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How do I remove hydrostatic water pressure?

John Beavers | Posted in General Questions on

We purchased a home in central Missouri recently and we see evidence of slab/footing movement. The home is built halfway down a hill. It has a walkout basement with a large patio off the walkout. The downhill side of the walkout has risen about an inch. There is a perimeter drain along the front and down the downhill side until the foundation wall begins the step down toward the patio, so it covers the front and half of one side. If I extend the drain to the patio will that help with the water pressure? I need to mention that the soil is clay.

A foundation repair company lasered the basement so that’s how I know the corner has risen. They want to install a sump pump and lay lateral lines to several locations under the interior slab. That means jack hammering the slab to install the pump and lines. I would like to be sure that it would help before cutting up the slab. Can anything be done outside to get rid of all the water? We have no water in the basement. The foundation is 10″ poured concrete.

John

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    John,
    The way to reduce hydrostatic pressure against a foundation wall is to install a good drainage system on the exterior of your foundation, at the level of the footing.

    This drainage system normally consists of perforated drainage pipe around the perimeter of your foundation, connected to a conductor pipe leading to daylight, to a sump, or to a distant drywell.

    The perforated drain pipe is usually surrounded by several inches of crushed stone, and the crushed stone is wrapped in a "burrito" of landscape fabric to keep out silt.

    Ideally, the excavation is backfilled with free-draining material like crushed stone or gravel, topped with a thin layer of clay-bearing soil, sloped away from the foundation.

    This type of drainage system sometimes includes a dimpled plastic drainage mat applied to the exterior of the foundation wall.

    If you have reason to believe that some of these elements are missing or clogged, it is probably cheaper to install an interior French drain. This is a trench around the perimeter of your basement, near the foundation walls, filled with crushed stone and a perforated drain pipe, leading to a sump.

    In all cases you need to be sure that the soil around your house is properly graded (sloping away from the house) and that roof water is collected in gutters connected to conductor pipes that convey the roof water far from the foundation.

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