Geology Reference
In-Depth Information
sewer lines with a shallow drilling program (Figure 14.9A). It was found that ice-rich clay
was present at several localities. During the subsequent trenching and excavations, it was
established that the original service mains had sunk at least 2 m from their initial 1967
installation. At one locality, the thaw bulb was approximately 6 m deep. Accordingly,
the new service mains were deliberately installed in thawed ground, and layers of 50 mm
thick extruded polystyrene insulation were placed between the service mains and the
underlying permafrost (Figure 14.9B). This has slowed further subsidence on this side of
the road but subsidence continues on the other side and the ongoing saga of Franklin
Avenue has yet to be completely eliminated.
Equally costly and site-specifi c measures have been undertaken in Dawson City, Yukon
Territory, to maintain some of the historic buildings in that town. The underlying problem
is that Dawson City was located, in 1898, on a restricted area of the fl ood plain of the
Yukon River. The site is underlain by silt and alluvial gravel. Despite nearly a century of
occupation, warm (between
1 °C) permafrost is present in much of the town
site to a depth of about 20 m. The presence of segregated and inactive (relict) ice wedges
means that the soils are thaw-sensitive and subject to settlement if disturbed.
The earliest buildings were log structures or frame buildings placed on squared timbers
laid at or near the surface. Virtually all the old buildings that still remain today have
settled differentially, necessitating periodic jacking and leveling with additional cribbing,
and/or eventual abandonment (Figure 14.10A). Since the early 1960s all new buildings in
Dawson have been constructed on wooden piles or gravel pads. In restoring some of the
historic buildings, Parks Canada (the Canadian Federal Government agency responsible
for heritage affairs) has tried to maintain the original levels of the buildings with respect
to the streets, ruling out the emplacement of thick gravel pads or the use of piles. Instead,
the silty ice-rich material has been excavated and replaced by thaw-stable granular mate-
rial to a depth of 5-7 m, and the historic buildings have been replaced in their original
positions supported by adjustable jacks (Figure 14.10B). These examples illustrate the
expense required to maintain the urban infrastructure.
In the Russian north, the urban infrastructure of many major cities is beginning to
cause major engineering and geotechnical problems. Many of these cities have populations
in excess of 100 000 persons and residential housing is in massive 5-6 story apartment
blocks constructed upon piles in permafrost. Frost-jacking, thaw subsidence around build-
ings, deterioration of water and sewage facilities, and a general lack of maintenance due
to fi scal concerns and poor management are now widespread. Unverifi ed statistics pro-
vided by L. Khrustalev (2000) predict the number of major buildings that, on sound
engineering practice criteria, will fail by 2030 (Table 14.1). In Vorkuta it is predicted that
50-60% of all major building constructed between 1950 and 1999 will have failed. Already,
9 out of 30 major buildings constructed in the 1990s are failing. In Yakutsk and Tiksi,
similar situations exist, where it is predicted that nearly all building constructed perior to
1990 will have failed by 2030 (see pp. 388).
−
3 °C and
−
14.5. WATER-SUPPLY PROBLEMS
Because permafrost acts as an impermeable layer, the movement of groundwater is
restricted to thawed zones or taliks (see Chapter 5). Given these hydrologic characteris-
tics, a diffi cult problem for many northern settlements is the provision of drinking water.
Supra-permafrost water is subject to near-surface contamination and intra-permafrost
water is often highly mineralized and diffi cult to locate. Therefore, the tapping of sub-
permafrost water is often essential.
