Page 55 - Geotechnical Engineering Soil and Foundation Principles and Practice

Special Problems with Sedimentary Rocks
50 Geotechnical Engineering

freezing. Because of the high costs associated with such contingencies, small
exploration tunnels often are driven to inspect for potential hazards.

Cherty Limestone
The famous white cliffs of Dover contain generous amounts of flint, which in
this case is a black, very hard silica rock. Flint is a synonym for chert, which can
be any color, but usually is tan or gray and has a waxy appearance. Flint also can
designate tools or arrowheads made by ‘‘flint knapping.’’ Skillfully dressed
‘‘Folsom points’’ are evidence of early man in the Americas.

Chert occurs as hard nodules or nodular layers in limestone, and dissolution of the
limestone therefore leaves a chert concentrate at the ground surface. Chert is
microcrystalline quartz but also often contains a noncrystalline, amorphous silica
component called opal. Opaline chert reacts with alkalis in Portland cement
concrete to make an expansive, destructive, oozing gel, and concrete aggregate
containing opaline chert is called reactive aggregate. The reaction can be controlled
by use of low-alkali cement or by the addition of a fine-grained pozzolan such as fly
ash to react preferentially with the alkali.

3.3.4 Evaporites
A class of sedimentary rocks that is even more soluble than limestone is called
evaporites. As the name implies, these are rocks that have been deposited through
evaporation of water. Because of their high solubility, evaporites rarely outcrop at
the ground surface, and then only in arid or semiarid regions. Two common
minerals in evaporites are gypsum and halite, which is rock salt. Gypsum is used
to make plasterboard.

Gypsum is both a rock name and a mineral name, where it identifies a hydrated
calcium sulfate. Gypsum crystals occur as secondary deposits in other sedimentary
rocks and soils. Sulfates in soils or in sea water cause unfavorable expansive
reactions with Portland cement concrete that can be minimized or controlled by the
use of sulfate-resistant cement.

Caverns form much more rapidly in evaporites compared with limestone. While
rare in soils, about one-third of the U.S. is underlain by evaporite deposits, mostly
in semiarid areas, and solution caverns occur in western Kansas and Texas, New
Mexico, Michigan, and in Nova Scotia.

Salt Domes
Salt layers under pressure from the overburden become plastic, and being less dense
than other rock tend to squeeze into large blobs that slowly rise and punch through
overlying rock layers to make salt domes. Salt domes are commercial sources for
salt and for elemental sulphur, and rock layers penetrated by the salt can form traps
for petroleum.

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