TM 5-820-3/AFM 88-5, Chap. 3
(2) Note 2. Sufficient granular backfill will be
Placing drains and culverts beneath pavements
placed beneath inlets and outlets to restrict frost
should be minimized to the extent possible. When
penetration to nonheaving materials.
this is unavoidable, the pipes should be installed
(3) Note 3. Design of short pipes with
before the base course is placed in order to obtain
exposed ends, such as culverts under roads, will
maximum uniformity. The practice of excavating
consider local icing experience. If necessary, extra
through base courses to lay drain pipes and other
size pipe will be provided to compensate for icing.
conduits is unsatisfactory since it is almost impos-
(4) Note 4. Depth of frost penetration in well-
sible to attain uniformity between the compacted
drained, granular, nonfrost-susceptible soil beneath
trench backfill and the adjacent material.
pavements kept free of snow and ice will be
b. No special measures are required to prevent
determined from data found in figure 3-5 of TM 5-
heave in nonfrost-susceptible subgrades. In frost-
818-2/AFM 88-6, chapter 4. For other soils and/or
susceptible subgrades where the highest ground-
surface conditions, frost penetrations will be deter-
water table is 5 feet or more below the maximum
mined by using conservative surface condition as-
depth of frost penetration, the centerline of the pipe
sumptions and methods outlined in TM 5-852-6/
should be placed at or below the depth of
AFM 88-19, Volume 6. In all cases, estimates of
maximum frost penetration. Where the highest
frost penetration will be based on the design freez-
ground-water table is less than 5 feet below the
ing index, which is defined as the average air-
depth of maximum frost penetration and the pipe
freezing index of the three coldest winters in a 30-
diameter is 18 inches or more, one of the following
year period, or the air-freezing index for the coldest
measures should be taken:
winter in the past 10-year period if 30 years of
(1) Place the centerline of the pipe at or
record are unavailable. Further information re-
garding the determination of the design freezing
below the depth of maximum frost penetration and
index is included in TM 5-818-2/AFM 88-6, chap-
backfill around the pipe with a highly free-draining
ter 4 and TM 5-852-6/AFM 88-19, Volume 6.
nonfrost-susceptible material.
(5) Note 5. Under traffic areas, and
(2) Place the centerline of the pipe one-third
particularly where frost condition pavement design
diameter below the depth of maximum frost pene-
is based on reduced subgrade strength, gradual
tration.
transitions between frost-susceptible subgrade
c. To prevent water from freezing in the pipe,
materials and nonfrost-susceptible trench backfill
the invert of the pipe should be placed at or below
will be provided within the depth of frost
the depth of maximum frost penetration. In arctic
penetration to prevent detrimental differential
and subarctic areas it may be economically infeasi-
surface heave.
ble to provide sufficient depth of cover to prevent
freezing of water in subdrains; also, in the arctic, no
2-7. Infiltration of fine soils through drainage
residual thaw layer may exist between the depth of
pipe joints.
seasonal frost penetration and the surface of
a. Infiltration of fine-grained soils into drainage
permafrost. Subdrains are of little value in such
pipelines through joint openings is one of the major
areas because, unless protected from freezing, they
causes of ineffective drainage facilities. This is a
are usually blocked with ice during the spring
serious problem along pipes on relatively steep
thawing period. Water freezing in culverts also
slopes such as those encountered with broken back
presents a serious problem in arctic and subarctic
culverts or stilling wells. Infiltration is not confined
regions. The number of such structures should be
to non-cohesive soils. Dispersive soils have a
held to a minimum and should be designed based
tendency to slake and flow into drainage lines.
on twice the normal design capacity. Thawing de-
b. Infiltration, prevalent when the water table is
vices should be provided in all culverts up to 48
at or above the pipeline, occurs in joints of rigid
inches in diameter. Large diameter culverts are
pipelines and in joints and seams of flexible pipe,
usually cleaned manually immediately prior to the
unless these are made watertight. Watertight
spring thaw. Drainage requirements for arctic and
jointing is especially needed in culverts and storm
subarctic regions are presented in TM 5-852-7/
drains placed on steep slopes to prevent infiltration
AFM 88-19, chapter 7.
and/or leakage and piping that normally results in
d. The following design notes should be consid-
the progressive erosion of the embankments and
ered for installations located in seasonal frost areas.
loss of downstream energy dissipators and pipe
(1) Note 1. Cover requirement for traffic
sections.
c. Culverts and storm drains placed on steep
loads will apply when such depth exceeds that
slopes should be large enough and properly vented
necessary for frost protection.
2-10
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