Homeowners planning a new septic system installation or an emergency drain field replacement across Hudson, Brighton, Eaton, or Greeley often call asking for a "perc test." Under Regulation 43, we excavate two 8-to-10-foot backhoe pits inside the proposed absorption zone so a Professional Engineer can inspect the soil horizons, identify iron-oxide "redox" mottling from seasonal high water tables, and confirm the mandatory 4-foot vertical separation above groundwater or bedrock. (If shallow bedrock or groundwater appears inside the top 48 inches, an engineered sand mound septic system is required.)
Why Open-Bottom Chambers Beat Pipe-and-Gravel in Colorado Clay
- Zero Stone Compaction or Silt Masking: Washed river rock crushes delicate clay soil pores at contact points and blocks over 50% of the infiltrative surface. Open-bottom chambers leave 100% of the trench floor unobstructed.
- Aerobic Respiration Prevents Black Biomat: Inside a hollow arch chamber, oxygen circulates freely across the entire soil interface—preventing the anaerobic black slime ("biomat") that causes squishy grass over leach fields.
- 30% to 40% Smaller Yard Footprint: Colorado Regulation 43 grants a formal sizing reduction factor for gravelless chambers—saving thousands on excavation and materials (see our 2026–2027 Colorado Septic System Cost Guide).
Frequently Asked Questions
Does Colorado still use traditional water percolation (perc) tests for septic permits?
No. Under Colorado CDPHE Regulation 43—enforced across Weld, Adams, Boulder, and Larimer counties—traditional hand-dug water perc holes have been replaced by excavated visual Soil Profile Test Pits (typically two 8-to-10-foot deep backhoe trenches) evaluated by a Professional Engineer (PE).
How much do soil test pits and stamped PE septic engineering drawings cost in Colorado?
At AB Septic, excavating the two 8-to-10-foot soil profile pits, coordinating the licensed Professional Engineer (PE) evaluation, and delivering your stamped OWTS engineering design typically ranges from $2,500 to $4,400 (compared to $3,200 to $5,500 at markup-heavy firms)—and you own your paid-for drawings 100%.
Why are open-bottom plastic chambers better than pipe-and-gravel in Colorado clay soil?
Washed gravel contains fine silt dust and crushes fragile clay soil pores at contact points, masking 50% to 70% of the soil surface. High-capacity open-bottom plastic chambers expose 100% of the trench floor to air and effluent and qualify for up to a 30% to 40% footprint reduction under county OWTS sizing tables.
How deep are septic soil test pits dug in Weld and Adams counties?
County regulations require excavating at least two test pits inside the proposed primary and replacement absorption footprint to a depth of 8 to 10 feet to verify that no seasonal high groundwater redox mottling or impermeable bedrock exists within the vertical separation zone.
Need Soil Test Pits, Stamped PE Design, or a Zero-Markup Installation Quote?
Austin Bitz (CO Master Installer #C10002733 | Weld #IL-2600008 | NAWT #19888ITC) provides transparent, zero-markup soil test pit excavation, PE engineering coordination, and chamber drain field installations across Weld, Adams, Boulder, Larimer, Morgan, and Arapahoe counties. Call 720-883-3390 today.