TM 5-8204/AFM 88-5, Chap 4
or
effective porosity is defined as the ratio of the
volume of the voids that can be drained under
Q=vA=kiA
gravity flow to the total volume of soil. Limited
Variables in the equations are defined in appendix
effective porosity test data for well-graded base-
D. According to Darcy's law the velocity of flow
coarse materials, such as bank-run sands and
and the quantity of discharge through a porous
gravels, indicate a value for effective porosity of
media are directly proportional to the hydraulic
not more than 0.15. Uniformly graded soils such
gradient. The flow must be in the laminar regime
as medium coarse sands, may have an effective
for this condition to be true.
porosity of not more than 0.25.
b. A thorough discussion of the Darcy equation
6-6. Backfill for subsurface drains.
including the limitations, typical values of perme-
ability, factors affecting the permeability, effects
should serve a dual purpose: it must prevent the
of pore fluid and temperature, void ratio, average
movement of particles of the soil being drained,
grain size, structure and stratification, formation
and it must be pervious enough to allow free water
discontinuities, entrapped air in water or void, de-
to enter the pipe without clogging it with fine par-
gree of saturation, and fine soil fraction can be
ticles of soil. The material selected for backfill is
found in TM 5-820-2/AFM 88-5, Chapter 2.
called filter material. An empirical criterion for
the design of filter material was proposed by Ter-
6-5. Drainage of water from soil. The quantity
zaghi and substantiated by tests on protective fil-
of water removed by a drain will vary depending
ters used in the construction of earth dams. The
on the type of soil and location of the drain with
criterion for a filter and pipe perforations to keep
respect to the groundwater table. All of the water
protected soil particles from entering the filter or
contained in a given specimen cannot be removed
pipe significantly is based on backfill particle sizes.
by gravity flow since water retained as thin films
adhering to the soil particles and held in the voids
b. The filter stability criteria for preventing
by capillarity will not drain. Consequently, to de-
movement of particles from the protected soil into
termine the volume of water that can be removed
or through the filter and the exceptions to this
from a soil in a given time, the effective porosity
criteria are discussed in chapter 5, TM 5-820-2/
as well as the permeability must be known. The
AFM 88-5, Chapter 2.
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