Page 45 - Geotechnical Engineering Soil and Foundation Principles and Practice

Special Problems with Sedimentary Rocks
40 Geotechnical Engineering

a finely layered bedding structure and readily shears along these planes. Third,
shale used as a fill soil gradually degrades and reverts to clay, causing delayed
difficulties and failures. While engineers may be content to call shale a soil, there
are abundant reasons why it also should carry a designation as shale.

The cause of the finely layered ‘‘shaley structure’’ of shale is compression under
large amounts of overburden coupled with geological time. This process of
densification is called consolidation, similar to the consolidation that occurs in clay
under a foundation load but to a much larger degree. Thus, whereas the void
content of soils is measured in tens of percent, the void content of a shale may be
only a few percent. As shale consolidates, the clay mineral platelets tend to
become oriented flat and parallel to one another and create the shale structure.
Claystones and siltstones are rocks having a similar grain-size composition that
are not so thinly bedded.

‘‘Consolidation’’ originally was proposed by an English geologist, Lyell, to
explain the compression of sediments in the sea. Nevertheless, petroleum
geologists refer to this process as ‘‘compaction,’’ even though in engineering
‘‘compaction’’ refers to mechanically induced densification, and differs from
consolidation because it squeezes out air instead of water.

Shales and Landslides
The planes of weakness in shales are highly conducive to landslides, particularly
if the rock layers have been tilted from deformations of the Earth’s crust.
This occurs as a result of plate collisions, faulting, folding, and mountain building.
Sometimes the Earth’s crust has been compressed so that rock layers are pushed
at the edges to form a series of symmetrical folds like ripples on water. Some
very large and tragic landslides are attributed to shale.

Shale Rebound from Unloading
Another property of shale that contributes to a catalogue of lamentable surprises
is a tendency to rebound and expand when pressure is removed by excavations.
Expansion is triggered if water is available to weaken the electrostatic bonding
in the clay. Excavations in shale therefore should not be allowed to remain open
any longer than absolutely necessary, and should be kept dry. This can be
accomplished by use of a thin layer of asphalt or Portland cement concrete.

Geologically old shale does not contain expansive clay minerals, and while
pressures from elastic rebound are relatively moderate, they can lift lightly loaded
floors while leaving nearby bearing walls intact. Damages are reduced if floors and
walls are kept separated. However, interior partition walls founded on a heaving
floor slab can be troublesome. Shale deposits that are Paleozoic or older generally
have been chemically modified or ‘‘deweathered’’ to more stable clay minerals.

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