2021 Kitsap County Stormwater Design Manual
2021 Kitsap County
Stormwater Design Manual

G.3.4 Large Pilot Infiltration Test (Large PIT)

A Large PIT will more closely simulate actual conditions for the infiltration BMP than a Small PIT and may be preferred at the discretion of the licensed professional if not already required per Table II-5.4. The testing procedure and data analysis requirements for the Large PIT are provided below. The report for this test shall include documentation of the testing procedure, analysis and results to assess infiltration feasibility and an explanation of the correction factor used to determine the design infiltration rate.

The Large PIT report shall be prepared by a licensed professional. The test method may be modified due to site conditions if recommended by the licensed professional and the reasoning is documented in the report. Refer to report submittal requirements in Volume II, Chapter 1.

G.3.4.1 Procedure

  1. Testing should occur between December 1 and April 1.

  1. The horizontal and vertical locations of the PIT shall be surveyed by a licensed land surveyor and accurately shown on the design drawings.

  2. Excavate the test pit to the depth of the bottom of the proposed infiltration BMP into the native soil. Note that for some proposed BMPs, such as bioretention and permeable pavement, this will be below the finished grade. If native soils will have to meet the minimum subgrade compaction requirement (for example, the road subgrade using permeable pavement), compact the native soil to that requirement prior to testing.

  3. Lay back the slopes sufficiently to avoid caving and erosion during the test. Alternatively, consider shoring the sides of the test pit.

  4. The size of the bottom of the test pit should be approximately 100 square feet. Where water availability is an issue, smaller areas may be considered, as determined by the licensed professional. Accurately document the size and geometry of the test pit.

Refer to Steps 4 through 9 as described in G.3.3 Small Pilot Infiltration Test (Small PIT) with the following modifications:

  • Step 5: Use a rigid 6‑inch diameter pipe with a splash plate.

  • Step 7b: Data may be recorded every 15–30 minutes.

    Keep adding water to the pit until 1 hour after the low rate into the pit has stabilized (constant flow rate; a goal of 5 percent or less variation in the total flow) while maintaining the same pond water level. The total of the pre‑soak time plus 1 hour after the flow rate has stabilized should be no less than 6 hours.

  • Step 8: After the flow rate has stabilized for at least 1 hour, turn off the water and record the rate of infiltration (the drop rate of the standing water) in inches per hour from the measuring rod data, until the pit is empty. Consider running this falling head phase of the test several times to estimate the dependency of infiltration rate with head.

  • Step 9: Mounding is an indication that a mounding analysis is necessary.

G.3.4.2 Data Analysis

Refer to the data analysis guidance for small PITs in G.3.3.2 Data Analysis.