In Modesto, the alluvial soils of the San Joaquin Valley can show surprising variability within a single lot — clay lenses interbedded with sandy loam, sometimes with a shallow water table after winter rains. We see infiltration rates that range from 0.1 in/hr in compacted clay to over 4 in/hr in the coarser pockets, which directly affects the sizing of dry wells, bioswales, and leach fields. Running a double-ring infiltrometer test at multiple locations captures that heterogeneity before design begins; skipping it often leads to basins that either pond too long or short-circuit through preferential flow paths. The California Stormwater Quality Association (CASQA) handbook recommends at least one test per 5,000 sq ft of drainage area, and Modesto's municipal code follows similar guidance. Before placing any infiltration BMP, we also recommend a permeability field test to verify vertical and lateral flow rates under actual moisture conditions.
In Modesto, the alluvial soils of the San Joaquin Valley can show surprising variability within a single lot — clay lenses interbedded with sandy loam.
Methodology and scope
A typical infiltration test sequence in Modesto starts with excavating a test pit to the proposed BMP invert depth, then placing the inner ring (12-inch diameter) and outer ring (24-inch) concentric against the soil. We pre-soak the annulus for at least 4 hours — or overnight in clay-dominant profiles — to simulate saturated conditions. After stabilization, we record the water level drop every minute for the first 10 minutes, then every 5 minutes for the next hour, using a hook gauge or pressure transducer. The steady-state infiltration rate is calculated per ASTM D3385-18, with corrections for sidewall friction if the soil is layered. In Modesto's older subdivisions, where fill soils from agricultural grading can be loose, we also run a density cone test alongside to check compaction levels that might affect percolation. The data feeds directly into HEC-HMS or SWMM models for stormwater sizing, or into the Uniform Plumbing Code (UPC) Appendix K formula for septic system design.
Technical reference image — Modesto
Local considerations
A common mistake in Modesto is running only one infiltration test per parcel, often placed in the visually driest corner. That single reading might miss a high-permeability sand seam that drains the entire basin in 30 minutes, or a low-permeability clay pocket that makes the pond ineffective. The result is a system that either overflows during a 10-year storm or never infiltrates enough to meet the 72-hour drawdown requirement. Designers then have to add underdrains or enlarge the facility — both costly retrofits. Testing at three to five locations across the proposed footprint, with the double-ring method applied at the actual invert depth, avoids this. For sites with suspected lateral flow, combining the infiltration test with a slug test in a piezometer provides the horizontal hydraulic conductivity that the single-ring alone cannot capture.
Double-ring infiltrometer (ASTM D3385-18) or Porchet (single-ring)
Ring diameters
Inner 12 in, outer 24 in (double-ring); 12–16 in (Porchet)
Pre-soak duration
4–24 hours depending on soil texture
Reading interval
Every 1 min (first 10 min), then every 5 min (next 60 min)
Reported value
Steady-state infiltration rate (in/hr) at field moisture
Applicable standards
ASTM D3385, CASQA BMP Handbook, UPC Appendix K
Associated technical services
01
Double-Ring Infiltrometer (ASTM D3385)
Standard test for stormwater BMP sizing in compliance with CASQA and Modesto municipal code. Includes pre-soak, 60-minute reading series, and report with steady-state rate, cumulative infiltration curve, and photos of soil profile.
02
Porchet (Single-Ring) Percolation Test
Simplified test for septic system or small-scale drainage design per UPC Appendix K. Single 12-inch ring excavated to proposed trench depth, with water level readings over 4 hours. Suitable for residential lots with uniform soil texture.
Applicable standards
ASTM D3385-18 (Standard Test Method for Infiltration Rate of Soils in Field Using Double-Ring Infiltrometer), California Stormwater Quality Association (CASQA) BMP Handbook — Infiltration Basin Design, Uniform Plumbing Code (UPC) Appendix K — Percolation Test for Septic Systems, IBC Chapter 18 — Stormwater Management and Soils Report Requirements
Frequently asked questions
How many infiltration tests are needed for a typical Modesto commercial lot?
For a 1-acre parcel, most Modesto stormwater reviews require at least four double-ring tests — one per each quadrant of the proposed basin. If the soil varies significantly (e.g., clay lenses next to sandy loam), we recommend six tests to capture the range.
What infiltration rate does Modesto require for a 72-hour drawdown?
Modesto's stormwater code typically requires a minimum design infiltration rate of 0.5 in/hr for the bottom of the basin, measured at the actual invert depth after pre-soaking. If the measured rate is below 0.2 in/hr, the design must include an underdrain system or a larger surface area. The double-ring test provides the steady-state rate used in the calculation, not the initial high rate observed during the first few minutes.
Can I use a single-ring Porchet test instead of a double-ring for Modesto's stormwater plan?
For most commercial and multi-family projects in Modesto, the city's engineering department prefers the double-ring method (ASTM D3385) because the outer ring minimizes lateral flow from the inner ring, giving a more accurate vertical infiltration rate. The single-ring Porchet test is acceptable only for single-family residential septic systems or small drainage improvements with less than 0.5 acres of contributing area.
