The following criteria for percolation test hole construction and testing have been adopted by the Sonoma County Public Health Service:
Number and Type of Test Holes
Private sewage disposal systems require one soil core hole and a minimum of three percolation test holes spaced uniformly through the area of the proposed leach field (including leach field expansion area). The percolation test holes shall be 6 to 8 inches (15 to 20 cm) in diameter and shall be of a depth as specified in Table 24, below. The proposed depth of percolation test holes must take into consideration whether subsequent grading and removal of soil would require deeper testing. The soil core hole shall be not less than 4 inches (10 cm) in diameter and shall be not less than 8 feet (2.4 m) in depth, if physically possible.
MINIMUM DEPTHS OF PERCOLATION TEST HOLES
Average Ground Slope (percent) Minimum Depth : (feet) : [meters)
10 3.0 1.0
11 - 15 3.6 1.1
16 - 20 4.4 1.3
21 - 25 5.1 1.6
26 - 30 5.9 1.8
After all test holes have been dug, loose and slumped material shall be removed from each hole and the walls of the holes carefully cleaned to expose the native in-place materials. At the bottom of each test hole, a 2-inch (5-cm) layer of clean pea gravel shall be placed, upon which is rested the end of a 3-inch (7.5-cm) diameter section of perforated pipe. The annular space between the outside of the pipe and the wall of the hole shall then carefully be backfilled with clean pea gravel.
Presoaking of Percolation Test Holes
On the day prior to conducting the percolation tests, each test hole shall be filled with clear water and then refilled at least once and as many times as may be necessary to simulate actual operating conditions.
Percolation-rate measurements shall be made on the day following the presoaking of the test holes, as follows:
1. When Water Remains from Presoaking:
Record the inches of water reamining in the hole, and adjust the water level to 12 inches (30 cm) above the bottom end of the pipe. Water level measurements are then taken from the top of the pipe to the water level hourly for the next six hours.
2. When No Water Remains from Preasoaking: Add clear water to ~ 30 cm) above the bottom end of the pipe. Measure the drop in water level hourly for the next six hours. When the water drains completely from the hole the first time, add water to a depth of 12 inches (30 cm); thereafter, add water to a depth of 12 inches (30 cm) whenever the depth of water in the pipe is less than 3 inches (7.5 cm).
3. When Hole is Dry After the First 30 Minutes:
Add clear water to a depth of 12 inches (30 cm), and take water level measurements every 10 minutes for a period of two hours. Add water to a depth of 12 inches (30 cm) after hole is completely drained the first time; thereafter, add water to a depth of 12 inches (30 cm) whenever depth of water in the pipe is less than 3 inches (7.5 cm).
The drop in water level occurring between the fifth and sixth measurements on the 6-hour tests and between the eleventh and
twelfth measurements on the two-hour tests is considered to be the stabilized percolation rate. The readings taken during the prior periods provide information for any modification of the stabilized percolation rate.
A detailed description of the soil profile shall be prepared from the soil core hole. The description shall include information on depth of topsoil, depth to ground water, if encountered, and notations on any clayey material, hardpan en, or rock strata.
All percolation test data and information from the soil core hole shall be submitted to the Sonoma County Public Health Service on forms provided. Test records submitted for lot splits, proposed subdivisions, and engineered plans for sewage disposal shall include written evaluation and certification by a registered civil engineer. Records submitted shall include a sketch or map showing the location of the percolation test holes and the soil core hole.
Land with a slope of 5 percent or less, and also that of greater than 5 percent if underlain by soils with a significant shrinkswell potential, require all percolation tests to be made: (1) during the period from November 1 to March 1, (2) after a total seasonal rainfall of 10 inches (25 cm) has occurred, and (3) immediately after 0.8 inch (2 cm) of rainfall has occurred during a 48-hour period. Rainfall figures shall be as reported by the Sonoma County Water Agency and based on readings taken at the Sonoma County Administration Center in Santa Rosa.
Leach fields can be constructed on filled land provided the following requirements are met:
1. A log of the soil materials and ground water level in the area of the proposed filled land be prepared.
Soil percolation tests which fail to show a percolation rate faster than 60 mpi (25 m/cm) at hole depths of 3 feet (1 m) may be retested in new holes in native soils to a depth of 2 feet (0.65 m). Percolation rates in the new holes must be greater than 60 mpi (25 m/cm).
3. All leaching trenches require a minimum depth of 3 feet (l-m) of permeable native soil, or a combination of not less than 2 feet (0.65 m) of native soil and sufficient permeable fill material to achieve a total depth of 3 feet (1 m).
The absorptive quality of the fill material shall be equal to, or better, than that of the native soil materials. Sand, gravel, rock, or any mixtures thereof, do not consti- tute acceptable fill material.
Site specifications for the fill shall include that all vegetation is to be removed and the ground surface prepared to permit a good mixing of the native soil materials and the fill materials.
6. Filled land shall be constructed in layers not over 8 inches (20 cm) thick to the same compaction as the native soil. A soil compaction test shall be made, certified by a registered civil engineer, and submitted to the Public Health Office.
7. The filled area shall equal 150 percent of the maximum area required for the leach field. The fill shall be of uniform depth extending to a distance of 15 feet (5 m) in any direction from the center of any proposed leach trench. Where installed on sloping ground, the fill shall provide a minimum horizontal distance of 15 feet (5 m) between the nearest leaching pipe and daylight. An additional area equal to 150 percent of the leach field area shall be reserved for future expansion. The expansion area is required to be filled in the manner described above at the time of construction of the disposal system or, in the case of minor subdivision (lot splits), of lots sized one acre (0.4 ha) or less.
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