New ACIP next to existing mat foundation

[danyul]

Next door is an existing mat foundation about 4.5 feet thick and embedded a couple feet. Designers are planning 20-80 foot deep auger cast piles very nearby. Maybe 5 feet away?

I’m concerned about any effects on the existing mat when we put in the ACIP.

Any suggestions?

 

[EireChch]

What diameter are the piles and what is the spacing?

Whats ACIP also? A... Cast In Place?

 

[danyul]

Auger cast in place. 2foot diameter drilled out then grouted then rebar placed inside.

Mostly single piles nearest the mat

 

[EireChch]

what does "mostly single piles nearest the mat" mean? Spacing is greater than 3D?

I think a 4.5ft mat would be ok, however id still do a hit and miss approach with concrete pouring immediately after they are drilled.

 

[jayrod12]

Honestly I'd be unconcerned. The piles are considered deep foundations and should barely stress the same soil as the mat foundation. In fact locally here we have to neglect the top few metres of skin friction when designing augercast piles.

 

[SlideRuleEra]

I agree with jayrod12, not a concern for the reasons he gave... and, IMHO, if the auger cast piling are point bearing definitely not a problem.

http://www.SlideRuleEra.net

 

[danyul]

Thanks for the responses.

The piles nearest the mat are single piles and not groups. So the spacing is relatively far between the piles.

I was thinking the existing mat is wide so the influence goes deep as well.

 

[jayrod12]

Deep in comparison to say a isolated pad, or strip footing, but not deep like a deep foundation. Have at 'er.

 

[SlideRuleEra]

I was thinking the existing mat is wide so the influence goes deep as well.

It does but the pressure is distributed over a larger area at depth. A traditional, simplified way to consider this concept is to assume the soil pressure footprint of the mat increases at an assumed 2(vertical) to 1 (horizontal) ratio:

http://www.SlideRuleEra.net

 

[danyul]

SlideRuleEra this is what i was worried about. The new piles will be going into the mat pressure zone that you showed.

 

[SlideRuleEra]

danyul - Yes, but the pressure from the mat deceases steadily with depth. Also, the piles 2' diameter and are 5+ feet (2.5 pile diameters) from the mat. A common rule of thumb is that piles spaced 3 pile diameter apart have only reasonable influence on each other. All of this is taking place underground in 3 dimensions:

The mat is "wide", any overlap of influence of piles at the perimeter and the mat is just a small part of the mat's soil contact area.

Load from the piles is "deep" while the mat is "shallow". The primary soil loading from each is at different elevations.

In general, the soil surrounding the piles (and the mat) is semi-infinite... interaction between the piles and mat, under the conditions above, is distributed over a large volume of soil... just "noise", not a problem.

http://www.SlideRuleEra.net

 

[danyul]

Thank you!

 

[ATSE]

I am not concerned about the existing mat on the ACIP work.
But I am concerned about the ACIP work (and resulting future foundation loading) on the existing mat.

Is the ACIP a displacement or partial displacement pile, or is it CFA (almost no cavity expansion lateral displacement)?
When I worked as a pile contractor, adding a whole punch of displacement piles on the site would raise up the grade a bit. Sometimes 1/2 foot - particularly in clay that did not densify.

 

[jayrod12]

ACIP is the same as a CFA pile. Essentially no displacement and no significant empty hole time.

 

[ATSE]

Jay - Minor disagreement on vernacular. Both FWHA and DFI recognize and differentiate ACIP-DD vs ACIP-CFA.
Both use the term "drilled displacement" and abbreviation DD.
Similar equipment, except for the auger, and DD requires a European-style rig with high crowd.
CFA is much more common and has been around since the 1960s.
All else being equal (never quite true), DD provides higher axial capacities.
For those unfamiliar with ACIP in general, this difference is usually lost.
DD auger:

 

[jayrod12]

Interesting. Thanks for that. Learned something new. We do not have those around here. We have ACIP-CFA and more commonly just standard drilled and cast the old fashioned way by throwing concrete in the top of the hole. On extra deep piles, or ones where squeezing of the holes is encountered, we usually sleeve them. Around here CFA is even rare because short term our sub-grades generally will hold shape long enough to get concrete down the hole.