It generally varies over depth as a function of insitu confining stress at a given depth. Note that larger confining stress reduces void ratio over the historical time. By the decrease of void ratio, undrained shear strength increases.

What affects undrained shear strength?

In reality, soil is partially drained, somewhere between the perfectly undrained and drained idealized conditions. The shear strength of soil depends on the effective stress, the drainage conditions, the density of the particles, the rate of strain, and the direction of the strain.

Does cohesion increase with depth?

It is shown that the rate of increase of cohesion with depth plays the same role as density plays in the bearing capacity of homogeneous cohesive-frictional soils. ... The analysis can also be applied to the stability of embankments on soft soils.