(3) Preparation of test hole. Carefully scratch the bottom and sides of the
hole with a knife blade or sharp-pointed instrument 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. Add 2 inches of coarse sand or fine gravel to
protect the bottom from scouring and sediment.
(4) Saturation and swelling of the soil. It is important to distinguish
between saturation and swelling. Saturation means that the spaces or separations
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 clay type soil, and is the reason for requiring a prolonged
soaking period. In the conduct of the test, carefully fill the hole with clear water to a
minimum depth of 12 inches over the gravel. In most soils, it is necessary to refill the
hole by supplying a surplus reservoir of water, possibly by means of an automatic
siphon, to keep water in the hole for at least 4 hours and preferably overnight.
Determine the percolation rate 24 hours after water is first added to the hole. This
procedure is to ensure 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 regardless of whether the test is 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 paragraph 1-13b(5)(c) after the
water from one filling of the hole has completely seeped away.
(5) Percolation-rate measurement. With the exception of sandy soils,
percolation-rate measurements are made on the day following the procedure described
in paragraph (4), above.
(a) If water remains in the test hole after the overnight swelling period,
adjust the depth to approximately 6 inches over the gravel. From a fixed reference
point, 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 after the overnight swelling period,
add clear water to bring depth of water in the hole to approximately 6 inches over the
gravel. From a fixed reference point, measure the drop in water level at approximately
30-minute intervals for 4 hours, refilling 6 inches over the gravel as necessary. The
drop that occurs during the final 30-minute period is used to calculate the percolation
rate. The drops during prior periods provide information for possible modification of the
procedure to suit local circumstances.
(c) In sandy soils (or other soils in which the first 6 inches of water
seeps away in less than 30 minute after the overnight swelling period), the time interval
between measurements is 10 minutes and the test is run for one hour. The drop that
occurs during the final 10 minutes is used to calculate the percolation rate.