§ 18.21.170. Percolation test procedures.

The percolation test is but one of many indicators of a site's future suitability to accept sewage for safe permanent disposal. Consequently, it should not be considered as the sole basis of designing an on-site sewerage system.

A.

Location and Number of Tests. A minimum of three test holes will be required with the holes uniformly spaced over the proposed absorption field site.

1.

Adjacent Groups. If the percolation rate results fall in adjacent groups, the absorption field may be designed using the test results from the hole with the slowest percolation rate or one additional hole may be dug, tested and all four results averaged. To properly average the results, each test result must be converted to minutes per inch, then added together. The sum is then divided by the number of tests. If tests in an area vary by more than twenty minutes per inch, variations in soil type are indicated and percolation rates should not be averaged.

2.

Nonadjacent Groups. If the percolation rate results fall in nonadjacent groups, the Absorption field may be designed using the test results from the hole with the lowest percolation rate or two additional holes may be dug, tested, and the results averaged.

3.

Alternative Procedure. In lieu of the previously mentioned procedure, four holes may be dug and tested and the results averaged at the same time to reduce the amount of time required to conduct the test.

B.

Type of Test Hole. Dig or bore a hole with a diameter of from six to twelve inches with vertical sides to the depth of the proposed absorption trench. The bottom of the hole must be at the same elevation as the proposed drainfield bottom.

C.

Preparation of Test Holes. Carefully

Table 18.21.160(D)

FLOW SHEET FOR SELECTNG PROPER SUBSURFACE

DISPOSAL METHODS

scratch the bottom and sides of the hole with a knife blade or sharp-pointed instrument in order to remove any smeared soil surfaces and to provide a natural soil interface into which water may percolate. Remove all loose material from the hole and carefully place approximately one inch of coarse sand or fine gravel into the bottom of the hole to protect the bottom from scouring.

D.

Saturation and Swelling of the Soil. It is important to distinguish between saturation and swelling. Saturation means that the void spaces between soil particles are full of water. This can be accomplished in a short period of time. Swelling is caused by intrusion of water into the individual soil particle. This is a slow process, especially in a clay-type soil, and is the reason for requiring a prolonged soaking period.

E.

Filling of Test Holes. In the conduct of the test, carefully fill the hole with clear water to a minimum depth of twelve inches. In most soils, it is necessary to refill the hole by supplying a surplus reservoir of water manually or by means of an automatic siphon, to keep water in the hole until saturation occurs (approximately twenty-four hours). Determine the percolation rate twenty-four hours after water is first added to the hole. This procedure is to insure that the soil is given ample opportunity to swell and to approach the condition it will be in during the wettest season of the year. Thus, the test will give comparable results in the same soil, whether made in a dry or in a wet season. In sandy soils containing little or no clay, the swelling procedure is not essential and the test may be made as described in subsection G of this section after the water from one filling of the hole has completely seeped away.

F.

Percolation Rate Measurement. With the exception of sandy soils, percolation rate measurements shall be made on the day following the procedure described in subsection E of this section. After the overnight swelling period, adjust the water depth to approximately twelve inches from the bottom. From a fixed reference point, measure the drop in water level over a thirty-minute period. This drop is used to calculate the percolation rate. If the rate is slower than thirty minutes per inch, continue with measuring the rate for an additional thirty minutes. The slower rate of the two consecutive one-half-hour tests should be used.

G.

Percolation Rate Measurement (Sandy Soils). In sandy soils (or other soils in which the first six inches of water seeps away in less than thirty minutes, after the overnight swelling period), the hole should be filled to a depth of six inches and that depth maintained by adding water for thirty minutes. After thirty minutes, the drop in water level should be measured over an additional ten-minute period and the percolation rate calculated from this measurement.

(Ord. 9448 (part), 1988)