content (w)Ww / Ws

18.9%

Unit Weights (kN/m³)

Moist Unit Wt (γ)

18.18

Dry Unit Wt (γd)

15.29

Saturated Unit Wt (γsat)

19.33

Volumetric Relationships

These parameters describe the relative amount of voids in the soil mass.

Void Ratio ( $e$ )

The ratio of the volume of voids to the volume of solids. It is a measure of the packing density of the soil.

Void Ratio

Ratio of the volume of voids to the volume of solids; a key measure of soil compactness.

e = \frac{V_v}{V_s}

Variables

Symbol	Description	Unit
$e$	Void ratio	-
$V_{v}$	Volume of voids	-
$V_{s}$	Volume of solids	-

Range: $0 < e < \infty$
Typical Values for Sands: $0.4 - 0.8$
Typical Values for Clays: $0.6 - 1.5$ (can be $> 2$ for soft clays)

Porosity ( $n$ )

The ratio of the volume of voids to the total volume, expressed as a percentage.

Porosity

Ratio of void volume to total volume, expressed as a percentage.

n = \frac{V_v}{V_t} \times 100\%

Variables

Symbol	Description	Unit
$n$	Porosity	-
$V_{v}$	Volume of voids	-
$V_{t}$	Total volume	-

Range: $0 < n < 100\%$

Relationship between $e$ and $n$ :

Porosity from Void Ratio

Converts void ratio to porosity directly.

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

Variables

Symbol	Description	Unit
$n$	Porosity	-
$e$	Void ratio	-

Void Ratio from Porosity

Converts porosity to void ratio directly.

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

Variables

Symbol	Description	Unit
$e$	Void ratio	-
$n$	Porosity	-

Degree of Saturation ( $S$ )

The ratio of the volume of water to the volume of voids, expressed as a percentage.

Degree of Saturation

Ratio of water volume to total void volume; indicates how much of the void space is filled with water.

S = \frac{V_w}{V_v} \times 100\%

Variables

Symbol	Description	Unit
$S$	Degree of saturation	-
$V_{w}$	Volume of water	-
$V_{v}$	Volume of voids	-

Dry Soil: $S = 0\%$
Saturated Soil: $S = 100\%$
Partially Saturated: $0\% < S < 100\%$

Relative Density ( $D_{r}$ )

A measure of the actual void ratio ( $e$ ) of a granular soil (sand or gravel) relative to its loosest ( $e_{max}$ ) and densest ( $e_{min}$ ) possible states. It is critical for assessing the strength, compressibility, and liquefaction potential of coarse-grained soils.

Relative Density (Void Ratio)

Quantifies the packing state of granular soils relative to their loosest and densest possible states, expressed using void ratios.

D_r = \frac{e_{max} - e}{e_{max} - e_{min}} \times 100\%

Variables

Symbol	Description	Unit
$D_{r}$	Relative density	-
$e$	In-situ void ratio	-
$e_{max}$	Maximum void ratio (loosest state)	-
$e_{min}$	Minimum void ratio (densest state)	-

Very Loose: $D_r < 15\%$
Medium Dense: $35\% \le D_r < 65\%$
Very Dense: $D_r \ge 85\%$

It can also be expressed in terms of dry unit weight ( $\gamma_d$ ):

Relative Density (Unit Weight)

Alternative form of relative density expressed in terms of measured dry unit weights from field and lab tests.

D_r = \frac{\gamma_{d(field)} - \gamma_{d(min)}}{\gamma_{d(max)} - \gamma_{d(min)}} \times \left( \frac{\gamma_{d(max)}}{\gamma_{d(field)}} \right) \times 100\%

Variables

Symbol	Description	Unit
$D_{r}$	Relative density	-
$\gamma_{d(field)}$	In-situ dry unit weight	-
$\gamma_{d(max)}$	Maximum dry unit weight (densest state)	-
$\gamma_{d(min)}$	Minimum dry unit weight (loosest state)	-

Weight Relationships

Water Content ( $w$ )

The ratio of the weight of water to the weight of solids, expressed as a percentage. Also known as moisture content.

Water Content

Ratio of the weight of water to the weight of solids, expressed as a percentage; also called moisture content.

w = \frac{W_w}{W_s} \times 100\%

Variables

Symbol	Description	Unit
$w$	Water content	-
$W_{w}$	Weight of water	-
$W_{s}$	Weight of solids	-

Range: $w \ge 0\%$
Can exceed 100% for organic soils and sensitive clays (meaning weight of water > weight of solids).

Specific Gravity ( $G_{s}$ )

The ratio of the unit weight of soil solids to the unit weight of water.

Specific Gravity

Ratio of the unit weight of soil solids to the unit weight of water; typically around 2.65–2.70 for most minerals.

G_s = \frac{\gamma_s}{\gamma_w} = \frac{W_s}{V_s \gamma_w}

Variables

Symbol	Description	Unit
$G_{s}$	Specific gravity	-
$\gamma_s$	Unit weight of soil solids	-
$\gamma_w$	Unit weight of water	-
$W_{s}$	Weight of solids	-
$V_{s}$	Volume of solids	-

Typical Values for Sand: 2.65
Typical Values for Clay: 2.70
Typical Values for Organic Soil: $< 2.0$

Unit Weight Relationships

Unit weight (or density) is the weight per unit volume.

Common Unit Weights

Moist (Bulk) Unit Weight

Weight of soil per unit total volume at a given moisture content; the standard field unit weight.

\gamma = \frac{W_t}{V_t} = \frac{(G_s + Se)\gamma_w}{1+e} = \frac{(1+w)G_s \gamma_w}{1+e}

Variables

Symbol	Description	Unit
$\gamma$	Moist unit weight	-
$W_{t}$	Total weight	-
$V_{t}$	Total volume	-
$G_{s}$	Specific gravity	-
$S$	Degree of saturation (decimal)	-
$e$	Void ratio	-
$w$	Water content (decimal)	-
$\gamma_w$	Unit weight of water	-

Dry Unit Weight

Used for compaction control.

\gamma_d = \frac{W_s}{V_t} = \frac{G_s \gamma_w}{1+e} = \frac{\gamma}{1+w}

Variables

Symbol	Description	Unit
$\gamma_d$	Dry unit weight	-
$W_{s}$	Weight of solids	-
$V_{t}$	Total volume	-
$G_{s}$	Specific gravity	-
$\gamma_w$	Unit weight of water	-
$e$	Void ratio	-
$\gamma$	Moist unit weight	-
$w$	Water content (decimal)	-

Saturated Unit Weight

Calculated when S = 100% (or S = 1.0).

\gamma_{sat} = \frac{(G_s + e)\gamma_w}{1+e}

Variables

Symbol	Description	Unit
$\gamma_{sat}$	Saturated unit weight	-
$G_{s}$	Specific gravity	-
$e$	Void ratio	-
$\gamma_w$	Unit weight of water	-

Effective (Submerged) Unit Weight

Unit weight of soil submerged below the water table, accounting for buoyancy; used in effective stress calculations.

\gamma' = \gamma_{sat} - \gamma_w = \frac{(G_s - 1)\gamma_w}{1+e}

Variables

Symbol	Description	Unit
$\gamma'$	Effective unit weight	-
$\gamma_{sat}$	Saturated unit weight	-
$\gamma_w$	Unit weight of water	-
$G_{s}$	Specific gravity	-
$e$	Void ratio	-

Fundamental Relationship

The 'Seven' Variables Relationship

A crucial mnemonic to remember the relationship between Saturation, Void Ratio, Water Content, and Specific Gravity.

Basic Phase Relationship

Useful for solving almost any phase relationship problem.

Se = wG_s

Variables

Symbol	Description	Unit
$S$	Degree of saturation (decimal)	-
$e$	Void ratio	-
$w$	Water content (decimal)	-
$G_{s}$	Specific gravity	-

Key Takeaways

Soil consists of Solids, Water, and Air. The Phase Diagram helps visualize these components.
Void Ratio ( $e$ ) and Porosity ( $n$ ) measure the volume of void space relative to solids or total volume, respectively.
Relative Density ( $D_r$ ) defines the packing state (loose vs. dense) of granular soils, crucial for assessing stability.
Degree of Saturation ( $S$ ) indicates how much of the void space is filled with water.
Unit Weights ( $\gamma, \gamma_d, \gamma_{sat}$ ) relate the weight of the soil to its volume and are critical for stress calculations.
The equation $Se = wG_s$ is the most powerful tool for solving phase relationship problems.

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NextSoil Composition - Examples & Applications

The Three-Phase System

Components of the Phase Diagram

Interactive Phase Diagram

Soil Phase Diagram (Three-Phase System)

Input Parameters

Calculated Ratios

Unit Weights (kN/m³)

Volumetric Relationships

Void Ratio (eee)

Void Ratio

Porosity (nnn)

Porosity

Porosity from Void Ratio

Void Ratio from Porosity

Degree of Saturation (SSS)

Degree of Saturation

Relative Density (DrD_rDr​)

Relative Density (Void Ratio)

Relative Density (Unit Weight)

Weight Relationships

Water Content (www)

Water Content

Specific Gravity (GsG_sGs​)

Specific Gravity

Unit Weight Relationships

Common Unit Weights

Moist (Bulk) Unit Weight

Dry Unit Weight

Saturated Unit Weight

Effective (Submerged) Unit Weight

Fundamental Relationship

The 'Seven' Variables Relationship

Basic Phase Relationship

Void Ratio ( $e$ )

Porosity ( $n$ )

Degree of Saturation ( $S$ )

Relative Density ( $D_{r}$ )

Water Content ( $w$ )

Specific Gravity ( $G_{s}$ )