Group effect for ACIP piles

[geotech2]

Can anyone provide some info about group effect for ACIP piles w/ spacing 3-6 D, not end bearing (in loose to medium dense sand)? Any reference books, papers (specific title, author if possible)?
I have a couple of papers/manuals that mention reduction factors for drilled piers, but not for ACIP, though similer.
Thanks in advance.

 

[GeoPaveTraffic]

No specific reference, but look at the group the same way you would a group of drilled shafts then look at it like a group of driven piles. ACIP piles behave somewhere between drilled shafts and driven piles.

 

[dmoler]

Sorry, have to disagree. There is no driving of piles involved in anyway with ACIP. The soil is augered, and there is stress relief like the drilled shafts for the surrounding soil. The grout installed with pressure only keeps the hole from collapsing, the pressure being, say, a 10 foot grout pressure head. I would only use the drilled shaft models for settlement, since there is no densification of surrounding soils due to driving.

 

[GeoPaveTraffic]

I understand your points, dmoler; however, the load tests that I have looked at show performance of ACIP piles to be between drilled shafts and driven piles. Therefore, I would expect the settlement to be between the two types as well.

That said, so much of the performance of ACIP piles depends on the skill of the installer and the soil types. I got no problem being conservative and using a more conservative analysis. Just don't be surprised if any measured settlemet is much less than predicted.

 

[geotech2]

Thanks for your inputs, GeoPaveTraffic and dmoler.

McVay et al. (1994) and O'Neill et al. (2002) has concluded that ACIP piles behave more like drilled shafts in sandy soil than driven piles and the FHWA method for drilled shafts yielded the closest prediction of compression capacity for ACIP piles.

For displacement type auger cast piles I have been involved, such as Berkel's APGD piles and Morris-Shea's DeWaal piles, I found they performed better than regular ACIP piles and most likely behave between drilled shafts and driven piles, from the load tests I observed.

When I posted my question I was hoping to find some specific references for ACIP group effect. I guess there is none.

Rather than analyzing pile group settlement in my case, I will probably just reduce the pile capacity using group efficiency as suggested by FHWA-HI-88-042 drilled shaft manual or CoE TI-88-02 Design of Deep Foundatoins.

 

[dmoler]

Are you asking about group bearing capacity or group settlement? They are not related in all cases. Simply reducing the individual pile design capacity does not mean that the settlement will be reduced. The pile group would simply be larger (more piles) but the same stress would be transferred and compress the sand subgrade.

Depending on the loadings, the equivalent pier or raft method presented in the Drilled Shaft Manuel might be a better reference with heavy loaded structures and large pile groups.

 

[geotech2]

Sorry for the late response, dmoler.

I agree that group capacity and group settlement are two different issues. They are not related if the total loading on pile cap is given and fixed. But they are related if total loading can be reduced depending on allowable capacity from pile.

I may confused you earlier since I know in my case settlement criteria won't be an issue.

Thanks for your comments, dmoler.