Page 28 - Geotechnical Engineering Soil and Foundation Principles and Practice

Igneous Rocks, Ultimate Sources for Soils
Igneous Rocks, Ultimate Sources for Soils 23

Soil scientists are agronomists whose main focus is on agriculture. The agronomic
definition of soil therefore is that it is the unconsolidated surface layer that can
support plant life. Thus the layer of rust on steel exposed to weathering is a soil
if it supports a growth of moss or lichens.

2.3 GEOMORPHOLOGY AND AIRPHOTO INTERPRETATION

2.3.1 Geomorphology
Geomorphology is the study of landforms, which provides important clues to the
composition of rocks and soils making up a landform. For example, a conical
shaped hill is the mark of a volcano, and if one is aware of the mechanics of
volcanoes, one can expect to find sloping layers of lava mixed with volcanic
rubble and ash.

2.3.2 Airphoto Interpretation
Geomorphic patterns readily can distinguish different soil parent materials
such as the type of bedrock, alluvial sands and clays, dune sands, etc., because
where the composition of these soils is closely related to the pattern of the
landform, that in turn relates to its method of formation. For example, collapsed
caverns have considerable engineering significance and identify the rock as
most likely being a limestone. The existence of a collapsed cavern or sink also is
revealed by stream drainage that disappears into the sink.

The study of geomorphology is aided by airphoto interpretation, which literally
gives a big picture. In recent years high-level satellite photography has become
readily available on the internet, and gives an even bigger picture but with less
detail.

Airphotos conventionally are photographed in strips along a flight path, with the
camera synchronized with ground speed to create about a 60 percent overlap
between adjacent photos. This offers an advantage for airphoto interpretation
because overlapping pairs can be viewed with a stereoscope, the left eye seeing
the left picture and the right eye seeing the right. This in effect puts the eyes where
the camera was, and gives an exaggerated three-dimensional view of the ground
surface, like peering down at a scale model.

Aerial photos usually are black and white, but also may be in color or made with
special films such as infrared, which shows green vegetation as white and water as
black. ‘‘False color’’ combines infrared and color photography to make infrared
reflection more obvious by printing it in red. This is useful for identifying open
water and assessing crops, and for military purposes such as to differentiate
between living vegetation and camouflage.

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