Manuel Melis Maynar
Catedrático
de Geotecnia
Catedrático
de Ferrocarriles
ESTAS PÁGINAS ESTÁN
DEDICADAS EXCLUSIVAMENTE A LOS ALUMNOS DE 6º CURSO DE FERROCARRILES DE LA ETS DE
CAMINOS DE MADRID.
ESTÁN PROTEGIDAS POR LA
CONSTITUCIÓN ESPAÑOLA, ARTÍCULO 20.c "Libertad de Cátedra".
LA UNIVERSIDAD ES
APRENDER A PENSAR, ESPÍRITU DE TRABAJO Y ESPÍRITU CRÍTICO AL
MÁXIMO NIVEL.
El túnel, ¿es una
ciencia? ¿es una parte de la Geotecnia? ¿es un arte?
Para los alumnos y los ingenieros
jóvenes que no lo hayan leído, incluyo aquí el famoso artículo del
Prof. Ralph B. Peck "ART AND SCIENCE IN SUBSURFACE ENGINEERING",
publicado hace más de 40 años, en GEOTECHNIQUE, en 1962. El artículo
sigue teniendo mucha vigencia, en mi opinión. Y es posible que la
Geotecnia, mal aplicada por expertos de gabinete, haya sido la causa
de la ruina de muchos túneles.
ART AND SCIENCE IN
SUBSURFACE ENGINEERING
by RALPH B.
PECK (Geotechnique, 1962)
Subsurface
engineering is an art; soil mechanics is an engineering science.
This distinction, often expressed but seldom fully appreciated, must
be understood if we are to achieve progress and proficiency in both
fields of endeavour.
Almost every week
the journals of foundation and earthwork construction describe at
least one failure. Yet, year after year, more and more workers have
been devoting their attention to soil mechanics and have created a
literature of impressive scope and extent. Can there be, as some
people have intimated, a correlation, a causal relationship, between
the growth of soil mechanics and the number of foundation and
subsurface mishaps?
To be sure, the
high incidence of failures or of unexpectedly costly jobs is a
consequence partly of the accelerated pace of construction, and
partly of the fact that soil mechanics has opened the door to more
complex and more daring jobs than would have been considered
feasible a few years ago. Nevertheless, a disturbingly large residue
of costly and unfortunate incidents remains to be explained, even in
circles where soil mechanics is by no means unknown. This situation
may have arisen out of our failure to discriminate between art and
science. In an age of scientific marvels, civil engineers have lost
sight of the accomplishments of the artist in his profession. We
would do well to recall and examine the attributes necessary for the
successful practice of subsurface engineering. These are at least
three: knowledge of precedents, familiarity with soil mechanics, and
a working knowledge of geology.
Of these, a
knowledge of precedents is by far the most important. Two centuries
ago, engineers were constructing widespread systems of canals
involving deep cuts in difficult ground; a century ago railway
engineers drove large and long tunnels in water-bearing sands below
ground-water level. These and many other works were completed, with
difficulty to be sure but nevertheless successfully. without benefit
of modern soil mechanics. The engineers of those days had little to
guide them but experience -not only their own experience, of course,
but also that of their contemporaries and, to the extent it had been
recorded, of their predecessors. If these men could accomplish so
much only on the basis of experience and their own native
intelligence, surely experience is a priceless asset of the
subsurface engineer even today.
The heritage from
the past every engineer may have by the simple act of reading. But
this heritage, although vital to his background, is necessarily
second-hand. To it, every engineer must add his own, first-hand
experience before he can attain professional competence. Yet the
mere passage of the years of professional life does not guarantee
the kind of experience needed to develop the artistry of the
proficient subsurface engineer. The experience must contribute to
professional growth; it must be carefully sought and selected. The
young man must see that he is assigned under competent supervision
to work of significance, of variety, and of increasing difficulty
and responsibility. Otherwise, after 20 years
of engineering activity he may not be able to claim 20 years oí
experience, but only that he has acquired one year's experience
twenty times.
The development of
a background of personal experience is not, then, a passive
activity. It requires conscious and persistent effort. It requires
the ability to observe what is happening, to retain the
observations, and to discriminate among items that are significant
and irrelevant.
The ability to
observe is by no means inborn. Many a young man, sent to make his
first field inspection, returns to the office to discover that he
cannot remember the details of much that he saw. His embarrassment
may Iead him to believe that he lacks a sixth sense that some
engineers must possess, so he may avoid future assignments of this
sort. If he follows this course, his future as an artist in the
practice of subsurface engineering is doomed; he has cut himself off
from his own experience. On the other hand, he may seek to develop
the art of observation and retention. In his hotel room in the
evening he sketches from memory the details of the bracing of an
open cut he just inspected. He discovers that he cannot remember
certain details; next day he corrects his sketches. He cultivates
the habit of keeping a professional diary with brief but accurate
descriptions of design and construction procedures. As he cultivates
these habits he discovers that his powers of observation and
retention become more acute; he begins to know what to look for and
what to ignore; he is on the way to becoming an experienced
individual.
Once the ability
to make the most of experience has been successfully cultivated,
even the young engineer can become truly experienced in surprisingly
few years. Personal experience is not a matter of elapsed time but
rather of the intensity with which the experience is pursued and
absorbed.
Finally, the young
subsurface engineer should seek experience with variety: in the
field, where he can observe at first-hand the methods and
consequences of construction operations in many types of soils and
rocks; in the design office, where he can learn the peculiar
problems of the designer and can come to appreciate the
interrelation between design and construction; and even in research,
where he can learn how the frontiers of knowledge are being pushed
back and can form a judgment about the difference in mental attitude
required for successful research as compared to successful
engineering practice.
No man can hope to
be truly an artist in the practice of subsurface engineering without
a rich background of personal experience, or without an adequate
knowledge of the experiences of his contemporaries and predecessors.
Therefore, experience is given first place among the three essential
attributes. Nevertheless, soil mechanics and geology are hardly less
important. What, precisely, is the function of each of these
disciplines in the art of subsurface engineering?
Soil mechanics, in
the first place, provides qualitative and quantitative data
concerning the stress-strain-time characteristics of earth
materials. This knowledge gives us a feeling for the behaviour of
soils, under idealized conditions, which may guide us in
anticipating behaviour under the more complex conditions in the
field. Similarly, the theories of soil mechanics provide insight
into behaviour under simple, ideal conditions. Yet, of what
practical value is this information?
First and
foremost, these aspects of soil mechanics form a framework that
helps engineers to organize, interpret and evaluate experience. The
miscellaneous collection of facts accumulated during a professional
lifetime would be of no value if it could not be organized and
brought to focus on new problems. Before soil mechanics there was no
rational framework to serve this purpose. As a result, even the most
renowned engineers occasionally made serious errors because they
tried to apply what appeared to be a precedent to a situation where
some vital factor was essentially different from those controlling
the behaviour of the precedent. All too often, generalizations were
drawn from experiences in a single soil deposit; there were no index
properties to warn that the deposit might, in fact, be unusual or
unique with respect to some controlling characteristic. With no
knowledge of the effect of seepage forces, failures were often
attributed to the type of soil rather than to relevant hydraulic
conditions. Soil mechanics brought order out of this chaos. Each
experience that comes to the attention of an engineer can now be
examined and categorized as to type of material, and as to its
relationship to the stress-strain-time characteristics of the
appropriate types of soil. It can also be examined in relation to
theoretical concepts. The experience may fit nicely into the
framework of soil mechanics, or it may appear to be an exception. In
either event, the body of knowledge known as soil mechanics provides
a convenient and logical basis for preserving the essential features
of the experience. In short, soil mechanics has made it practicable
to utilize the vast amount of precedent and experience already
accumulated and still to be obtained. Its importance in this
connexion cannot be overemphasized.
The everyday
procedures now used to calculate bearing capacity, settlement, or
factor of safety of a slope, are nothing more than the use of the
framework of soil mechanics to organize experience. If the
techniques of soil testing and the theories had not led to results
in accord with experience and field observations, they would not
have been adopted for practical, widespread use. Indeed, the
procedures are valid and justified only to the extent that they have
been verified by experience. In this sense, the ordinary procedures
of soil mechanics are merely devices for interpolating among the
specific experiences of many engineers in order to solve our own
problems which we recognize to fall within the limits of previous
experiences.
In addition,
however, the subject of soil mechanics provides the means by which
we can often go beyond the limits of our own experience or that of
others. It points the way to new solutions of old problems, or to
the solution of previously unsolved problems. It is, in this
respect, a means for extrapolating our experience. Of course, such
extrapolation involves a measure of uncertainty until the pertinent
experience becomes available. But even here, soil mechanics guides
us as to what we should observe to check our procedures as we
execute the work.
These are the
vital functions of soil mechanics. They fully justify all the
attention that has been paid to the subject. But it is clear that
soil mechanics is no substitute for experience. Its great role is in
making experience more meaningful.
Geology, the third
ingredient, is as basic to subsurface engineering as is soil
mechanics. Possibly its most significant role is to make us aware of
the departures from reality inherent in our simplifying assumptions.
Whereas the theories and computational procedures of soil mechanics
would be impracticable without simplifying assumptions regarding the
properties of the subsurface materials, nature is not simple. The
geology of a site must be understood before any reasonable
assessment can be made of the errors involved in our calculations or
predictions. Indeed, in some instances the geologic structure or the
results of geologic processes may completely override all
considerations of soil mechanics. The nature and orientation of the
relict joints in a residual soil may govern the stability of the
sides of an excavation for a foundation, quite irrespective of the
properties of the soil between the joints and quite at variance with
predictions of theory based on assumptions of homogeneity.
Geology also, like
soil mechanics, provides a means for correlating our experience, but
on a regional or physiographic basis. Regional studies of foundation
conditions have proved very useful to the practising engineer. They
pertain to areas in which experiences should be similar; hence, the
conclusions are valid only if the physiographic units have been
established upon a sound basis of geologic similarity.
Finally, whether
we realize it or not, every interpretation of the results of a test
boring and every interpolation between two borings is an exercise in
geology. If carried out without regard to geologic principles the
results may be erroneous or even ridiculous. Conversely, if done
with a keen perception of local geologic conditions, the results are
likely to be much more reliable. It is hardly necessary to labour
the point that intelligent subsurface exploration is impossible
without a working knowledge of geology.
The highest level
of artistry in the practice of subsurface engineering is found in
the man who, in addition to a sound training in civil engineering,
has cultivated a background of pertinent experience, correlated and
extended by means ot the two sciences – soil mechanics and geology.
The background of the expert in the practice of subsurface
engineering and that of the expert in soil mechanics, then, are by
no means the same. The distinction is important. Possibly it can
best be illustrated by an analogy to the practice of medicine and to
medical science (*) The analogy is especially instructive because
the practice of the art of medicine is almost everywhere considered
to be an endeavour of professional stature.
(*)
This analogy was first suggested by Mr M. M. FitzHugh, who had
observed Dr Terzaghi's approach to problems of design and
construction of an unprecedented shipway for which Mr FitzHugh was
responsible (see Trans. Amer. Soc. Civ.
Engrs, 112:298-324.)
When we are ill,
we visit the doctor with the expectation that he will diagnose our
illness correctly and, if possible, provide a treatment that will
cure us. He begins his investigation with questions about our
history, our family, and our environment, and about our recent
symptoms. Then he passes to a qualitative physical examination. He
thumps our chest and listens to the sound, he uses his stethoscope
to hear further sounds, he taps our knees with a rubber mallet to
ascertain our reactions, he holds down our tongue and looks down our
throat. None of these activities can be considered scientific. They
do not provide numerical values of any physical quantities that can
be used directly in diagnosis, but they provide qualitative data
that have been correlated by experience with the behaviour of
thousands of individuals.
Next, the
physician performs or requests the performance of certain physical
tests that provide quantitative data. Some of these tests are
routine, such as the determination of our weight, height and blood
pressure. Some, such as blood counts and sedimentation rates, must
be done by trained technicians. A few may require higher degrees of
technical training, such as the performance of basal metabolism
tests. These tests are based upon scientific studies but they are
not in themselves scientific. They provide numerical values which
have much the same function as index properties in soil mechanics.
Their usefulness lies in the correlation of the numerical values
with the behaviour of many human beings.
After this routine
the doctor may be ready to make his diagnosis. This is the first
important result for which we have retained his services. We should
like the diagnosis to be correct. We may have chosen a particular
doctor because of his reputation as an expert diagnostician. But
what makes the difference between the expert diagnostician, the
artist in his profession, and the less talented man?
The true artist in
his profession considers al the information he has obtained, digests
and studies it in the light of his training and experience, and
arrives at a diagnosis which he regards as a tentative hypothesis
regarding our ailment. It is obvious that the more varied personal
experience the doctor has had, the better are the chances that he
can recognize our ailment. Furthermore, if he conscientiously
studies current medical literature, he is better prepared for the
diagnosis. If he is well versed in medical science and in the
academic studies he pursued as a younger man, this, too, will add to
his skill as a diagnostician. Yet, we recognize that the simple
addition of all these attributes does not produce the true artist.
The same experiences are more meaningful to some individuals than to
others. The artist may notice and be influenced by intangibles of
which the ordinary practitioner may not even be conscious: the
colour of our skin, the way an eyelid droops, the way we walk as we
enter his office. The sum total of all these impressions contributes
to the diagnosis.
But the diagnosis
is at best only a hypothesis that must be tested. The artist in the
medical profession accomplishes this by prescribing a treatment. If
his diagnosis is correct, the treatment may result in a cure. If his
diagnosis is not correct, the treatment is designed to produce
reactions that in themselves will lead to a better diagnosis. So,
the doctor gives his orders and his prescription, and tells us to
return after a week. If we are faithful and do so, he inquires about
our feelings and reactions, he may repeat some of his qualitative
tests and he may even repeat some of the quantitative tests. On the
basis of the second examination he may conclude that his tentative
hypothesis was in error, but if he has planned his treatment well,
our reaction has provided him with the information for a much better
diagnosis. He may now be able to prescribe with greater confidence a
treatment that will lead to a cure. If our case is particularly
difficult, he may have to follow his second hypothesis with a third
or a fourth, always refining the treatment and studying the reaction
until our response is what he anticipated. He is then finally sure
of his diagnosis.
Of course, the
true artist is more likely to arrive at a correct diagnosis the
first time or to arrive more quickly at a correct diagnosis than his
less expert colleagues. But the method is valid even for the man of
lesser ability, provided of course that we survive the period of
experimentation. If our illness is critical, we will be well advised
to consult the best possible diagnostician in the hope that he can
arrive at the proper solution in time.
The similarity of
this procedure to the observational or learn-as-you-go method of the
subsurface engineer is obvious. In dealing with the foundations for
earth dams, and with difficult foundation or excavation problems,
the subsurface engineer has, indeed, found the observational
procedure to be his most powerful tool.
Let us now look
briefly at medical science. It may surprise us to find that many of
the. outstanding workers in medical science are not M.D.'s at all,
but Ph.D.'s and D.Sc.'s in a variety of fields. They may be
biologists, biochemists, physical chemists, psychologists or even
solid state physicists. When we look, for example, at the broad
attack on the cancer problem today, we see scientists of all types
and technicians in all branches of science working individually or
as members of teams on a variety of problems that may seem only
remotely related to cancer.
Many gifted and
highly trained people are investigating problems that interest them
from the scientific point of view. Yet these people are rarely
medical practitioners. They might be very poor diagnosticians.
Indeed, many of them could not obtain the necessary licence to
practice the art of medicine. But they contribute mightily to the
body of medical science that filters into practice as its
implications become of practical value.
The medical
practitioner is not uneducated in science. He has a background of
courses in anatomy, physiology, chemistry, physics, and many other
scientific subjects. A large part of his education is built on the
results of medical science, but he is not a scientist and his
scientist friends are not practitioners of the art of medicine.
In the application
of the observational procedure the engineer relies heavily on the
scientific background to which he has been exposed in mechanics and
hydraulics as well as soil mechanics and geology. He may in some
instances not perform a single calculation based upon theory, but
the theoretical relationships among the variables associated with
different phenomena have become ingrained in his intellect by long
study and have in a sense become second nature. He also brings to
bear his broad knowledge of construction and design practices, and
his skill and knowledge in the interpretation of geologic phenomena.
All these aspects of his background are brought to focus on the
individual problem with which he deals. He recognizes as does the
medical doctor that no two foundation or earthwork jobs are
identical any more than any two human beings are identical. He must
know what is common about the properties of soil deposits and he
must know what is specific to a particular deposit. His skill in
planning the exploratory programmes and in devising field
observations that will lead to better hypotheses constitutes one of
his most valuable assets.
Although in his
attack on a problem the subsurface engineer makes liberal use of
soil mechanics and geology, his professional work is neither soil
mechanics nor geology. It is a synthesis of these and a host of
other aspects of his background without which he could not
successfully practice the art.
The expert in the
engineering science of soil mechanics, on the other hand, is in fact
a scientist. He may be a theoretician interested in the behaviour of
idealized materials. He may be an experimentalist vitally concerned
with the relationships among the variables that seem to control the
physical properties of soils. He may be intrigued by the forces that
act between soil particles. He may be a physical chemist, a
pedologist, or a mechanical engineer. His contributions to soil
mechanics may find application in the art but they do not constitute
the practice of engineering. The distinction is as clear as that
between the science and the art of medicine.
It is apparent
that a high degree of professional attainment in the science of soil
mechanics is no warranty of success in the practice of the art of
subsurface engineering. Conversely, some of the most expert
practitioners in the art of foundation engineering would be at a
loss to make a significant contribution to soil mechanics.
Occasionally the same individual combines talents in both the
science and the art. Such individuals are rare, but soil mechanics
might not have born or found application had not Terzaghi been of
this dual character. Certainly, Terzaghi has always clearly
recognized the distinction between the science of soil mechanics and
the art of subsurface engineering.
The analogy that
has been developed has implications with regard to engineering
education. There is today a strong and even a growing feeling that
engineering education should consist of education in engineering
science. Soil mechanics fits into this category. On the other hand,
the art of subsurface engineering may be relegated to a seminar or
may not be taught at all. Some people in academic circles are of the
opinion that an art cannot be taught.
The medical
profession does not share this delusion. It believes that the art of
medicine can be taught. It sees that its students obtain good
backgrounds in science and in laboratory techniques. But it also
trains its novices in the accumulated empirical knowledge of their
profession and in the art of diagnosis and clinical practice. It
does not by any means believe that classroom training alone can make
a good diagnostician but it recognizes that the approach to the
problems of diagnosis is a separate and important aspect of medical
education. It knows that training in science alone could not
possibly produce men capable of becoming expert in the art of the
profession of medicine.
Indeed the medical
profession is not greatly concerned with the training of medical
scientists. The majority of medical scientists are trained not in
medical schools but in universities, usually colleges of liberal
arts and science. While there is much cross-fertilization of ideas
among workers in medical science, training in this field is
essentially training in some branch of science.
There is no point
in debating the relative importance of the worker in the science of
soil mechanics and of the practitioner of the art of subsurface
engineering. Both belong to worthy professions, but these
professions are fundamentally as different as are the art and
science of medicine. The increasing importance of science and the
increasing adaptation of science into the art of subsurface
engineering fully justify the training of engineering students in
soil mechanics. But the practice of the art of foundation
engineering requires far more than a background in soil mechanics.
Much of this background can and should be taught at the college
level. A working knowledge and an appreciation of geology are
essential. Some concept of the process of design, possibly obtained
in courses in steel or concrete design, is also a necessary
ingredient. Furthermore, the essence of the observational procedure
can be taught and can be illustrated by effective examples. If soil
mechanics is substituted for the whole of the broader aspects of
this professional background, the education of the subsurface
engineer is poor indeed.
The real artist in
the practice of foundation engineering is one of the finest products
of the engineering profession. He is today a rare individual. It is
as absurd to believe that he will develop automatically, provided he
is thoroughly trained in soil mechanics while in college, as it is
to believe than an expert medical doctor will develop automatically
if a man is taught only medical science.
It is granted that
the scientific aspects of engineering subjects can be learned most
readily at the college level whereas experience accumulated through
the years adds increasingly to a man's professional ability. But
unless our educational system provides a knowledge of the other
disciplines that also contribute to proficiency in practice, and
unless the engineering student is given guidance as to what
experience he should seek and how to utilize this experience, he may
not develop his potentialities.
Finally, it is
suggested that research in subsurface engineering need not be
confined to soil mechanics. There is room for research in a broader
sense, into the interrelationships among soil mechanics, geology and
engineering practice, and into the techniques for most successfully
attacking problems made especially difficult by the complexities of
nature. The current emphasis on research in soil mechanics as such
only demonstrates the necessity for increased attention to the other
ingredients for successful practice.
Is it true, then,
that there is a causal relationship between the growth of soil
mechanics and the number of foundation and subsurface mishaps?
Certainly it is, to the extent that soil mechanics is considered a
substitute for the whole art of subsurface engineering. As long as
engineers are content to make recommendations for design and
construction solely on the basis of borings, soil tests, and
calculations; as long as virtuosity in theory is considered more
praiseworthy than artistry in practice; as long as education
glorifies mathematical science to the exclusion of our heritage of
empirical knowledge; as long as research at the desk or in the
laboratory is regarded as being of a higher order than that pursued
in the field - as long as these conditions exist or to the extent
that they exist, the practice of subsurface engineering may suffer
at the hands of soil mechanics. Let us hope that this is a passing
phase in a transition from pure empiricism to the highest
professional artistry, in which soil mechanics may play its proper,
necessary, and worthy role.
Ralph B. Peck, Class of 1934
Leader in Soil Mechanics
1912 -
An acclaimed international expert in the field of soil mechanics,
Ralph Peck has helped to change the face of the Earth through his
discoveries of the way soils behave. Through his work on the Chicago
subway in the early 1940s he emerged as one of the undisputed
leaders in the development and practice of soil mechanics and
foundation engineering.
As a distinguished professor at the School of Engineering of the
University of Illinois, he conducted field and laboratory research
on stabilization of railroad beds and embankments, the mechanics
of earth dams, the stability of retaining walls, and the settlement
of foundations. Peck has served as a consultant for major foundation
projects throughout the world, from the Trans-Alaska Pipeline, to
rapid transit systems in Chicago, San Francisco, and Washington, to
dams in Turkey and Greece, to the Dead Sea dikes in Israel.
President Ford awarded Peck the National Medal of Science in 1974. |
