Soil Nail Head

Hi,

This is more of a construction question on the installation of the soil nail.

Many references call out for the shotcreting of the soil nail wall to be carried out in 2 stages. The first stage is shotcrete until the base plate. Then snug tight the nail bolt and second stage is another concrete layer to cover the soil nail head.

 Question is can the shotcreting be done in one and final stage which means shotcrete and cover the soil nail head in one go for a 7 inches (175mm thk) shotcrete wall?

Thanks for any reply

I don't see how you can shotcrete effectively under a nail's base plate.  I like to shotcrete around the already installed nails, seat the base plates firmly into the shotcrete for full bearing, then install and snug tighten the nuts.  For a permanent wall, the final facing would be applied afterward.

In my opinion this is more of an aesthetics question.  A permanent wall can be installed with one layer of shotcrete, assuming that you design the facing to the appropriate factors of safety.  A typical sequence that I have used is as follows:
    Install the nails
    Shoot the facing with reinforcement
    Place the bearing plate and nut        
        Then shoot a hump over the bearing plate and nut.
The purpose of the hump is to provide corrosion protection only.  The question is will the owner be willing to accept a wall with humps on the facing.  This detail has been used on many projects such as power plants where the owner is not too overly concerned with the aesthetics of the final product.   One item of caution when building the wall is that you need to be careful that the shotcrete will stay on the wall when you shoot the humps.  To this end it is typical to shoot the cover for the nail heads at the end of the day or the next day.  If you are using accelerator in the shotcrete mix this will help with the installation.

Thanks for your opinion.

No doubt it is easy to shotcrete around the soil nail without the base plate, the soil nail head has to be masked to protected from the shotcrete concrete so that the base plate and the nut can be installed easily later.

In order to hide the soil nail, I intend to have a single THICK layer about 8 inches.

So I wonder is there other major problem if I will to adopt one single stage shotcrete process instead of 2?

Another question is that the design and placement of the rebars. Where is the saggging and hogging moment? At midspan is the sagging moment occurs away from the soil side and hogging moment closest to the soil?

 

To mask the nail during shotcreting, just slide a length of pvc pipe over the nail.  I had a subcontractor not use the pipe even after I suggested it.  I'd love to have the money he spent cleaning grout off the bars so he could thread on the nuts.

I don't see any reason at all that you can't do it in one hit.  A soil nail is a passive reinforcement so the plate isn't really doing anything except giving you a decent size shear perimeter to secure the facing.  Some people see an analogy with soil anchors and think that the nails need to be tensioned but that's simply not correct.

If you space the plate say 80mm off the soil face (perhaps using two nuts) and secure the mesh reinforcement behind the plates, you should be able to shotcrete the face and embed the soil nail head and plate in the concrete.  175mm is a bit thin and won't leave you much cover to play with, and you will have to size the head to keep the shear stress in the concrete within acceptable limits, but it is certainly do-able.  

And what do you really save by doing it like that?  It takes little time to install a plate and a nut.  I say shotcrete, place the plates while the shotcrete is still workable, tighten the nut, and then, if you want, apply additional shotcrete to cover the nails.  It is not necessary to stop shotcreting.  And, you will have proper bearing under the plate.

Why not use a cement mortar for leveling behind the nail's base plate (fast setting such as Williams Wil-X cement grout); set your base plate and then you can shotcrete in one-go.

Because it costs more to do that?