Page 51 - Geotechnical Engineering Soil and Foundation Principles and Practice

Special Problems with Sedimentary Rocks
46 Geotechnical Engineering

it is continuous downward, and not a disconnected piece of rock or ‘‘float’’ resting
in clay.

Clay Pockets in Limestone
Surface water penetrating limestone follows along vertical fractures, creating
solution cavities. As shown in Fig. 3.3, these often are filled from above with
residual soil. The vertical orientation of clay pockets and their random nature
is such that they may be missed by exploration drilling borings and come as an
unpleasant surprise when excavations are opened for construction of foundations.
The surprise often can be averted by inspecting nearby outcrops and roadcuts,
as in Fig. 3.3. Failing that, geophysical measurements such as earth resistivity or
the use of ground-penetrating radar can be quite helpful. Such tests may be
performed by geotechnical engineers or by geologists.

Case History
A tunneling machine essentially is a horizontal drill that is pushed forward
by hydraulic pistons that push against opposing plates expanded against the
sides of the completed tunnel boring. Therefore in order for the machine to
advance it must have competent rock to push against. Exploration borings
were conducted every 50 ft (15 m) along a tunnel route and showed no clay
pockets, but the machine found one that was between the borings. The
machine was stopped and has no capability for backing up, so a tunnel was
started from the other end so that the machine could be pulled forward,
and the tunnel completion date was moved back one year. A geophysical
survey probably would have prevented this.

Figure 3.3
A thin layer of residual soil covers this limestone, but weathering has proceeded along vertical
fractures to make solution cavities that are infiltrated with soil, creating pockets of soft clay that will
not support a foundation.

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.