A hole 6" to 8" in diameter is excavated into the interval chosen to receive retention basin water. The depth of the hole will vary (depending on site conditions and preliminary basin design) but should be at least 10 feet below the elevation of the bottom of the proposed basin. Materials encountered in the hole should be logged continuously by a soils engineer or engineering geologist. Surficial deposits should be classified according to the Unified Soil Classification System (USCS). Bedrock should be described in accordance with standard geologic practice. The sidewalls should be vertical, the bottom should be as free of loose material as possible and covered with two inches of 1/2" to 3/4" gravel to prevent scouring upon addition of water. If the hole is likely to cave, it should be cased. The annular space between perforated casing and sidewall should be filled with gravel.
The percolation boring should be presoaked 24 hours in advance of the test. The hole should be filled with clear water to the bottom elevation of the proposed basin. The water level should not be closer than 4 feet to the ground surface.
At the start of the percolation rate test, the water level should be at the bottom elevation of the proposed basin. From a fixed reference point, the water level drops are measured to the nearest 0.5 inch with respect to the time of drop. After each measurement the water level is raised to the starting elevation.
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