Design Change: Soil Nail Wall into Tieback Wall

[jonnywalker]

Background:

We were hired to design the structure of a soil nail wall for a geotech company. They gave us all the of the loads and the anchor locations. The soil nail wall was designed per ACI as mentioned in the below threads and was to have a shotcrete temp facing and a concrete permanent facing.

It went through a county hired review firm here in California and they want it to be stressed tendons...which would change it to a tieback wall.

per threads

http://www.eng-tips.com/viewthread.cfm?qid=59653&page=8

http://www.eng-tips.com/viewthread.cfm?qid=252994&page=8

http://www.eng-tips.com/viewthread.cfm?qid=124597&page=405

http://www.eng-tips.com/viewthread.cfm?qid=43703&page=617

and this companies publications

http://www.schnabel.com/files/publi...ntractor Designed Tiedback Retaining Wall.pdf

Questions:

How does this change my design? Can I still use shotcrete and a concrete face or do I have to incorporate a soldier pile system as mentioned in the above threads? Or is this simply going to change my design loads at each tendon location because of the stressing and the design of the structure would be the same?

Thank you for your time. All insight is greatly appreciated.

 

[PEinc]

Soil nails are passive, tension members. They pick up load when the ground deforms around them. They are not usually preloaded. However, on some projects, the nails may be tensioned a small amount to take out some of the "slack" in the system or to better hold in place the temporary shotcrete facing if the soil is relatively loose (which probably isn't good material for a soil nail wall).

Soil nails are not tieback anchors. The entire nail length is bonded to the soil similar to a reinforcing bar in concrete. Soil nails do not have an unbonded length like tieback anchors. Therefore, if you pre-load a nail, you may be loading only the portion of the nail immediately behind the facing, but in front of the failure plane. Therefore, the pre-load may not reach the area of the nail that provides the majority of the wall support.

http://www.PeirceEngineering.com

 

[jonnywalker]

Right. We have the tieback bonded length and unbounded length provided by our geotech. There is a lot of information about the geotechnical side of the design, however it wasn't totally clear how to find the prestressing force- I found that it should be the design load plus losses but it is not clear how to determine these losses. Also, should the wall facing be designed as a continuous beam with supports at the anchors and point loads for the prestressing forces? If so where would these point loads act?

Thank you for your post!

 

[PEinc]

How you design the wall depends on whether you have a soil nail wall or a tiedback wall. The designs are very different. Re-read the threads you referenced above.

Check out FHWA's, AASHTO's, and CALTRANS' bridge and anchored wall design manuals. Check out FHWA and PennDOT design manuals for soil nail wall design guidance. I would expect CALTRANS to have a similar soil nail wall manual too. You need more help than can be provided in a forum.

http://www.PeirceEngineering.com

 

[dcarr82775]

I have designed anchored (tiebacks) walls with a shotcrete facing, you do not need soldier beams. For an anchored wall the design of the facing is very different as is the design of the anchor itself.

The FHWA facing design is a bit of a black box that makes many assumptions that are not obvious (I think some are wrong, but that is another story). When I use a shotcrete facing for an anchored wall I design the facing as a 2 way slab using the ACI318 direct design method with loading half of the apparent pressure.

 

[PEinc]

Concrete facings (CIP or shotcreted) for tiedback soldier pile walls are usually designed as one-way slabs spanning between soldier piles. Facings for soil nail walls are usually designed as two-way slabs spanning horizontally and vertically between nails. The designs are very different.

http://www.PeirceEngineering.com

 

[dcarr82775]

I might not have been clear, but what I was describing was for an anchored wall without soldier beams.

 

[PEinc]

What do you mean by "an anchored wall without soldier beams?" By anchors, do you mean soil nails or tieback anchors? What is supporting the soil? Are you referring to a soil nail wall?

http://www.PeirceEngineering.com

 

[dcarr82775]

It looks like a soil nail wall, but instead of nails it uses tensioned anchors (free/bonded length, apparent pressures, etc.) so to me it becomes and anchored wall. Once the nails become anchors the FHWA equations are no longer applicable in my opinion.

 

[born2drill]

FHWA has manuals on anchored soldier pile & lagging walls as well as soil nail walls (Geotechnical Engineering Circulars #4 & #7).

For this hybrid system, I would think the facing would have to be very thick and much more heavily reinforced than a traditional soil nail wall. I'm picturing something like the concrete anchor blocks used in landslide stabilization. Either that, or the tiebacks would need to be very closely spaced. Either way it doesn't sound like a more economical solution than using traditional methods.

 

[PEinc]

I agree fully.

http://www.PeirceEngineering.com

 

[dcarr82775]

Yes I know the FHWA has equations for all sorts of stuff, and some of them are absolute crap.

I was saying the FHWA equations for soil nail wall facing wouldn't apply for the original post once the 'nails' become tensioned. FHWA doesn't have design criteria for what the original poster was asking I was simply telling him how I have done them.

I agree, not the cheapest system out there, but sometimes the most economical doesn't work/fit/whatever and something different is needed.