Test of Unit Weight of Soil | Properties of Soil | Soil Mechanics

  Test of Unit Weight of Soil  

Test of Unit Weight of Soil can be determined using the following methods. (To calculate density 1. Calculate unit weight, 2. Density=Unit Weight/Gravity)

1. Water Displacement Method.
2. Submerged Mass Density Method.
3. Core Cutter Method.
4. Sand Replacement Method.
   

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1. Water Displacement Method

• This method is suitable for cohesive soil which is highly sticky in nature.
• In this method sample of soil is trimmed into more or less uniform shape and weight(W1).
• Sample is the coated with paraffin wax (to prevent the entry of water into the soil) and is again weight (W2).
• 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.

Weight of paraffin wax= W2-W1.

Volume of paraffin wax(Vp)=\(\frac{W2-W1}{γ_p}\)

(\(ρ_p=.908 g/ml\) )

Volume of water displacement Vw= Volume of coated soil.

Vw=Volume of soil(V)+ volume of paraffin wax(Vp).

Vw=V+Vp.

V=Vw-Vp

Unit Weight =\(\frac{Weight of soil}{Volume of Soil}\)

Unit Weight =\(\frac{W1}{V}\)

V= Vw-\(\frac{W2-W1}{γ_p}\)

Unit Weight =\(\frac{W1}{Vw-\frac{W2-W1}{γ_p}}\)

2. Submerged Mass Density Method

• This method is suitable for cohesive soil which is 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(Vp)=\(\frac{W2-W1}{γ_p}\)

(\(ρ_p=.908 g/ml\) )

Submerged weight of coated specimen(W3)= W2-Fb.

Fb= Force of Buoyancy= Vw\(γ_w\)

W3=W2-Fb

W3=W2-Vw\(γ_w\)

Vw=\(\frac{W2-W3}{γ_w}\)

Vw=Volume of soil(V)+ volume of paraffin wax(Vp).

Vw=V+Vp.

V=Vw-Vp.

V=\(\frac{W2-W3}{γ_w}\)–\(\frac{W2-W1}{γ_p}\)

Unit Weight =\(\frac{Weight of soil}{Volume of Soil}\)

Unit Weight =\(\frac{W1}{(\frac{W2-W3}{γ_w})-(\frac{W2-W1}{γ_p})}\)

3. Core Cutter Method

• It is field method which is suitable to be used for cohesive soils.
• This method is generally not suitable for gravel, sand and dry soil.
• It is generally used for soft silt and clay.
• This method consist of a core cutter of volume 1000cc (Diameter=10cm, Height=13cm) is pushed into the ground and then core containing soil is taken out from the ground.
• Weight of core with soil sample (W2) and empty core(W1) is measured & V=1000cc. Hence Unit Weight is given as:

Unit Weight=\(\frac{Weight of Soil}{Volume}\)=\(\frac{W2-W1}{V}\).

4. Sand Replacement Method

• Since core cutter method in not suitable for hard & gravel soil, sand replacement method is used in this case.
• It is field method.
• A small pit is excavated and the excavated soil sample is weighted.
• 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.
• It is suitable for gravel, sand and dry soil.
• the bulk unit weight is calculated as:

Unit Weight=\(\frac{Weight of Soil}{Volume}\)

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