Perk Test Procedure
The percolation test holes shall be spaced uniformly over the proposed absorption field site area. A minimum of three test holes are required. If only three to five tests are performed the design percolation rate for the absorption system is the slowest rate from all the holes tested. If six or more percolation tests are performed, the design percolation rate for the absorption system is the average of all holes tested as determined by the formula below.
An 8 to 12 inch diameter hole shall be dug or bored to the proposed depth of the absorption field. This will normally be approximately 4 feet deep. The walls shall be vertical, to expose a natural soil surface, the sides and bottom shall be scraped with a sharp pointed instrument and the loose material shall be removed from the hole. Coarse sand or gravel shall be placed in the bottom of the hole to prevent it from scouring and sealing.
The purpose of pre-soaking is to have the water conditions in the soil reach a stable condition similar to that which exists during continual waste water application. The minimum time of pre-soaking varies with soil types and moisture content, but must be sufficiently long enough so the water seeps or soaks away at a constant rate. The following pre-soaking instruction are usually sufficient to obtain a constant rate.
- A. In sandy or gravely soils place 24 to 30 inches of water in the hole and allow it to seep away. Fill the hole again with 24 to 30 inches of water and if the water seeps away quicker than ten minutes per inch this indicates that the soil is very permeable and the percolation rate shall be recorded as ten minutes per inch.
- B. In other soils where the water remains longer than the ten minutes per inch rate, additional saturation is necessary. If this is the case allow the water to soak in the hole for eight hours or preferable over night. This will allow the soil to swell and become saturated before the actual measurements are taken.
4. Percolation Rate Measurement
The water level should be adjusted to 12 to 18 inches above the gravel initially and after each measured time interval. Refill the hole with water to the 12 to 18 inch level after each measurement to maintain the same hydraulic loading for each measurement. Time the water level drop in minutes so we know how many minutes it takes the water to drop one inch. The test may be terminated when the water drop is consistent for three consecutive measurements in each percolation hole.
The percolation rate for each hole is calculated as follows
- Time interval (minutes) divided by Final water level drop (inches) equals Number of minutes to drop one inch