(2) Rising Head Test. In a saturated zone with sufficiently
permeable materials, this test is more accurate than a constant or a falling
head test. Plugging of the pores by fines or by air bubbles is less apt to
occur in a rising head test. In an unsaturated zone, the rising head test
is inapplicable.
(3) Falling Head Test. In zones where the flow rates are very high
or very low, this test may be more accurate than a constant head test. In
an area of unknown permeability the constant head test should be attempted
before a falling head test.
(4) Pumping Test. In large scale seepage investigations or
groundwater resource studies, the expense of aquifer or pumping tests may be
justified as they provide more useful data than any other type of test.
Pump tests require a test well, pumping equipment, and lengthy test times.
Observation wells are necessary. A vast number of interpretive techniques
have been published for special conditions.
(5) Gravity and Pressure Tests. In a boring, gravity and pressure
tests are appropriate. The segment of the boring tested is usually 5 to 10
feet, but may be larger. A large number of tests must be conducted to
achieve an overall view of the seepage characteristics of the materials.
The zone of influence of each test is small, usually a few feet or perhaps a
few inches. These methods can detect changes in permeability over
relatively short distances in a boring, which conventional pump or aquifer
tests cannot. Exploration boring (as opposed to "well") methods are
therefore useful in geotechnical investigations where inhomogeneity and
anisotropy may be of critical importance. Results from pressure tests using
packers in fractured rock may provide an indication of static heads, inflow
capacities, and fracture deformation characteristics, but conventional
interpretation methods do not give a true permeability in the sense that it
is measured in porous media.
c. Percolation Test. The percolation test is used to ascertain the
acceptability of a site for septic tank systems and assist in the design of
subsurface disposal of residential waste. Generally, the length of time
required for percolation test varies with differing soils. Test holes are
often kept filled with water for at least four hours, preferably overnight,
before the test is conducted. In soils that swell, the soaking period
should be at least 24 hours to obtain valid test results.
(1) Type of Test. The percolation test method most commonly used,
unless there are specified local requirements, is the test developed by the
Robert A. Taft Sanitary Engineering Center as outlined in the Reference 18,
Public Health Service Health Manual of Septic Tank Practice, by HUD. A
specified hole is dug (generally 2 feet square), or drilled (4 inches
minimum) to a depth of the proposed absorption trench, cleaned of loose
debris, filled with coarse sand or fine gravel over the bottom 2 inches, and
saturated for a specified time. The percolation rate measurement is
obtained by filling the hole to a prescribed level (usually 6 inches) and
then measuring the drop over a set time limit (usually 30 minutes). In
sandy soils the time limit may be only 10 minutes. The percolation rate is
used in estimating the required leaching field area as detailed in Reference
18.
7.1-108
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