Soil Characteristics and Properties (Engineering): Density, Specific Gravity, Unit Weight

Table of Contents

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If you are studying civil engineering, understanding “soil” is essential.

While concrete is also important, knowing the engineering properties of soil is crucial for understanding bearing capacity and ground settlement.

First, soil can be classified into gravel, sand, and clay based on particle size.

To understand the engineering properties of soil, we need to consider it as a mixture of particles (gravel, sand, and clay), water, and air.

Let’s dive right in!

1. Particle Size Distribution Curve: Gravel, Sand, Clay

As mentioned earlier, “soil” can be categorized into gravel, sand, and clay based on particle size.

Particles larger than 0.075mm are classified using sieve analysis, while those smaller than 0.075mm are classified using hydrometer analysis.

The distribution of gravel, sand, and clay varies by region due to environmental differences.

Thus, we can assess the particle size distribution for soil samples from different regions, classifying them into well-graded and poorly-graded soils.

Well-graded soil : Particles are distributed across a wide range.

Poorly-graded soil : Most particles are of a similar size.

To evaluate particle size distribution, the following parameters are used:

1-1. Effective Size (D10)

The particle size at which 10% of the soil sample passes through.

It’s useful for calculating permeability and drainage capabilities.

1-2. Coefficient of Uniformity (Cu)

$C_{u}={\frac {D_{60}}{D_{10}}}$

The ratio of the particle size at which 60% of the sample passes through (D60) to the effective size (D10).

1-3. Coefficient of Curvature (Cc)

$C_{g}={\frac {D_{30}^{2}}{D_{10}\times D_{60}}}$

For good gradation, generally, Cu is greater than 4 or 6, and Cc is between 1 and 3.

(These criteria can vary based on classification systems.)

Even in the current diagram, it can be seen that the coefficient of uniformity of well-graded soil is much larger.

Soil mechanics uses sieve analysis and hydrometer analysis in conjunction with Atterberg limits and use systems like the Unified Soil Classification System (USCS) or the AASHTO Soil Classification System to classify soils .

2. Engineering Properties

To understand the engineering properties of soil, it’s essential to consider it as a mixture of particles (gravel, sand, and clay), water, and air.

Soil naturally forms a three-phase system of solid, liquid, and gas.

These properties can be depicted as follows:

2-1. Specific Gravity (Gs)

Specific gravity is defined as the ratio of the unit weight of soil particles (γs) to the unit weight of water (γw) at 4°C.

$G_{s}={\frac {\gamma _{s}}{\gamma _{w}}}={\frac {\rho _{s}g}{\rho _{w}g}}={\frac {\rho _{s}}{\rho _{w}}}$

Since the density of water (ρw) is 1000 kg/m³, this ratio indicates how much denser the soil is compared to water.

It’s a dimensionless constant and typically, higher specific gravity means denser soil.

2-2. Void Ratio (e) and Porosity (n)

Void Ratio (e) : The ratio of the volume of voids (Vv) to the volume of solids (Vs).

{\displaystyle e={\frac {V_{v}}{V_{s}}}}

Porosity (n) : The ratio of the volume of voids (Vv) to the total volume (V).

The void ratio and porosity can be related through specific formulas.

<Relation between Void Ratio and Porosity>

2-3. Degree of Saturation (S)

The ratio of the volume of water (Vw) to the volume of voids (Vv).

This indicates how saturated the soil is.

{\displaystyle S={\frac {V_{w}}{V_{v}}}\times 100(\%)}

2-4. Moisture Content (w)

The ratio of the weight of water (Ww) to the weight of solids (Ws).

Assuming the weight of air Wa is zero, W = Ws + Ww.

This measures how much water is present in the soil relative to the solid particles.

Here, weight refers to mass multiplied by gravitational acceleration, g.

{\displaystyle \omega ={\frac {W_{w}}{W_{s}}}\times 100(\%)}

2-5. Unit Weight (γ)

The weight of soil per unit volume, commonly expressed in kN/m³.

It can also be expressed in terms of dry unit weight (γd) when water is completely removed from the soil.

As mentioned earlier, soil is a three-phase system consisting of solids, water, and air. Therefore, the weight of the air is considered to be zero, and in an extremely dry state, all water can be removed.

The dry unit weight (ϒd), which is the weight per unit volume of soil excluding water, can be expressed as follows.

2-6. Relative Density (Dr)

For sands, this measures the compactness or looseness.

In the case of sand, compaction can remove air, which changes the volume and reduces the void ratio. Therefore, the relative density can be defined by the following equation.

$D_{r}={\frac {e_{max}-e}{e_{max}-e_{min}}}\times 100(\%)$

$D_{r}={\frac {\gamma _{dmax}}{\gamma _{d}}}\cdot {\frac {\gamma _{d}-\gamma _{dmin}}{\gamma _{dmax}-\gamma _{dmin}}}\times 100(\%)$

$e_{max}$ : max. void ratio, $e_{min}$ : min. void ratio, e : void ratio

$\gamma _{dmax}$ : max. dry unit weight, $\gamma _{dmin}$ : min. dry unit weight, $\gamma _{d}$ : dry unit weight

$e_{max}$ , $e_{min}$ : Hitting the soil does not reduce the volume of the solid. This is a difference caused by a change in Vv.

$\gamma _{dmax}$ , $\gamma _{dmin}$ : As the air in the soil escapes, the volume changes and the unit weight also changes.

3. Relationships Among Unit Weight, Void Ratio, Moisture Content, and Specific Gravity

Various definitions explained earlier can be interconnected if we assume the volume of solids (Vs) to be 1 :

3-1. Unit Weight

3-2. Dry Unit Weight

3-3. Degree of Saturation

The equation for degree of saturation is crucial because it is frequently used to determine soil properties and appears often in problems.

It includes both weight and volume considerations.

By understanding these definitions and relationships, we can assess the engineering properties of soils, which is essential for further studies in soil mechanics.

Thank you for reading.

References :

1. 김상균, 이문규, 조재국. (2014). 토목기초공학. 홍릉과학출판사.

2. 홍성철, 박승주, 송철호. (2009). 토질공학. 자유아카데미.

3. 김진호, 김재동, 윤병진. (2013). 토목기초공학 개론. 박영사.

4. https://ko.wikipedia.org/wiki/%ED%9D%99