One of the first few initial steps in the design process for a Septic system (new or replacement) or a stormwater / gray water infiltration system is to investigate and test the soils where the proposed system will be located. The type of soils and any evidence of groundwater or shallow bedrock can greatly impact the type of septic or infiltration system that can be used and in some cases whether or not the soils are even suitable for a septic or infiltration system.
Soil Observation Testing
A soil investigation / assessment starts with excavation locations within the proposed site. These test pits, which are commonly dug using an excavator, are typically three feet wide and dug to a depth of 6 feet or until ledge rock is found (whichever occurs first). The purpose of these observation pits is to record the soil profile and determine the limiting factors for the design. This is important because different layers or types of soil each influence water and effluent differently. The color, texture, composition, and consistency of the horizon are recorded. Other important factors such as root depth, groundwater, and mottling are all recorded. This information is used to determine the depth of the most restrictive layer which is then used as the basis for the design calculations. The most common restrictive layers are seasonal high groundwater, glacial till or hardpan layers, and ledge rock. These different restrictive layers inhibit movement of effluent and water and need to be accounted for in any successful design. Deep observation pits are always backfilled after the soil profiles are recorded to prevent safety risks.
Percolation Tests
The second part of the soil investigation is the percolation test. The test, which is performed in a post hole that is approximately six inches wide with a varying depth, is used to measure how quickly water dissipates from the hole. The depth of the percolation test is determined by the depth of the restrictive layer. A pre-soak is first performed by adding twelve inches of water and allowing the test hole to either go dry or run for a period of one hour. The purpose of the pre-soak is to allow sufficient soil-water contact time. During the pre-soak, swelling clays that may be present in the soil will expand resulting in reduced void spaces within the soil. Sufficient presoaking also allows the advancing capillary wetting front, which controls the rate of water flow in unsaturated soils, to move far enough away from the test hole so that an apparent equilibrium flow rate is achieved. Following the pre-soak, the percolation test hole is then refilled to twelve inches of water. The water level is then measured at ten minute intervals to determine the rate that the water is absorbed into the ground. The final measurement at the end of one hour is used to determine the percolation rate and is expressed as the amount of time it takes for the water level to drop one inch. The percolation test hole is also backfilled after the completion of the testing and final restoration is completed.