Page 14 - Geotechnical Engineering Soil and Foundation Principles and Practice

Introduction
Introduction 9

Also in 1856, two other concepts were introduced that play an important role in
soil engineering. These are Darcy’s Law defining gravitational flow of water
through porous media such as soils, and Stokes’ Law describing the equilibrium
velocity of solid particles settling in liquids. Stokes’ Law is used for measurement
of fine soil particle sizes from their settlement rates.

Another important contribution in the field of soil pressures is that of
Sir Benjamin Baker in 1881. Baker observed that the slip surface for a bank
failure is not planar as indicated by the Rankine and Coulomb analyses, but
incorporates vertically oriented cracks at the upper end. Decades earlier, in 1846,
the internal structure of landslides was investigated in the field by Alexandre
Collin, whose detailed cross-sections showed a curved instead of a flat slip
surface. Collin’s work unfortunately escaped the attention or was ignored by later
workers, who perhaps were more taken with the classical theory, and was
rediscovered only in recent years.

In 1871, Otto Mohr devised a simple graphical means for presentation and
analysis of stress data, and ‘‘Mohr’s circle’’ has become an invaluable tool for
the modern geotechnical engineer. Mohr later confirmed and generalized the
Coulomb failure criterion by relating to experimental findings, and Coulomb’s
earlier description of components of shearing strength sometimes is referred to as
the ‘‘Mohr-Coulomb failure envelope.’’

Another nineteenth-century contribution that was destined to become extremely
useful in modern soil engineering was the solution by a mathematician,
Boussinesq (pronounced Boo-sin-esk). By use of elastic analysis, in 1885 he
showed that stresses from a point load on the surface of soil should dissipate in
three-dimensional space much like ripples from a stone thrown in water. Although
soil is far from being an ideal elastic material, pressure measurements indicate that
the Boussinesq solution is appropriate for determining pressures from founda-
tions and for computing lateral pressures on retaining walls from loads applied at
the surface of the soil backfill.

1.11 SOIL MOISTURE STUDIES

Early in the twentieth century soil scientists employed by the U.S. Depart-
ment of Agriculture were active in the study of the mechanics of soil moisture.
Among these was Briggs, who suggested a classification of soil moisture.
Concurrently Buckingham proposed the concept of capillary potential and
conductivity, which has led to a better understanding of the forces responsible for
the retention and movement of capillary water in soils. Haines and Fisher
developed an important concept of soil cohesion resulting from capillary forces
or suction of water in soils that tends to pull soil grains together and increase
friction.

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