Practical and Academic Geotechnical Reflections, 2018

JOURNAL July
2018
Geotechnical

Practical and Academic
Geotechnical Reflections
Success of geotechnical projects depends on several coupling
factors such as academic preparation, continuing education and
professional experience; all of which, when well acquired, allow
for success and application of what is called “engineering judgment.”
All three factors become more relevant when one considers that
geotechnics is not a precise science but instead, an art. As the
legendary geotechnical engineer and professor, Ralph B. Peck 1 (5)
mentioned in one of his last memorable conferences, “geotechnics is
an imprecise art.”
It is possible that for lack of academic knowledge, poor
experiences and even worse, an engineer who has been negligent
in their basic duties, the following typically occur: (a) geotechnical
exploration is insufficient, (b) application of basic geotechnical
concepts is misled, (c) foundation design redundancy is not applied,
and (d) instrumentation is not considered or does not have the
appropriate scope to fulfill project needs.
The goal of this article is to present orderly, important and
practical geotechnical aspects (as listed above) to consider when
involved in a project design or existing construction project in
layman’s terms and without deepening into extended numerical
theories. When these aspects are not accounted for properly or focus
is missed during project development, generally the project quality
and human safety could be compromised, causing loss of life and/
or loss of property. We are not exonerated from human factors and
the geotechnical failures associated with it, but we strive to diminish
them.
The goal of this article is to
present orderly, important
and practical geotechnical
aspects to consider when
involved in a project design
or existing construction
project in layman’s terms
and without deepening into
extended numerical theories.
By Jose N. Gomez, PE,
D.GE, F.ASCE
ECS Florida LLC
1
Low numbers in parenthesis refer to references given at the end of the article.
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Geotechnical Exploration. This is
perhaps one of the most needed activities
in practical geotechnics. Quite often the
geotechnical engineer is pressured by
projects’ incumbents to reduce the scope of
an exploration plan, alleging that budgets
are too low. You may be forced to reduce the
number of borings or test pits and/or their
depths without any valid technical support or
geotechnical judgment. As such, a complete
soil profile (the “x-ray” of subsoil) with basic
properties to develop appropriate designs
might not be accomplished, thus leaving
gaps that will affect a foundations’ design.
Professor R. Peck (5)
, a disciple of K. Terzaghi
(known as the father of Soil Mechanics)
also emphasized the need to know and
differentiate the types of soils to reach
interpretations according to existing subsoil
conditions.
It isn’t a secret for any member of an
engineering team that during the construction
of a project, there can be a negative impact
associated with poor subsoil exploration
scope. As a result, one is exposed to more
contingencies, which can generate significant
changes in the project cost and schedule.
Sufficiently complete geotechnical
exploration campaigns and soil laboratory
tests supported by traditional practices of
the Soil Mechanics should be attained under
all circumstance to meet project needs.
Exploration scopes may also be extended,
if applicable, during the construction stage
Figure 1. Fine soils: typical shear strength envelopes.
Figure 1. Fine soils: typical shear strength envelopes
of the project by recommending additional
field explorations and laboratory tests to be
able to adjust the designs by observational
engineering.
Basic Geotechnical Concepts. There are
cases when review of geotechnical studies
are needed due to slope failures, unexpected
foundation behaviors, or concerns generated
by owners who have some experience in the
field. These concerns usually are generated
because the projects are somehow over—
or under—designed. The typical issues
here are the strength properties assigned
to fine soils such as clays, based on their
classification from the point of view of
formation. Normally consolidated clays (NC)
are usually saturated and soft, while preconsolidated
clays (PC) and/or residual soils
are partially saturated and stiff. The strength
of these clays are very sensitive to moisture
conditions and to the rate of application of
the foundation load, which consequently
affects the dissipation of pore pressure (water
in the soil structure). The point here is that
the same fine-grained soil, depending on the
initial moisture and loading conditions, will
have differing strength responses.
Deficiencies in the understanding of clay
strength are sometimes noticed in young
engineers, which then become inappropriate
habits. This arises due to lack of support
and guidance from more experienced
engineers who do not share the need for (a)
identification of either NC or PC clay type, or
(b) the determination of strength properties.
There have been cases of working under
unconsolidated undrained conditions (UU)
in partially saturated PC clays that greatly
underestimate its shear strength. As shown
in Figure 1, the designer was most likely
working on the “safe” or “conservative” side.
What really happened was that this concept
did not apply or wasn’t correct; especially if
one has data to prove otherwise. The result
of this erroneous assignment of the strength
to a fine soil could lead in practice to having
a more expensive foundation solution,
e.g., piles instead of a shallow foundation
with footings. The actual ultimate bearing
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Figure 1. Fine soils: typical shear strength envelopes
Figure 2. Concrete diaphragm wall, slope failure; misled selection of LEP
capacity (qult) of a PC clay using the
general bearing capacity developed by
Terzaghi and Meyerhof can be of the
order of 75 tn/m² (Ø = 21˚, c = 3.5 tn/
m², B=1.5m), whereas if the criteria of
an NC clay is wrongly used under the
UU condition (cu = qu/2 = 2.0 tn/m²),
qult could be less than 15 tn/m² (80%
less or more).
Estimation of lateral earth
pressures (LEP) distribution on a
temporary bracing structure also
deserves attention. Retaining walls
(rigid structures) support triangular
Rankine or Coulomb LEPs, whereas
temporary bracing-shoring structures
are flexible and support different
LEP distribution shapes based on
empirical analyses. It turns out that
the latter develops LEPs from the
ground surface, unlike the former.
Within empirical formulas, it is also
necessary to differentiate between
continued on next page
Figure 2. Concrete diaphragm wall, slope failure; misled selection of LEP.
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Figure 3. Base failure; wrong excavation sequence.
Figure 3. Base failure; wrong excavation sequence (EITHER WORKS or BOTH)
fine soils (clays) and coarse or granular soils,
since the differences in magnitude and the
shape of the LEP distributions are important.
Figure 2 shows the case of a landslide (2)
where the LEP in the lower part of the
excavation was considered for a granular soil
rather than a clayey soil that was encountered
afterwards… thus generating a LEP up
to four times larger than those that were
considered. Obviously, the system collapsed.
Designs concepts must not be changed
without knowledge or technical support,
even if requests come from high management
levels. Some examples are: reducing safety
factors; ignoring events such as earthquakes,
floods, hurricanes; reducing excavation
support elements; among others (redundancy,
see below). Modifying excavation methods
from sections to continuous modalities
under the pretext of speeding up the work
(see Figure 3) could end in failure. In short,
several examples could be given that often
lead to generating serious problems in
projects.
Lack of Redundancy. The classic
article of professor J. Osterberg (4)
deepens the
philosophy of redundancy in geotechnical
engineering. This concept is often forgotten
when designing (e.g.) key and repetitive
elements, on which the stability of a work
depends. This is the case for systems
involving tiebacks, piles, pumping systems,
among others, where if “a single element”
which is an integral part of several fails, the
Figure 4. Excavation slide due to tiebacks failure (see Figure 2)
entire system could collapse. The landslide
that occurred on Calle 72(2), where, due
to the failure of one or two 25-ton tiebacks
out of many caused an entire 17-meter deep
excavation to collapse (Figures 2 and 4) is an
example. As a recommendation, one should
habitually ask oneself, “what happens if an
element fails within a well mathematically
calculated and stable system” and “what
measures can be anticipated to insure the
stability of a work?” This is what is now
called Plan B. A simple and illustrative
example is the case of a foundation with
piles when the calculation result indicates
that only one pile per foundation support
element is required. It is better to decrease
(e.g.) the diameter of the pile a little and to
contemplate two or three smaller diameter
piles in each support rather than depending
on a single element for stability.
Lack of Instrumentation. It is necessary
to insist on designing and implementing
instrumentation programs in projects,
regardless of the “magnitude or size” of the
project. Both the “large” and “small” projects
will require it. As Professor R. Marsal
said when it came to teaching, “there is no
difference between small or large projects;
all of them need studies and solutions to be
thoroughly analyzed.” (3)
Instrumentation
is the only tool that is available to verify
the project design considerations and to
know the timely behavior of the works
during and after construction. This is an
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activity that can usually be classified as
“Cinderella” in engineering projects; its cost
is insignificant compared to the total value
of the project, but its benefits are priceless
if it prevents a project from collapsing
(with the consequences worse than can be
imagined). With the implementation of a
good instrumentation program, we can safely
sleep in peace.
Conclusions. The author wanted to
highlight and alert some aspects of the
general practice of applied geotechnics,
which when not considered or omitted, could
lead to geotechnical situations difficult to
manage and control. Damage and even loss
of human life can occur easily when these
concepts are not put into focus on every
project. Finally, let’s not forget the eminent
Professor G. Sowers (6)
(author’s professor
1982-83), who emphasized the human factors
and the geotechnical failures with these
words: “we do not exonerate ourselves from
failures, but surely, we can reduce them;
by training us, updating our knowledge
frequently and controlling our emotions, we
practice safer engineering.”
References
(1) Focht, (1994) “Lessons Learned from
Missed Prediction;” 29th Terzaghi Lecture,
Journal of Geotechnical Engineering, Vol
120 No. 10, ASCE, October.
(2) Gómez S., Jose N., (1999), “Analysis
and Solution of Stabilization of a Landslide
with Reinforced Landfill,” XI Panamerican
Conference on Soil Mechanics and
Geotechnical Engineering, Fox do Iguassu,
Brazil.
(3) Marsal, Raúl J. (1986). “Teaching and
Exercise of Civil Engineering,” Conference
Fernando Espinosa, Edited by the Federal
Electricity Commission and the College of
Civil Engineers of Mexico.
(4) Osterberg Jorj O. (1989). “Necessary
Redundancy in Geotechnical Engineering;”
2nd Terzaghi Lecture, Journal of
Geotechnical Engineering, Vol 115. No. 11,
ASCE, pp. 1511 - 1531, November.
Figure 4. Excavation slide due to tiebacks failure (see Figure 2).
(5) Peck, Ralph B. (1997). “Gaining
Ground,” Civil Engineering Magazine,
ASCE, pp. 54-56; December.
(6) Sowers, George F. (1993). “Human
Figure 4. Excavation slide due to tiebacks fail
Factors in Civil and Geotechnical
Engineering Failures;” Journal of
Geotechnical Engineering. Vol. 119, No. 2,
ASCE, pp. 238-256, February.
About the Author:
Jose N. Gómez, PE, D.GE, F.ASCE,
is Vice President and Senior Principal
Geotechnical Engineer with ECS Florida
LLC. He has worked as a consulting
geotechnical engineer for over 38 years
and as adjunct professor for 25 years. He
is the incoming vice-chair of the GMEC
and current president of FES Palm Beach
chapter.
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