When the level of ground water increases, shear strength will ___________?

When the level of groundwater increases, the effective stress within the soil is reduced, which directly influences the soil's shear strength. The shear strength of soil is primarily determined by its effective stress, according to the formula:

[ \tau = c + \sigma' \tan(\phi) ]

where ( \tau ) is the shear strength, ( c ) is the cohesion of the soil, ( \sigma' ) is the effective normal stress, and ( \phi ) is the angle of internal friction.

As groundwater rises, the total stress remains constant, but the pore water pressure increases, leading to a decrease in effective stress (i.e., ( \sigma' = \sigma - u ), with ( u ) being the pore water pressure). Since shear strength is dependent on effective stress, a reduction in effective stress results in a decrease in shear strength.

Therefore, when groundwater level increases, the overall result is a decrease in the shear strength of the soil, making the correct answer decrease.