What is Soil Infiltration Testing?
A soil infiltration test is a type of field test conducted by an engineer, geologist, or soils scientist to determine the infiltration rate of a particular area’s soils and to explore the subsurface conditions underneath the soil or pavement surface. The “infiltration rate” refers to the speed at which water enters into the soil.
Soil infiltration testing is typically conducted early in the planning and design process to determine if an infiltration-based design is suitable for a specific site and at what locations within the site. Testing enables a successful and effective storm-water management design that incorporates a suitable infiltration rate for design calculations. Infiltration testing also helps investigate the subsurface conditions below existing surfaces/pavement, and identifies existing soil horizons (layers), as well as any limiting features, historic conditions, etc.
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Process of Infiltration Testing:
​Boreholes (test holes), typically 6-10 inches in diameter and 5-10 feet deep, are drilled using an auger into the area of the proposed design. This boring (drilling) method is used to minimize disturbance and maintain pedestrian/vehicular traffic. After the test holes are pre-soaked, each test hole is filled with 6-12 inches of water. As this water slowly sinks (infiltrates) down into the ground, measurements of the water level are made inside the hole. A minimum of 8 readings are necessary to complete the testing process. The change in the water level that occurs in the test hole during the final testing interval, expressed in inches per hour, represents the percolation rate for that test location.
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Soil Infiltration Test Requirements:

• Testing must be conducted or overseen by a qualified Professional Engineer, Registered Geologist, or Certified Engineering Geologist licensed in the state
• The depth of the test must correspond to the facility depth. If a confining layer, or soil with a greater percentage of fines, is observed during the subsurface investigation to be within 4 feet of the bottom of the planned infiltration system, the testing should be conducted within that confining layer.
• Tests must be performed in the immediate vicinity of the proposed facility. Exceptions can be made to the test location provided the qualified professional can support that the strata are consistent from the proposed facility to the test location. The test must be conducted in the twenty-four months prior to the date the plans were submitted for review.
• Typically, 1-4 borings/test holes are tested per project site, with one hole being 10’ deep and the others 5’ deep. A depth of 10’ is recommended to document the presence of any seasonally high groundwater table, bedrock, or other potential limiting conditions.
• The overall duration of the test is at least 3 hours, and most project sites require 8-10 hours for testing.