New pile foundation 1

[calculor1]

Have a situation where were modifiying an industrial building. The existing foundation consists of very large pile caps supported by timber piles, as can be seen in the attached sketch. Unfortunately my new columns also supported on piles are approximately 20 to 36" away from the edge of these large pile caps. I am proposing to cut a seat in the existing pile and then cast my new pilecap. Does this seem like a reasonable solution or does anybody have another solution?

 

[SlideRuleEra]

calculor1 - Don't even think about trying to notch the existing concrete. Here are two good reasons:

1. Trying to remove existing reinforced concrete with the precision shown in the sketch is virtually impossible (and trying to do so is very expensive).

2. If the existing piling are close to the edge of the slab (very likely), then creating the notch will weaken the cap directly above these existing piling.

Additionally, the proposed new piling layout will apply a good portion of the new 500 kip load to the existing pile cap. It is unlikely existing piling have the reserve capacity to pickup this new load in addition to current loads.

IMHO, position your new piling to carry the ENTIRE 500 kip / column load, without relying on the any support from the existing pile cap. This may not be easy, but is should be possible.

Connect the new pile cap to the existing pile cap by scabbling the existing pile cap (the full 3' depth) then drilling horizontal holes. Rebar dowels can be epoxied into the holes.

Soak the scabbled area with water (for at least 24 hours) before placing the new concrete. If this step is done correctly, the bond between the new and existing concrete is as good as, or better than a modern bonding agent.

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[calculor1]

Sliderule I should clarify, the existing pile cap once supported a turbine which has since been removed, therefore the existing piles (76 of them) are no longer supporting anything other than the cap. My problem is that I need 4 piles per column, I only have 20 to 36" space between the edge of the pile cap and center of new pile cap basicly I can only place the piles eccentric to the column which I believe is a worst option than cutting a seat as shown in the sketch. BTW I have 6" diameter casin with a 14"x14" bearing plate at top of casing which makes it difficult to satify pile spacing requirements. Any suggestions?

 

[SlideRuleEra]

calculor1 - Here is a suggestion:

1. Demolish the strip of the existing pile cap that covers the first row of existing timber piles (this would include the complete assumed tributary area for all of this first row).

2. Cut off the the exposed timber piling in this first row below grade so that they no longer can carry any load.

3. Design a new pile plan, 2 rows. The inner row will have to be spaced between the remaining, cut-off timber piling. The outer row will have a sufficient number of piling so that the load on ALL the new piling is "reasonably" equal.

4. Note that the new pile cap can be any thickness you need, not necessarily limited to the existing 3' thickness.

5. If you want to tie the new and existing caps together, drill and dowel into the edge of the existing pile cap. This "may" not really be necessary, depending on possible lateral loading. If the two caps are tied together there could be a problems with differential settlement - that 500 kips / column is a lot.

A sketch of this suggestion is posted below.

This may seem like a lot of "extra" work... but compared to successfully cutting the proposed notch (or the consequences of NOT being able to do so), this suggestion is both "easy & inexpensive". There are a lot more piling, but once a pile installation mobilization has been performed, adding additional piling is usually cost effective.

If you need for me to go into details about why the notch is a really BAD idea, I'll be glad too.

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[PEinc]

SlideRuleEra, I agree with you. Good answer.

 

[escrowe]

Very thoughtful!