Page 57 - Geotechnical Engineering Soil and Foundation Principles and Practice

Special Problems with Sedimentary Rocks
52 Geotechnical Engineering

blackboards before plastics. More severe metamorphism can change shale into
schist, a rock that glitters from its content of mica, which is commonly mistaken
for gold.

Metamorphic rocks are less abundant than igneous or sedimentary rocks,
occurring close to igneous activity. Large spotty areas are exposed in continental
shield areas. Metamorphism can be expected to occur close to any igneous
intrusions, including those from volcanoes.

Mica Schist
As shale is the most abundant sedimentary rock and is easily metamorphosed,
the most common metamorphic rock is mica schist. Heat and pressure transform
the clay minerals into mica, which has a similar crystal structure but a much larger
grain size, so individual flackes of mica are visible to the unaided eye. Mica, being
close to clay minerals in composition, is fairly resistant to weathering, so soil
developed from weathering of mica schist often contains abundant mica that
is springy and difficult to compact. Such soils therefore are given a separate
engineering classification based on their fractious physical properties.

Marble
True marble is metamorphosed limestone. Most is imported from Italy.
Some commercial ‘‘marble’’ is not metamorphic but is dense limestone that takes
a polish.

Quartzite
One of the hardest natural stones is sandstone that has been metamorphosed into
a dense pattern of interlocking quartz grains, appropriately called quartzite.
Quartzite is highly resistant to weathering. It forms a superior concrete aggregate,
but crushing is expensive and wears on equipment.

Metamorphosed Igneous Rocks
The most common metamorphic rock from an igneous source is ‘‘gneiss’’
(pronounced ‘‘nice’’), which looks like granite but has a flow structure. Some
granite also is considered to be metamorphic because it appears to retain some
sedimentary rock structure. Basalt that undergoes metamorphism can recrystallize
into a dark-colored layered rock that also is called schist, but does not contain
abundant mica.

3.5 FAULTS AND EARTHQUAKES

Definition
A fault is a fracture along which there has been slipping. Faults can occur in any
kind of rock. Grinding action along a fault creates soil-like fault gouge. The more
grinding that has occurred, the thicker the zone of pulverized and broken rock

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