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# Skin Friction Capacity (Piles in Cohesionless Soil)...

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I am working Problem 510 in the "NCEES Civil Exam Sample Problems and Solutions- Geotechnical Afternoon problems" which is asking us to calculate the skin friction resistance of a timber pile driven in sand (cohesiononless soil).

I am trying to verify whether the book has published a mistake or whether I have misunderstood the concept of pile skin friction capacity. In my opinion, each of the (3) sections of soil that the pile passes through needs to be multiplied by the depth of each pile section. The solution in the NCEES Problems book is not doing this.

Any light you can shed on this issue is appreciated and thank you for your time.

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I am working Problem 510 in the "NCEES Civil Exam Sample Problems and Solutions- Geotechnical Afternoon problems" which is asking us to calculate the skin friction resistance of a timber pile driven in sand (cohesiononless soil).

I am trying to verify whether the book has published a mistake or whether I have misunderstood the concept of pile skin friction capacity. In my opinion, each of the (3) sections of soil that the pile passes through needs to be multiplied by the depth of each pile section. The solution in the NCEES Problems book is not doing this.

Any light you can shed on this issue is appreciated and thank you for your time.

I don't have the problem for reference , but the force developed by skin friction is based on the contact surface area between the pile and the soil providing the pile. So if it passes thorugh different strata then the frictional support/resistance for that strata is based only on the height of that soil layer. Is this your question?

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I don't have the problem for reference , but the force developed by skin friction is based on the contact surface area between the pile and the soil providing the pile. So if it passes thorugh different strata then the frictional support/resistance for that strata is based only on the height of that soil layer. Is this your question?

No, what I'm referring to is simply converting soil pressure distributed load into its resultant. If you have a pile passing through a 3 ft deep section of top soil with 100 pcf density, then the soil pressure is districuted as a triangular load with the pressure at the bottom of the 3' pile section = (3 ft)*(100 pcf) = 300 psf.

Then, if you want to find the resultant of the pressure triangle: Area = 1/2*base*height = (1/2)*(300 psf)*(3 ft) = 450 plf. It is my opinion that the NCEES solution has skipped this step, though I have to give them the benefit of the doubt.

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Preparation Hell,

The equation that is presented is correct, but you are entirely correct to question it because it does not stress (no pun intended) the point at which the resultant force is acting. I have a text entitled, "Geotechnical Engineering Pile Design and Construction," authored by Ruwan Rajapakse, P.E. (A Very Good Text for Pile Design). It states that the OVERBURDEN PRESSURE at a point of interest, say the contact point between different soil types, is used because it is a CONSERVATIVE estimate of the stresses applied to the pile along the length of that pile up to that point.

So the practice is based on conservative estimations of the pressure - not a singular, unique analytical solution. If this response doesn't satisfy your question, please let me know and I can go into more detail.

JR

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Preparation Hell,

The equation that is presented is correct, but you are entirely correct to question it because it does not stress (no pun intended) the point at which the resultant force is acting. I have a text entitled, "Geotechnical Engineering Pile Design and Construction," authored by Ruwan Rajapakse, P.E. (A Very Good Text for Pile Design). It states that the OVERBURDEN PRESSURE at a point of interest, say the contact point between different soil types, is used because it is a CONSERVATIVE estimate of the stresses applied to the pile along the length of that pile up to that point.

So the practice is based on conservative estimations of the pressure - not a singular, unique analytical solution. If this response doesn't satisfy your question, please let me know and I can go into more detail.

JR

I could not find this equation in chap 38 of :sux: or Das, Ultimate skin friction using Mohr-Coloumb failure criteria?

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Das provides the equation by categorizing this method as skin friction within sandy soil (e.g. cohesionless). It turns out that the equation is developed by utilizing the Mohr-Coloumb failure critieria but he does not go into detail for the derivation of this equation. It turns out that many of these equations are derived partly from theoretical basis and partly from emperical observations.

The question I would ask - do you plan on taking Civil with Geotech PM? If you aren't taking Geotech PM, then the subject of deep foundations is most likely outside the specifications provide in the NCEES guide. Not withstaning that statement, if you are interested in reading more about piles - try this website http://sbe.napier.ac.uk/projects/piledesig...de/chapter1.htm. It provides pretty good coverage - practical and theoretical for pile design theory.

I have the Das Foundations of Geotechnical Design text. I will dig it out and find the page that covers this equation for you.

JR

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Das provides the equation by categorizing this method as skin friction within sandy soil (e.g. cohesionless). It turns out that the equation is developed by utilizing the Mohr-Coloumb failure critieria but he does not go into detail for the derivation of this equation. It turns out that many of these equations are derived partly from theoretical basis and partly from emperical observations.

The question I would ask - do you plan on taking Civil with Geotech PM? If you aren't taking Geotech PM, then the subject of deep foundations is most likely outside the specifications provide in the NCEES guide. Not withstaning that statement, if you are interested in reading more about piles - try this website http://sbe.napier.ac.uk/projects/piledesig...de/chapter1.htm. It provides pretty good coverage - practical and theoretical for pile design theory.

I have the Das Foundations of Geotechnical Design text. I will dig it out and find the page that covers this equation for you.

JR

thanks JR