Determintion of unit weight

Definition of Bulk Density or "B.D."

Soil weight is referred to as soil bulk density. Density is the mass of material contained within a given volume. This is the idea that a given size of box may be heavy or light, depending upon what kind of material it contains. If the box were filled with wood, it would be light when compared to having it filled with lead. The weight of water is the reference for density measurements: 1 gram of water=1 cubic centimeter (cc), & 1 cc water=1 ml. & 1 cubic foot of water=62.4 lbs.

The bulk density of a soil is the mass of dry soil per unit of "bulk" (total volume of soil or soil particles & pore space).

B.D. = mass of oven dry soil (grams) ÷ total volume of soil (cm3)=grams / cc

The bulk density takes into account the total soil volume (the space occupied by the solid particles plus the space occupied by the air of the pores or pore space). To determine the mass of the soil--since air does not have any significant weight--we can just weigh the oven dry soil on a balance. The volume of the soil can be determined by pouring the soil into a graduated cylinder and measuring the volume that it occupies.

The problem with this procedure is that structural aggregates may be crushed or compacted once they are removed from the

soil and placed into the cylinder. A better procedure is one in which the aggregates could be removed from the soil and frozen exactly the way they were in the soil. Plastic fixatives enable us to do this.

A large aggregate has been removed from the soil and is being fixed by dipping it into a saran solution.

This will make the clod impervious to water.

The weight of our clump of soil can be obtained by hanging it on the balance or placing it on

the metal weighing tray.

The volume of the clod also needs to be determined. This can be obtained by weighing the sample again, only this time the sample will be in water. The weight of the

sample now will be less, since the sample will be buoyed up by the amount of water it is displacing.

Archimedes most famous theorem gives the weight of a body immersed in a liquid, called Archimedes' principal. If you want to know more about the mathematician who determined that this procedure would work, go to Archimedes .

Thus, by subtracting the weight of the clod in water from its weight in air, we obtain the weight of the water displaced by the clod, which equals the volume of water

displaced by the clod (because 1 cm³ of water=1 gm. of water) and is equal to the volume of the soil clod.

Clod Method of BD Determination =Mass of Clod ÷ Volume of water displaced

Volume of water displaced=weight of water or the (gms clod in air) - (gms clod in water)=(grams or volume of water displaced)

Cylinder method of BD Determination=Mass of oven dry soil (gms) ÷ total volume of soil (cm³)

Another method of determining bulk density is by using soil cores. These are obtained with a probe that forces a cylinder into the soil. The soil can be removed from this cylinder and cut into sections and placed in another cylinder for transportation back to the laboratory. Care needs to be taken to avoid compacting the soil during and after

obtaining the core.

BD of soil (using core method)={(mass of soil + mass of core) - (mass of core} ÷ (volume of core)

volume of core=Pi (3.1416) r² × hr=radius of core ; h=the height of core.

For the cores in the lab, r=2.45cm (½ of Diameter); h=5 cm

V=3.1416 × 2.45² × 5=94.3 cm3;

Bulk density is a fairly easy determination that will yield information about the soil that will be significant for determining the soil's potential for plant growth or a building foundation. Remember it is the oven dry mass/volume soil. or g/cc