The correct answer is: Unit weight of soil decreases due to submergence in water.
The unit weight of soil is the weight of a unit volume of soil. It is expressed in units of pounds per cubic foot (lb/ft3) or kilograms per cubic meter (kg/m3). The unit weight of soil is affected by the following factors:
- The void ratio of the soil: The void ratio is the ratio of the volume of voids to the volume of solids in the soil. A higher void ratio means that there is more space between the soil particles, and the soil will have a lower unit weight.
- The water content of the soil: The water content of the soil is the ratio of the weight of water to the weight of solids in the soil. A higher water content means that the soil particles are surrounded by more water, and the soil will have a lower unit weight.
- The effective stress on the soil: The effective stress on the soil is the difference between the total stress on the soil and the pore water pressure. A higher effective stress means that the soil particles are more tightly packed together, and the soil will have a higher unit weight.
When soil is submerged in water, the water pressure in the pores of the soil increases. This increase in pore water pressure reduces the effective stress on the soil, and the soil particles become more loosely packed. As a result, the unit weight of soil decreases when it is submerged in water.
Option A is incorrect because the unit weight of wet soil is less than the unit weight of dry soil.
Option B is incorrect because the dry unit weight of soil is always greater than the total unit weight of soil. This is because the total unit weight of soil includes the weight of both the solids and the water in the soil, while the dry unit weight of soil only includes the weight of the solids.
Option C is incorrect because the unit weight of soil decreases due to submergence in water.