Here are some points to be remembered while calculating the effective stress in the region of the soil where the capillary rise of water takes place.
Capillary action and Effective stress:
Capillary rise in the soil strata may saturate the soil and it has the effect of the effective stress. If the water table is below the soil strata saturated due to the capillary action then the effective stress is increased by an amount equal to the depth of the water table multiplied by the density of the water risen to that soil strata.
So the water pressure becomes negative here so, instead of decreasing the effective stress, this increase the effective stress in the saturated region by an amount equal to the surcharge weight of the water of the same depth.
If 'h' is the capillary rise of water above the water table, then the soil will have its effective stress increased by an amount equal to "h.dw".. where dw is the density of the water.
If the water table reaches the soil strata, the capillary meniscus gets destroyed and so the negative water pressure will disappear.