TM 5-852-4/AFM 88-19, Chap. 4
adherence of the ice to the pier, either from the
may already exist before construction and a condition of
increasing accumulation of ice above the water level or
substantial seepage flow through the soil under the
by lowering of the water level with decreasing stream
culvert location may already have been established. For
flow. On the other hand, raising of the water level in the
this reason a natural drainage site is nearly always
spring can cause a large upward thrust on the pier from
preferable. If the culvert is cut into an area which has
the buoyancy of ice adhering to the pier or bearing
not previously carried such flow, a new thermal regime
against connecting members.
In tidal areas the
will begin to develop; if the culvert is cut into permafrost
adherence of sea ice sheets to piers or piles at low tide
containing ground ice, thaw will occur in summer into the
may not only cause spalling or other direct structural
soil surrounding the culvert structure from the effects of
damage but may cause them to be jacked up by the
both the heat in the water flowing at the bottom of the
buoyant force of the ice in the succeeding high tide. In
culvert and the exposure to above-freezing air
coastal Connecticut, piles for a waterfront structure were
temperatures.
Since the surface of ponded water
pulled out of the ground in one winter by this action.
exposed to the sun in arctic and subarctic areas may
When such uplift is possible, piers, caissons, or piles
reach temperatures as high as about 70F in the
should be kept entirely smooth, without projection or
summer, heat input from the water may be substantial. If
cross-bracing, in the levels at which ice may act, and
permafrost containing ground ice underlies the culvert,
must have sufficient depth of embedment or other
catastrophic settlement can be produced in a single
anchorage to resist such uplift forces as can develop.
summer. Not only may the culvert structure be damaged
g. If the foundation is supported on frozen
by loss of support, but the water may begin to pass
under the structure instead of through it, leading to even
ground, short duration forces such as wind gusts are
more rapid collapse. If the flow is derived primarily from
likely to be of little consequence because design for long
snow melt or emergence of seepage from thawing
term stability will have automatically introduced large
ground, it may be close to 32 F. If the water before
factors of safety relative to short term loadings.
reaching the culvert is experiencing net heat loss by
However, any loadings which can act relatively
radiation to the sky, it may even contain frazil ice
consistently over substantial periods must be taken into
particles.
account in the design load assumptions. In all cases
c. In truly arctic areas it is possible to
careful analysis of frost heaving forces is required (para
construct a stable culvert cut into permafrost by placing
4-31) and a safety factor against heave must be provided
sufficient non-frost-susceptible backfill under and around
(para 4-8h). As shown in figure 4-41, which represents a
the culvert structure so that thaw will be confined to this
relatively stable bridge for subarctic conditions, bridge
material in summer, with thawed material refreezing in
foundations tend to be continuously in motion due to
the following winter. Required depth of gravel may be
seasonal effects. If progressive frost heave occurs, or if
computed by analysis methods outlined in TM 852-
allowable bearing stresses are overestimated and
6/AFM 88-19, Chapter 6". It will be necessary to
settlement occurs, movements may be progressive and
determine not only the shaded air temperature but also
much greater in magnitude, as well as differential.
the temperature of the water flowing in summer in the
Because of these ever-present possibilities, types of
culvert and to compute thaw penetrations separately for
bridge span structures which are especially sensitive to
these conditions using techniques for two-dimensional
foundation movements should not be employed in arctic
radial heat flow analysis. The only accurate way of
and subarctic areas unless stable foundations can be
determining the temperature of water which is flowing in
provided with complete certainty.
a culvert is by actual measurement during a summer
4-13. Culverts.
season. When in doubt, the designer should make
a. Criteria for design of flexible and rigid pipe
certain that any error is on the safe side. Insulation may
culverts, for required depths of placement of culverts and
sometimes be economically substituted for part of the
of required depths of cover below pavements are given
8
non-frost-susceptible material. In marginal permafrost
in TM 5-820-2/AFM 88-5, Chapter 1 and TM 5-820-
9
areas, however, it may not be possible to achieve
3/AFM 88-5, Chapter 3 . The problem of icing in culverts
17
essentially complete freeze-back in winter and thaw will
is discussed in TM 5-852-7/AFM 88-19, Chapter 7 .
then be progressive. Insulation can slow but not prevent
b. The problems of thermal stability of culvert
this. In this case, if unacceptable settlement would
foundations are somewhat similar to those of bridges in
otherwise result, the designer should consider use of
that the presence and flow of water causes a special
only established drainageways where reasonable
thermal regime under the structure. If the culvert is a
thermal stability has been naturally achieved, use of pile-
large one and carries water flow during most of the year,
supported bridges instead of culverts, or complete
the frost penetration pattern may be substantially altered
excavation of the thaw-susceptible materials and replace
locally.
If the culvert is constructed in a natural
drainageway, a special, relatively stable thermal pattern
4-155
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