What Depth of Foundation for a Brick Wall?

Significance

• The brick wall’s foundation serves as an anchor for the structure’s horizontal and vertical loads. While the foundation's width is directly related to the horizontal footing area, the depth determines the vertical surface area bracing against the soil. Strong foundations provide vertical support and create enough friction against soils to minimize the wall’s movement. Also, a wider and deeper foundation helps prevent the wall from sliding, overturning or being pulled out of the ground due to lateral forces. When a brick wall's foundation is built at the proper width and depth, it is able to safely support the weight of vertical and horizontal loads during events such as earthquakes.

Water Level

• You should determine the soil's water level prior to constructing a brick foundation wall. Expansive soil with a high water level may cause the wall's foundation to move, resulting in damage to the overall structure. Likewise, you should locate the water table depth to determine if the soil condition is suitable for building and supporting heavy loads. The U.S. Department of Energy “Builder’s Foundation Handbook” states that a water table depth of 8 feet or less from the surface is unsuitable for foundation walls at the basement level. Draining and pumping the water may be too expensive. A high water table also restricts the wall foundation's design, as soil with significant moisture swells and shrinks over time.

Soil Type

• Soil properties influence the depth that will enable your wall to resist structural loads. Coarse-grained soils such as gravel and sand lack moisture and contain particles that do not hold together well. As a result, the dryness of the soil can cause a brick foundation's walls to shift horizontally.

  On the other hand, fined-grained soils such as silts and clays provide a high level of cohesiveness. According to “Excavation, Trenching and Shoring Safety and OSHA’s Excavation Standard,” a high clay content provides greater stability for trench walls. Soils containing grass, leaves, branches and other organic materials are considered unstable and are not recommended for brick walls or any other foundation. Consult a geotechnical or soil engineer to find out your property’s soil type and whether it will impact the building depth for the wall's foundation.

Loads and Soil Condition

• The structural loads supported by brick walls will also determine proper depth for the wall's foundation. For example, measurements for the foundation depth should be based on the number of floors in the building. Loads should also be examined in conjunction with the soil’s condition. The Association of Bay Area Governments publication “Foundations” advises builders to look out for signs where soil has been disturbed or excavated. Soils that have been previously dug up are considered dangerous, since layered soils provide weak support for foundation walls. If you suspect that the soil is too expansive, “Foundations” authors Richard Chylinski and Timothy P. McCormick suggest consulting an architect or engineer for determining the structural integrity of foundations built less than 24 inches deep. This also applies to foundations supporting buildings with one, two or three floors.