Unit weight of any soil is the weight of soil sample to the volume of that sample.
Unit weight is necessary for the calculation of overburden pressure on soil particles at any depth within soil. There are several methods for the determination of unit weight of soil in its natural or compacted state.
Lets understand first method, core cutter method.
Core cutter is a cylindrical tube type container which is opened from both top and bottom side. It is sharp at one side so that it can be plunged into ground to take out soil sample.
To begin with first we determine the dimensions of the core cutter so that we can know its volume. We measure its internal diameter and height and calculate its volume.
Next we determine the weight of core cutter and say this weight as W1.
In the field we clean and level a small area from where soil is to be tested. Place the core cutter with its sharp edge on the surface and over the core cutter place a dolley which is nothing but another similar cylindrical tube type structure but smaller in length which acts as an open cap to the core cutter.
Then we use a rammer and give blows on the the dolley so that the whole arrangements plunges into the ground and only about 15 mm of dolley is visible above the ground.
Then remove the soil surrounding core cutter and take it out from the ground. Remove the dolley and trim all the extra soil from the top and bottom surface using a knife.
Now weigh this core cutter which has soil sample into it and note weight W2.
With these two weights we can calculate weight of soil sample as W2 minus W1.
Now unit weight being weight divided by its volume can be calculated as W2 minus W1 divided by volume of core cutter, which is also the volume of soil.
Now from this core cutter if we take out a small sample and determine the water content of the soil using any of the water content determination methods, then we can also calculate dry unit weight Yd of this soil using this formula.
Here Y is unit weight of soil that we determined in this test
and w is water content.
This method can not be used if soil is gravelly as gravel soil has large particle size and when we lift the core cutter, soil will not stay in to it and will fall down.
This method is also not applicable if soil is hard because core cutter may not be able to plunge into the ground or it may bring faulty sample.