Terzaghi Back to Future
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Transcript of Terzaghi Back to Future
ORIGINAL PAPER
Terzaghi: back to the future
Opening address to the 2006 IAEG conference
John Burland
Received: 29 October 2006 / Accepted: 25 November 2006 / Published online: 20 January 2007� Springer-Verlag 2007
Abstract Terzaghi is generally acknowledged as the
father of soil mechanics. The paper draws attention to
his appreciation of an understanding of the geological
profile in terms of its engineering significance. His
contribution is considered in terms of the modern
practice of modelling and his own emphasis on the
importance of experience and judgement. The ‘‘geo-
technical triangle’’ is discussed as a useful way of
bringing together the important aspects of the geo-
technical engineer’s work.
Keywords Terzaghi � Geotechnical triangle �Modelling � Engineering judgement
Resume Terzaghi est generalement reconnu comme
le pere de la mecanique des sols. L’article attire
l’attention sur sa demarche d’interpretation d’un profil
geologique en termes de proprietes geotechniques et
consequences pour des travaux d’engineering. Sa con-
tribution est analysee par rapport a la pratique actuelle
de la modelisation, considerant son attachement a un
jugement d’expert eclaire, soutenu par des references
aux retours d’experience. Le « triangle geotechnique »
est presente comme une base utile a la confrontation
des differentes approches necessaires au metier de
l’ingenieur geotechnicien.
Mots cles Terzaghi � Triangle geotechnique �Modelisation � Expertise
Introduction
This paper looks at the development of our knowledge
of ground conditions and their importance for con-
struction. Karl Terzaghi is frequently referred to as the
father of soil mechanics, but he was very much more
than that. If we regard the term ‘‘geotechnics’’ or
‘‘geotechnical engineering’’ as embracing engineering
geology, rock mechanics and soil mechanics, then
Terzaghi is truly the father of geotechnical engineering.
The paper reflects on Terzaghi’s struggles to develop
and establish the discipline, the art and the science of
geotechnical engineering. His influence on our current
thinking is considered in terms of the geotechnical
triangle, which highlights the importance of not only
the individual aspects of ground profile, observed
behaviour and appropriate model, but also the inter-
action between these and well-winnowed experience.
Attention is drawn to the value of the geotechnical
triangle as an educational tool and the importance of
encouraging young people into geotechnical engineer-
ing as a challenging and exciting career.
Terzaghi—back to the future
Terzaghi was borne in Prague in 1883. Dick Goodman
has written a most illuminating and thoroughly
researched narrative of his life ‘‘Engineer as Artist’’.
He does not come across as a very loveable person, but
geniuses seldom do. The late Professor Sir Alec
Skempton knew him well and distinguished between
his professional life (in which he was harsh and brutal)
and his private social life in which he showed intense
interest and charm in his dealings with others.
J. Burland (&)Department of Civil Engineering, Skempton Building,Imperial College, South Kensington Campus,London SW7 2AZ, UKe-mail: [email protected]
123
Bull Eng Geol Environ (2007) 66:29–33
DOI 10.1007/s10064-006-0083-9
Though he read Mechanical Engineering at the
Technical University of Graz he was much more
interested in geology. He was keen on climbing and it
is related that he made every climbing expedition into
a joyous adventure in field geology. He switched to
civil engineering and went to work for a firm special-
ising in hydroelectric power generation. Although his
main activity was in the design of reinforced concrete,
the planning of the structures was of course intimately
involved with geology. Frequently he found the guid-
ance of the expert geologists of the time unhelpful. He
encountered many cases of failure—significantly
mainly due to lack of ability to predict and control
groundwater. Piping failures were abundant and also
slope failures, bearing capacity failures and excessive
settlements.
Recognising the obvious influence of geological
factors he concluded that it was necessary to collect as
many case records as possible so as to correlate failures
with geological conditions. It is well known that he
then spent two intense years (1912–1914) in the wes-
tern United States, observing and recording. These
two years ended in disillusionment and depression—of
course compounded by the early part of the First
World War.
The following quote from his Presidential Address
to the Fourth International Conference on Soil
Mechanics and Foundation Engineering sums up his
mood at that time:
‘‘At the end of 2 years I took my bulky collection
of data back to Europe, but when I started sep-
arating the wheat from the chaff I realised with
dismay that there was practically no wheat. The
net result of 2 years of hard labour was so dis-
appointing that it was not even worth publishing
it’’.
So much for geology on its own. So much for precedent
and case histories on their own.
To quote Goodman, the problem lay in the fact that:
‘‘.... the names geologists give to different rocks
and sediments have developed mainly from a
scientific curiosity about the geologic origin of
these materials, whereas Terzaghi was aiming
towards discerning the differences in their engi-
neering properties’’.
Shortly after his appointment to the Royal Ottoman
Engineering University in Constantinople in 1916,
Terzaghi began to search the literature for insights into
the mechanical behaviour of the ground. He became
increasingly frustrated. What he witnessed was a steady
decline in accurately recorded observations and
descriptions of behaviour from the 1880s. This was
replaced by myriads of theories postulated and pub-
lished without adequate supporting evidence. This
experience must have been uppermost in his mind
when, in his Presidential Address referred to previ-
ously, he stated the following:
‘‘In pure science a very sharp distinction is made
between hypothesis, theories, and laws. The dif-
ference between these three categories resides
exclusively in the weight of sustaining evidence.
On the other hand, in foundation and earthwork
engineering, everything is called a theory after it
appears in print, and if the theory finds its way
into a text book, many readers are inclined to
consider it a law’’.
Thus Terzaghi was emphasising the enormous impor-
tance of assembling and examining factual evidence to
support empirical procedures. He is also bringing out
the importance of instilling rigour. This is often equa-
ted with mathematics but there is at least as much
rigour in observing and recording physical phenomena,
developing logical argument and setting these out on
paper clearly and precisely.
In 1918 Terzaghi began to carry out experiments on
forces against retaining walls. He then moved on to
piping phenomena and the flow beneath embankment
dams. He used Forchheimers flownet construction to
analyse his observations and apply them in
practice—methods which were themselves adapted
from the flow of electricity. We see here the interplay
between experiment and analytical modelling.
Over this period Terzaghi came to realise that
geology could not become a reliable and helpful tool
for engineers unless and until the mechanical behav-
iour of the ground could be quantified—this required
systematic experimentation. On a day in March 1919,
and on a single sheet of paper, he wrote down a list of
experiments which would have to be performed.
Terzaghi then entered an intense period of experi-
mental work in which he carried out oedometer tests
and shear tests on clays and sands and developed his
physical understanding of effective stress, excess pore
water pressures and time-rate of consolidation—the
birth of Soil Mechanics.
To make headway with modelling the consolidation
phenomenon analytically, he turned to the mathemat-
ics of heat conduction. Again there is the interplay
between experiment and analytical modelling.
30 J. Burland
123
The geotechnical triangle
Geotechnics is a difficult subject and is regarded by
many engineers as a kind of black art. I used to think
that this was due to the nature of the ground and the
fact that it is a two or even three phase material—much
more complex than the more classical structural
materials of steel, concrete and even timber.
However, after careful study of the views expressed
by Terzaghi, and from my own experience, I came to
the conclusion that the main problem was due to a lack
of appreciation of the number of aspects that have to be
considered in tackling a ground engineering problem.
Examining Terzaghi’s struggles towards establishing
the subject, it is clear that there are four distinct but
interlinked aspects:
• The ground profile including groundwater condi-
tions
• The observed or measured behaviour of the ground
• Prediction using appropriate models
• Empirical procedures, judgement based on prece-
dent and ‘‘well-winnowed experience’’.
The boundaries between these four aspects fre-
quently become confused and one or more of them is
often completely neglected.
The first three may be depicted as forming the
apexes of a triangle (Fig. 1) with empiricism occupying
the centre (Burland 1987). Associated with each of
these aspects is a distinct and rigorous activity.
(a) The ground profile Establishing the ground
profile is the key outcome of the site investigation. In
this context, the ground profile is the description in
simple relevant engineering terms of the successive
strata together with the groundwater conditions and
their variation across the site. Also it is vital to
understand the geological processes and man-made
activities that formed the ground profile, i.e. its genesis.
I am convinced that nine times out of ten, the major
design decisions can be made on the basis of a good
ground profile. Similarly, nine failures out of ten result
from a lack of knowledge about the ground
profile—often the groundwater conditions.
(b) The observed or measured behaviour of the
ground This activity involves observation and
measurement. It includes laboratory and field testing,
field observations of behaviour including movements,
groundwater flow and the development and extent of
pollution plumes. It certainly includes modern satellite
methods of earth observation.
(c) Appropriate modelling The term modelling is
being used increasingly and the engineering geologist is
very familiar with the process of developing geological
models. Modelling is the process of idealising or
simplifying our knowledge of the real world and
assembling these idealisations appropriately into a
model which is amenable to analysis and hence
prediction of response—to analyse is to idealise.
The modelling process has not been completed until
the response has been validated and assessed. This may
involve a number of iterations. Thus the process of
modelling is very much more than simply carrying out
an analysis. A model can be a very simple conceptual
one; it can be a physical 1 g model or a centrifuge
model; it can be a very sophisticated numerical model.
By using the term ‘‘model’’ we are emphasising the
THE GEOTECHNICAL TRIANGLEALL ASPECTS ARE DISTINCT BUT INTERLINKED
EACH ACTIVITY HAS ITS OWN DISTINCT METHODOLOGY AND ITS OWN RIGOUR
GOOD GROUND ENGINEERING REQUIRES THAT THE GEOTECHNICAL TRIANGLE IS KEPT INBALANCE
ALL ASPECTS SHOULD BE PROPERLY CONSIDERED
GROUNDPROFILE
OBSERVEDBEHAVIOUR
APPROPRIATEMODEL
PrecedentEmpiricism
Well-winnowedexperience
Geological or Man-made ProcessesGENESIS
Groundexploration
and Description
IDEALISATION -Conceptual, Physical
or Analyticalmodelling -
VALIDATION
Observation,Measurement,
Lab & Field Testing
IdealisationValidation
aV
lidationIdealis
ta ion
Fig. 1 The geotechnicaltriangle
Terzaghi: back to the future 31
123
idealisation process and de-mystifying the analytical
process. The geotechnical triangle helps in this.
(d) Empirical procedures and experience With
materials as complex and varied as the ground,
empiricism is inevitable and it is (and will always
remain) an essential aspect of geotechnical
engineering. Many of our design and construction
procedures are the product of what I have termed
‘‘well-winnowed experience’’. That is, experience that
results from a rigorous sifting of all the facts that relate
to a particular empirical procedure or case history.
In summary we see that within the geotechnical
triangle (Fig. 1) there are four key aspects, each asso-
ciated with distinct types of activity, with different
outputs. Each activity has a distinct methodology, each
has its own rigour, each is interlinked with the others.
Terzaghi’s approach reveals a coherence and integra-
tion which is reflected in a balanced triangle.
Interaction between geotechnical engineersand civil/structural engineers
There are very real difficulties in communication be-
tween geotechnical engineers and civil/structural
engineers. How often have I heard the anguished cry of
the structural engineer: ‘‘why can’t you simply give me
the spring constants for the foundations’’? I have come
to realise that, at the heart of the problem, there are
profound differences in the approach to modelling the
real world situation (Burland 2006).
For routine modelling the civil engineer specifies the
material and the geometry. The idealisation process is
relatively straightforward. Activity is concentrated at
the right hand corner of the triangle in the analytical
process which gives a false idea of precision. The
geotechnical triangle helped me to realise what was
going on and to explain to civil engineers what it is that
we geotechnical engineers are doing.
Consider the example of a civil engineer working on
an ancient building. Figure 2a shows an isometric of
the West Tower of Ely Cathedral which was strength-
ened in 1973/1974 as described by Heyman (1976).
Figure 2b shows the geotechnical triangle but with
some descriptions changed to represent the key activ-
ities undertaken by the structural engineer.
For the ground profile at the top of the triangle we can
insert the structure of the building and its materials. To
establish these requires the most careful examination
and investigation. As with the ground, small disconti-
nuities and weaknesses can play a major role in deter-
mining the overall response. It is also vital to establish
the way the building was constructed and the changes
that have taken place historically—we might call this the
genesis of the building and it is analogous to the geo-
logical processes that have formed the ground profile.
At the bottom left of the triangle are the properties
of the materials and the observed behaviour of the
building. This aspect requires observation, field mea-
surement, sampling and testing. At the bottom right of
the triangle there is the need to develop appropriate
predictive models that take account of the form and
structure of the building, its history, its material prop-
erties and known behaviour—an almost identical
requirement for the ground.
There is a whole spectrum of models that can be
developed, ranging from the intuitive and conceptual
right through to highly sophisticated numerical models.
The key is to appreciate the inevitable idealisations
that have to be made and the limitations that they
impose. Finally, as in ground engineering, well-win-
nowed experience is of supreme importance and well-
documented case records are invaluable.
It is evident from the foregoing that, even if engi-
neers were in possession of unlimited analytical power,
the uncertainties in both the soil and the structure are
so great that precision in the prediction of behaviour
would be unlikely to improve significantly. As in so
many fields of engineering, modelling is only one of the
many tools required in designing for soil-structure
interaction. In most circumstances the real value of
modelling will be in assisting the engineer to place
bounds on likely overall behaviour, in understanding
the mechanisms of behaviour and in beneficially
modifying that behaviour if necessary.
Education matters
The geotechnical triangle was originally conceived as
an educational aid—as a means of illustrating to stu-
dents and practising engineers the distinctive activities,
and their interactions, involved in geotechnical engi-
neering.
The triangle illustrates the way that the scientific
method can be applied when using observations,
measurements and experience in the formulation of
predictive models. It has also proved valuable in
developing balanced syllabuses for teaching various
aspects of geotechnical engineering. Students get very
confused when dealing with results that may be based
on experiment, others that may be essentially empirical
and still others that are based on analysis of an idea-
lised model. The geotechnical triangle serves to
develop confidence in the interactions between these
distinct methodologies.
32 J. Burland
123
It is important to ensure that rigour is retained
in all the distinctive activities, that they are kept in
proper balance and that the interactions between
them are clearly understood (see also Fookes
1997).
Changing the public perception
‘‘Civil Engineering...see boring’’
This statement is from a well-known British telephone
directory and I am afraid is an only too accurate per-
ception of the general public of engineering.
There are many reasons for this but a major one is
that we fail to convey to the public, and to children in
particular, the reality of what working as a geotechni-
cal engineer really involves. Most construction projects
are unique, each offers very special challenges from
conception, through planning and design and then in
construction. Overcoming these challenges can be very
demanding, requiring ingenuity, determination, clear
thinking and many other human attributes.
Yet we present our projects in an unbelievably bland
way. Our technical papers do not properly reflect the
challenge and drama of our work. The perception is
projected that we turn a handle and out pops another
tunnel or dam. It is my experience and belief that the
public, and young people in particular, relate very strongly
to the drama of overcoming adversity, dealing with the
unexpected, responding to challenges and set-backs.
If we start to show more openly the real challenges
of geotechnical engineering as it interfaces with
Mother Nature, I believe that our public profile will
rise and our profession will be seen to be the challenge
and responsibility that it really is. In this way we can
attract the right sorts of young people to it.
Far from being boring, geotechnical engineering is
creative, challenging, has its struggles and very real
human dramas. The responsibilities that we carry are
immense. We are serving communities and working
with our fragile natural environment.
We need to convey this to our students, to the gen-
eral public, and to young people in particular. We must
bring our profession alive to them as it truly is for us.
References
Burland JB (1987) The teaching of Soil Mechanics: a personalview. In: Proceedings of the 9th European conference onsoil mechanics and foundation engineering, Dublin, vol 3, pp1427–1447
Burland JB (2006) Interaction between structural and geotech-nical engineers. Struct Eng April:29–37
Fookes PG (1997) Geology for engineers: the geological model,prediction and performance. QJEG 30:293–424
Heyman J (1976) The strengthening of the West Tower of ElyCathedral. Proc Inst Civ Eng 60:123–147
Fig. 2 a The West Tower of Ely Cathedral (after Heyman 1976).b The geotechnical triangle adapted for the stabilisation of anhistoric building
Terzaghi: back to the future 33
123