Concrete Surfaces in Post-Construction Basement Excavations in Bay Area Homes

When inspecting older houses in the San Francisco Bay Area, I occasionally encounter basements that appear to have been excavated after foundation installation. These basements typically house utilities such as water heaters and furnaces, and are often characterized by a rear foundation wall that has been cut to provide stair access and a concrete wall and ledge extending to the interior side of the foundation wall to protect the soil's angle of repose—the natural slope at which soil remains stable without additional support.

The Deceptive Appearance of Concrete Support

The concrete installation near the foundation resembles a massive concrete pour or bench foundation—a foundation added adjacent to an exposed foundation base or footing to prevent movement. However, this appearance of significant concrete reinforcement is often deceiving. While I cannot quantify the exact frequency, many of these installations consist of concrete as thin as one inch thick in critical areas.

Understanding the Construction Method

Excavating soil to create these basements produced steep cuts-in-grade—nearly vertical soil edges requiring support to prevent cracking, sloughing, or structural failure. To stabilize these exposed soil faces, contractors applied concrete over the cut surfaces. Because excavations were typically tapered to prevent collapse of the top edge, this concrete varied in thickness from as little as one inch at the bottom to six inches at the top. The flat soil area between the protected edge and the existing foundation also received a thin concrete covering.

In modern construction, significant cuts-in-grade would be supported by properly engineered retaining walls with adequate footings. These retaining walls would be designed to handle the lateral earth pressure of soil behind the walls. Professional engineering is required for walls exceeding certain height thresholds.

Contemporary walls and concrete installations would include steel reinforcement within the concrete for improved structural performance and incorporate drainage systems to reduce water intrusion in below-grade areas. Neither of these details can be determined through visual inspection alone.

Cut in grade next to concrete retaining wall — Exposed Cut in Grade

View of concrete covering where concrete has been cut and removed — Exposed Edge Where Concrete Thin at Bottom

Performance and Long-Term Reliability

Based on over twenty years of periodic observations, these systems appear to function adequately when undisturbed. In the limited instances where I have observed the concrete disturbed—typically during conversion to proper basement living spaces—several installations showed signs of soil movement related to the modification. While these installations fall short of current construction standards and likely never received official approval, they appear to serve their intended purpose.

However, this assessment comes with an important caveat: I lack comprehensive data regarding long-term failure rates or the total number of installations that may have already failed.

Concrete covering either damaged or not fully installed — Either Partial Covering or Damaged

Bottom of concrete pour, showing lack of footing — No Footing at Bottom

Side view where concrete was damaged during modification — Concrete Damaged Where Cut

Recommendations for Property Owners

If you are purchasing a home or currently own a property with this type of installation, I recommend professional evaluation by a qualified structural engineer or geotechnical contractor. This evaluation should assess both the current condition and long-term stability of the installation.

Additionally, inspection after any significant seismic event is recommended. Look for new cracks in the concrete surfaces, signs of soil movement, or changes in the basement's structural integrity. Any concerning changes should be examined by a qualified contractor and repaired as needed.

Conclusion

While these post-construction basement excavations represent non-standard construction methods, many continue to provide functional utility spaces decades after installation. However, their structural adequacy depends heavily on initial installation details and the ongoing stability of the surrounding soil. Professional evaluation remains the best approach for ensuring safety and determining whether additional work is necessary.

Glossary

Illustration of angle of repose — Angle of Repose Illustration

Angle of Repose:: The natural slope at which soil remains stable without additional support. The required slope for stability depends on soil type. Any excavation steeper than the angle of repose risks foundation stability and requires repair.
Cut-in-Grade:: An excavated area where soil has been removed, creating a steep or vertical soil face that may require stabilization.
Lateral Earth Pressure:: The horizontal pressure exerted by soil against structures. This pressure is a critical consideration in geotechnical engineering as it affects soil consolidation, strength characteristics, and the design of retaining walls, basements, and foundations.