Geotechnical torsion resistance of drilled shaft

[davout777]

Hello,
I have a question regarding the geotechnical torsional resistance of a drilled shaft.
After calculating the skin friction (force), do we need to subtract any axial loads (shaft self-weight and any applied loading on top) to get a net resistance force, and then use this force time shaft radius r to get the torsion resistance. Or, directly use skin friction times r?
Thank you!!

 

[HTURKAK]

do we need to subtract any axial loads (shaft self-weight and any applied loading on top) to get a net resistance force, and then use this force time shaft radius r to get the torsion resistance. Or, directly use skin friction times r?

Neither one..Consider that ( for granular soils ) the friction btw conc. surface and soil may be calculated based on at rest pressure . The developing frictional resistance for unit area would be vectorial sum of horizontal and vertical friction components. Horizontal component could be used to calculate torsional resistance calculation.
You may consider a mathematical model (total horizontal friction volume has a conical shape along the height ) then take the integral.
We are waiting you to post your model and findings based on your calculations.

 

[davout777]

Thank you, HTURKAK.
I understand your model. But, in geotech, we usually just take the approach of getting a total skin friction, uniformly along the shaft surface.
The approach comes with a unit skin friction/pressure, which catches the vertical earth pressure already. Then times the shaft surface area to get the total skin friction.
The only question I want to know is should the axial loading/weight be reduced from this total skin friction before a torque resistance calc.

 

[ANE91]

Your post got me curious, so I looked into it:

FHWA Manual doesn’t address it. None of the traditional alpha and beta methods that I know of account for this. Seems the mechanism is not well understood.

Stumbled across this — https://eng.auburn.edu/files/centers/hrc/ir17-3-torsional-stallings.pdf. Nowak is a big name in the field of reliability, Anderson is an geotech legend, and FDOT does amazing work on deep foundations. You can probably get away with using one of these methods; seems like everyone else is doing it.

 

[davout777]

Your post got me curious, so I looked into it:

FHWA Manual doesn’t address it. None of the traditional alpha and beta methods that I know of account for this. Seems the mechanism is not well understood.

Stumbled across this — https://eng.auburn.edu/files/centers/hrc/ir17-3-torsional-stallings.pdf. Nowak is a big name in the field of reliability, Anderson is an geotech legend, and FDOT does amazing work on deep foundations. You can probably get away with using one of these methods; seems like everyone else is doing it.

Thank you, ANE91.
I am actually pretty familiar with these methods that different DOTs use. These are all methods to get the ultimate torsion resistance, but none of them addresses the details in design. I am assuming the axial load is not subtracted from the ultimate resistance.

 

[HTURKAK]

we usually just take the approach of getting a total skin friction, uniformly along the shaft surface.

-Your approach could be assumed reasonable for piles driven through clay, friction may be equal to or less than, undrained shear strength of the clay. The same reasoning is not true for granular soils which friction depends on coefficient of lateral earth pressure, effective overburden pressure ,and friction angle between the soil and pile wall.

. I am assuming the axial load is not subtracted from the ultimate resistance.

Your assumption is not reasonable. Consider the unit area skin friction capacity will be the same for horizontal and vertical movement. Total frictional resistance would be vectorial combination of the horizontal and vertical components .

 

[davout777]

-Your approach could be assumed reasonable for piles driven through clay, friction may be equal to or less than, undrained shear strength of the clay. The same reasoning is not true for granular soils which friction depends on coefficient of lateral earth pressure, effective overburden pressure ,and friction angle between the soil and pile wall.

Your assumption is not reasonable. Consider the unit area skin friction capacity will be the same for horizontal and vertical movement. Total frictional resistance would be vectorial combination of the horizontal and vertical components .

Hi, HTURKAK, if we are now only considering the torsion/twisting of the pile, should we subtract the vertical loading/shaft weight from the skin resistance, that's the question I want to know. Thank you.

 

[HTURKAK]

Hi, HTURKAK, if we are now only considering the torsion/twisting of the pile, should we subtract the vertical loading/shaft weight from the skin resistance, that's the question I want to know. Thank you.

The answer is yes but this is not linear subtraction. If fs= total friction at the area of discussion , ft= available torsional friction fv=vertical friction;

fs^2= fv^2+ ft^2 or ft= SQRT( fs^2- fv^2 ).

My opinion ..

 

[davout777]

The answer is yes but this is not linear subtraction. If fs= total friction at the area of discussion , ft= available torsional friction fv=vertical friction;

fs^2= fv^2+ ft^2 or ft= SQRT( fs^2- fv^2 ).

My opinion ..

So, if we are only talking about t-direction, it should not have anything to do with the v-direction loading, right? The torsion resistance should only depends on your side friction in t-direction.