Basement Foundation Wall Collapses
As we have entered Spring it seems appropriate to discuss basement foundation wall collapses. While these types of claims can occur throughout the year, we are most often retained to investigate these claims in early Spring.
With basement foundation walls it is first important to understand that these walls function as both a bearing support wall for the building superstructure above as well as an earth retaining wall separating the soil on the outside of the wall from the living/interior space on the inside of the wall. These two (2) functions result in vastly different loading being applied to the foundation wall. The superstructure above applies gravity loads at the top of the wall while lateral soil pressure, soil vibration, and hydrostatic pressure from groundwater apply forces to the exterior below grade portion of the wall.
When a basement foundation wall collapses, we are often asked to investigate and report on the cause of the collapse, in addition to emergency handling support and repair design. Sometimes the cause of the collapse may be obvious, such as a vehicle impact event or improper original design or construction. Other times the cause of the collapse is less obvious, and a careful investigation is required. Often, multiple less obvious causes work in combination to cause the ultimate failure and collapse. Some examples of these causes are outlined below:
Lack of Lateral Support
In common residential foundation design, lateral support is provided for the basement foundation wall at the bottom, typically by a concrete slab, and at the top, typically by the main floor structural framing. Both elements are required to ensure the intended loading/support relationship. During construction or renovation, a floor framing or the basement slab may be temporarily removed or modified, compromising the lateral support for the wall. This can lead to a catastrophic collapse of the wall. Temporary bracing should be provided to support the foundation at these vulnerable times.
Through age-related deterioration or rot after improper treatment of water damage for example, floor framing can weaken, compromising the lateral support for the wall. When this happens, the wall may start to shift at the top as an early warning sign before ultimately collapsing.
Poured concrete with steel reinforcement is much stronger than concrete without steel, while concrete block is even weaker. Block walls are often seen to crack at mid height and start to move inwards, since there is no lateral support at that height on the foundation.
Surcharge Loading
Foundation walls built as per the requirements of the Ontario Building code are designed to resist lateral soil pressure, soil vibration, and hydrostatic pressure. When unforeseen loads are applied to a basement foundation wall, the wall can be weakened, crack, and/or collapse. One of the most common unforeseen loads would be surcharge loading caused by heavy equipment or vehicles operating immediately adjacent to the foundation wall. This is especially common during construction and renovation work. When heavy equipment or vehicles operate next to a basement foundation wall, the adjacent soil is compacted, amplifying the lateral soil pressure on the wall. As a rule of thumb, it is best to keep heavy equipment and vehicles away from foundation equal distance to the depth of the foundation below grade.
Hydrostatic Pressure
Hydrostatic pressure is water in the soil applying a lateral force to the exterior face of the foundation wall. Hydrostatic pressures peak during periods of heavy rain or snow melt in the spring, which is why we see the most claims for basement foundation wall collapses during that time. If a foundation wall is built as per the requirements of the Ontario Building Code, it should be adequate to withstand typical hydrostatic pressures.
When hydrostatic pressure is found to be the ultimate cause of a basement foundation wall collapse, other factors are usually in play as well, such as lack of lateral support, age-related deterioration or preexisting damage. In these cases, hydrostatic pressure is sufficient to cause a collapse of an already structurally compromised wall.
Often in older homes, below slab sump pumps are installed to keep groundwater below foundation level, thus preventing or limiting hydrostatic forces on older foundation walls more susceptible to water penetration and/or a hydrostatic pressure failure. Sump pump failures allow an increase in the exterior pressure on the wall and can lead to leaks and collapses. Blocked foundation drains (weeping tile) can have the same impact.
These are just three possible causes of a foundation wall collapse, and they often combine to cause the ultimate collapse. Early warning signs such as foundation shifting, bowing, cracks, or water penetration should prompt an inspection of the foundation to hopefully prevent such a collapse from occurring.
The staff at Caskanette Udall Consulting Engineers specialize in foundation assessments and remedial measures to prevent foundation collapses. When collapses do occur we provide fast emergency support for shoring design, a structural scope of repair and drawings for building permit to repair the damage.
