Unit weight of Soil

The bulk Unit weight of soil is defined as the ratio of the total weight of the soil sample and its total volume.

                                            ${{\gamma }_{t}}=\frac{W}{V}=\frac{{{W}_{s}}+{{W}_{w}}}{{{V}_{s}}+{{V}_{w}}+{{V}_{a}}}$

Unit weight of soil solids is defined as the weight of soil solids per unit volume of solids alone.

                                            ${{\gamma }_{s}}=\frac{{W}_{s}}{{V}_{s}}$

The dry unit weight of soil is defined as the weight of soil solids (or dry soil) per unit volume of soil.

                                            ${{\gamma }_{s}}=\frac{{W}_{s}}{V}$

The saturated unit weight of soil is defined as the bulk unit weight of soil mass in the saturated condition.

                                            ${{\gamma }_{sat}}=\frac{\text{Weight of saturated soil}}{\text{Volume of soil}}$

The submerged unit weight of soil is defined as the submerged unit weight of soil solids per unit Volume. When soil exists below groundwater, the buoyant force acts on the soil solids and reduces its unit weight.

                                            ${{\gamma }_{sub}}=\frac{{{W}_{sub}}}{V}={{\gamma }_{sat}}-{{\gamma }_{w}}$

Different methods to find unit weight of soil

1. Core Cutter Method

   
   It is field method which is based on IS 2720 Part 29. A core cutter is a cylindrical seamless steel tube of internal diameter 100 mm and 130 mm long with 3 mm thick wall. Steel Dolley has 25 mm height with an internal diameter of 100 mm and a wall thickness of 7.5 mm with a lip to enable it to be fitted on the top of the core cutter.
   
   
   
   
   
   

   Procedure:

   1. Weigh the empty core cutter accurately, say ${W}_{c}$
   2. Remove the top layer of soil of an area of approximately $30 {cm}^{2}$.
   3. Place the dolly on the soil and rammed it down vertically into the soil layer until only 15 mm of the dolly protrudes above the surface.
   4. Dug out the cutter of the surrounding soil carefully and trim flat the soil core.
   5. Weigh the cutter containing soil, say ${W}_{s}$
   6. Remove the soil core from the cutter and take a representative soil sample in an airtight container for its water content determination $w$.
   
   

                         $\text{Bulk unit weight, }{{\gamma }_{t}}=\frac{{{W}_{s}}-{{W}_{c}}}{{{V}_{c}}}$
                         $\text{Dry unit weight, }{{\gamma }_{d}}=\frac{{{\gamma }_{t}}}{1+w}$




2. Water Displacement Method

   
   In this method Volume of the soil specimen is determined. The soil mass is coated with paraffin wax to make soil specimen impervious to water as when soil comes in contact with water it disintegrates.
   
   

   Procedure:

   
   
   1. Trim the soil specimen into a more or less regular shape and weigh it, say $W$
   2. Coat the specimen with thin layer of paraffin wax by dipping it in molten wax and weigh it after it cools down, say ${W}_{t}$
   3. Immerse the waxed specimen in a water displacement container. And the volume of water that comes out of the flow tube will be the volume of the waxed specimen, say ${V}_{t}$.
   4. Peel off the wax out of the specimen for determination of its volume from its mass and its density ${\rho}_{p}$
   5. Take a representative soil sample in an airtight container for its water content determination $w$.
   
   

                         $\text{Volume of soil specimen, V}={{V}_{t}}-\frac{{{W}_{t}}-W}{{{\rho }_{p}}}$
                         $\text{Bulk unit weight of soil,}{\gamma}_{t}=\frac{W}{V}$
                         $\text{Dry unit weight, }{{\gamma }_{d}}=\frac{{{\gamma }_{t}}}{1+w}$




3. Sand Replacement Method

   
   This is a field method based on IS 2720 Part 28.
   
   

   Procedure:

   
   
   1. Excavate a small area and weigh the excavated soil precisely.
   2. Place a calibrated cylinder containing sand over the excavated area and fill the pit with sand.
   3. Find the volume of the pit from the calibrated cylinder.
   4. Take a representative soil sample in an airtight container for its water content determination $w$.
   
   
   
   Once the volume and water content is known, bulk unit weight and dry unit weight is calculated as shown in the core cutter method.

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