Why do foundations fail?

And when is foundation repair necessary? A foundation’s main job is to hold up the house. Often, however, the biggest load on the foundation is not the weight of the house, but the pressure of the soil around the foundation.

Called the lateral load, it is the amount of pressure exerted on basement walls and can vary widely. It depends on many factors, including what type of soil surrounds the foundation of your home, how wet that soil is and how deep the basement/foundation extends beneath the surface.

Coarse-grained, well-drained soils like sand and gravel apply the least lateral load, while fine silts and clays, especially if saturated, apply more load.

The deeper the basement, the greater the pressure. The designer and builder must understand the importance of these factors in order to design and build a foundation that can stand up to lateral loads. Since soil engineering is not an exact science and loads are hard to predict accurately, all foundations should have a large, built-in margin of safety.

The Effect of Water

Soils are mixtures of mineral particles, water, and air. They’re not quite solid and not quite liquid, but they can flow. The more water a soil contains, the more it behaves like a fluid. As water lubricates the small soil particles, they slip more easily past each other.

Clays are particularly affected by moisture: A clay embankment that stands up by itself when dry may turn to mud and slump like pudding when wet. That’s why effective drainage systems are a critical element when building in clay soils.

Groundwater exerts its own pressure on basement walls as it seeps through the earth. The water pressure against a basement when the soil is thoroughly soaked is called hydrostatic pressure. It can force moisture through pores in the basement wall and even crack or buckle the wall itself. Here again, proper drainage is the remedy.

Digging Deeper Increases Pressure

Digging-equiptmentHydrostatic pressure and soil pressure increase with depth. Just as a submarine experiences more stress on the hull as it dives deeper, a basement wall faces more lateral pressure deep underground than it does near the surface. Other things being equal, deep foundation walls need more strength than shallow ones.

Cracked, Bowed or Bulging Walls

Horizontal, open cracks are typically a sign that your basement wall is beginning to push in or even buckle. Horizontal cracks are often caused by frost heave (expansion of water when it freezes) against the wall or heavy wet soil from hydrostatic pressure. These types of cracks may denote a structural issue and should be addressed immediately.

Vertical cracks in poured concrete walls may be a result of settling. These types of cracks often occur inside the corner of a window opening or beam pocket. Vertical cracks in block basement walls is often a sign of blunt trauma to the outside wall often done during the back fill process.

Step cracks usually appear from minor settling in block basements. Fine step cracks in isolated areas are usually not structural. These may require minor repair to prevent moisture from leeching into your basement.

Open step cracks are more of a concern. This means that more substantial settling has taken place. If you see dirt, mud, sediment or stains on the wall or floors around the cracks, this means the soil outside is pushing through the cracks. This should be addressed immediately.

Bowed or bulging basement walls means your wall is buckling from the pressure exerted on it by the outside dirt and water. If this is not treated, eventually your basement wall may collapse and further damage may be caused.

A bowed wall typically can be repaired using structural support and wall bracing. It is also imperative to reduce the pressure on the outside wall. This typically requires the replacement of the exterior drainage system to reduce the soil moisture and hydrostatic pressure being exerted on the wall.

Cracked basement floors, where part of the slab appears lower than the rest, is a sign of floor settling. Large open cracks in the floors are gateways for water and insects to infiltrate your home and should be addressed.

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