Another walk-through of an actual project, redacted. This one was a custom home on a sloped Kaysville lot, where the architect's plan called for a walk-out basement on the downhill side and a flat back yard above. To make that work we needed to engineer a tiered retaining wall system: a lower wall at the walk-out level holding back the slope, and an upper wall about 12 feet behind it creating a flat terrace. The interaction between the two walls was the most interesting part of the engineering.
The site
Lot was on a roughly 12% slope, soils were the typical bench clay with moderate expansion potential per the soils report. The walk-out elevation was about 9 feet below the upper terrace elevation, which the architect wanted as a flat outdoor living area.
The wall configuration
- Lower wall: 8.5 ft tall (measured from bottom of footing to top), reinforced concrete cantilever, about 60 ft long, set roughly 3 ft from the basement walk- out
- Upper wall: 4.5 ft tall, segmental retaining block with geogrid reinforcement, set 12 ft behind the lower wall, creating the flat terrace
Why two walls instead of one
We could have designed a single 13-ft wall to hold back the full elevation difference, but the engineering and the construction cost would have been significantly higher (taller walls require disproportionately more reinforcement and bigger footings). Two walls with a setback between them broke the load into two manageable pieces.
The surcharge problem
The catch with tiered walls is that the lower wall sees the surcharge from the upper wall and the soil it retains. The 12-ft setback was enough to reduce the surcharge effect (rule of thumb: at least twice the lower wall height of setback), but not eliminate it. We designed the lower wall for a modest surcharge from the upper terrace, plus a uniform live load from anything that might end up on the upper terrace (furniture, BBQ, planters).
The lower wall's reinforcement schedule reflected this — #5 bars at 12" o.c. vertical and #4 bars at 16" o.c. horizontal, plus a heel-extension on the footing for sliding resistance.
Drainage
Drainage is what kills retaining walls. We designed a full drainage package:
- Free-draining gravel backfill behind the lower wall
- 4-inch perforated drain pipe at the bottom of the gravel, sloped to a daylight outlet on the side of the property
- Surface grading to direct water away from the upper terrace
- Filter fabric between the gravel and the surrounding soil to prevent fines from clogging the drainage
Without that drainage, hydrostatic pressure during a wet spring could have added 60+ psf of horizontal load to the wall — enough to cause real problems over time.
The permit and the build
Stamped permit set delivered three weeks after the initial call. Kaysville plan check came back with two minor questions (a callout about the geogrid spacing on the upper wall and a confirmation of the daylight outlet location), both answered same day. Contractor built it the following summer. Two winters in, no movement, no cracking, no drainage issues.
The fee picture
- Engineering scope: $2,400 fixed fee
- Site visit and existing-condition documentation: included
- Plan check responses: included
- Construction-phase questions: included
For a wall system this size, that's about a third of what a big firm would have charged. And the homeowner had a direct line to the engineer the entire project. See our foundations and retaining walls page for more.