SKIN FRICTION vs ADHESION 2

[BAGW]

Hi,

I am confused about the term skin friction and the adhesion. Are they the same? Is skin friction used in sand and adhesion in clays?

Thanks

 

[IRstuff]

https://www.sciencedirect.com/topics/engineering/skin-friction

https://www.finesoftware.eu/help/geo5/en/adhesion-of-soil-01/

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[BAretired]

I am confused about the term skin friction and the adhesion. Are they the same? Is skin friction used in sand and adhesion in clays?

I don't believe they are the same, but I'm not too clear on adhesion. I don't recall that term being used in subsurface investigations. We use a lot of friction piles in my area (Alberta) and the predominant soil here is clay. Soil reports typically give values for skin friction, not adhesion.

I read both of the references by IRstuff. The article on skin friction seems excellent, but I wasn't able to get a clear understanding of adhesion.

The value of adhesion a is usually considered as a fraction of the soil cohesion c.

From that statement, adhesion would apply to clay but not to sand because sand has no cohesion, but the table of typical cohesion and adhesion values shed very little light on the matter.

EDIT: The term "skin friction" might apply to sandstone, but usually not to loose sand as drilling pile shafts in loose sand would be difficult, probably requiring steel casing during the drilling and gradual removal during the concreting.

Perhaps this question would be better moved to "Geotechnical engineering general discussion"

BA

 

[IRstuff]

see:

https://civilengineeringbible.com/subtopics.php?i=91

That said, friction is somewhat a function of the cohesiveness of the soil side and the material inserted into the soil.

https://www.researchgate.net/publication/308376814_Skin_Friction_between_Soil_and_Pile_Materials

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[r13]

My street smart explanation of adhesion is "how sticky" the cohesive material is when struct to another solid substance. Sure, there is more to it in technical terms, and I could be wrong.

Shear friction is the friction/surface forces between two objects, which are in contact, but without bonding, sliding/moving in opposite directions. I guess this is more well understood.

 

[WARose]

It's always confused me too.....but my best guess on it: In the α [alpha] method for side capacity of piles (very popular with clays) the side resistance is a function of shear strength of the clay. (Which varies with depth, however there is typically a limit used with this method.) The beta method (used with piles in sandy soils) is more of a classic, sliding resistance type concept. (Although modified by testing.)

In Coduto's text (i.e. 'Foundation Design: Principles and Practices') he makes clear (even with the α method) that both should be thought of as "a classical sliding friction problem". (As opposed to any surface "sticking" to another.)

 

[GeoEnvGuy]

Skin friction and adhesion are related to dissimilar materials shear strength. Is it just as simple as skin friction is for sand and adhesion is for clay, for most practical applications that is how it is analyzed but technically no. The best way to picture it is if you throw playdoh at a wall it will then slide down(skin friction) and elongate(adhesion). If the playdoh were to slide down and not elongate then only skin friction, if it were to just peel off and fall then adhesion only.

 

[human909]

Some fairly different responses here. I'll add my 2cents which may or may not be helpful. (Not my strongest area.)

According to basic coulomb model of soil mechanics skin friction increase linearly with effective stress.
Adhesion is a constant regardless of effective stress.

Of course reality doesn't quite fit the basic model but the two aspects of friction and adhesion are still significantly different.

https://en.wikipedia.org/wiki/Soil_mechanics

 

[GeoPaveTraffic]

Here is my explanation after 30+ years working as a geotechnical engineer. Note that this discussion is for piles with only static loads or very small cyclic loads relative to their static capacity. Piles subjected to cyclic loads are a complete different beast and should be designed very carefully.

Piles, regardless of the soil and/or rock in which they bear, are generally designed as either friction piles or end bearing piles. In this thread we are only talking about friction piles.

Friction piles develop their load capacity based on resistance along the sides of the pile, i.e. capacity is related to the side area of the pile. The side resistance usually changes with depth depending on soil type and one or more of the following: groundwater level or levels, soil strength, and over burden pressure.

In sands the side resistance is calculated, in almost all cases, is based on the friction between sand and the pile. This resistance is normally based on the strength (phi angle) of the sand and the effective over burden pressure at a given depth. Therefore the side resistance changes both with soil strength and depth. Most piles bearing in sand are designed as friction piles and settlement is rarely a problem.

In clays the side resistance is calculated based on the interaction between the clay and the pile. This resistance is generally related to the undrained shear strength (cohesion) of the clay and the value of the interaction is often termed adhesion. The adhesion cannot be greater than the cohesion and there are many methods/opinions on what is the correct/appropriate relationship between cohesion and adhesion. Generally the overburden pressure has only a small impact on the resistance in clays, at least the way the resistance is typically calculated. Piles are rarely founded in soft or medium stiff clays and settlement can be a problem and should always be considered.

Soil conditions with both sand and clay require careful consideration since the different stiffnesses in the different soil layers can really affect where the load goes and how the piles behave.

Mike Lambert

 

[BAGW]

Thank you..