If the water table rises upto ground surface, then the A. effective stress is reduced due to decrease in total stress only but pore water pressure does not change B. effective stress is reduced due to increase in pore water pressure only but total stress does not change C. total stress is reduced due to increase in pore water pressure only but effective stress does not change D. total stress is increased due to decrease in pore water pressure but effective stress does not change

The correct answer is A.

Effective stress is the stress that acts on the soil particles and is responsible for the strength and deformation of the soil. It is calculated as the difference between the total stress and the pore water pressure.

When the water table rises, the pore water pressure increases. This results in a decrease in effective stress, as the total stress remains the same. The decrease in effective stress can lead to a decrease in the strength and an increase in the deformation of the soil.

Option B is incorrect because the total stress does not change when the water table rises. Option C is incorrect because the pore water pressure increases when the water table rises. Option D is incorrect because the effective stress decreases when the water table rises.