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Piling
Terminology
from
"Basics of Foundation Design", Second Expanded
Edition, 1999, by Bengt H. Fellenius,
Dr. Tech., P. Eng.
There
is an abominable proliferation of terms, definitions, symbols,
and units used in papers and engineering reports written by
the piling community. Not only do the terms vary between authors,
many authors use several different words for the same thing
in the same paper, which makes the papers and reports difficult
to read and conveys an impression of poor professional quality.
More important, poor use of terminology in an engineering
report could cause errors in the design and construction process
and be the root of a construction dispute and, ultimately,
the report writer may have to defend the report in a litigation.
Fig. 1 illustrates the main definitions and preferred piling
terms.
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Fig.
1. Example of Definitions and Preferred Terms
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Upper End of a Pile
One of the most abused terms is the name for the upper and lower
ends of a pile. Terms in common use are, for the upper end,
"top", "butt", and "head", and
for the lower end, "end", "tip", "base",
"point", "bottom", and "toe".
The
term "top" is not good, because, in case of wood
piles, the top of the tree is not normally the 'top' of the
pile, which can and has caused confusion. Also, what is meant
by the word "top force"? Is it the force at the
'top of the pile' or the maximum (peak) force measured somewhere
in the pile? "Butt" is essentially a wood-pile term.
"Head" is the preferred term. For
instance, "the forces were measured at the pile head".
Lower End of a Pile
With regard to the term for the lower end of a pile, the word
"tip" is easily confused with "top", should
the latter term be used - the terms are but a typo apart.
A case-in-point is provided by the 3rd edition (1993) of the
Canadian Foundation Engineering Manual, Page 289, 2nd paragraph.
More important, "tip" implies a uttermost end, usually
a pointed end, and piles are usually blunt-ended.
The term "end" is not good for two reasons: the
pile has two ends, not just one, and, more important, "end"
has a connotation of time. Thus, "end resistance"
implies a "final resistance".
"Base" is not a bad term. However, it is used mainly
for shallow footings, piers, and drilled-shafts. "Point"
is often used for a separate rock-point, that is, a pile shoe
with a hardened tip (see!) or point. Then, before driving,
there is the point of the pile and on the ground next to the
pile lies the separate rock-point, making a sum of two points.
After driving, only one, the pile point remains. Where did
the other one go? And what is meant by "at a point in
the pile"? Any point or just the one at the lower end?
The preferred term is "toe", as it
cannot be confused with any other term and it can, and is,
easily be combined with other terms, such as "toe resistance",
"toe damping", "toe quake", etc.
The word "bottom" should be reserved for use as
reference to the inside of a pile, for instance, when inspecting
down a pipe pile.
The Pile Shaft
Commonly used for the part of the pile in between the head
and toe of the pile are the terms "side", "skin",
"surface", and "shaft". The terms "skin"
and "shaft" are about as frequent. "Side"
is mostly reserved for stubby piers, and "surface",
although the term is used, it is not in frequent use. The
preferred term is "shaft" because
"skin" is restricted to indicate an outer surface
and, therefore, if using "skin", a second term would
be necessary when referring to the actual shaft of the pile.
Other Preferred Piling Terms
A word often causing confusion is "capacity", especially
when it is combined with other words. "Capacity"
of a unit, as in "lateral capacity", "axial
capacity", "bearing capacity", "uplift
capacity", "shaft capacity" and "toe capacity",
is the ultimate resistance of the unit. The
term "ultimate capacity" is a tautology to avoid,
although it cannot be misunderstood. However, the meaningless
and utterly confusing combination terms, such as "load
capacity", "design capacity", "carrying
capacity", "load carrying capacity", even "failure
capacity", which can be found in many papers, should
not be used. (I have experienced a court case where the main
cause of the dispute turned out to originate from the designer's
use of the term "load capacity" to mean capacity
while the field people believed the designer's term to mean
"allowable load". As a factor of safety of 2 was
applied, the piles were driven to twice the capacity necessary
with predictable results). Use "capacity"
as a stand-alone term and as a synonym to "ultimate
resistance". Never use an adjective with "capacity".
Incidentally,
the term "ultimate load" can be used as a substitute
for "capacity" or "ultimate resistance",
but it should be reserved for the capacity evaluated from
the results of a static loading test.
As to the term "resistance", it can stand alone,
or be modified to "ultimate resistance", "mobilized
resistance", "shaft resistance", "toe
resistance", "static resistance", "initial
shaft resistance", "unit toe resistance", etc.
Obviously, combinations such as "skin friction and toe
resistance" and "bearing of the pile toe" constitute
poor language. They can be replaced with, for instance, "shaft
and toe resistances", and "toe resistance"
or "toe load", respectively. "Shaft bearing"
as well as "toe bearing" are acceptable terms.
Resistance develops when the pile forces the soil: "positive
shaft resistance", when loading the pile in compression,
and "negative shaft resistance", when loading in
tension. The term "skin friction" by itself should
not be used, but it may be combined with the 'directional'
words "negative" and "positive": "Negative
skin friction" is caused by settling soil and "positive
skin friction" by swelling soil. A compilation of the
preferred terms is given in Fig. 1, above.
The terms "load test" and "loading test"
are often thought to mean the same thing. However, the situation
referred to is a test performed by loading a pile, not a test
for finding out what load that is applied to a pile. Therefore,
"loading test" is the semantically
correct and the preferred term.
Arguing for the term "loading test" as opposed to
"load test" may suggest that I am a bit of a fusspot.
After all, the semantically correct term for one of my favorite
desserts is "iced cream", not "ice cream"
(but compare "iced tea"). By any name, though, the
calories are as many and a rose would smell as sweet. On the
other hand, the laymen, call them lawyers, judges, or first
year students, do subconsciously pick up on the true meaning
of "load" as opposed to "loading" and
are unnecessarily confused.
While the terms "static loading test" "static
testing" are good terms, do not use the term "dynamic
load testing" or worse: "dynamic load test".
Often a capacity determination is not even meant by these
terms. Use "dynamic testing" and,
for instance, "capacity determined by dynamic testing".
When presenting the results of a loading test, many authors
write "load-settlement curve" and "settlement"
of the pile. The terms should be "load-movement
curve" and "movement".
The term "settlement" must be reserved to refer
to what occurs over long time under a more or less constant
load smaller than the ultimate resistance of the pile. The
term "deflection" instead of "movement"
is normally used for lateral deflection. "Compression",
of course, is not a term to use instead of "movement"
as it means "shortening".
In fact, not just in piling terminology, but as a general
rule, the terms "movement", "settlement",
and "creep" mean deformation. However, they are
not synonyms and it is important not to confuse them.
When there is a perfectly good common term understandable
by a layman, one should not use professional jargon. For example,
for an inclined pile, the terms "raker pile" and
"batter pile" are often used. But "a raker"
is not normally a pile, but an inclined support of a retaining
wall. As to the term "batter", I have experienced
the difficulty of explaining a situation to a judge whose
prior contact with the word "batter" was with regard
to "battered wives" and who thought, no, was convinced,
that "to batter a pile" was to drive it abusively!
The preferred term is "inclined".
The
word "set" means penetration for one blow, sometimes
penetration for a series of blows. Sometimes, "set"
is thought to mean "termination criterion" and applied
as blows/inch! The term "set" is avoidable jargon
and should not be used.
The word "refusal" is another example of confusing
jargon. It is really an absolute word. It is often used in
combinations, such as "practical refusal" meaning
the penetration resistance for when the pile cannot reasonable
be driven deeper. However, "refusal" used in a combination
such as "refusal criterion" means "the criterion
for (practical) refusal", whereas the author might have
meant "termination criterion", that
is, the criterion for when to terminate the driving of the
pile. Avoid the term "refusal" and use "penetration
resistance" and "termination criterion", instead.
Terms such as "penetration resistance", "blow-count",
and "driving resistance", are usually taken to mean
the same thing, but they do not. "Penetration resistance"
is the preferred term for the effort required to advance a
pile and, when quantified, it is either the number of blows
required for the pile to penetrate a certain distance, or
the distance penetrated for a certain number of blows.
"Blow-count" is a casual term and should be used
only when an actual count of blows is considered. For instance,
if blows are counted by the foot, one cannot state that "the
blow-count is so and so many inches per blow", not even
say that it is in blows/inch, unless words are inserted such
as, "which corresponds to a penetration resistance of…"
Obviously, the term "equivalent blow-count" is a
no-good term.
"Driving resistance" is an ambiguous term, as it
can be used to also refer to the resistance in terms of force
and, therefore, it should be avoided.
Often, the terms "allowable load" and "service
load" are taken to be equal. However, "allowable
load" is the load obtained by dividing the capacity with
a factor of safety. "Service load" is the load actually
applied to the pile. In most designs, it is smaller than the
"allowable load". The term "design load"
is undefined and should be avoided.
The term for describing the effect of resistance increase
with time after driving is "set-up" (soil set-up).
Do not use the term "freeze" (soil
freeze), as this term has a different meaning for persons
working in cold regions of the world.
Avoid the term "timber pile", use "wood pile"
in conformity with the terms "steel pile" and "concrete
pile".
Do not use the term "reliability" unless presenting
an analysis based on probabilistic principles.
Brief Compilation of Some additional Definitions
and Terms Related to Piling
| Caisson
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A large, deep foundation unit other than a driven or bored
pile. A caisson is sunk into the ground to carry a structural
unit. |
| Capacity |
The
maximum or ultimate soil resistance mobilized by a foundation
unit. |
| Capacity,
bearing |
The
maximum or ultimate soil resistance mobilized by a foundation
unit subjected to downward loading. |
| Capacity,
geotechnical |
See
capacity, bearing. |
| Capacity,
lateral |
The
maximum or ultimate soil resistance mobilized by a foundation
unit subjected to horizontal loading. |
| Capacity,
structural |
The
maximum or ultimate strength of the foundation unit. |
| Capacity,
tension |
The
maximum or ultimate soil resistance mobilized by a foundation
unit subjected to tension (upward) loading. |
| Cushion,
hammer |
The
material placed in a pile driving helmet to cushion the
impact (formerly called "capblock"). |
| Cushion,
pile |
The
material placed on a pile head to cushion
the impact. |
| Downdrag |
The
downward movement on a deep foundation unit due to negative
skin friction and expressed in terms of settlement. |
| Dragload |
The
load transferred to a deep foundation unit from negative
skin friction. |
| Dynamic
method of analysis |
The
determination of capacity, impact
force, transferred energy, etc,
of a driven pile using analysis of measured
strain-waves induced by the driving of the pile. |
| Dynamic
monitoring |
The
recording of strain and acceleration induced in a pile
during driving and presentation of the data in terms of
stress and transferred energy in the pile
as well as of estimates of capacity. |
| Factor
of safety |
The
ratio of maximum available resistance or of the capacity
to the allowable stress or load. |
| Foundation
unit, deep |
A
unit that provides support for a structure by transferring
load or stress to the soil at depth considerably larger
than the width of the unit. A pile is the
most common type of deep foundation. |
| Foundations |
A
system or arrangement of structural members through which
the loads are transferred to supporting soil or rock. |
| Groundwater
table |
The
upper surface of the zone of saturation in the ground. |
| Impact
force |
The
peak force delivered by a pile driving hammer to the pile
head as measured by means of dynamic monitoring
(the peak force must not be influenced by soil resistance
reflections). |
| Load,
allowable |
The
maximum load that may be safely applied to a foundation
unit under expected loading and soil conditions
and determined as the capacity divided by
the factor of safety. |
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Load, applied or
load, service |
The
load actually applied to a foundation unit. |
| Neutral
plane |
The
location where equilibrium exists between the sum of downward
acting permanent load applied to the pile and dragload
due to negative skin friction and the sum
of upward acting positive shaft resistance
and mobilized toe resistance. The neutral
plane is also where the relative movement between the
pile and the soil is zero. |
| Pile |
A
slender deep foundation unit, made of wood,
steel, or concrete, or combinations thereof, which is
either premanufactured and placed by driving, jacking,
jetting, or screwing, or cast-in-situ in a hole formed
by driving, excavating, or boring. A pile can be a non-displacement,
a low-displacement, or displacement type. |
| Pile
head |
The
uppermost end of a pile. |
| Pile
impedance |
Z
= EA/c, a material property of a pile cross section determined
as the product of the Young's modulus (E) and area (A)
of the cross section divided by the wave speed
(c). |
| Pile
point |
A
special type of pile shoe. |
| Pile
shaft |
The
portion of the pile between the pile head
and the pile toe. |
| Pile
shoe |
A
separate reinforcement attached to the pile toe
of a pile to facilitate driving, to protect the lower
end of the pile, and/or to improve the toe resistance
of the pile. |
| Pile
toe |
The
lowermost end of a pile. (Use of terms such
as pile tip, pile point, or pile end in
the same sense as pile toe is discouraged). |
| Pore
pressure |
Pressure
in the water and gas present in the voids between the
soil grains minus the atmospheric pressure. |
| Pore
pressure, artesian |
Pore
pressure in a confined body of water having a
level of hydrostatic pressure higher than the ground surface. |
| Pore
pressure, hydrostatic |
Pore
pressure varying directly with a free-standing
column of water. |
| Pore
pressure elevation, phreatic |
The
elevation of a groundwater table corresponding
to a hydrostatic pore pressure equal to
the actual pore pressure. |
| Pressure
|
Omnidirectional
force per unit area. (Compare stress). |
| Settlement |
The
downward movement of a foundation unit or soil layer due
to rapidly or slowly occurring compression of the soils
located below the foundation unit or soil layer, when
the compression is caused by an increase of effective
stress. |
| Shaft
resistance, negative |
Soil
resistance acting downward along the pile shaft because
of an applied uplift load. |
| Shaft
resistance, positive |
Soil
resistance acting upward along the pile shaft because
of an applied compressive load. |
| Skin
friction, negative |
Soil
resistance acting downward along the pile shaft as a result
of downdrag and inducing compression in
the pile. |
| Skin
friction, positive |
Soil
resistance acting upward along the pile shaft caused by
swelling of the soil and inducing tension in the pile. |
| Stress |
Unidirectional
force per unit area. (Compare pressure). |
| Stress,
effective |
The
total stress in a particular direction minus the pore
pressure. |
| Toe
resistance |
soil
resistance acting on the pile toe. |
| Transferred
energy |
The
energy transferred to the pile head and determined as
the integral over time of the product of force, velocity,
and pile impedance. |
| Wave
speed |
The
speed of strain propagation in a pile. |
| Wave
trace |
A
graphic representation against time of a force or velocity
measurement. |
Units
In the SI-system, all parameters such as length, volume, mass,
force, etc. are to be inserted in a formula with the value
given in its base unit. If a parameter value is given in a
unit using a multiple of the base unit, e.g., 50 MN - 50 meganewton,
the multiple is considered as an abbreviated number and inserted
with the value, i.e., "mega" means million and the
value is inserted into the formula as 50*106. Notice
that the base units of hydraulic conductivity (permeability),
k, and consolidation coefficient, Cv, are m/s and
m2/s, not cm/s or cm2/s, and not m/year
or m2/hour, respectively.
When indicating length and distance in the SI-system, use
the unit metre and multiples millimetre (mm) or kilometre
(Km). Avoid using the unit centimetre (cm).
For area, square centimeter (cm2) can be used when
it is alone. However, never in combined terms (for example,
when indicating stress). The unit for stress is multiple of
newton/square metre or pascal (N/m2 or Pa). Combination
units, such as N/mm2 and MN/cm2 violate
the principle of the international system (SI) and can be
the cause of errors of calculation. That is, prefixes, such
as "M" and "m", must only be used in the
numerator not in the denominator. Notice also that the unit
"atmosphere" (at =100 KPa) is an aberration to avoid.
Notice, the abbreviated unit for "second" is "s",
not "sec"! - a very common and unnecessary mistake.
The units "newton", "pascal", "joule"
etc. do not take plural ending. It is logical and acceptable
to omit the plural ending for all other units in the SI-system.
The terms "specific weight" and "specific gravity"
were canceled as technical terms long ago, but they are still
found in current professional papers. "Specific weight"
was used to signify the weight of material for a unit volume.
However, the proper terms are "solid density"
and "unit weight" (the units are mass/volume
and force/volume, respectively). The term "specific gravity"
was used to mean the ratio of the density of the material
over the density of water (dimensionless). The internationally
assigned term for this ratio is "relative density",
which term, unfortunately, conflicts with the geotechnical
meaning of the term "relative density" as a classification
of soil density with respect to its maximum and minimum density.
For the latter, however, the internationally assigned term
is "density index".
Soils can be moist, but the measurements and term for the
amount of water in a soils ample is "water content",
not "moisture content".
When writing out SI-units, do not capitalize the unit. Write
"67 newton, 15 pascal, 511 metre, and 96 kilogramme".
Moreover, while the kilogramme is written kg - it is really
a single unit (base unit) although this is belied by its symbol
being composed of two letters. For true multiple units, such
as kilonewton and kilometre, the "kilo" is a prefix
meaning 1,000. When abbreviating the prefix of these, it is
acceptable, indeed preferable, to capitalize the prefix letter:
"KPa", "KN", etc., instead of writing
"kPa", "kN", but be consistent. Notice,
"kg" should be considered as one symbol; it always
requires lower case "k".
If your text uses SI-units and the original work quoted from
a paper used English, make sure to apply a soft conversion
and avoid writing "30.48 metre", when the original
measure was "100 feet", or maybe even "about
100 feet". Similarly, "about one inch" is "about
20mm" or "about 30 mm", while the conversion
of"2.27 inches" is "57.7 mm".
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