TM 5-814-3/AFM 88-11, Volume III
(1) Six or more tests Will be made in separate test holes uniformly spaced over the proposed absorption
field site.
(2) Dig or bore a hole with horizontal dimensions of 4 to 12 inches and vertical sides to the depth of
the proposed trench.
(3) Carefully scratch the bottom and sides of the excavation with a knife blade or sharp-pointed instru-
ment to remove any smeared soil surfaces and to provide a natural soil interface into which water may
percolate. Add 2 inches of coarse sand or fine gravel to the bottom of the hole. In some types of soils the
sidewalls of the test holes tend to cave in or slough off and settle to the bottom of the hole. It is most likely
to occur when the soil is dry or when overnight soaking is required. The caving can be prevented and more
accurate results obtained by placing in the test hole a wire cylinder surrounded by a minimum 1-inch layer
of gravel of the same size that is to be used in the tile field.
(4) Carefully fill the hole with clear water to a minimum depth of 12 inches above the gravel or sand.
Keep water in the hole at least 4 hours and preferably overnight. In most soils it will be necessary to augment
the water as time progresses. Determine the percolation rate 24 hours after water was first added to the hole.
In sandy soils containing little clay, this prefilling procedure is not essential and the test may be made after
water from one filling of the hole has completely seeped away.
(5) The percolation-rate measurement is determined by one of the following methods:
(a) If water remains in the test hole overnight, adjust the water depth to approximately 6 inches
above the gravel. From a reference batter board as shown in figure 64, measure the drop in water level over
a 30-minute period. This drop is used to calculate the percolation rate.
(b) If no water remains in the hole the next day, add clean water to bring the depth to approximately
6 inches over gravel. From the batter board, measure the drop water level at approximately 30-minute
intervals for 4 hours, refilling to 6 inches over the gravel as necessary. The drop in water level that occurs
during the final 30-minute period is used to calculate the percolation rate.
(c) In sandy soils (or other soils in which the first 6 inches of water seeps away in less than 30
minutes after the overnight period), the time interval between measurements will be taken as 10 minutes and
the test run for 1 hour. The drop in water level that occurs during the final 10 minutes is used to calculate
the percolation rate. Figure 6-5 will be used to determine the absorption area requirements from percolation
rate measurements. Tile fields are not usually economical when drop is less than 1 inch in 30 minutes.
6-6
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