Infiltration: Testing

Four Step Process

Soil infiltration testing is a four-step process to obtain the necessary information for stormwater management planning and design. The four steps include:

1. Background Evaluation

• Based on available published and site specific data;
• Includes consideration of proposed development plan;
• Used to identify potential BMP types, locations and soil test locations;
• Done prior to field work; and
• On-site soil tests may be done to identify/screen potential BMP locations.

2. Test Pit or Soil Boring Observations

• Includes multiple testing locations;
• Provides an understanding of sub-surface conditions; and
• Identifies limiting conditions (e.g., aquitard, bedrock or water table elevations).

3. Infiltration Testing

• Must be conducted on-site;
• Various testing methods are available; and
• Different testing methods for screening versus verification purposes.

4. Design Considerations

• Determination of a suitable infiltration rate for design calculations; and
• Consideration of desired BMP drawdown time.

Test Pit or Soil Boring Observations

Test pits or soil borings provide information regarding the soil horizons and overall soil conditions both horizontally and vertically in that portion of the site. Multiple observations can be made across a site at a relatively low cost and in a short time period.

Test pit excavations or soil borings should extend to a depth of between 2.5 - 5 m below ground surface or until bedrock or fully saturated conditions are encountered. It is important that the tests provide information related to conditions at least 1.5 m below the proposed bottom elevation of the infiltration BMP. Test pit trenches should be benched at 1 m depth intervals for access and infiltration testing.

At each test location, the following conditions should be noted and described:

• Soil horizons (upper and lower boundary);
• Soil texture and colour for each horizon;
• Color patterns (mottling) and observed depth;
• Depth to water table (if encountered);
• Depth to bedrock (if encountered);
• Observations of pores or roots (size, depth);
• Estimated type and percent coarse fragments;
• Hardpan or other limiting layers; and
• Strike and dip of soil horizons.

The number of test pits or soil borings varies depending on site conditions and the proposed development plan. General guidelines are as follows:

• For infiltration BMPs with footprint surface areas from 50 - 900 m², a minimum of two test pits or one test pit and two soil borings are required at, or within 10 m of the proposed location to determine the suitability and distribution of soil types present;
• For infiltration BMPs with footprint surface areas > 900 m², a minimum of one test should be conducted for each 450 m² of footprint area. Tests should be conducted equidistant from each other to provide adequate characterization of the area;
• For linear infiltration BMPs (e.g. bioswales, enhanced grass swales, infiltration trenches, exfiltration trenches) a minimum of one test should be conducted within each soil mapping unit present along the proposed BMP location. Soil borings should be conducted every 50 m and a test pit should be conducted every 450 m; and
• For sites with multiple infiltration BMPs, each with footprint surface area < 50 m², a minimum of one test pit is required and one soil boring per infiltration BMP location is recommended.

Infiltration Testing

• Double ring infiltrometer
• Single Ring Infiltrometer
• Constant-head well permeameter
• Falling-head well permeameter
• Cased-borehole
• Falling Head Test
• Constant Head Test
• Basin Flooding Test

The preferred testing period is during April and May. If testing is conducted between June 1 and December 1, the soil should be soaked for 24 hours prior, to simulate field saturated conditions.

• If uniform soil texture is observed in the proposed location, at least three percolation test holes should be prepared. If the soil texture changes within the area, make at least two test holes in each soil texture. All test holes should be uniformly spaced across the proposed area.
• Test holes should be between 150 - 200 mm in diameter, and penetrate to the depth of the proposed BMP. Observations should be made on the extracted soil and variations recorded.
• The lowest 300 mm and the bottom of the test pit should be scarified, and the loose material removed.
• A mesh bag containing about 1 L of pea gravel may be lowered into the pit, to prevent scouring of the base. Attaching a line to the bag allows it to be removed and reused.
• Carefully pour 6 L of clean water into the base of the pit. A hose is recommended to prevent washing out or erosion of the sides.
• In sandy soils, where half the depth of water percolates within 30 minutes. Otherwise, calculate the infiltration rate

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Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

The upper surface of the zone of saturation, except where the surface is formed by an impermeable body.

Subsurface water level which is defined by the level below which all the spaces in the soil are filled with water; The entire region below the water table is called the saturated zone.

The rate at which stormwater percolates into the subsoil measured in inches per hour.

Ground depression acting as a flow control and water treatment structure, that is normally dry.