Written by: carmstrong
Building type:
Single-family residence
Issues:
Beam pocket blow-out at top of foundation wall
Diagnosis:
Top of foundation wall is experiencing lateral movement from the backfill
Solution:
Recommend installing an interior buttress to provide intermediate support to foundation wall
The Problem and Diagnosis
When drywall began cracking in the finished basement of a residential home, the homeowners contacted SBSA’s staff for an evaluation. We performed an evaluation of the exterior and interior of the home, as well as a relative level survey. Level surveys help determine if there’s any vertical foundation or floor movement occurring in the basement and the garage.
During the investigation, SBSA discovered multiple exterior locations where the main floor beams were punching through the exterior foundation. This is a common sign that the top of the foundation wall is rotating inwardly as a result of lateral loads from the soils. After evaluating the interior distress and level survey results, SBSA concluded that the damage to the home was a result of this lateral wall movement, not vertical foundation movement.
Two counterforts were installed along this foundation wall as part of the original foundation design. Typically 3 – 4 feet in length, counterforts are installed on the exterior of the foundation, perpendicular to the wall near its mid-span. As the lateral loads from the soil try to push the foundation wall in, the counterforts are designed to hold the wall back. In this case, the counterforts were not sufficiently preventing the walls from moving inward. It was this inward movement that was causing the cracking in the drywall that the homeowners were seeing.
The Repair
To repair this condition, SBSA recommended abandoning the exterior counterfort design and replacing it with an interior buttress. Buttresses are wall supports identical to counterforts except they are installed on the interior of the foundation. While buttresses are more inconvenient and intrusive for the homeowners—since they encroach on the finished space—SBSA prefers using buttresses for supporting foundation walls in Colorado.
Since Colorado has expansive soils, any heave of the soils around a counterfort will cause the counterfort to push against the exterior of the foundation wall, increasing—rather than decreasing—the inward rotation of the foundation walls. When soils heave around a buttress, the buttress pushes back against the interior of the foundation wall in the opposite direction as the loads are imposed by the exterior soils.
Cutaway view showing a counterfort and buttress.
Green arrows show forces from soil heave from below; blue arrows show forces from lateral soils. Counterforts help to resist these forces, reducing the amount of deflection.
Green arrows show forces from soil heave from below; blue arrows show forces from lateral soils. Together, the deflection of the wall increases if only a counterfort is used.