Debonded piles

miss no stars

Was (making an attempt at) reading a journal article and noticed that it mentioned that pile foundations had been debonded within a particular influence zone using "the double casting method" as they wished to avoid load being transferred to nearby tunnels.

If it's not too much trouble, could someone please explain what debonded piles are and what the double casting method is? I can't find any explanation of it online.

henryporter

Not an Engineer but presumably it means that the piles were cast in two - an inner pile and an outer sleeve. Again presumably the two sections separated or 'debonded' - similar to the way floor screeds debond from concrete subfloors

FesterBeatty

Henryporter has essentially answered the question. There are effectively 2No. methods of transferring the load to the ground (or bedrock) via the pile:

1) End bearing - which means that the pile sits into the bedrock and acts as a column.

2) Skin Friction - meaning that the pile does not have to sit onto rock but is effectively gripped into the surrounding ground with the load bearing capacity being a function of the piles surface area and the coefficient of friction. (Same principal as if you were hammering a timber stake into the ground - the deeper it goes the more force it takes to drive it down further, until it has eventually reached the required load capacity)

Naturally you would expect type 2 above to literally try to drag the surrounding ground downwards thus potentially imposing additional loads on adjacent substructures such as tunnels. Whereas a sleeved or debonded pile (i.e a pile that is not in contact with the surrounding soil) would transfer load directly to the bedrock and not impose any frictionally induced loads on the ground, and would therefore not affect the tunnel in question.


Hope that makes sense.;)

dan_d

How exactly would it transfer the load directly to the bedrock if it's a friction pile, ie essentially not sitting on bedrock?

I don't quite get how that would look/work.

FesterBeatty

Piles don't always have to go to rock - i.e piles that gain their capacity using the principal of skin friction (where friction coefficients associated with the soil allow). As i said above, its as if you hammer a timber stake into the ground, its not sitting on rock but after a few minutes you wont be able to hammer it in further.

http://bored-piles.com/wp-content/uploads/2009/09/boredpilestype1.jpg

dan_d

Yes I know that. Actually I think I kind of get what you said initially - I took it up wrong I think. When you referred to an de-bonded pile transferring load directly to the bedrock, were you talking about the end bearing pile being de-bonded, as opposed to the friction pile? Do they de-bond friction piles...is that possible?In my head it doesn't seem to be...

FesterBeatty

Yes only an end bearing pile can be debonded. Skin friction piles rely on the 'bond' between pile surface area and the soil. So if a skin friction pile was debonded it would no longer have any 'skin friction' and would punch right through the subsoil causing the building to partially or completely collapse :eek:

miss no stars

That's great! So simple once it's explained lol. Thanks for that!

dan_d

FesterBeatty wrote: »

Yes only an end bearing pile can be debonded. Skin friction piles rely on the 'bond' between pile surface area and the soil. So if a skin friction pile was debonded it would no longer have any 'skin friction' and would punch right through the subsoil causing the building to partially or completely collapse :eek:

That's exactly the picture that was in my head:D