A recent mixed-use development near the South Platte River required verification of fill thickness before foundation design could proceed. The site lay within an area mapped as Holocene alluvium, where buried organic layers and undocumented historical fills are common. The exploratory test pit program exposed the contact between the artificial fill and the natural sandy gravel at 4.2 feet depth, resolving a discrepancy in the pre-existing desktop study. In Denver, where the geological transition between the Pierre Shale bedrock and the overlying Quaternary deposits can occur abruptly across a single city block, direct visual inspection through test pits delivers stratigraphic certainty that borehole logs alone cannot provide. The method allows the geotechnical team to observe soil fabric, moisture conditions, and the presence of coarse cobbles that SPT drilling might miss or disturb during sampling.
A test pit transforms the subsurface from an interpreted log into an exposed cross-section that can be measured, photographed, and sampled at precisely the locations the engineer selects.
Methodology and scope
Local considerations
Chapter 18 of the IBC, as adopted by the City and County of Denver, requires that the allowable bearing pressure be based on an adequate exploration of the subsurface materials. Relying solely on SPT borings spaced at 50-foot intervals can miss isolated pockets of compressible clay or buried construction debris that directly underlie a spread footing. A test pit exposes a continuous vertical section, revealing thin seams of expansive claystone weathered from the Denver Formation that might go undetected in split-spoon samples. In the Montbello and Green Valley Ranch areas, where swelling soils are well documented, observing the in-situ moisture profile across the pit face helps the design team decide whether footings need to be deepened or whether a moisture-conditioned fill pad is adequate. The risk of not performing a test pit program is that a foundation designed on incomplete information encounters a soft spot that was never sampled, leading to post-construction settlement and costly litigation.
Applicable standards
ASTM D2487-17e1: Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System), IBC 2021 Chapter 18: Soils and Foundations (adopted by Denver Building Code), ASCE 7-22 Section 11.4: Site Classification Procedure, OSHA 1926 Subpart P: Excavation Safety (trench shoring and classification)
Associated technical services
Stratigraphic Verification Pits
Excavation to the planned foundation depth plus one foot, with detailed logging of soil strata, color, moisture, and consistency using USCS terminology. Photographic documentation of each pit wall with a North arrow and scale.
In-Situ Density and Compaction Testing
Sand cone or nuclear gauge density tests performed on the pit floor and at intermediate depths as the excavation proceeds. Results are correlated with laboratory maximum dry density to compute relative compaction.
Groundwater and Percolation Assessment
Observation of the stabilized groundwater level and, where required for stormwater infiltration design, a falling-head percolation test conducted within the test pit before backfilling.
Typical parameters
Frequently asked questions
How deep can a test pit be excavated safely in Denver soils?
Under OSHA 1926 Subpart P, an unshored vertical pit in Type C soil — which describes much of the sandy alluvium along the South Platte and Cherry Creek corridors — is limited to 4 feet. With an engineered trench box or by sloping the walls to 1.5H:1V, depths of 12 to 15 feet are routinely achieved in Denver projects. The actual limit depends on the soil type encountered and the presence of groundwater.
What is the typical cost range for a test pit program in Denver?
A single test pit investigation, including mobilization, excavation with a rubber-tired backhoe, logging, sampling, and backfill compaction, typically falls between US$430 and US$900 depending on depth, access constraints, and the number of in-situ tests performed. A program of three to five pits across a typical commercial lot is the most common scope we price.
Can you collect undisturbed samples from a test pit?
Yes, though the technique differs from borehole sampling. In cohesive strata such as the weathered claystone of the Denver Formation, a thin-wall Shelby tube can be pushed horizontally into the pit wall to obtain a relatively undisturbed specimen. For granular soils, bulk samples are collected and sealed in plastic bags to preserve the natural moisture content for laboratory classification and Proctor testing.