Page 37 - Geotechnical Engineering Soil and Foundation Principles and Practice

Igneous Rocks, Ultimate Sources for Soils
32 Geotechnical Engineering

Pozzolans
Volcanic ash is important as a pozzolan, named after the town of Pozzuoli located
on the Bay of Naples, Italy. Volcanic ash can occur wherever there has been
volcanic activity; for example, the eruption that created what is now Yellowstone
Park threw out ash that was carried eastward by prevailing winds for many
hundreds of kilometers. As the ash cooled rapidly it is mainly composed of
glass that the Romans discovered reacts with hydrated lime, Ca(OH) 2 , to make
a hydraulic cement—that is, a cement that sets under water. The famous
Roman aqueducts were cemented with pozzolanic mortar, and the still-standing
roof of the Parthenon is composed of pozzolanic concrete. Any fine siliceous
material such as industrial fly ash that reacts with lime now is referred to as
a pozzolan.

Pozzuoli, Naples, and the ancient Roman city of Pompeii are close to the
still active Mt. Vesuvius, and in recent years the town of Pozzuoli has risen
approximately 4 m (13 ft) due to underground magmatic activity, causing the town
to be relocated.

2.5.9 Bentonite and Montmorillonite
Volcanic ash is highly susceptible to chemical weathering because of the
disordered atomic arrangement of the constituent glass. Chemical weathering
can convert entire layers of volcanic ash into a white, sticky, highly expansive clay
called bentonite. Because of its large water-absorbing properties bentonite is
utilized in drilling mud and as a sealant for lagoons and basements. The dominant
clay mineral in most bentonite is montmorillonite, named after the town of
Montmorillon in France.

Montmorillonite is one of a group of minerals called smectites that form in many
soils and can be very troublesome in engineering. Smectites lose part of their
crystalline water on drying and reabsorb it upon wetting, causing cycles of
shrinking and swelling that can lift and destroy vulnerable structures. More
geotechnical engineering problems are caused by smectite-rich expansive clays than
by any other type of soil. On the other hand, small percentages of smectite in a soil
can benefit plant growth by acting as temporary storage for water and plant
nutrients.

2.5.10 Plateau Basalts

On an area basis, the most abundant lava flows are not volcanic cones, but
occur where lava emerges from a fissure and can cover thousands of square
kilometers. The Columbia Plateau of Washington, Oregon, and Idaho consists
of repeated lava flows averaging about 30 m (100 ft) thick, and covering an area of
2
2
about 520,000 km (200,000 m ). Another prominent example is the Deccan
Plateau of west central India.
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