Pile Design and Construction Practice, Fifth edition

When inspecting the geological conditions in the shafts full precautions should be taken
against the collapse of the walls or base of the drill hole and against the presence of
explosive or asphyxiating gases (see Section 11.3.3). The advice of an experienced mining
engineer should be sought before exploring abandoned workings from the pile borehole.
9.4 Piling in frozen ground
Miscellaneous piling problems 449
9.4.1 General effects
In most parts of the UK the depth of penetration of frost into the ground does not exceed
600 mm, and consequently frozen soil conditions are not detrimental to piled foundations.
However, in countries lying in the northern latitudes with continental-type climates the
penetration of frost below the surface gives rise to considerable problems in piling work. In
the southern regions of Canada and in Norway the frost penetrates to depths of 1.2 to 2.1 m.
In far-northern latitudes the ground is underlain by great depths of permanently frozen soil
known as ‘permafrost’. About 49% of the land mass of the former USSR is a permafrost
region, which generally lies north of latitude 50�. The depth of permafrost extends to 1.5 km
in some areas. Permafrost regions are also widespread in Northern Canada, Alaska,
and Greenland.
In areas where frost penetration is limited to a deep surface layer overlying non-frozen
soil, the effect on pile foundations is to cause uplift forces on the pile shaft and on the pile
caps and ground beams. These effects occur in frost-susceptible soils, i.e. soils which exhibit
marked swelling when they become frozen. Frost-susceptible soils include silts, clays, and
sand–silt–clay mixtures. Swelling of these soils occurs when water in the pores migrates into
layers or lenses and becomes frozen. The increase in volume when the ice lenses form
results in a heave of the ground surface. When these soils are frozen onto the shafts of piles,
or onto the sides of pile caps and ground beams, the uplift forces tend to lift the foundations.
The soil heave also causes uplift forces to develop on the undersides of the pile caps and
ground beams. The uplift forces on the sides of the sub-structure are referred to as
‘adfreezing’ forces and measures to prevent the upward movement of piled foundations are
described in the next section of this chapter.
The foundation problems presented by permafrost are much more severe, because of the
extreme conditions of instability of this material within the depths affected by piling work.
The permanently frozen ground is overlain by an ‘active layer’ that is subject to seasonal
freezing and thawing. In the winter adfreezing occurs on foundations sited within frostsusceptible
soils in the active layer. In summer there is rapid and massive collapse of thawing
ice lenses in the active zone. Severe freeze-thaw conditions in highly frost-susceptible soils
can result in the formation of dome-shaped ice-caverns as much as 6 m high above the
permafrost. The thickness of the active layer is not constant, but varies with cyclic changes
in the climate of the region, with changes in the cover of vegetation such as mosses and
lichens, and with the effects of buildings and roads constructed over the permafrost.
The laws governing the physical, chemical and mechanical properties of frozen soil have
been reviewed by Anderson and Morgenstern (9.9) , and Andersland and Ladanyi (9.10) provide
extensive soil mechanics data for frozen ground conditions with worked examples of a
variety of foundation support systems.
Tsytovich (9.11) has described three modes of formation of permafrost: these are when
water-bearing soils are frozen through, when ice and snow are buried, and when ice is formed