TM 5-852-4/AFM 88-19, Chap. 4
offer adequate resistance against being pulled upward in
progressive reduction of embedded length. Maintenance
winter.
requirements are substantial and periodic replacement of
o. Figure 4-90e illustrates a pile foundation
piling and even changes of alignment are required. At
some bridge sites, the stubs of several "generations" of
with flanged sleeves to isolate the piles from frost heave
piling which have been successively abandoned may be
forces. The sleeves may be omitted if adequate pile
seen, d. Especially careful and detailed subsurface
embedment in permafrost or other provisions against
exploration should be carried out at proposed bridge
uplife are provided. Sleeves and other techniques for
locations to assure the most favorable bridge alignment
providing heave force isolation are discussed in greater
and positioning of the foundations and to provide the
detail in paragraph 4-3/.
p. The timber crib foundation shown in figure
information required for thorough and painstaking
foundation design of these vital facilities. Positions of the
4-90f has been used successfully for pole lines in difficult
permafrost table and any residual thaw zones should be
marginal permafrost terrain. Pole lines are further
13
carefully determined. The amount of any excess ice in
discussed in TM 5-852-5 .
the ground should be carefully ascertained using
4-12. Bridge foundations.
refrigerated drilling techniques, and the subsurface
a. Foundations of bridges which do not cross
temperature conditions, should be determined.
water bodies should be designed using the previously
e. The presence of frozen ground should not
described criteria for walls and retaining structures (para
be assumed to preclude scour; fine-grained permafrost
410), footings and piers (para 4-7), and piling (para 4-8),
soils are often very highly susceptible to erosion upon
as applicable.
thaw and the effects of floods may be substantial,
b. Bridges over water bodies in permafrost
particularly directly adjacent to piers placed in the
areas tend to involve difficult special problems because
streams. Also, the gouging action of floating ice often
the permafrost conditions are substantially altered near
has a powerful eroding effect on stream banks. Since
and under the water. As shown in figure 4-92, the
stream flow and thermal regime patterns may change
permafrost table tends to be depressed under a water
substantially and unpredictably over the life of a bridge,
body; under a major water body, permafrost may be
and hydrologic data are often grossly inadequate, bridge
absent except very close to the shore. Temperatures of
foundations should be placed at depths which are
permafrost near and under the water also tend to be
conservatively safe with respect to usual criteria for
warmer; especially in areas of marginal permafrost,
safety against undercutting. Stationary ice sheets"', ice
permafrost temperatures at the ground levels in which
jams, and ice sheets and ice cakes moving with various
foundations are supported may be very close to or at the
velocities of flowing water or blown by wind and with
melting point. Water moving in thaw zones beneath and
various angles of attack, can exert very substantial
adjacent to the water body may cause an extremely
pressures on foundations placed in the water.
complex and uncertain thermal regime pattern. Little or
Foundations and piers should be positioned and shaped
no capacity for natural freezeback of piles may be
so as to minimize the effects that these forces can exert
available and the tangential adfreeze strength that can
on exposed members of the foundation and designed
be safely relied on may be very low. Footing-type
with sufficient armor and strength to resist the forces
foundations encounter substantial risk of settlement from
78,79
140
and Dunham
which may then still occur. Michel
slight changes in the subsurface thermal regime.
present useful discussions of this problem. Davis has
c. For all these reasons highway and railroad
reported on rock fill and sheet pile construction exposed
bridge foundations in permafrost areas have been a
54
to sea ice in Thule Harbor . Techniques for design of
continuing source of difficulties. In marginal permafrost
structures against ice forces are in early stages of
areas, designs of stable piers and abutments are among
development.
the most challenging engineering problems which may
f.
Icing or the progressive accumulation of ice
be encountered in permafrost areas. In order to support
in winter by freezing of seepage or stream flow on the
these facilities on relatively stable frozen materials, pile
48,50
surface is unlikely to contribute any structural loadings to
. However,
foundations are commonly employed
the foundation if the ice rests directly on the ground,
because of uncertain or incomplete freezeback, the
although it may significantly reduce the hydraulic
frequency of serious frost heaving of pile bridge
capacity of the bridge opening. However, if the ice build-
foundations has been very high. On the Alaska Railroad
up occurs on floating ice, a downward thrust on the
for example, wood piling of many of the bridges is
180
foundation may be exerted in winter because of
heaved every year. Pewe has reported that this heave
reaches as much as 14 in./yr and the elevation of the
track is seriously disturbed, making necessary reduction
of speed to avoid uncoupling of cars or shifting of cargo.
Tops of piles are trimmed off each summer, resulting in
4-152
Previous Page
