Excavation earth retaining structural system

[GalileoG]

I recently received shop drawings which shows an earth retaining structural system that I am not familiar with. The excavation is for the underground floors of a building. I would really appreciate it if someone can tell me the name of this structural system. It is essentially unreinforced concrete piles side by side that overlap 6” to each other and every 4rth concrete pile contains a structural steel member within it.

The intermediate unreinforced concrete piles (3 side by side) between the piles which contain the steel section are not embedded very deep into the soil (maybe 1’) as compared to the piles with the steel section. The piles are 3’ in diameter. Is this standard practice? Also, how can unreinforced concrete piles transfer one-way bending between the piles with the steel support? The concrete piles with the steel section are 11’ apart approximately. Is the 6” overlap between each pile (I suppose they drill when the concrete is still fresh) have sufficient bond/shear transfer capabilities?

Thank you.

 

[dcarr82775]

I have designed similar systems, but in those every other pier was reinforced. The soil arches between the reinforced piers so the load on the unreinforced piers is not very large. Even with the 8' or so gap between reinforced piers the load isn't full soil pressures, but I agree there isn't a load path for bending between the reinforced piers. Is there a shotcrete facing is installed in lifts? If so it would easily be detailed to tranfer those loads.

What so the calcs say? If they say nothing, I would require re-submittal with calcs showing how the system works

 

[GalileoG]

dcarr82775,

Thank you for the response. Would you know what the name is for this system?

There is no shotcrete. Calcs only show the capacity of the piles, nothing about load transfer between across the three unreinforced pile.

 

[GalileoG]

oops, I meant that the calcs only show the capacity of the steel section (reinforced pile), nothing about the unreinforced piles.

 

[msquared48]

If there is soil arching between the steel reinforced piles, then the three intermediate pile do two primary things here:

1. Retain any minor sluffing between the two steel pile, and

2. Serve as restraint against lateral buckling for the steel pile.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[GalileoG]

msquared48, thanks. How can I determine if the arching effect will take place?

 

[dcarr82775]

M^2,

The soil arches which reduces the load on the intermediate piers to something less than the full soil load, but they still need to span horizontally to the reinforced piers. They are glorified lagging so you still need some ability to span horizontally.

GalileoG,

The soil will arch. I was thinking that if the retained soil has a fair amount of cohesion it may be self supporting between the piers

 

[bmrossetti]

GalileoG,

This type of system is called a secant pile wall.

Hope this helps.

Brandon

 

[fancypants]

You are describing a soldier pile type system. I have used a similar system for deep receiving and and bore pits. I have never seen the 6 inch overlap and have always used HP's inside the pile. The 3 foot diameter is about right. This is especially true when the steel is in the pile as you need to be able to get a trimmy all around the steel section.

The comments are arching is correct. Will arching occur? Depends on the soil structure. Sand would just run on you where something clayey may arch. This information has to come from the Geotechnical engineer.

 

[hokie66]

Sounds like some sort of hybrid system. As bmrossetti said, the system using overlapping piles is called a secant pile wall. But as dcarr said, secant pile walls have alternate piles which are reinforced. You drill the alternate "soft" piles first, then drill the reinforced piles, creating the overlaps each side. The ones I have been involved in used reinforced concrete for the "hard" piles, but steel sections would work.