Joint in secant piles

[killi_82]

Hi folks,

Need some helps, question about bored pile connection.
Is the connection between the piles reliable? can it sustain the load that act on the section with no shear connector installed as composite (concrete and steel) does?
attached is the concern issue.

thanks,

 

[HouseBoy]

I would expect the connection to be fine but I suppose one could calculate the "punching shear" based on the area of concrete that remains in place to resist the force.

I do wonder how those "infill" piers in the illustration will be drilled??

I am familiar with "Tangent Walls" which consist of socketed piers that are spaced TWO pier diameters apart.
The infilled piers are unreinforced concrete and are actually positioned slightly "uphill" from the socketed piers.

 

[killi_82]

Hi HousBoy,

first they create the primary piles, and then drilled the other piles as called secondary piles.
They sometimes delay the development strength of the primary piles (to be more than 28 days) to get more easy drilling.

however, my concern is, the primary piles and secondary piles has different stiffness, so that when they response the loading, both structure will deflect differently (one pile is stiffer than the other).
Without implementing the shear connector, thus the transfer of forces between those piles will rely on the friction they have (please see my sketch above).

Has someone justify that this friction or interlocking area is reliable, such that we can consider it is monolith, no further checking is required.?

 

[HouseBoy]

Let's say the piers are one diameter (say 30") and based on your illustration, there is a gap of less than 30" between the piles.
It does not seem to me that it would be easy to drill a 30" pier in the location where you are showing the second set of "infill" piers.

Maybe I am not interpreting your illustration correctly. Which piers are installed first - those with the round cages or those with the rectangular cages?

Perhaps you are saying that the concrete at the edges of the first set of piers is drilled out by the second set of piers. I do not have experience with that construction detail and I would not expect that to be easy to do. I do have experience with installations that are similar to this but there are differences as I indicated in my previous reply.

For the case where the piers are of different stiffness, I would expect that the stiffer pier will attract the load in proportion to its relative stiffness and the less stiff piers will go along for the ride.

Regarding the shear transfer, I think it is a matter of one way shear calculated for the "infilled" pier. It also seems that some (possibly significant) friction will exist too but I think that depends on the roughness of the surfaces and THAT will depend on the sequence of the installation.

In my experience, the "primary" piers are socketed (usually into bedrock) and the infilled piers are not. They (the infill piers) are used only to prevent the soil that is above the bedrock from slipping thru the gap (sometimes referred to as "plug piers") and they are plain concrete. See attached photo.

 

[GeoPaveTraffic]

Secant wall are uncommon, but not rare. In many cases the secondary piles are completely unreinforced. Since load is attracted by the stiffer member, load is taken by the primary piles by arching. I've never seen an implementation like the one shown using rectangular cages in the secondary piles, but should work if you really need that much wall stiffness. Note that the secondary piles are installed first.

As for drilling the primary piles, it can be difficult and requires the right equipment and a good operator. But it can be done.

Mike Lambert

 

[killi_82]

Hi HouseBoy,

The first pier installed is the part with rectangular rebar cage which I called primary piles, and the other with circular rebar cage is secondary one.
The term is mentioning which come first rather than the configuration of the rebar itself.

It is not easy, but it can be done as sketched.


I agree with your statement,

I need to know, how reliable the friction capacity is.

However, I found a paper that this interlocking capacity also depend on the time you commence the secondary piles, they measured in hours,in which the longer it takes the more likely the joint will fail.
But this is not the real case I need, though it close to my expectation.
In my case, this structure will be used for permanent structure, which support 3 levels of underground slab, something like basement structure.

nice pict, I understand what you meant.

Thank you