Relative Density of Granular Soils

What you’ll learn

Understanding the Relative Density of Granular Soils

What is Relative Density in Granural Soils?

Relative Density (DR), or also known as density index is used to describe the state of compactness of granular soils compared to their loosest and densest possible states.

It can be expressed using this formula:

$D_r= \frac{e_{max}-e}{e_{max}-e_{min}}$

where:
$e = void \: ratio \: of \: the \: soil \: (in-situ)$
$e_{min} = void \: ratio (densest)$
$e_{max}= void \: ratio (loosest}$

We can also express Relative Density in terms of dry unit weight:

Where:
$\gamma_{dmin} = dry \: unit \: weight (loosest)$
$\gamma_d = dry-unit \: weight \: (in-situ)$
$\gamma_{dmax} = dry \: unit \: weight (densest)$

Relative Density Classifications

Designation	Dr (%)
Very Loose	0 – 15
Loose	15 – 35
Medium Dense	35 – 70
Dense	70 – 85
Very Dense	85 – 100

Significance in Geotechnical Engineering

Bearing Capacity: Relative density directly influences the bearing capacity of foundations on granular soils. Denser soils can support higher loads with less settlement. Engineers use relative density values to estimate safe bearing pressures and design appropriate foundation systems.
Settlement Prediction: The compressibility of granular soils is strongly related to their relative density. Loose sands undergo significant settlement under loading, while dense sands exhibit minimal compression. Accurate relative density assessment allows engineers to predict settlement behavior reliably.
Liquefaction Susceptibility: Loose saturated sands (low relative density) are highly susceptible to liquefaction during seismic events. Relative density is a key parameter in liquefaction potential assessments. Soils with Dr < 50% in seismically active regions are considered at risk and may require ground improvement.

Suitability Number

What is Suitability Numbers?

The suitability number is a dimensionless parameter that evaluates soil based on its grain size distribution and plasticity characteristics. It provides a numerical rating that indicates how appropriate a soil is for use as construction material, particularly for highway subgrades, embankments, and fill materials.