Soldier piles with lagging using helical piles

[joe26]

Hi everyone,

I am a geotechnical EIT and I have been asked to design a temporary shoring for an excavation depth of 10 feet below the existing ground surface. The owner and contractor have insisted on using helical piles as soldier piles with wood lagging. The geotechnical report says that there is very dense mixture of till, silt and clay down to 4 feet below the existing grade and compact sand below that. It also says that minor seepage was encountered in the sand layer. I do not have a lot of experience designing shoring systems so any help would be appreciated. I started using the California trenching and shoring manual and I believe it offers some good advice. I was however not able to compute the embedment depth required below the dredge line. Also the helical piles that are to be used have a circular section (5.6" dia).

 

[PEinc]

Usually, sheeting walls that are only 10 feet high are cantilevered, not tied back, unless there is bedrock near subgrade that makes soldier beam installation very expensive.
Helical anchors can be difficult, if not impossible, to install in very dense or stiff soils. Boulders, cobbles, and other obstructions can be enough to rule out using helical anchors.
If the dense till extends only down 4' from original grade, the helical anchors should be in the compact sand. Talk to an anchor installer to get an opinion if the soil profile is OK for helical anchors.
Some of the helical anchor manufacturers can provide you with software to design helical anchors and piers. Or, you can write your own program.
CALTRANS, FHWA, AASHTO, and other organizations have manuals that will show how to design the sheeting wall, including embedment.
For a 10' high, tiedback wall, sum the moments about the anchor location. Solve for the embedment depth that gives moment equilibrium. Increase the calculated embedment depth by at least 20%.
When calculating the passive soil pressure, don't forget to consider buoyant soil weight if there is ground water below subgrade. The seeping water above subgrade should drain through the lagging. If it doesn't, you should not use soldier beams with lagging and you may need to add hydrostatic pressure.

http://www.PeirceEngineering.com

 

[dcarr82775]

Are you wanting to use the helical piers as the soldier beams? Do not do it. The helix disturbs the ground passing through. The pile will have to move significantly to resist any lateral earth pressures.

 

[joe26]

Thank you for the input guys. PEinc - The wall would be cantilevered since this is a residential house and we do not have permission to drill onto the neighboring property and due to the shallow depth of excavation. I considered using rakers to provide more support against deflection but it would greatly reduce the construction space required for the foundation footings. Just as dcarr82775 said, I am trying to use the helical piles as soldier piles (not tiebacks) with wood lagging between them but now I understand that the helix would disturb the soil and I might get significant deflection on the piles (this cannot happen because the neighboring property is brand new and any damage to the pavement would need to be remediated). Also they want to remove the helical piles after the completion of the construction, which would prevent me from using a whaler to somewhat keep the helical piles in place. I don't know any other cases where helical piles have been used in place of soldier piles.

 

[PEinc]

I misunderstood. I did not realize that the vertical helical would serve as the soldier beams. I have seen signed and sealed plans for a NJ job that used helical piers as soldier beams. The vertical helical soldier beams also had helical tiebacks to keep the helical pier soldiers from falling over. I would NOT do that. Sounds very shaky to me. Cheap (if they go in far enough), but shaky.

Is the cantilevered wall to be temporary or permanent? You could drill in pipe piles as your vertical soldier beams. Then, you could attach lagging to the front of the pipe piles. Or, you could just drill in WF or HP soldier beams.

http://www.PeirceEngineering.com

 

[joe26]

The contractor has insisted that they have done this once before and everything has worked out fine. This is just for temporary shoring and they want to unscrew the helical piles after the job is done. I was thinking to specify that any excavation work would have to commence at least 2 weeks after the helical piles are installed so that there would be some time for soil settlement. I am being very conservative in my design (3' spacing c/c)for the piles with and embedment depth of 12' below the dredge line. I am also increasing the lagging thickness to 4" as from my understanding non-cohesive soil would exert more pressure on the lagging.

 

[dcarr82775]

IMO, if the contractor did it before and nothing happened it was because the soil held itself up, the helicals were not doing much of anything. I notice you are an EIT, what does your supervising PE think of all this? Don't bother with thicker lagging, 3" would be fine, but I would run, do not walk, away from the idea of a cantilevered helical pier.

 

[RFreund]

"Convince" them they need to install the horizontal tie backs as well. As noted the soil is distrubed and can lead to large deflections (I think AB Chance

http://www.abchance.com/

has some information in thier design manual on this). They can unscrew the horizontal tie backs also so all should be well.



EIT

http://www.HowToEngineer.com

 

[PEinc]

I'm with dcarr82775 on this one. 1. the soils may be too dense, stiff, or cobbled for helicals. 2. I would use helical piles as cantilevered soldier beams unless they were large diameter pipe (but then they would be even harder to install). 3. Tied back, closely spaced helical piles might work but you would be increasing the number of helicals that would need to install (and remove?) in possibly difficult soils, and you would need permission to screw the tieback anchors into a neighbor's property. I would install drilled-in steel WF or HP soldier beams and backfill the drill holes entirely with low strength, lean mix concrete or flowable fill. I also believe that 4" lagging boards will add nothing but cost.

http://www.PeirceEngineering.com

 

[PEinc]

Sorry, I meant to say that I would never use helical piles as cantilevered soldier beams.

http://www.PeirceEngineering.com

 

[joe26]

Thank you for all the input guys. It is much appreciated. My supervising P.Eng is the one who told me to use helical piles (he has interest in a helical pile company) so you can see my situation. Tiebacks cannot be installed because we don't have permission from the neighbor. I told him to go with regular HP soldier beams because of potential deflection but he doesn't want to listen. I have presented him the idea of using battered helical piles (since he wants to use just helical)to act as rakers. We will see now.

 

[PEinc]

Talk to someone from A.B. Chance to see if the design can be done. If not, show your boss. Case closed.
If Chance thinks the design can be done (and work!), then good luck with your boss. You will need it.

http://www.PeirceEngineering.com

 

[RFreund]

I've talked with AB before, but please do talk with them. They say that it can be done (I was in a similar situation). But when I talked with them about deflection and such their answer did not suffice me. I believe they told me cantilever has been done before. However in my case we received permission to install across the property line as I did not feel comfortable with the cantilever and deflection (mostly due to the fact that there was a nearby structure), so it was a tied back system. I thought I posted on this in the past. I will try to see if I can find the old thread. If you do talk to AB please post back with your findings it would be great to hear.

I'd like to tell you that in my case everything worked great. However it didn't. I think it would have been just fine. However the contractor did not drill the helical piers to the specified embeddment. He stopped about 18" below the dredge line. Sure enough we got a major rainstorm (on the order of 25 year reoccurrence if I remember correctly). The bottom bulged/pushed out. Luckily there was no damage. The contractor they had install the system seemed to be a fly by night kinda guy and we actually didn't know about the incident until the shoring was removed and the project was completed.

EIT

http://www.HowToEngineer.com