Page 304 - Geotechnical Engineering Soil and Foundation Principles and Practice

Compaction
Compaction 299

13.9 STRENGTH AND MODULUS OF COMPACTED SOIL SPECIMENS

13.9.1 Another Emphasis
Compaction specifications not only are intended to control future volume changes
of a soil, they also may be intended to increase the soil strength. It sometimes
is assumed that higher density means higher strength, but this trend is trumped by
the moisture content: compacting on the dry side of the OMC may leave the soil
such that it can collapse under its own weight when wet with water, or too
vigorous compaction on the wet side can shear and remold the soil.
Most compacted soil is used in embankments for roads, highways, or earth dams.
In these applications, soil in the embankments not only should resist volume
changes, it must have sufficient shearing strength that side slopes are stable and do
not develop landslides. Railroad embankments, many constructed a century ago,
usually were not compacted and, significantly, still require continual maintenance
and repair. It therefore is important that the shear strength characteristics of the
compacted soil should be determined.

Compacted soils must be strong enough to support structures and minimize
settlement that not only can affect the integrity of the structure but also that
of connecting utility lines; a broken gas or water line or sewer that slopes the
wrong way is more than just an inconvenience.

13.9.2 Strength of Compacted Soils
Since compacted soil is a manufactured, quality-controlled product, its suitability
for particular applications often is based on experience with the same or similar
soils under similar applications. However, if a compacted soil is to be used for
founding heavy structures that are outside of the range of experience, the soil
strength and compressibility should be measured at the worst-case densities and
moisture contents that are within specification limits, and those values used in
design. After a soil is compacted it can be drilled, tested, and analyzed using
methods presented in later chapters.

The simplest option is to subject compacted density specimens to an unconfined
compression test, where ends are loaded until the specimen fails. This test
is discussed in Chapter 18.

13.9.3 Laboratory ‘‘K Test’’
Specimens compacted for laboratory density determinations usually are
discarded, but they can easily and quickly be tested for strength and

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