Technically speaking, hydrostatic pressure is
“The pressure exerted by a fluid at equilibrium at a given point within the fluid, due to the force of gravity. Hydrostatic pressure increases in proportion to depth measured from the surface because of the increasing weight of fluid exerting downward force from above.” – Dictionary.com
But what does this mean for someone doing a construction project? What forces does a construction manager (or industrious home do-it-yourselfer) have to consider when waterproofing below-grade or even below-water walls and foundations?
This means, in part, that a product simply being water-repellant or even waterproof in common testing in a laboratory is not the same as that product keeping out water in a wall or foundation that will be exposed to water.
How does hydrostatic pressure impact waterproof surfaces?
Hydrostatic pressure is not constant in a given spot over time. This is because the nature of hydrostatic pressure means that it changes based on the depth of the water.
Why? There are two variables at work. First is the water itself. Second is the depth of the water or, more precisely, the pressure exerted by the weight of all the water above it.
You have experienced this pressure before if you have ever dived into deep water, like the deep end of a pool, or if you have gone a few body lengths below the surface when snorkeling or scuba diving.
If you have done those things, you have felt hydrostatic pressure against your eardrums. The additional pressure of the weight of the water above compressed your ear drum. At first it just muffled the sounds, but then, if you dove deep enough, the pressure even may have caused some pain.
This increased pressure works against your waterproof eardrum membrane in much the same way as it works against a waterproof wall membrane.
Eventually, if the water gets deep enough and the pressure gets high enough, the pressure causes a fissure or rupture.
Or, even if there is not a large leak, water can still be forced through the tiny pores and pockets that are part of a concrete pour. Over time, this water gets forced in and adds to the degradation of the wall and the foundation.
This is why your building project must account for water around the foundation. And it must take into account the effects of hydrostatic pressure.