Page 299 - Geotechnical Engineering Soil and Foundation Principles and Practice

Compaction
294 Geotechnical Engineering

Figure 13.11
Serious problems caused by collapse of soil compacted on the dry side of the OMC: (a) earth dam
piping; (b) pavement collapse; (c) fracture and leak of a gas or water pipe: gas flowing through loose
trench backfill soil and into a building can cause fire or explosion.

Broken water mains can cause locally severe underground erosion, so the
dramatic collapse of the street is sure to make the 6 o’clock news.

Gas line breaks can be more tragic, as escaping gas follows the path of least
resistance through loose trench backfill into a building or basement, where it can
cause a fire or explosion. Pipes crossing through soil in a trench should be
supported by some type of beam.

Collapse of soil within an earth dam can lead to serious consequences by allowing
‘‘piping,’’ or flow of water through the dam, leading to erosion and failure.

13.7.3 Preventing Collapse of Compacted Soil

Figure. 13.12 shows volume changes after compacting and wetting. The area of
zero volume change is shown shaded, and for the most part falls within
a box drawn for the specification limits of OMC 2 percent, and a compacted
unit weight between 95 and 100 percent of the standard Proctor density. Thus,
compaction within these specification limits normally will result in a soil that is
relatively stable against future volume changes. An exception is expansive clays,
which are discussed later in this chapter.

Where even a small amount of collapse cannot be tolerated, as in the case of
the core material in an earth dam, the specification limits for moisture content
often are increased to between the OMC and the OMC plus 2 percent to as high as
4 percent. However, compaction on the wet side weakens the soil and makes it
more susceptible to overcompaction and development of excess pore water
pressure.

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