Properties of Soils

Soil phase system: 

• Unsaturated soil mass consists of solids +water + air. It is three phase system
• Fully saturated soil consists of solids +water. It is two phase system
• Fully dried soil consists of solids + air. It is Two phase system.

Three phase system (Partially saturated soil)

Two phase system (Fully saturated soil)

Two phase system (Fully Dried soil)

Void ratio:  is the ratio of the volume of voids (V_v) to the volume of soil solids (V_s), and is expressed as a decimal.

• Range: Can have any value greater than zero.
• Some times ‘e’ may also be greater than 1.
• Void ratio for coarse grained soils is generally less than that for fine grained soils.
• When spherical grains are arranged in cubical array, the maximum possible e=O.91
• When spherical grains are arranged diagonally, the least void ratio, e = 0.35.

Porosity (n): is the ratio of the volume of voids to the total volume of soil (V ), and is expressed as a percentage.

 

Void ratio and porosity are inter-related to each other as follows: 
 and 

•  Range: 0 < n < 100%
• Relation between ‘n’ and ‘e’ is n = e/1+e

Degree of saturation (Sr): The volume of water (V_w) in a soil can vary between zero (i.e. a dry soil) and the volume of voids. This can be expressed as the degree of saturation (S) in percentage. 

For a dry soil, S = 0%, and for a fully saturated soil, S = 100%.

•   For fully saturated soils, Sr = 1 or 100 %
• For dry soils, Sr = O

Air content (a_c): is the ratio of the volume of air (V_a) to the volume of voids. 

•  for a saturated soil, a = O and
• for a dry soil, a = 1.

 Percentage air voids (n_a) : is the ratio of the volume of air to the total volume.

•  for a saturated soil, (n_a)= 0 and for a drySoil, (n_a)=n

Water content (or moisture content),w: The ratio of the mass of water present to the mass of solid particles is called the water content (w), or sometimes the moisture content. 

Its value is 0% for dry soil and its magnitude can exceed 100%.

•  Range: can have any value greater than equal to 0
• Some times it can be greater than 100%

Bulk unit weight of soil: is a measure of the amount of solid particles plus water per unit volume. 

Dry unit weight of soil:  is a measure of the amount of solid particles per unit volume. 

 Unit weight of solids: ys =Wt. of solids /VoL of solids 

• While ‘ys  is constant for a given soil the not constant.

Saturated unit weight (‘ysat):

• It is bulk unit weight when soil is fully saturated
•  It is also equal to total weight of soil when fully saturated per unit of total volume.

Submerged unit weight (‘Ysub or ‘y’)

•  It is the submerged weight of soil solids per unit of total volume of soil.
• Y = ‘Ysat - Yw
• It is based on Archmedis principle.

Specific Gravity of soil solids or soil(G or Gs):

•  It is the ratio of weight of a given volume of soil solids at a given temperature to the weight of an equal volume of distilled water at that temperature.
• As per IS code standard temperature for measuring ‘G is 27°C
• G=ys/yw
• For a given soil, G €remains constt. Generally G for inorganic soils lies between 2.60 - 2.85.

 Apparent or Mass or Bulk specific gravity soil:

Gm = Y/ Yw        for partially saturated soil

Gm=  Yd/ Yw      for dry soil

Gm = Ysat / Yw    for saturated soil

Gm is not constant.

Gm<G

Some important relationships 

1. 
2. 
3. 
4.

5.
6.
7.

Relationship between volumes of soil with respect to void ratio

V1 / V2 = 1+e1/1+e2

 Water content determination:

Oven drying method: This is the most accurate method among all.

Temperature to be maintained in the oven is 105° - 110°C for 24 hours

•  for highly organic soils a low temperature of about 60°C is preferable.
•  if Gypsum is present, temperature should  not be more than 80°C but for long  time

Specific gravity determination:

 Pycnometer method: It can be used for all types of soils

Oven dried soil sample is used.

• w1= wt. of pycnometer / empty container
• W2 = wt. of pycnometer + dry sample
• W3 = wt. of pycnometer + dry sample+water
• W4 = wt. of pycnometer + full water