A common mistake we see in Denver is treating a retaining wall as a pure structural element and ordering rebar before touching the soil. The front range isn't uniform. Denver sits on the Pierre Shale and Denver Formation — claystone that swells when wet and shrinks when dry, putting seasonal pressure behind any wall. At our lab, we start retaining wall design with the ground itself: drilling to refusal, measuring plasticity, and calculating at-rest versus active pressures. Without that, even a well-reinforced wall rotates or cracks at the stem within three freeze-thaw cycles. We combine field SPT drilling data with lab consolidation curves because Denver's overconsolidated clays don't behave like textbook soils, and ignoring that fact has cost more than one contractor a rebuild before the warranty expires.
Denver's overconsolidated claystone can develop lateral pressures closer to at-rest conditions than active — designing without that data leads to walls that tilt within the first wet season.
Methodology and scope
Local considerations
Compare a wall in Cherry Creek's sandy alluvium with one in Green Valley Ranch's expansive clay, and you're dealing with two entirely different risk profiles. Cherry Creek drains well, so hydrostatic buildup isn't the main concern — but the loose sands need careful compaction behind the stem or you get settlement cracking at the coping. Green Valley Ranch sits on high-plasticity clays that swell enough to add lateral pressure equivalent to several extra feet of soil surcharge. We've measured swell pressures exceeding 8 ksf in that area. The Denver building code requires a geotechnical investigation for any wall over four feet, but the real threshold for detailed retaining wall design is lower when the site is within mapped swelling-soil zones. Skipping lab testing in those neighborhoods means the wall's heel might be undersized, and the drainage blanket too thin to relieve seasonal moisture cycling.
Applicable standards
IBC 2021 Section 1807 (Earth Retaining Structures), ASCE 7-22 Chapter 3 (Lateral Soil Pressures), ASTM D1586-18 (Standard Penetration Test), ASTM D4318-17e1 (Atterberg Limits), ASTM D4767-11(2020) (Consolidated-Undrained Triaxial), City of Denver Building Code Amendments (expansive soil provisions)
Associated technical services
Site Investigation & Soil Profiling
SPT borings to refusal or 20 feet, logged per ASTM D2488, with groundwater observation. We map the Denver Formation/Pierre Shale contact and identify any paleochannel fill that could act as a preferential drainage path behind the wall.
Laboratory Strength & Swell Testing
Triaxial CIU or CD tests on undisturbed Shelby tube samples to define the Mohr-Coulomb envelope. Swell-consolidation testing per ASTM D4546 when the retained soil is high-plasticity claystone, which is common west of Broadway.
Lateral Earth Pressure Recommendations
We provide active, at-rest, and passive coefficients based on lab-measured phi and cohesion, not generic tables. For walls with restrained movement, we default to at-rest pressures and specify the drainage aggregate gradation needed to prevent hydrostatic buildup.
Typical parameters
Frequently asked questions
How much does a retaining wall geotechnical study cost in Denver?
For a typical residential or small commercial retaining wall in Denver, our geotechnical investigation and lab testing program ranges from US$1,030 to US$4,310, depending on wall height, number of borings, and whether triaxial or swell testing is required. A 4-to-6-foot wall on an average Denver lot is usually at the lower end; walls over 10 feet with surcharge from a roadway need more testing and fall toward the upper end.
What lab tests are essential for retaining wall design in Denver's expansive soils?
At minimum, Atterberg limits and particle-size distribution tell us the soil classification and give a first warning on expansion potential. For walls over 4 feet, we recommend a triaxial test to get real phi and cohesion values, plus a swell-consolidation test if the site is in a mapped Denver expansive zone. These feed directly into lateral pressure calculations and heel width decisions.
How deep do you drill for a retaining wall investigation?
We typically drill to a depth equal to twice the wall height below the proposed footing, or until refusal on bedrock — whichever is shallower. In Denver's west neighborhoods, refusal on claystone can happen at 3 to 6 feet, and we'll core into it to confirm it's competent and not just a weathered crust.
Can you design the wall reinforcement and structural details?
Our scope is the geotechnical side — soil parameters, lateral earth pressures, bearing capacity, and drainage recommendations. The structural engineer uses our report to design stem thickness, reinforcement, and footing dimensions per ACI 318 and IBC. We stay in close coordination with the structural team throughout the project.
What's different about retaining wall design in Denver compared to other cities?
Two things drive Denver retaining wall design: swelling claystone and a 36-inch frost depth. The claystone applies pressure even without water if it wets up, so we design for at-rest conditions more often than in non-expansive areas. The frost depth means the footing has to sit below 3 feet, and the backfill drainage needs to extend well below that to prevent ice lensing behind the wall.