Spreadbored Pile Spacing 1

[dik]

I have a current project where the equipment loading requires a small slab. The slab is small enough that the pile spacing is less than the 3x spacing stipulated in the geotech report. The soil is cohesive, a sandy silt.

Is there a method of safely approximating the proximity of the piles to the end bearing capacity? The bells do not overlap. I suspect the spacing is related to the 'pressure bulbs' overlapping at the bottom of the spreadboreds. Is there a means of accommodating this? It doesn't seem possible to get info from the geotech that did the original report. The factored equipment weight (as DL) is of the magnitude of about 100K in an area, of the slab, of approx 6' x 8'. The centres of the 24" dia spreadbored piles is 1' from the edge of the slab.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[SlideRuleEra]

dik - Should not be a problem. There are several formulas for pile efficiency, but they are all for friction piling, not end bearing. Spacing of end bearing piling can be less than friction piling because the "point load" from each pile is horizontally contained by the surrounding bearing layer which has significant strength. Spacing of 2x is often considered acceptable for end bearing:

This layout is even better, recommended 3x spacing is available on the 8' side. This results in two groups, two piles in each group... the two piles in each group tend to interfere with each other for only a small portion of their circumference.

Also, since the groups are "small" and the piles are bored (not driven displacement piling) there will be no problem with pile heave.

I would not be concerned, 100k should not be a problem for four properly designed piling with this spacing.

Note: I was not familiar with the the term "spreadbored", but a little time on Google makes me believe this what I would call a "bell caisson":

 

[dik]

I agree about the 2x for end bearing and 3x for friction piles, but the geotech report stipulates 3x... I've already extended the slab by 1' on each side to accommodate the 3x. I was thinking of the overlap at a 45deg angle for 3x and using that value for the 2x, but didn't know if there were any other issues.

First time playing draftsman for a couple years...

Thanks, SRE.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[SlideRuleEra]

...the geotech report stipulates 3x...

Well, then provide it... forget the belled caissons and use a true pile group with 3x spacing to mobilize the entire 6' x 8' soil block down to bearing elevation. See if the Geotech will go with a dozen 8" drilled shafts at 24" on center:

 

[dik]

It's a problem site... watertable is high and soil sloughs... they may even have to sleeve the piles. The soil becomes a tad better after 10', and spreadbored piles go down to 15'.In spite of this, the geotechnical report recommends spreadbored piles... I'm always just the janitor's kid, as my brother used to say, doing jobs the janitor doesn't want to do... I've cautioned the client about site soil conditions...

Installing those 12 piles would be a lot more costly than widening the slab by 2', I suspect.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[SlideRuleEra]

Problem site, high water table and ground that sloughs... that's every day around here.
If a slab 8 ft. x 8 ft. will solve the problem... why didn't you do that to start with... cheapest concrete ever poured. I assumed there was no space for a larger slab...
or
Drive a dozen, or so, HP8x36 and be done with it, a pile group is better anyway (spread the load more uniformly and redundancy).

 

[dik]

It's the footprint of the new equipment, to be installed in an existing facility with a bunch of other existing equipment...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[PEinc]

This looks like a good micropile project. Small quantity of piles, small loads, small diameter micropiles, small drilling equipment, small footing.

http://www.PeirceEngineering.com

 

[dik]

thanks... I don't know if I can increase the slab size... it's been bounced back to the client. I'll look into micropiles... my last experience with them was that they were pricey. Driven piles are out of the question. The only geotech report I have recommends spreadbored piles for other different work, and the client doesn't want to get another report.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik