Hydrostatic pressure should be determined by a civil engineer prior to waterproofing design. A general rule in determining hydrostatic pressure is that it increases linearly with depth, which produces a triangular horizontal loading pattern where pressure is exerted from below the structure, above the structure and at the wet face of the structure. The formula to determine hydrostatic pressure at a depth of 10 feet is 62.4 x 10 = 624 psf in all directions.
Hydrostatic pressure occurs at a building component anytime the water table raises above the component and it is directly related to ground water levels. Water rises in most soils by capillary action. The rise can be as extreme as 11.5 feet in soils made of small particles (clay and silt) to zero in granular (gravel) soils that have large spaces between the particles. When ground water levels are at the highest points – in the spring, after flooding from rains, heavy run-off from walls or from clogged drains – the hydrostatic pressure rises. These changes in pressure can occur on an hourly basis. The designer should specify waterproofing to meet the highest pressures under extreme conditions. This can be calculated by one of three methods:
P = wd on top slab
P = w(dth) lateral pressure at the base of the wall
P = w(dth) upward resistance on the slab-on-ground
Slabs-on-ground that are subjected to hydrostatic pressure must be properly designed to resist uplift from hydrostatic pressure. This can be accomplished in one of the following methods:
1. Increased slab weight (to counterbalance upward hydrostatic pressure)
2. Reinforcing the slab for flexural resistance and anchoring it to foundations and
3. Tying the slab to rock anchors
Installing under-slab drains and footing drains that are directed to the storm water system can also accomplish prevention of hydrostatic pressure.
The intensity of the hydrostatic pressure is also an important consideration in the selection of
waterproofing materials. When subjected to continuous hydrostatic pressures, membranes with low moisture-absorbance rates (butyl rubber) tend to perform well. High moisture-absorbance membranes can be subjected to swelling, disbanding and wrinkling in these conditions.
Wrinkled membranes are subject to an increased risk of puncture. Intense hydrostatic pressure can also force membranes in concrete voids exposing the membrane to cracks from stress created by flex allowing openings for moisture infiltration.
Hydrostatic pressure can be harmful to a building's foundation and not every product is up to the task of handling it.
This makes it an ideal product for above and below grade waterproofing, as it is proven to be able to handle the elements.