Percolation Test - Soil Properties: Testing, Measurement, and Evaluation - page 247

2 in. of coarse sand or fine gravel to protect the bottom from scouring
and sediment.
(4) 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 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 in. 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 syphon, to keep water in
the hole for at least 4 h and preferably overnight. Determine the
percolation rate 24 h 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, 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 under item
(5)(C), after the water from one filling of the hole has completely
seeped away.
(5) With the exception of sandy soils, percolation-rate measure-
ments shall be made on the day following the procedure described
under item (4) above.
A. If water remains in the test hole after the overnight swelling pe-
riod, adjust the depth to approximately 6 in. over the gravel. From
a fixed reference point, measure the drop in water level over a
30-min 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 the depth of water in the hole to
approximately 6 in. over the gravel. From a fixed reference point,
measure the drop in water level at approximately 30-min intervals
for 4 h, refilling 6 in. over the gravel as necessary. The drop that
occurs during the final 30-min period is used to calculate the per-
colation 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 in. of water
seeps away in less than 30 min, after the overnight swelling
period), the time interval between measurements shall be taken as
10 min and the test run for 1 h. The drop that occurs during the
final 10 min is used to calculate the percolation rate.
It will be noted that the foregoing method provides a slightly
different procedure for computing percolation rate. In any event, per-
colation rate is reported in either inches per hour or minutes per inch,
or both.