UNIT WEIGHT OF SOIL :- Unit weight of soil can be determined using following methods.

Core-Cutter Method

• It is field method.
• Core cutter of known volume 1000 CC is pushed into the ground and then core containing soil is taken out from the ground.
• Weight of with soil sample (W2) and empty core (W1) is measured. Hence bulk unit is given as

       γb = Weight of soil / Volume

             = W2 – W1 / V

• The measuring water content of given sample, dry unit weight can be Calculated as

        γd = γb / 1 + W

• This method generally not suitable for gravels, sand and dry soil.
• This is generally using for soft silt and clay.

Water Displacement Method

• This method is generally used for choesive soil that are highly sticky in nature.
• In this method sample of soil is trimmed into more or less uniform shape and weighed ( W1).
• Sample is then coated with paraffin wax and is again weighd (W2), Coating prevent entry of water into sample.

• The coated specimen of soil is immersed slowly in a container, that is completely filled with water and volume of water displaced by coated specimen is Vw.

  Volume of paraffin wax = W2 – W1 / γpw

                                                = W2 – W1 / Gpw γw

  Vw = Volume of soil + Volume of paraffin wax

  Volume of soil = Vw – Volume of paraffin max

                              = Vw – W2 – W1 / γpw

  Unit weight of soil = Weight of soil /       Volume of soil

                                 = W1 / Vw – { W2 – W1 /γpw }

Submerged Density Method

• This method is used for cohesive soils which are very highly sticky in nature.
• In this method submerged mass of coated specimen W3 is noted to find volume of water displaced by soil + Paraffin wax.

Weight of paraffin wax = W2 – W1

Volume of paraffin wax = W2 – W1 / γpw

                                              = W2 – W1 / Gpw γpw

Submerged weight of coated specimen = W2 – Fa

Fb = Force of buoyancy = Vwδw

W3 = W2 – Fb

        = W2 – Vwδw

Vw = Volume of soil + Volume of paraffin wax

       = W2 – W3 / γw

Volume of soil = Vw – Vpw

                            = W2 – W3 / γw – W2 – W1 / γpw

Unit weight of soil

          = W1 / (W2 – W3 / γw – W2 – W1 / γpw )

Sand Replacement Method

• It is field method.
• A small pit is excavated and the excavated soil sample is weighed.
• A calibrated cylinder containing sand is placed over the excavated pit and is filled with sand.
• Volume of the pit is obtained from the calibrated cylinder.
• The bulk unit weight is calculated as

       γb = Weight / Volume

• It is suitable for gravel , sand and dry soil.