Environmental Engineering Reference
In-Depth Information
this material system responds to expansion- contraction cycles in that portion of the ice-
water mass which regularly changes phase. It is further influenced by the presence of a
frozen, impermeable substrate - the ground ice itself - beneath a seasonal active layer in
what is commonly described as the
periglacial
environment. The term is misused when
the existence of permafrost is equated with the proximity and climatic influence of
glaciers, as the increase in permafrost intensity and activity in Alaska
away
from the
cordilleran glaciers demonstrates. It is more appropriate to regard glaciation and
periglaciation as sharing persistent cold temperatures and permanent ice, albeit in
different amounts, forms and regimes, than to regard them as sharing geographical
proximity.
Freeze-thaw cycles stimulate material expansion and contraction simultaneously.
Ground ice styles associated with freezing were described earlier. Initial ground
expansion on freezing is driven by a 9 per cent volumetric increase of freezing water,
accentuated by migration of pore water to the freezing plane in porous substrates. This
establishes corresponding dewatering contraction elsewhere which accentuates the
thermal contraction associated with falling temperatures. Thawing effectively reverses
these processes, and the combined impact of repetitive stress cycles is responsible for the
cryofracture of rock and the cryoturbation of unconsolidated materials (see Chapter 13).
THE PERMAFROST LANDSYSTEM
Erosion and deposition are less easily ascribed to permafrost than other geomorphic
environments except for cryofracture (frost weathering). This clearly erodes exposed
bedrock (and comminutes clasts in sediments). Its products are distributed as depositional
or residual landforms, or are fed into the mass wasting system. These processes dominate
upland permafrost landscapes (see Chapter 25) with a distinctive suite of residual and
detrital landforms (Figure 15.12, Plates 25.6 and 25.7). Tors represent the surviving
bedrock surface on summits and plateau breaks-of-slope. The permafrost erosion surface
may be represented by an altiplanation terrace, influenced by the structure and depth of
the active layer. Massive and resistant rocks such as granite, grits and tuffs produce the
best tors. Rock debris litters the landscape and is crudely stratified on plateau surfaces,
with an upper pavement of large angular boulders over a layer of fines washed down by
meltwater and sheltered from deflation.
The general form of permafrost slope systems is described in Chapter 25 as a variation
on general models of mass wasting and slope evolution (Chapter 13). Cryofracture on
summits and slopes is one source of slope
