unit weight of soil

Unit Weight of Soil | Significance & Types of Unit Weight of Soil


Soil is formed by weathering of rocks. It contains minerals, organic matters, gases, liquids, organisms, fossils, etc. It has its texture, porosity, colour, structure and chemistry depending upon the composition of its content.

Civil engineering has a close relationship with soil or earth surface. Every work of civil engineering starts with the soil. Soil mechanics plays a vital role to manage every earthwork in bridges, buildings, etc.

For various soil works, we should know about its profile, unit weight, density, strength, etc and we are discussing the unit weight of soil in this post.


Unit Weight of Soil

Unit weight of soil is defined as the ratio of the total weight of soil to the total volume of soil. It is also referred to as Specific weight.


Unit weight of soil = Weight of soil(W) / Volume of soil(V)

The soil has void spaces which may be filled with water or air. So, the weight of water and air are also included in the total weight and volume of soil while calculating the unit weight of soil.

Unit weight can be expressed as KN/m³  in SI unit and g/cc in the CGS unit.


Significance of Unit Weight of Soil in civil engineering

~ It is important to study the strength, durability and bearing capacity of the soil.

~ For large scale projects, the specific weight helps to study soil deformation. Thus, we can use precautions to prevent the failure of structures.


Types of Unit Weight of Soil

There are five types of unit weight based on water/air filled in voids of soil.

a. Bulk unit weight

b. Dry unit weight

c. Saturated unit weight

d. Sub merged unit weight /Buoyant unit weight

(Note: Total volume depicts: Volume of (solid +liquid + air) )


a. Bulk unit weight:

It is the ratio of the total weight of moist soil mass to the total volume of soil.

It is denoted by λ t .

λ t =  Wmoi. /V


W mois. = Weight of moist soil

V= Total Volume


b. Dry unit weight:

It is the ratio of the weight of soil in a dry state to the total volume of soil. It is denoted by λ d .

Water in voids gets evaporated and soil remains dry. Thus, the air is filled in voids.

Degree of saturation is zero for dry unit weight of soil.


λ d = W d /V


Wd = Dry weight of soil

V = Total Volume


c. Saturated unit weight:

It is the ratio of the total weight of saturated soil to the total volume of soil. Its degree of saturation is 100%.

It is denoted by λ sat .

It is given as;

λ sat = W sat. / V


W sat. = Saturated weight of soil

V= Total volume


d. Sub-merged unit weight:

It is the effective soil mass per unit volume of soil sub-merged below the water table. Void space of soil is filled with water and soil surface is also covered with water.

Here, the water applies an upward force to soil particles. The upward force is known to be a buoyant force.  It is based on the Archimedes principle.  So, the total force applied is equal to the weight of soil displaced by water. It is also known as Buoyant unit weight.

It can be used to calculate the effective stress of soil.

It is denoted by λ b or λ ‘ .


λ b = W sub / V


W sub= Effective Soil weight


It can also be defined as the difference between the saturated unit weight of soil and unit weight of water.

 λ b = λ sat – λ w


λ w = 9.81 KN/m 3 or 1 g/cc


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