Deformation, Mountain Building, and the Continents - The Changing Earth: Exploring Geology and Evolution - page 251

Geo-Focus

Engineering and Geology

As you might expect, engineering
geologists apply the principles and
concepts of geology to engineering
practices. Geologists with this spe-
cialized training may be involved in
slope stability studies and in studies of
acceptable areas for power plants, for
highways in mountainous regions, for
tunnels and canals, and for structures
to protect riverbank and seashore
communities.
A good example is the concern
prior to building the Mackinac
(pronounced “mack-in-aw”) Bridge, a
huge suspension bridge that connects
the Upper and Lower Peninsulas of
Michigan (

Geologic
engineers are in-
variably involved
in planning
for large-scale
structures such
as bridges, dams,
power plants,
and highways,
especially in
tectonically
active regions.
For example,
the bridge in
◗

Figure 2 is only
a short distance
from the San
Andreas fault, so
engineers had to
take into account the near certainity
that it would be badly shaken during
an earthquake. Many other sturctures
on or near the San Andreas fault were
constructed when codes were much
less stringent, and now they are being
retrofi tted to make them safer during
earthquakes.
Being aware of a problem and tak-
ing remedial action sometimes come

◗ Figure 1 Before the Mackinac Bridge that connects the Lower
and Upper Peninsulas of Michigan could be built, geologists and
engineers had to determine whether the Mackinac Breccia could
support such a large structure.

◗

Figure 1). Geologists and
engineers were aware that some of the
rock in the area, called Mackinac
Breccia, was a collapse breccia, or rub-
ble that formed when caverns
collapsed. The concern was whether
the breccia or any uncollapsed caverns
beneath the area would support
the weight of the huge piers and
abutments for the bridge. Obviously,
the project was completed successfully,
but detailed studies were done before
construction began.

too late. For instance, engineers were
aware that the Santa Monica Freeway
in the Los Angeles area would likely be
damaged during an earthquake and
retrofi tting was scheduled for February
1994. Unfortunately, the Northridge
earthquake struck on January
17, 1994, and part of the freeway
collapsed. Of course, this and other
similar events provide important

Joints
Besides folding, rocks are also permanently deformed
by fracturing. Joints are fractures along which no move-
ment has taken place parallel with the fracture surface
(

Many domes and basins are so large that they can be
visualized only on geologic maps or aerial photographs. The
Black Hills of South Dakota, for example, are a large oval
dome (Figure 10.13b). One of the best-known basins in the
United States is the Michigan Basin, most of which is buried
beneath younger strata so it is not directly observable at the
surface. Nevertheless, strike and dip of exposed strata near
the basin margin and thousands of drill holes for oil and gas
show that the strata are deformed into a large basin.
Unfortunately, the terms dome and basin are also used to
distinguish high and low areas of Earth's surface, but as with
anticlines and synclines, domes and basins resulting from
deformation do not necessarily correspond with mountains
or valleys. In some of the following discussions, we will have
occasion to use these terms in other contexts, but we will try
to be clear when we refer to surface elevations as opposed to
geologic structures.

Figure 10.14), although they may open up; that is,
they show movement perpendicular to the fracture. Coal
miners used the term joint long ago for cracks in rocks that
they thought were surfaces where the adjacent blocks were
“joined” together.
Remember that rocks near the surface are brittle and
therefore commonly fail by fracturing when subjected
to stress. In fact, almost all near-surface rocks have joints
that form in response to compression, tension, and shear-
ing. They vary from minute fractures to those extending for
many kilometers and are often arranged in two or perhaps
three prominent sets. Regional mapping reveals that joints

◗