Engineering Geology

E n g i n e e r i n g G e o l o g y
freezing produces visible layers of ice of various thicknesses. Ice segregation in soil also
takes place under cyclic freezing and thawing conditions.
Mechanical Properties of Frozen Soil
The presence of masses of ice in a soil means that as far as engineering is concerned, the
properties of both have to be taken into account. Ice has no long-term strength, that is, it flows
under very small loads. If a constant load is applied to a specimen of ice, instantaneous elastic
deformation occurs. This is followed by creep, which eventually develops a steady state.
Instantaneous elastic recovery takes place on removal of the load, followed by recovery of
the transient creep.
The relative density influences the behaviour of frozen coarse soils, especially their shearing
resistance, in a manner similar to that when they are unfrozen. The cohesive effects of the ice
matrix are superimposed on the latter behaviour, and the initial deformation of frozen sand is
dominated by the ice matrix. Sand in which all the water is more or less frozen exhibits a brittle
type of failure at low strains, for example, at around 2% strain. The water content of coarse
soils is converted almost wholly into ice at a very few degrees below freezing point. Hence,
frozen coarse soils exhibit a reasonably high compressive strength only a few degrees below
freezing, and there is justification for using this parameter as a design index of their performance
in the field, provided that a suitable factor of safety is incorporated. The order of increase
in compressive strength with decreasing temperature is shown in Figure 5.14.
On the other hand, frozen clay, in addition to often containing a lower content of ice than
sand, has layers of unfrozen water (of molecular proportions) around the clay particles.
These molecular layers of water contribute towards a plastic type of failure. In fact, in fine
sediments, the intimate bond between the water and clay particles results in a significant
proportion of soil moisture remaining unfrozen at temperatures as low as -25∞C. The more
the clay material in the soil, the greater is the quantity of unfrozen moisture. As far as the
unconfined compressive strength of frozen clays is concerned, there is a dramatic increase
in strength with decreasing temperature. In fact, it appears to increase exponentially with the
relative proportion of frozen moisture. Using silty clay as an example, the amount of moisture
frozen at -18∞C may be only 1.25 times that frozen at -5∞C, but the increase in compressive
strength may be more than fourfold.
Because frozen ground is more or less impermeable, this increases the problems due to thaw
by impeding the removal of surface water. What is more, when thaw occurs, the amount of
water liberated may greatly exceed that originally present in the melted out layer of the soil
(see below). As the soil thaws downwards, the upper layers become saturated, and since
244