Page 43 - Geotechnical Engineering Soil and Foundation Principles and Practice

Source: GEOTECHNICAL ENGINEERING
3 Special Problems with Sedimentary Rocks

3.1 SEDIMENTARY ROCKS

Rocks that have been weathered, eroded, and transported are first deposited as
loose sediments that given time and favorable conditions eventually will solidify
into sedimentary rocks. Sediments and sedimentary rocks form a relatively thin
blanket covering about three-fourths of the area of the continents—a blanket has
some holes where igneous mountains and shield areas stick through.

Regardless of their present positions on continents, most sedimentary rocks are
marine in origin, having been deposited in former shallow seas. Only limited areas
of continents were submerged at any one time, leaving adjacent higher ground as
the sources for sediments.

The advances and retreats of shallow seas were relatively slow, and enabled
a sandy beach deposit to be transformed into a continuous layer of sand. As a sea
retreated, the layer of sand would cover a layer of clay that was being deposited in
deeper water offshore. In this way a series of alternating layers of sand and
clay would be deposited.

Other layers are composed of chemically precipitated calcium carbonate that is
deposited farther offshore and is destined to become limestone. Other types of
limestone originate in shallow water as accumulations of shells. Sedimentary rock
layers therefore tend to be sequential, involving varying amounts of sandstone,
limestone, and shale.
Sedimentary rocks also vary widely in resistance to weathering and erosion,
so harder rocks tend to cap flat hills as softer rocks are exposed on the sides.
This is the classic form called a mesa, or tableland. If layers of sedimentary rocks
are tilted on edge, the harder rock layers will form high ridges and softer rocks

38 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2007 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.