Page 78 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
Soils That Are Sediments 73

During the final retreat large blocks of ice may stagnate and be incorporated into
the moraines. Later when the blocks of ice melt they leave steep-sided depressions
called ‘‘kettle lakes.’’ Kettle lakes are common in the northern U.S. and in the
British Isles.

4.4.14 Two Kinds of Till
Terminal and recessional moraines are not overridden by a glacier so they are not
overconsolidated. Glacial till that is not overconsolidated sometimes is called
superglacial till, not because it is super but because it was deposited by melting
from the top. A close association with water draining from the melting ice causes
superglacial till to be more sandy than subglacial till, and to contain irregular
pockets and layers of sand.

Superglacial till, being less dense than subglacial till, more readily weathers, so
a distinction also may be made on the basis of soil color, brown on top of gray.

4.4.15 Ground Moraine
The uniform layer of glacial sediment left during a steady retreat of an ice
front is called a ground moraine. Ground moraines have a gently to moderately
rolling topography with internal drainage, meaning that streams collect into pools
and have no exit. The resulting wet conditions in the swale areas give a ground
moraine a mottled appearance on aerial photographs. Ground moraines also can
show fingerprint-like patterns caused by seasonal retreats of the ice front.

Erosion by surface runoff water gradually removes soil from the shallow hills
of a ground moraine and deposits it in adjacent swales, where the soil tends to
be wet, clayey, and highly compressible. Swale soils also can contain expansive
clay minerals, so drainage may dry them out and make them vulnerable to later
rewetting.

4.4.16 Peat in the Swales

Peat is vegetation that has grown and died in bogs, and is protected from decay by
being under water. Peat is common in ground moraine areas. As peat is mostly
water, it is a very difficult soil for the engineer. Piles or piers are used to support
structures such as bridges, and road embankments for roads may be built high
enough that they can sink and displace the peat until it reaches solid soil at the
bottom. The process is speeded up by drilling through the embankment and
placing dynamite charges in the peat layer.

Another approach is to float an embankment on foamed plastic. The least
expensive alternative may be to simply go around the bog. Most important is to
recognize areas of peat in time to influence location, design, and construction.

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