The correct answer is D. Effective stress is the stress at particles contact, a physical parameter that can be measured, and important because it is a function of engineering properties of soil.
Effective stress is the stress that acts on the soil particles and is responsible for their movement. It is calculated by subtracting the pore water pressure from the total stress. Effective stress is important because it is the driving force for soil deformation and failure. It is also used to calculate the bearing capacity of soil and the stability of slopes.
Effective stress can be measured in a number of ways, including the use of piezometers, tensiometers, and pressure cells. Piezometers measure the pore water pressure, tensiometers measure the suction in the soil, and pressure cells measure the total stress.
The engineering properties of soil that affect effective stress include the soil’s void ratio, permeability, and compressibility. The void ratio is the ratio of the volume of voids to the volume of soil solids. The permeability is the ability of the soil to transmit water. The compressibility is the ability of the soil to change its volume under load.
Effective stress is a critical concept in soil mechanics and is used in a variety of engineering applications.