TM 5820-4/AFM 885, Chap 4
CHAPTER 6
SUBSURFACE DRAINAGE
topographic survey and ground explorations. The
6-1. General.
sampling and exploratory work can be made more
a. The water beneath the ground surface is de-
rapid and effective after analysis of aerial pho-
fined as subsurface water. The free surface of this
tographs has developed the general soil features.
water, or the surface on which only atmospheric
The location and depth of permanent and perched
pressure acts, is called the groundwater table.
groundwater tables maybe sufficiently shallow to
Water is contained above an impervious stratum
influence the design. The season of the year and
and hence the infiltration water is prevented from
rainfall cycle will measurably affect the depth to
reaching a groundwater table at a lower eleva-
the water table. In many locations information
tion. The upper body of water is called perched
may be obtained from residents of the surround-
groundwater and its free surface is called a perched
ing areas regarding the behavior of wells and
water table.
springs and other evidences of subsurface water.
b. This water infiltrates into the soil from sur-
The soil properties investigated for other pur-
face sources, such as lakes, rivers and rainfall, and
poses in connection with the design will supply
some portion eventually reaches the groundwater
information that can be used for the design of the
table. Groundwater tables rise and fall depending
drainage system. It may be necessary to supple-
upon the relation between infiltration, absorp-
ment these explorations at locations of subsurface
tion, evaporation and groundwater flow. Seasonal
drainage structures and in areas where soil in-
fluctuations are normal because of differences in
formation is incomplete for design of the drainage
the amount of precipitation and maybe relatively
system.
large in some localities. Prolonged drought or wet
periods will cause large fluctuations in the
6-3. Laboratory tests. The design of subsurface
groundwater level.
drainage structures requires a knowledge of the
following soil properties of the principal soils en-
6-2. Subsurface drainage requirements. The
countered: strength, compressibility, swell and
determination of the subsurface soil properties and
dispersion characteristics, the in situ and com-
water condition is a prerequisite for the satisfac-
pacted unit dry weights, the coefficient of perme-
tory design of a subsurface drainage system. Field
ability, the in situ water content, specific gravity,
explorations and borings made in connection with
grain-size distribution, and the effective void ra-
the project design should include the following
tio. These soil properties may be satisfactorily de-
investigations pertinent to subsurface drainage.
termined by experienced soil technicians through
A topographic map of the proposed area and the
laboratory tests, The final selected soil properties
surrounding vicinity should be prepared indicat-
for design purposes may be expressed as a range,
ing all streams, ditches, wells, and natural res-
one extreme representing a maximum value and
ervoirs. The analysis of aerial photographs of the
the other a minimum value. The true value should
areas selected for construction may furnish val-
be between these two extremes, but it may ap-
uable information on general soil and ground-
proach or equal one or the other, depending upon
water conditions. An aerial photograph presents
the variation within a soil stratum.
a graphic record of the extent, boundaries, and
surface features of soil patterns occurring at the
6-4. Flow of water through soils.
surface of the ground, The presence of vegetation,
a. The flow of water through soils is expressed
the slopes of a valley, the colorless monotony of
by Darcy's empirical law which states that the
sand plains, the farming patterns, the drainage
velocity of flow is directly proportional to the hy-
pattern, gullies, eroded lands, and evidences of the
draulic gradient. This law is expressed in equation
works of man are revealed in detail by aerial pho-
form as:
tographs. The use of aerial photographs may sup-
V=ki
plement both the detail and knowledge gained in
61
Previous Page
