Page 13 - Geotechnical Engineering Soil and Foundation Principles and Practice

Introduction
8 Geotechnical Engineering

In 1691 Bullet, of the French Royal Academy of Architecture, presented the first
theory of lateral earth pressures based on the principles of mechanics. It was he
who introduced the concept of a ‘‘sliding wedge’’ of soil against a retaining wall.
He assumed that the slip angle was 458, which later was shown to be an
oversimplification.

Coulomb in 1773 applied the principle of maxima and minima to the sliding-
wedge theory to determine the most critical wedge angle, and showed that it
depends in part on internal frictional properties of the soil. His formulas, while
now recognized as simplifications, still are in use today and are the basis for many
computer programs. It therefore is important to know the assumptions and
limitations of a computer program prior to committing it to use.

Another important contribution by Coulomb was to recognize the concept that
shearing resistance of soil involves two separate components, cohesion and
friction. An empirical formula embodying these components now is almost
universally accepted and used in geotechnical engineering practice, and is referred
to as the Coulomb equation.

Coulomb’s interest and insight into soil problems were inspired by his profession
as a military engineer. After retiring as a result of ill health he invented the torsion
balance while seeking a prize for inventing a frictionless navigational compass.
He did not win the prize, but found a better use for his instrument by measuring
the faint attractive and repulsive forces caused by electrical charges. He at first
assumed that the attractions were inversely proportional to the separation
distance, but his experiments then established that they relate to the square of the
distance. Couomb’s inverse square law governs not only electrostatic attractions,
but also gravitational forces and, ironically, navigation of orbiting satellites.

1.10 NINETEENTH-CENTURY DEVELOPMENTS

In 1856 Rankine, in his treatise ‘‘On the Stability of Loose Earth,’’ employed the
concept of soil internal friction to retaining wall problems. His analysis gave a
quasi-hydrostatic distribution of pressure that agreed with Coulomb’s analysis
based on a sliding wedge, but by application of the theory of conjugate stresses
Rankine concluded that the resultant pressure on a wall acts parallel to the surface
of the backfill instead of horizontally. The contributions of Rankine and
Coulomb are regarded as classic and have served engineers well over the years, but
are now known to be simplifications that are not precisely realized in engineering
practice. For example, both theories predict that the resultant of pressure on a
retaining wall acts at one-third of the height of the wall, whereas measurements
indicate that it is higher because of partial support of the soil by wall friction and
arching action. The consequent increase in overturning moment is covered by the
factor of safety.

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