In these trying times, it’s nice to know that there are some things that stay consistent. The sun rises in the East and sets softly in the West. The birds sing to each other as they have for eons. And it is raining like a son-of-a-*$&#@ here in Pittsburgh. Pittsburgh is one of the “Dreariest Cities in America.” Please…please…hold your applause. On average, Pittsburgh has 140 days per year of precipitation. It gets a little wet around these parts and it can affect how designers protect a building against the intrusion of groundwater.
A typical beautiful day in Western Pennsylvania.
You may have read the term “hydrostatic pressure” when skimming through geotechnical reports. Hydrostatic basically means “resting water”. As the water level rises, it creates pressures which can force water into areas where it otherwise might not go. This is because water is heavy.
It can even crush cars!
Ideally, you want to build the foundations of your building above the water table. The water table is the upper level of an underground surface in which the soil or rocks are permanently saturated with water. The water table fluctuates both with the seasons and from year to year because it is affected by climatic variations and by the amount of precipitation used by vegetation.
Now, to make things a little bit more confusing. Almost, every project will see hydrostatic pressure at some point, maybe after a big rain or a snow melt (Intermittent Hydrostatic Pressure). But typically, once the rain stops or the snow stops melting, the water table will lower back to normal. When a foundation wall is above the water table and may only see some intermittent hydrostatic pressure, a designer may add some kind of perimeter drainage system to help keep the structure dry. This could be a Trench Drain, a sump pump system, or a French Drain (that’s just a normal drain…with tongue).
However, some projects require that the foundation walls sit below the water table (constant hydrostatic pressure). If the project has walls that will sit below the water table, then there’s really no point in designing drainage, is there? Because there is a constant source of water, pumping it or directing it elsewhere is tantamount to Sisyphus pushing the rock up the hill for eternity. It is a fruitless endeavor. The water will always be there, no matter what you do. Take a look at the CETCO Waterproofing Membrane details below. The one marked “Non-Hydrostatic” shows a drainage board, likely directing the water to a drainage system of some kind. The one marked “Hydrostatic” doesn’t have a drain board, because there is no where for the water to drain to!
Remember that water is the #1 source of construction litigation today. So to combat water, you need to think like water. Understanding the difference between intermittent and constant hydrostatic pressure is a great starting point.