Page 24 - Geotechnical Engineering Soil and Foundation Principles and Practice

Igneous Rocks, Ultimate Sources for Soils
Igneous Rocks, Ultimate Sources for Soils 19

Figure 2.1
The geological
cycle with
shortcuts and
subcycles.

to changes caused by heat and pressure, but if the same rock is exposed to air,
acid rain, and frost action, it weathers.

Saprolite is from the Greek for decayed stone, and designates weathered rock
that still retains the original rock structure. Saprolite looks like rock but has a
distinguishing feature in that it falls apart when struck with a shovel.

Weathering proceeds from the ground surface downward, creating a mantle
of residual soil that with increasing depth is transitional to saprolite and to
unweathered rock. Borings in residual soil generally show a gradual transition
from soil to soil containing rock fragments, to saprolite and solid rock.

2.1.4 Boulders Cut and Shaped by Weathering

It sometimes is assumed that large boulders are round because they have a
history of rolling along in a stream, which might sound reasonable except that
it is not easy to move a boulder that is as big as a house. Stone Mountain in Georgia
is round and at last report was not going anywhere, so another explanation
should be sought for rounding. Observations of granite rock outcrops reveal
boulders that have been isolated and shaped by chemical weathering penetrating
along intersecting fractures. This is illustrated in Fig. 2.2. The round shape is
encouraged because weathering penetrating the surface causes the rock to expand
and flake off like layers of an onion. This origin of boulders is important because
borings intended to establish a depth to competent rock may instead hit a boulder
that is floating in a soil matrix. More than one boring is required, or the boring
should be extended deep by coring to assure that the rock is connected. Soils derived
from igneous rocks are discussed later in this chapter.

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