Fundamental Soil Mechanics - Mathematics and Statistics for the Environment - page 314

12.7 SOIL COMPRESSIBILITY
When a vertical load such as a building or material stockpile is placed above a soil layer, some
settlement can be expected. Settlement is the vertical subsidence of the building (or load) as the soil
is compressed. Compressibility refers to the tendency of soil to decrease in volume under load. This
compressibility is most significant in clay soils because of the inherent high porosity. Although the
mechanics of compressibility and settlement are quite complex and beyond the scope of this text,
the reader should know something about the actual evaluation process for these properties, which
is accomplished in the consolidation test. This test subjects a soil sample to an increasing load. The
change in thickness is measured after the application of each load increment.
12.8 SOIL COMPACTION
The goal of compaction is to reduce void ratio and thus increase the soil density, which, in turn,
increases the shear strength. This is accomplished by working the soil to reorient the soil grains
into a more compact state. If water content is within a limited range (sufficient enough to lubricate
particle movement), efficient compaction can be obtained. The most effective compaction occurs
when the soil placement layer (commonly called lift ) is approximately 8 inches. At this depth, the
most energy is transmitted throughout the lift. Note that more energy must be dispersed, and the
effort required to accomplish maximum density is greatly increased when the lift is greater than 10
inches in thickness. For cohesive soils, compaction is best accomplished by blending or kneading
the soil using sheepsfoot rollers and pneumatic tire rollers. These devices work to turn the soil into
a denser state. To check the effectiveness of the compactive effort, the in-place dry density of the
soil (weight of solids per unit volume) is tested by comparing the dry density of field-compacted
soil to a standard prepared in an environmental laboratory. Such a test allows a percent compaction
comparison to be made.
12.9 SOIL FAILURE
Construction, environmental, and design engineers must be concerned with soil structural implica-
tions involved with natural processes (such as frost heave, which could damage a septic system)
and changes applied to soils during remediation efforts (e.g., when excavating to mitigate a haz-
ardous materials spill in soil). Soil failure occurs whenever it cannot support a load. Failure of
an overloaded foundation, collapse of the sides of an excavation, or slope failure on the sides of
a dike, hill, or similar feature is termed structural failure . The type of soil structural failure that
probably occurs more frequently than any other is slope failure (commonly known as a cave-in ).
A Bureau of Labor Statistics review of on-the-job mishaps found that cave-ins occur in construc-
tion excavations more frequently than you might think, considering the obvious dangers inherent
in excavation.
What is an excavation? How deep does an excavation have to be to be considered dangerous?
The answers to these questions could save your life or help you protect others when you become
an engineer involved with safety. An excavation is any manmade cut, cavity, trench, or depression
in the Earth's surface formed by earth removal. This can include excavations for anything from a
remediation dig to sewer line installation. No excavation activity should be accomplished without
keeping personnel safety in mind. Any time soil is excavated, care and caution are advised. As
a rule of thumb (and as law under 29 CFR 1926.650-652), the Occupational Safety and Health
Administration (OSHA) requires trench protection in any excavation 5 feet or more in depth. Before
digging begins, proper precautions must be taken. The responsible party in charge (the competent
person, according to OSHA) must