Once you have a clogged exterior footing drain, water begins to build up more and more around your house as the backfill becomes saturated. This added weight creates an enormous amount of force (known as hydrostatic pressure) on the walls. This pressure pushes the water into the basement through cracks and joints, with water most commonly being pushed through the floor-wall joint. Hydrostatic pressure on the walls also pushes water vapor through the porous concrete of your home, where the drier air inside of your basement sucks it up and makes your basement air more humid.
Properly designed and constructed footing and foundation walls are often strong enough to hold up against the forces of nature for many years. However, the combined pressures of gravity, soil swelling with water, freezing and thawing, hydrostatic pressure, and tree roots, among other forces, are all working together to break through. Sooner or later, a crack will form, a seal will break, or something else will give way. Exterior drains don't work- you're going to need to find another way to protect your basement.
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