The encased falling head procedure is performed with a 6-inch
casing that is embedded approximately 6 inches into the native soil.
The goal of this field test is to evaluate the vertical infiltration
rate through a 6-inch plug of soil, without allowing any lateral infiltration.
The test is not appropriate in gravelly soils or in other soils where
a good seal with the casing cannot be established.
1. Embed a solid 6-inch diameter casing into the native soil at the
elevation of the proposed facility bottom. Ensure that the embedment
provides a good seal around the pipe casing so that percolation will
be limited to the 6-inch plug of the material within the casing.
a. This method can also be used when testing within hollow stem augers,
provided the driller and tester are reasonably certain that a good
seal has been achieved between the soil and auger.
2. Fill the pipe with clean water a minimum of 1 foot above the soil
to be tested, and maintain this depth for at least 4 hours (or overnight
if clay soils are present) to presoak the native material.
a. Any soil that sloughed into the hole during the soaking period should
be removed.
b. In sandy soils with little or no clay or silt, soaking is not necessary.
c. If after filling the hole twice with 12 inches of water, the water
seeps completely away in less than 10 minutes, the test can proceed
immediately.
3. To conduct the first trial of the test, fill the pipe to approximately
12 inches above the soil and measure the water level.
a. Alternative water head heights may be used for testing provided the
presaturation height is adjusted accordingly.
b. The level should be measured with reference to a fixed point. Record
the exact time.
c. Measure the water level at 10-minute intervals for a total period
of 1 hour (or 20-minute intervals for 2 hours in slower soils) or
until all of the water has drained.
d. In faster draining soils (sands and gravels), it may be necessary
to shorten the measurement interval in order to obtain a well-defined
infiltration rate curve.
i. Constant head tests may be substituted for falling head tests at
the discretion of the professional overseeing the infiltration testing.
ii.
Successive trials should be run until the percent change in
measured infiltration rate between 2 successive trials is minimal.
iii.
The trial should be discounted if the infiltration rate between
successive trials increases.
iv.
At least 3 trials must be conducted.
v. After each trial, the water level is readjusted to the 12 inch level.
vi.
The average infiltration rate over the last trial should be
used to calculate the unfactored infiltration rate.
vii.
Alternatively, the infiltration rate measured over the range
of water head applicable to the project stormwater system design may
be used at the discretion of the professional overseeing the testing.
viii.
The final rate must be reported in inches per hour.
The following information should be included in the infiltration
testing report. The infiltration testing report should be attached
to the project's stormwater management report:
1. Statement of project understanding (proposed stormwater system).
2. Summary of subsurface conditions encountered.
3. Summary of infiltration testing including location and number of
tests and testing method used.
4. Discussion of how the tests were performed (i.e. pipe type or diameter
or test pit dimensions).
5. Infiltration testing results in inches per hour.
6. Recommended design infiltration rate including factors of safety.
7. Groundwater observations within exploration and an estimate of the
depth to seasonal high groundwater.
8. Site plan showing location of infiltration tests.
9. Boring or test pit logs.
a. The logs should include an associated soil classification consistent
with ASTM D2488-00, Standard Practice for Classification for Description
and Identification of Soils (Visual-Manual Procedure).
b. The logs should also include any additional pertinent subsurface
information, such as soil moisture conditions, depth and description
of undocumented or engineered fill, soil color and mottling conditions,
soil stiffness or density, and approximate depth of contact between
soil types.