Page 8 - Geotechnical Engineering Soil and Foundation Principles and Practice

Introduction
Introduction 3

Figure 1.1
Soils are
particulate and
respond to
external load by
developing
friction at the grain
contacts. As a
result the K ratio of
horizontal to
vertical stress is
quite variable, and
depends on the
stress history and
loading conditions.

6. When soil particles are disturbed they can exhibit a kind of mob behavior,
jamming into each other like elbows on a New York subway. This can either
markedly increase or decrease the soil strength. A decrease in strength
explains why landslides continue to move until they find a more level
geometry. Soils can be tricky.
7. Horizontal pressure from soil is an important consideration for design of
retaining walls, but also is highly variable. Whereas the K ratio of horizontal
to vertical stress in a liquid is 1.0, and for a rigid solid is 0, in soils the ratio
depends on the resistance of particles to sliding. This is illustrated in Fig. 1.1.
K for soils typically varies from 0.2 to 0.5, but can be much higher if high
horizontal stress is inherited from overburden that has been removed by
erosion.
8. Horizontal pressure depends on whether a soil is pushing or is being pushed.
For example, soil piled against a retaining wall usually will exert much less
pressure than if it is being pushed by a bulldozer. The reason for this is
illustrated by changing directions of the friction arrows in the lower part
of Fig. 1.1—whichever stress is higher, vertical or horizontal, determines
the directions of the friction arrows and the K ratio. A bulldozer that is
designed on the basis of the wrong K will have a built-in anchor. Soils can
be very tricky.
9. An unsaturated soil derives part of its strength from the pull or suction
of capillary water—the same pull that draws water up into a fine straw
as shown at the right in Fig. 1.2. Then when the soil becomes saturated it
loses this part of its strength. Some wind-blown loess soils that have never been
saturated therefore collapse under their own weight if they become saturated.
10. Compressing a saturated soil causes stress to be carried in part by
grain-to-grain contact and in part by pressure from the pore water, as

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.