While both tests involve analyzing soil, they serve entirely different purposes and are not interchangeable for structural work. The primary differences include:

• Primary Purpose: A perc test (percolation test) measures how quickly water drains through soil, which is used solely for sizing septic system leach fields. A geotechnical report is a comprehensive subsurface investigation used to assess soil properties for structural foundation design.
• Data Measured: A perc test only measures percolation rates and groundwater levels. In contrast, a geotechnical report measures soil bearing capacity, composition, density, moisture content, frost depth, and corrosion potential.
• Structural Requirements: For projects involving helical piers, a perc test is insufficient because it provides no data on load-bearing characteristics. A geotechnical report is mandatory for determining pier depth, capacity, and the specific installation methods required by building codes and ASTM standards.
• Cost and Scope: Perc tests are generally cheaper (typically $200–$900) and involve saturating holes with water. Geotechnical reports are more expensive (ranging from $1,500 to $3,500+) because they require professional soil borings, laboratory analysis, and engineering recommendations.