Fundamental Soil Mechanics - Mathematics and Statistics for the Environment - page 308

To clear up this confusion, let's view that handful of soil from a different—but much more basic
and revealing—perspective. Consider the following descriptions of soil to better understand what
soil is and why it is critically important to us all:
1. A handful of soil is alive, a delicate living organism—as lively as an army of migrating
caribou and as fascinating as a flock of egrets. Literally teeming with life of incomparable
forms, soil deserves to be classified as an independent ecosystem or, more correctly stated,
as many ecosystems.
2. When we reach down and pick up an handful of soil, exposing the stark bedrock surface,
it should remind us, maybe startle some of us, that without its thin living soil layer Earth is
a planet as lifeless as our own moon.
If you still prefer to call soil dirt, that's okay. Maybe you view dirt in the same way as E.L.
Konigsburg's character Ethan does:
The way I see it, the difference between farmers and suburbanites is the difference in the way we feel
about dirt. To them, the earth is something to be respected and preserved, but dirt gets no respect. A
farmer likes dirt. Suburbanites like to get rid of it. Dirt is the working layer of the earth, and dealing
with dirt is as much a part of farm life as dealing with manure: neither is user-friendly, but both are
necessary (Konigsburg, 1996, p. 64).
12.3 SOIL BASICS
Soil is the layer of bonded particles of sand, silt, and clay that covers the land surface of the Earth.
Most soils develop multiple layers. The topmost layer ( topsoil ) is the layer in which plants grow. This
topmost layer is actually an ecosystem composed of both biotic and abiotic components—inorganic
chemicals, air, water, decaying organic material that provides vital nutrients for plant photosynthe-
sis, and living organisms. Below the topmost layer (usually no more than a meter in thickness), is the
subsoil , which is much less productive, partly because it contains much less organic matter. Below
that is the parent material , the bedrock or other geologic material from which the soil is ultimately
formed. The general rule of thumb is that it takes about 30 years to form one inch of topsoil from
subsoil; it takes much longer than that for subsoil to be formed from parent material, the length of
time depending on the nature of the underlying matter (Franck and Brownstone, 1992).
12.3.1 s oil For C onstruCtion
By the time most students reach the third or fourth year of elementary school, they are familiar with
the Leaning Tower of Pisa, and many are also familiar with Galileo's experiments with gravity and
the speed of falling objects dropped from the top of the tower. This 12th-century bell tower has been
a curiosity for literally millions of people from the time it was first built to the present. Eight stories
high and 180 feet tall, with a base diameter of 52 feet, the tower began to lean by the time the third
story was completed, and leans about 1/25 inch further each year.
How many people know why the tower is leaning in the first place—and who would be more than
ordinarily curious about why the Leaning Tower leans? If you are a soil scientist or an engineer,
this question has real significance and requires an answer. In fact, the Leaning Tower of Pisa should
never have acquired the distinction of being a leaning tower in the first place. The problem is that
the Leaning Tower of Pisa rests on a non-uniform consolidation of clay, and the ongoing process of
leaning may eventually lead to failure of the building.
As you might have guessed, the mechanics of why the Leaning Tower of Pisa leans is what this sec-
tion is all about. More specifically, it is about the mechanics and physics of the soil—important factors
in making the determination as to whether a particular building site is viable for building. Simply put,
these two factors are essential in answering the question “Will the soils present support buildings?”