Page 49 - Geotechnical Engineering Soil and Foundation Principles and Practice

Special Problems with Sedimentary Rocks
44 Geotechnical Engineering

Figure 3.2
Mesa County, Colorado. Hard sandstone caps the flat hills, and a more resistant layer also is seen
in the high bank along the river. These features are obvious in this airphoto because there is no
vegetative cover, but the same relationship also exists in forested or cultivated areas. Width, 2.9 km
(1.8 miles). (USDA photo.)

Mesas are obvious where vegetation has been held in check by a dry climate,
but also occur in humid climates, where the edges are muted. The more readily
erodible rock exposed on the sides of a mesa typically is shale or a softer sand-
stone. Therefore a search for hard aggregate will lead to the top layer of a mesa
and not the sides.

Cementation and Weathering of Sandstone
Sandstones weather back to loose sand as cementing materials are weakened or
leach out. The cements that bond sand into sandstone are, in increasing order
of strength, clays, iron oxides (in which case the sandstone is red), calcium
carbonate, and quartz. Quartz-cemented sand in particular can be quite hard
as quartz grains grow by crystallization so the enlarged crystals interlock.

3.3.3 Limestone
Limestone is the light-colored rock that occurs in ledges that outcrop in roadcuts
or along stream valleys. Limestone is quarried and is a valuable aggregate used in
engineering. However, most limestone is not satisfactory for such purposes, and

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