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

G.3.3 Small Pilot Infiltration Test (Small PIT)

The testing procedure and data analysis requirements for the Small 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 Small 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.3.3.1 Procedure

  1. Excavate the test pit to the depth of the bottom of the proposed infiltration BMP. In the case of bioretention, excavate to the lowest estimated elevation at which the imported soil mix will contact the underlying soil. For permeable pavement, excavate to the elevation at which the imported subgrade materials, or the pavement itself, will contact the underlying soil. If the underlying soils (road subgrade) will be compacted, compact the underlying soils prior to testing. Note that the permeable pavement design guidance recommends compaction not exceed 90 to 92 percent.

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

  2. The size of the bottom of the test pit should be 12 to 32 square feet. Accurately document the size and geometry of the test pit.

  3. Install a device capable of measuring the water level in the pit during the test. This may be a pressure transducer (automatic measurements) or a vertical measuring rod (minimum 5 feet long) marked in half‑inch increments in the center of the pit bottom (manual measurements).

  4. Use a rigid pipe with a splash plate or some other device on the bottom to convey water to the bottom of the pit and reduce side‑wall erosion and excessive disturbance of the pit bottom. Excessive erosion and bottom disturbance may result in clogging of the infiltration receptor and yield lower than actual infiltration rates. The rigid pipe may be a 3‑inch-diameter pipe for pits on the smaller end of the recommended surface area, or a 4‑inch pipe for pits on the larger end of the recommended surface area.

  5. Pre‑soak period: Add water to the pit so that there is standing water for at least 6 hours. Maintain the pre‑soak water level at least 12 inches above the bottom of the pit.

  6. Steady state period:

    1. At the end of the pre‑soak period, add water to the pit at a rate that will maintain a depth of 6–12 inches above the bottom of the pit over a full hour. A rotameter can be used to measure the flow rate into the pit. The depth should not exceed the proposed maximum depth of water expected in the completed BMP.

    2. Every 15 minutes during the steady state period, record the cumulative volume and instantaneous flow rate (in gallons per minute) necessary to maintain the water level at the same point (between 6 inches and 1 foot) on the measuring rod or pressure transducer readout. The specific depth should be the same as the maximum designed ponding depth (usually 6 to 12 inches).

  7. Falling head period: After 1 hour, turn off the water and record the rate of infiltration (the drop rate of the standing water) in inches per hour every 15 minutes using the pressure transducer or measuring rod data, for a minimum of 1 hour or until the pit is empty. A self‑logging pressure sensor may also be used to determine water depth and drain‑down.

  8. At the conclusion of testing, over‑excavate the pit to determine if the test water is mounded on shallow restrictive layers or if it has continued to flow deep into the subsurface. The depth of excavation varies depending on soil type and depth to the hydraulic restricting layer and is determined by the engineer or certified soils professional (refer to Table II-5.4). The soils professional should judge whether a mounding analysis is necessary. Minimum investigation depths are provided in G.2 Subsurface Investigation.

G.3.3.2 Data Analysis

Calculate and record the initial Ksat rate in inches per hour in 30‑minute or 1‑hour increments until 1 hour after the flow has stabilized. Use statistical/trend analysis to obtain the hourly flow rate when the flow stabilizes. This would be the lowest hourly flow rate.

Adjust the measured infiltration rate using the correction factor (CF) described in G.4 Calculation of Design Infiltration Rate of the Native Soils to estimate the design infiltration rate.