Block walls have been used in basement construction for years. In unsupported basement block walls, concrete blocks are laid to form the wall, which supports the building structure (Figure 1). Back-filling of the soil at the exterior of the wall forms the grade, which diverts water away from the wall. The wall system is relatively easy to install and low in initial cost.

Maintenance of the wall requires keeping water away from the foundation and sealing water leakage through joints. With lack of maintenance, back-fill settling and excessive water transportation to the foundation can occur, causing the block wall to fail from hydraulic pressure (Figure 2).

Water pressure places a significant load against the wall, causing a deflection inward toward the basement interior. An average water pressure of 0.5 psi (72 psf) places a total force of almost three tons against a 10-foot by 8-foot concrete block wall. The inward deflection of the wall forms a slight void in the soil, which aggravates the condition by causing additional settling. This funnels more water toward the foundation, increasing pressure, inward deflection, and the soil void.

Without proper maintenance, final failure of the wall occurs as depicted in Figure 3. Typically, the blocks cave inward along with a significant amount of soil. Many times, the resulting loss of the foundation wall requires immediate shoring to prevent the building from collapsing downward into the basement.

Such a failure occurred in the basement shown in Figure 4. It had rained heavily the night before, and the insured heard the wall collapse during the night. Temporary shoring was required to hold the house in place.

Figure 5 is a view of the corner of the block wall, which shows long-term settling of a sidewalk near the wall that collapsed (as indicated by the arrow). This is evidence that the wall had deflected progressively over time until final failure occurred, because the increasing soil void against the foundation results in grade level settling.

Failure of concrete block walls also can result from design defects. For example, if a long wall is constructed without proper support, it is more prone to failure from hydraulic pressure. Typical supporting structures for a concrete block wall are shown in Figure 6. Pilasters (built-in supports) can be constructed in the wall but do take up room and, often, are not desired by home owners. Interior walls from basement rooms also can support concrete block walls and reduce the chances of block-wall failure from hydraulic pressure. A long wall without such support is an invitation to failure from hydraulic pressure.

When investigating block wall claims, the age of the building is of interest. Old buildings are candidates for such failures because of long-term deterioration and lack of maintenance. Look for signs of long-term settling, leading to water drainage toward the foundation. The condition of gutters and downspouts also can provide clues of water drainage. Poorly maintained gutters allow storm water to drain down the exterior of the foundation wall, causing hydraulic pressure and wall failure. Patching of interior cracks is evidence of long-term deflection inward. Finally, the wall may have been improperly installed or designed, either of which, ultimately, could lead to failure.

Charles C. Roberts Jr., Ph.D., PE, is a consulting engineer based in Big Rock, Ill. He is primary author of the reference work “Technical Notebook: Forensic Aspects of Claims,” released by ClaimsBooks.

Special Reports /

Failure Analysis of Block Walls

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