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Shaft resistance of piles in rock

Shaft resistance of piles in rock

Shaft resistance of piles in rock

(OP)
Hi all,

I have a query regarding the design of piles in rock. How would I quantify the shaft resistance of a R.C. pile embedded in rock? I am particularily interested in a situation where the pile end bearing alone would not be enough to safely transfer the design load and therefore shaft resistance would need to be utilized.


Thanks in advance...

RE: Shaft resistance of piles in rock

For a start most would do through a shaft friction factor. The friction angle not to exceed that of the broken rock at the friction layer, and no cohesion to be taken; only if grouted as in micropiles some adhesion would be counted, I think.

RE: Shaft resistance of piles in rock

You should coordinate with the Geotech on the project and ask for recommendations for minimum embedment and shear strength values is rock.  But typically, these values will be approximately half of what the allowable end bearing resistance is, given the minimum embedment.

If the budget allows and the project scope warrants, an Osterberg load test could/should be done to verify and value engineer the shafts.

RE: Shaft resistance of piles in rock

After re-reading post, I may have misunderstood.  Are these drilled shafts or micropiles in rock?  What diameter range are you looking at?

RE: Shaft resistance of piles in rock

(OP)
Yes, these are bored piles infilled with reinforced concrete. I anticipate that a pile size of 900mm Diameter would be used. Boreholes indicate following:

Ground-5m, made ground
5m-8m, Stiff clay, C=80kN/m²
below 8m,Rock, allowable bearing pressure of 1000kN/m²

Service load to be applied to pile is about 1300kN. I think a 900mm dia pile would have capacity of 636kN solely from end bearing only. Can the toe of the pile be under-reamed to increase the end bearing area? If a 1300mm dia bulb at the pile toe was achieved then the end bearing capacity would be 1327kN.

Maybe all this is unneccesary and the shaft resistance will provide the additional capacity required???



 

RE: Shaft resistance of piles in rock

Check with your Geotech.  For clay soil, it is usually not advisable to combine end bearing values with skin friction values but this may be different with rock.  

BA

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