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What makes the history of soil mechanics fascinating is that it is not only a chronicle of mere discovery it is a story of trial and error, of collapsed towers, of successful foundations, and the growing realization that the ground beneath our feet is far more complicated than anyone ever imagined.
Imagine
you are standing in front of a big entity, such as a tower, a dam, or an
historic monument, and someone turns to you and asks the following simple
question:
“What’s
holding all of this up?”
Most people look at the walls, the stone, the concrete, or the steel. But the real answer sits quietly beneath our feet. Every structure, from ancient pyramids to modern skyscrapers, starts with one common element: soil. For thousands of years, contractors worked with it without fully understanding it. Some structures stood for centuries, while others cracked or settled, or leaned dramatically off balance. Today, students learn about soil mechanics in the classroom, but for early engineers, the soil itself was the teacher.
Soil is the
uncemented aggregate of mineral grains and decayed organic matter with liquid
and gas in the empty spaces between the solid particles. Soil is used in various
construction projects and as a structural foundation material. Therefore, civil
engineers must study the behavior of soil, its origin, grain size distribution,
shear strength, compressibility and load bearing capacity. Soil engineering is
the application of the principles of soil mechanics to practical problems.
Soil Engineering Prior to
the 18th Century
No one
really knows when people first started using soil as a building material. It
goes back so far that the earliest record is long lost. What we now think of as
soil engineering didn’t start to take shape until the early 18th century, as
noted by Skempton in 1985. Before that, the field was driven by practical
experience. Builders learned by trying things, observing what worked, and
adjusting through experiments rather than relying on any scientific framework.
Using that approach, they built many structures. Some failed over time, while
others are still standing.
Recorded
history tells us that ancient civilizations flourished along the banks of
rivers, such as the Nile (Egypt), the Tigris and Euphrates (Mesopotamia), the
Huang Ho (Yellow River, China), and the Indus (India). Dykes dating back to
about 2000 B.C. were built in the basin of the Indus to protect the town of
Mohenjo-daro (in what became Pakistan after 1947). During the Chan dynasty in
China (1120 B.C. to 249 B.C.) many dykes were built for irrigation purposes.
There is no evidence that measures were taken to stabilize the foundations or
check erosion caused by floods (Kerisel, 1985). Ancient Greek civilization used
isolated pad footings and strip-and-raft foundations for building structures.
Beginning around 2750 B.C., the five most important pyramids were built in
Egypt in a period of less than a century (Saqqarah, Meidum, Dahshur South and
North, and Cheops). This posed formidable challenges regarding foundations,
stability of slopes, and construction of underground chambers.
In
some cases, the foundation pressure exceeded the bearing capacity of the soil
and thereby caused structural damage. One of the famous examples of problems
related to soil-bearing capacity in the construction of structures prior to the
18th century is the Leaning Tower of Pisa in Italy. Construction of
the tower began in 1173 A.D. when the Republic of Pisa was flourishing and
continued in various stages for over 200 years. The structure weighs about
15,700 metric tons and is supported by a circular base having a diameter of 20
m (66 ft). The tower has tilted in the past to the east, north, west and, finally
to the south.
Recent
investigations showed that a weak clay layer exists at a depth of about 11 m (36
ft) below the ground surface. Compression of this layer caused the tower to
tilt. It became more than 5 m (16.5 ft) out of plumb relative to its 54 m (179
ft) height. The tower was closed in 1990 because it was feared that it would
either fall over or collapse. It has recently been stabilized by excavating
soil from under the north side of the tower. About 70 metric tons of earth were
removed in 41 separate extractions that spanned the width of the tower. The
tower now leans 5 degrees.
Leaning Tower of Pisa, Italy (Courtesy of Braja M. Das, Henderson, Nevada)
The second example involves two towers in bologna, Italy, that
were built in the 12th century. The Left tower is usually referred
to as the Garisenda Tower. It is 48 m (157 ft) in height and weighs about 4210
metric tons. It has now tilted about 4 degrees. The tower on the right is the
Asinelli Tower, which is 97 m high and weighs 7300 metric tons. It has tilted
about 1.3 degrees.
Tilting of Garisenda
Tower (left) and Asinelli Tower (right) in Bologna, Italy
After encountering many foundation-related
problems during construction over the centuries, engineers and scientists began
to study the properties and behaviors of soils starting in the early part of
the 18th century. Based on the emphasis and the nature of study in soil
engineering, the time span extending from 1700 to 1927 can be divided into four
major periods (Skempton, 1985):
1. Pre-classical (1700 to 1776 A.D.)
2. Classical soil mechanics—Phase I (1776 to 1856 A.D.)
3. Classical soil mechanics—Phase II (1856 to 1910 A.D.)
4. Modern soil mechanics (1910 to 1927 A.D.)
1.
Pre-classical (1700 to 1776 A.D.)
This period marks the early
stages of geotechnical engineering as it began moving from practical craft to a
field with scientific direction. Engineers and scholars started to question why
soils behaved the way they did, instead of relying solely on trial and error.
During these years, key observations were made about earth pressure, slope
stability, and the behavior of retaining structures. Although the understanding
was still limited, the ideas that emerged in this period laid the groundwork
for the more formal theories that followed.
In 1717 a French royal engineer,
Henri Gautier (1660–1737), studied the natural slopes of soils when tipped in a
heap for formulating the design procedures of retaining walls. The natural
slope is what we now refer to as the angle of repose. According to this study,
the natural slopes of clean dry sand and ordinary earth were 31° and 45°,
respectively. Also, the unit weight of clean dry sand and ordinary earth were
recommended to be 18.1 kN/m³ (115 lb/ft³) and 13.4 kN/m³ (85 lb/ft³),
respectively. No test results on clay were reported.
In 1729, Bernard Forest de
Belidor (1671–1761) published a textbook for military and civil engineers in
France. In the book, he proposed a theory for lateral earth pressure on
retaining walls that was a follow-up to Gautier’s (1717) original study.
2. Classical soil mechanics—Phase I (1776 to 1856 A.D.)
In the pre-classical period,
practically all theoretical considerations used in calculating lateral earth
pressure on retaining walls were based on an arbitrarily assumed failure
surface in soil. In his famous paper presented in 1776, French scientist
Charles Augustin Coulomb (1736–1806) used the principles of calculus for maxima
and minima to determine the true position of the sliding surface in soil
behind a retaining wall. In this analysis, Coulomb used the laws of friction
and cohesion for solid bodies.
In 1820, special cases of
Coulomb’s work were studied by French engineer Jacques Frederic Francais
(1775–1833) and by French applied mechanics professor Claude Louis Marie Henri
Navier (1785–1836).
3. Classical soil mechanics—Phase II (1856 to 1910 A.D.)
One of the earliest and most
important publications is one by French engineer Henri Philibert Gaspard Darcy
(1803–1858). In 1856, he published a study on the permeability of sand filters.
Based on those tests, Darcy defined the term coefficient of permeability (or
hydraulic conductivity) of soil, a very useful parameter in geotechnical
engineering to this day.
Sir George Howard Darwin
(1845–1912), a professor of astronomy, conducted laboratory tests to determine
the overturning moment on a hinged wall retaining sand in loose and dense
states of compaction.
4. Modern soil mechanics (1910 to 1927 A.D.)
Around 1908, Albert Mauritz
Atterberg (1846–1916), a Swedish chemist and soil scientist, defined clay-size
fractions as the percentage by weight of particles smaller than 2 microns in
size. He realized the important role of clay particles in a soil and their
plasticity. In 1911, he explained the consistency of cohesive soils by defining
liquid, plastic, and shrinkage limits.
In October 1909, the 17-m (56-ft)
high earth dam at Charmes, France, failed. It was built between 1902 and 1906.
A French engineer, Jean Fontard (1884–1962), carried out investigations to
determine the cause of failure. In that context, he conducted undrained double-shear tests on clay specimens (0.77 m² in area and 200 mm thick) under constant
vertical stress to determine their shear strength parameters. The times for
failure of these specimens were between 10 and 20 minutes.
Comments
If you’d like to follow‑up on the
developments in soil mechanics after 1927 A.D., including the life of Karl
Terzaghi the foundation of modern soil mechanics, and his contributions to
concepts like effective stresses, shear strength, Consolidation,
Elastic theory and stress distribution. Feel free to like this post and let
me know in the comments section. If there's enough interest, we’ll explore his
work and influence in the next part of this series. Your feedback will help
shape the next part of this series. Thankyou.
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