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Definitions, symbols and units

Piling Terminology - http://www.geoforum.com/info/pileinfo/terminology.asp
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.


Fig. 1. Example of Definitions and Preferred Terms

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 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.
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.

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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