soil nails v. soil screws for retaining walls

[fattdad]

Soil nails include a tendon that is enveloped by grout that develops shear along the grout-soil interface. Soil screws include a shaft with helicies (sp) at varying locations. Between the helicies there is limited shear capacity along the shaft-soil interface.

Consider the image below:

Consider potential failure modes:

I would think that the soil screw doesn't work quite the same.

Comments?

f-d

¡papá gordo ain’t no madre flaca!

 

[cvg]

I'm not an expert, but my 2 cents...

1) face failure seems to be a structural failure of the concrete facing, mostly un-related to the soil nail or screw. If you are able to successfully install the nail and facing, then face failure should not happen.

2) screws may have more uniform tension along nearly the entire length of the rod, as the anchoring helix is generally on the end.

3) screws do not impart as much stress into the looser surface zones, instead they only anchor in the competent material where the helices are.

 

[lallatin]

You are ignoring the fact that helical anchors do not have to be intalled at a declining angle.

 

[fattdad]

Not sure why you made that statement. I'm neither ignoring that nor discussing it. My inquiry was about failure modes for two identically installed elements, soil nail v. soil screw. I do acknowledge your statement, however. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[PEinc]

Soil screws have helices spaced along the entire length of the "nail," not just at the end. Refer the A.B. Chance's Soil Screw Retention Wall System Design Manual.

 

[InDepth]

Helical anchors are different from Soil Nails!

1) Helical anchors usually develop their resistance through bearing, and essentially act like a compression anchor. Sometimes you can substitute tiebacks for helical anchors in a bulkhead type system. You could use Helical anchors in a soil-nail grid pattern, but you would have to have a bulkhead face and an anchor zone beyond the failure plan (for an appropriate soil slope stability FS for local and global). Helical anchors usually aren't grouted, so you wouldn't develop large skin friction along the rod (usually smooth steel along main shaft).

2) Soil Nails create a large stable block of earth and develop their resistance along the anchor (Thus the requirement to meet ASTM standard indentations like Rebar).

The way I usually use helicals are in "tendon" type applications.

Hope this helps.

 

[PEinc]

InDepth,

No offense intended, but it seems to me that you steered this thread toward helical tieback anchors instead of the original topic of soil screws.

1. Soil screws are similar to helical tieback anchors except that soil screws have helices spaced along the entire length of the nail, not just in the lead section(s) beyond the failure plane. Therefore, you don't need skin friction along the length of the soil screw. Instead, there are multiple helices each with bearing capacity. A wall with soil screws, helical tieback anchors, soil nails, or tieback anchors still needs a "bulkhead face" and tendon length behind the failure plane.

2. Soil screws also create a large stable block of earth and develop resistance (in bearing) along the anchor.

3. Helical anchors are usually used in tendon type applications as tieback anchors or as helical piers (piles). Soil screws are used for soil nailing.

 

[fattdad]

So, it seems there may be some consensus that soil screws and soil nails have a similar soil-structure interaction and would make little difference, if properly executed/designed, in the overall performance.

f-d

¡papá gordo ain’t no madre flaca!

 

[PEinc]

Both provide soil support in front of and behind the failure plane. Both hold the sliding soil to the stable soil behind the failure plane and help provide a stable soil mass to resist overturning and sliding.

 

[STVU]

Also note that soil screws are "active" resisting elements in that once installed they are holding back the excavation height. They also need no grouting.

However, soil nails are "passive" in that only when the shored height deflects slightly, do the nails start to hold back. These need grouting and have to be stressed to a design and test load to check capacity.

 

[InDepth]

I learn something new everyday. Thanks for the clarification.

 

[hokie66]

May be clarification for you, but as a reader of this thread, I am confused how these screws can be "active", yet nails are "passive". If both are engaging along the length, I wouldn't see any difference. If they are helical anchors only engaged near the end, I see the difference.

 

[STVU]

Sorry no intention for confusion. If you can get a copy of PTI's- "Recommendations for prestressed rock and soil anchors"
it will clear somethings.

Soil nails transfer the stresses of the nail (usually #8 thru #10 rabar)to the soil by friction along the drill hole perimeter and length. After construction is completed, if the soil behind the soil nail wall does not move, no load exists on the nail, so there are no stresses and therefore we call the nail is "passive".

Take tiebacked walls for example, the soldier piles have to be stressed prior to moving downward too much of a depth. So the tiedback strands or threaded bars are "active" right form the beginning.

You can get more information from the SNAILZ PDF manual found here:

http://www.dot.ca.gov/hq/esc/geotech/request.htm

 

[fattdad]

Isn't the "SNAILZ" program unique to soil nails?

Thanks everybody for your thoughts so far!

f-d

¡papá gordo ain’t no madre flaca!

 

[STVU]

fattdad-

Yes that manual and the software is only for soil nail walls (free program).

 

[fattdad]

Herein lies my ultimate question: Considering the desing assumptions of the SNAILZ program would it be appropriate to use the program for nail design but then substitute screws for construction? I honestly don't know, but I can see areas for doubt.

f-d

¡papá gordo ain’t no madre flaca!

 

[PEinc]

No. The Snail program is written for grouted nails. Check with A. B. Chance to see if they have a program for screw nails.

 

[sure8]

I am looking for detail explainations for Snailz outputs.

 

[PEinc]

Good luck.

 

[jheidt2543]

Just wondering about another difference:

When helical anchors are installed there is a dial on the drill that measures the torque on the helical shaft as it is installed. There are tables that relate the torque to axial load capacity, so you know as you are installing the anchor the approximate holding capacity.

Is this true for soil nails? I would think you are unsure of their actual capacity until the grout has set up and you test a few - is this true?