Page 77 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
72 Geotechnical Engineering

have been found in glacial deposits, and back-tracking has led to the discovery of
valuable diamond-bearing kimberlite rocks in the Northwest Territories.

4.4.10 Glacial Outwash

Glacially derived water deposits that are carried beyond the limits of glacial
advances are called outwash. These are mainly sand and gravel and are important
sources of aggregate for use on roads and in concrete.

4.4.11 Glacial Till

The most abundant glacial deposit is glacial till, which is deposited by slow
melting of ice such that there is little sorting by running water. Till deposited by
modern glaciers is a mud that readily flows under its own weight, and gradually
settles into a solid mass as the the soil loses water and consolidates.

Subglacial till has been been run over and compressed into a hard mass by
the weight of the glacier. In engineering terms such a soil is said to be
overconsolidated, meaning that it has been consolidated under a pressure that is
in excess of that which exists today. The pressure involved in overconsolidation
is called the overconsolidation pressure, also called the preconsolidation pressure.
This is an important measure in foundation engineering because it represents
a pressure that can be replaced without causing appreciable settlement. It is
surcharge imposed by the weight of a glacier.

4.4.12 Overconsolidation Pressure and Ice Thickness
The overconsolidation pressure determined from laboratory tests may not
represent the maximum pressure imposed by the weight of glacial ice because of
restricted drainage as water is trapped between soil on the bottom and ice on the
top. This is another evidence for the existence of positive pore-water pressure that
aided glacial movement. The overconsolidation pressure determined from
laboratory testing may better reflect the maximum ice thickness where the glacier
has overridden a porous rock such as limestone.

4.4.13 Retreat of an Ice Front
Retreat of a glacier does not mean that the glacier backed up, but signifies retreat
of a glacial margin when the rate of melting exceeds the rate of ice advance.
During the transition from advance to retreat, the rate of advance temporarily
equals the rate of retreat, so the ice front is stationary even though the ice still is
moving. This causes a large pile-up of sediment called a terminal moraine. Periodic
surges during the final retreat result in hills called recessional moraines that have a
similar origin.

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