***Skin Friction & Pile Question***

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maximus808

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When finding the Skin Friction Capacity of a pile if it is penetrating through multiple soil layers.....(for example, first layer being clay and second layer being sand) how do you find the effective vertical stress? The POW pile problems use specific methods to finding the Qs. However, when doing a problem from the NCEES hand book in the Geo Depth (I believe #518 or so) the problem states to ignore the clay layer at top regarding friction. The solution then takes the vertical stress at the tip or bottom of the pile and divides it by 2? I don't understand why you need to divide the vertical stress by 2. Is it because it is within the 20B critical depth region as is triangular pressure distribution. If that's the case then why is the clay layer not divided by two as well? Or maybe it's because the problem states to ignore it? A typical case would call for both layers to be divided by two? Could you please provide some insight on Skin Friction and especially the vertical stress acting on the pile regardless of clay or sand. Thanks.

 
maximus808 said:
When finding the Skin Friction Capacity of a pile if it is penetrating through multiple soil layers.....(for example, first layer being clay and second layer being sand) how do you find the effective vertical stress? The POW pile problems use specific methods to finding the Qs. However, when doing a problem from the NCEES hand book in the Geo Depth (I believe #518 or so) the problem states to ignore the clay layer at top regarding friction. The solution then takes the vertical stress at the tip or bottom of the pile and divides it by 2? I don't understand why you need to divide the vertical stress by 2. Is it because it is within the 20B critical depth region as is triangular pressure distribution. If that's the case then why is the clay layer not divided by two as well? Or maybe it's because the problem states to ignore it? A typical case would call for both layers to be divided by two? Could you please provide some insight on Skin Friction and especially the vertical stress acting on the pile regardless of clay or sand. Thanks.
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I struggled with this question (No. 518) too. My interpretation is you are concerned with the skin friction along the pile from the 5 ft depth to the 35 ft depth. The vert pressure at 5 ft is 600 psf and at 35 ft it is 4050 psf, so the average vert press Po = (600+4050)/2 =2350 psf.

This answer may seem simple, since I just restated what you said, but what we are concerned with is the average Po x 9.75phi x pile surface area.

As to why they said to ignore the friction resistance of the soft clay layer, my guess since in the eqn Qs=Kht(Po)tan(.75phi)H(perimeter), phi for clay might be zero or close to it.

The interesting part about this question is the pile is in tensile capacity, Tult, and the solution is the same as for Qs. If you have Das'Principles of Foundation Engineering 5th ed, the eqn used is on page 504.

 
so if there are multiple sand layers, do you find the overburden stress at midpoint of the last layer similar to settlement or do find the average of each layer and sum together? Thanks.

 

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