Page 17 - Geotechnical Engineering Soil and Foundation Principles and Practice

Introduction
12 Geotechnical Engineering

a soil classification for use by the U.S. Army in World War II, later named
the ‘‘Unified Classification,’’ and now used by most foundation engineers.
Casagrande also made improvements in laboratory tests, including a mechanical
device to measure the liquid limit that is based on a cog-wheel invention by
Leonardo da Vinci. This device is now standard equipment in all soil mechanics
laboratories.

1.16 LANDSLIDES

Field investigations, soil sampling, and testing gained new impetus and respect
after a series of landslides was investigated by a committee appointed by the
Swedish Royal Board of State Railways and chaired by Wolmar Fellenius.
The results, published in 1922, included a simple method of analysis that remains
the basis for a variety of modern computerized methods for evaluation of slope
stability. In 1948, a textbook by MIT professor Donald W. Taylor contained
considerable original material for the analysis of slope stability, with charts
that were developed with the aid of his graduate students.

Starting in the l950s, contributions towards a better understanding of clays,
soil compressibility, and landslides came from Norway and the Norwegian
Geotechnical Institute, led by Laurits Bjerrum and N. Janbu. In England,
A. W. Bishop and A. W. Skempton respectively presented a new slope stability
model and a theory explaining how slope failures in clays may be delayed
for many decades. Purdue University pioneered the use of computers to solve
slope stability problems, publishing a computer program that is widely used and
is the basis for later copyrighted programs.

Engineering geologists use a different but more general approach to landslides
by using their occurrences as a basis for landslide susceptibility maps that are
particularly useful in planning. The geotechnical engineer should be aware of the
availability of these maps in order to perform an intelligent investigation of
a specific site.

1.17 FOUNDATIONS

As previously mentioned, while many bearing capacity theories have been
proposed and used, the theory that forms the basis for most modern investiga-
tions is that of Terzaghi. In Canada, G. G. Meyerhoff extended and modified
Terzaghi’s bearing capacity theory, and at Duke University, A. Vesic suggested
modifications based on model studies with sands. In the 1960s, T. W. Lambe of
MIT introduced a new approach to the prediction of settlement called the ‘‘stress
path method.’’ One difficulty was the inability to accurately measure lateral
soil stresses in the field, and the 1970s saw the introduction of new methods

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