Groundwater Pressure and Basements

Groundwater Pressure and Basements

Waterproofing articles

Why do basements and cellars suffer issues of water ingress? Simplistically, this is as a result of HYDROSTATIC PRESSURE coming to bear, but what is hydrostatic pressure? When the ground becomes SATURATED (all voids full of water), the weight of that water in a standing column exerts hydrostatic pressure.

As an example, if we were to take a paper cup and fill it up with water, that water would then apply hydrostatic pressure to the base and sides of the container.

If we imagine forming a small hole in the side of that container at low level, and one at high level, WATER WOULD PRESSURE AND SQUIRT OUT of the lower hole further than it would through the higher level hole.

This demonstrates both the effect of pressure resulting from water in a standing column, and that the deeper the water, the greater the pressure (more height = more weight = more pressure).

Also note that when we pour water into a container, it fills from the bottom up, so pressure always bears in the base of a container first, and rises as it fills.

This can be comparable to that which happens in the ground around a basement. Note that a variety of factors can result in water being present within the ground, and that is another post in itself, but a common cause can be water PERCHING (standing) on or within impermeable ground (clay for example), such that the ground above, then becomes SATURATED.

Where ground external of a basement structure becomes saturated, the basement structure is then comparable to the paper cup - if there are holes within it, water will pour through!

So this is a basic explanation of what hydrostatic pressure is, and how this can result in basement water ingress. Simple right?

NB: Hydrostatic pressure can be a significant force, often underestimated - see following photos.

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