Geology Reference
In-Depth Information
provision of large drainage ditches adjacent to the road. Icings may also block culverts
placed beneath road embankments and, by diverting snowmelt waters, initiate washouts
in the thaw period. The costs of icing control and/or remedial measures can be consider-
able; for example, van Everdingen (1982) provides a conservative estimate of $20 000.00
for icing control at one locality studied on the Alaska Highway, Yukon, during the 1979-
80 winter.
Traditional bridge construction methods are also a problem in permafrost regions.
Traditionally, these are usually constructed using piles inserted across the river or stream
channel. While in warmer climates the chief problem of piles is to obtain suffi cient bearing
strength, in permafrost regions the problem is to keep the piles in the ground because
frost action tends to heave them upwards. Since heaving becomes progressively greater as
the active layer freezes, it follows that the thicker the active layer the greater is the upward
heaving force. In discontinuous permafrost, where the active layer may exceed 2 m in
thickness, frost heaving of piles assumes critical importance. In parts of Alaska, for
example, old bridge structures illustrate dramatically the effects of differential frost heave
(see Péwé, 1983c) because it is not uncommon for a thawed zone to exist beneath the river
channel. Thus, piles inserted in the stream bed experience little or no heave and piles
inserted within permafrost on either side of the river are also unaffected. However, the
piles adjacent to the river bank experience repeated heave since they are located in the
zone of seasonal freezing. As a result, uparching of both ends of the bridge may occur.
In order to prevent these problems, alternative structures involving minimal pile
support are now used. One example, that of the Eagle River Bridge on the Dempster
Highway, northern Yukon (Figure 14.11A), is described here. The bridge was constructed
in the late 1970s by Canadian Army engineers. It consists of a single 100 m long steel span
with footings on the north side placed in permafrost. Drilling prior to construction had
indicated that permafrost, present on the north bank to a depth of
90 m, had a tempera-
ture of
3 °C. A deep near-isothermal talik existed beneath the Eagle River channel. On
the opposing slope, the permafrost was only 8-9 m thick and marginal in temperature
(
0.4 °C).
In order to maintain the delicate permafrost conditions and to provide structural integ-
rity, 15 steel piles were inserted at each abutment (Figure 14.11B). Conventional adfreeze
analysis indicated the optimum depth of emplacement of each pile was
5 m. However,
because of the warm permafrost at the south side, piles were driven to a depth of 30 m.
On the north side, where the permafrost was colder, piles were driven to a depth of 12 m.
The piles were backfi lled with sand slurry to promote adfreeze. A further complexity was
that construction had to be carried out during the winter (1976-77), in order to minimize
surface terrain damage. Subsequent monitoring indicates that the piles experienced
minimal heave, the thermal regime of the permafrost was maintained, and the bridge
structure has performing satisfactorily (Johnston, 1980).
Several railways, constructed across permafrost terrain in Alaska and northern Canada,
have also experienced costly maintenance problems on account of frost heave adjacent to
bridge structures and thaw settlement along the railbed itself (Ferrians et al., 1969; Hayley,
1988; Hayley et al., 1983).
One illustration is provided by the Hudson Bay railway, which extends through
northern Manitoba to the port of Churchill. During nearly 60 years of operation, thaw
settlement of the railway embankment and destruction of bridge decks by frost heave have
been perpetual problems. Test sections installed with heat pipes proved effective, but
costly, measures that minimized thaw subsidence (Hayley, 1988). However, the numerous
transitions from frozen to unfrozen terrain in discontinuous permafrost terrain made
installation of such techniques along the entire 820 km route impractical. In Alaska, the
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