Page 50 - Geotechnical Engineering Soil and Foundation Principles and Practice

Special Problems with Sedimentary Rocks
Special Problems with Sedimentary Rocks 45

limestone bedrock has some peculiarities that can present serious problems for
foundation engineers.

Why Call It Limestone?
The name limestone comes from its use to make lime for mortar, which dates from
ancient times. The dominant mineral in limestone is calcite, or calcium carbonate,
CaCO 3 . When calcite is heated to several hundred degrees Celsius it decomposes
and releases carbon dioxide, CO 2 , leaving quicklime, CaO. Quicklime retains a
pebble shape until it is hydrated by adding water to make hydrated lime, Ca(OH) 2 .
The name ‘‘quick’’ can be attributed to the heat released by the rapid hydration.

Other Minerals in Limestone
While the most common calcium carbonate mineral in limestone is calcite, another
mineral with the same chemical composition accumulates in shells of invertebrate
marine animals. This mineral is ‘‘aragonite,’’ which can be much harder than calcite
due to crystallite orientations and possible interactions with organic matter.
‘‘Coquina’’ is soft limestone that is composed almost entirely of shells and may be
used for surfacing roads. Coral reefs are another example of limestones that are
made up of skeletons of marine animals.

Dolomite
This is a closely related carbonate rock in which some of the calcium has been
replaced during geological time by magnesium to make Ca.Mg(CO 3 ) 2 . Dolomite
generally is older than limestone and less desirable as concrete aggregate.
Dolomite is both a rock name and a mineral name.

Concretions and Caliche
Calcium carbonate is slightly soluble in water that is made acidic by the presence
of CO 2 . Carbon dioxide released by combustion of carbon compounds dissolves
in atmospheric water to make the acid rain that is so devastating to monuments
made of limestone, and contributes to global warming. Carbon dioxide also is
released into topsoil by microorganisms. Rain infiltrating downward through
topsoil therefore can become acidic and dissolve carbonate minerals that then are
redeposited as rounded nodules or ‘‘concretions.’’ In arid and semiarid climates
the occasional movement of water upward and subsequent evaporation leaves
a crusty deposit of calcium carbonate near the ground surface called caliche.
Caliche sometimes is used as gravel on roads.

Residual Soils Developed in Limestone
Competent limestone rock is suitable for supporting building foundations,
but a major goal of exploration is to establish its competence. Weathering of
limestone, in contrast to weathering of shale or sandstone, leaves relatively little
residue after the calcium carbonate is simply dissolved away. Surface soils on
limestone typically are clayey, and incorporate increasing percentages of rock
fragments with depth, being transitional to solid rock. Foundation borings in
limestone normally are continued deep enough into the rock to ensure that

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