soil nailed wall 1

[cvg]

I'm looking at the possibility of constructing a near vertical, soil nailed, shotcrete wall within an ephemeral stream. The wall would retain the channel bank under / near a bridge. It would be extended down to the expected scour depth. Soils are generally silty and clayey sands and gravels. Is this a good idea?

 

[PEinc]

I would not even try that. The biggest enemy to a soil nail wall is water. Will there be ground water or stream water that will prevent an open, vertical cut from remaining stable? Probably.

Will you be able to shoot and maintain shotcrete on a vertical surface that is saturated or has water running out of the excavation face? Probably not.

Scouring at the toe of a soil nail wall will also be a big problem.

Can you use sheet piling instead?

 

[cvg]

project is in Phoenix Arizona. Groundwater is not a problem. Stream will not be flowing during construction. We would likely reinforce the shotcrete with steel wwf and extend below surface 10-15 feet for anticipated scour depth. Our other option is to excavate and construct some sort of retaining wall, either a gravity / counterfort wall or install drilled shafts for the new bridge abutment and connect a wall to them.

 

[PEinc]

You really have not given anywhere near enough information for anyone to give a meaningful comment. You gave no information about the bridge, its foundation, its location to the wall and stream, the subsurface conditions, etc. Do you have a geotechnical engineer involved with the project? If not, you should hire one with highway experience.

 

[cvg]

Yes, we are consulting a geotechnical engineer, but I am hoping for another opinion.

groundwater is several hundred feet deep

It rains just 7 inches per year (avg) and last year only 3 inches. Chance of flowing water during construction is remote.

We have already determined that maintaining a near vertical wall long enough to shoot the shotcrete should not be a problem.

The wall will be protected against scour by constructing a footing down to the predicted scour depth.

We will be looking at alternatives to increase capacity under the bridge along with widening the bridge. One alternative is to leave the bridge abutments and construct a vertical wall. A retaining wall would be very expensive and shotcrete/soil nailing might be a cheaper alternative. The second alternative is to lengthen the bridge by constructing new abutments farther from the edge of the existing river. Alternatives also include attaching the vertical wall to the drilled shafts to provide additional lateral support. The concern is that if the ground behind the shotcrete ever gets saturated, would that require excessive number or length of soil nails such that the cost would become excessive?

 

[jdmm]

cvg

A soil nail wall is a good solution where the wall will be entirely in excavation. Groundwater can be contolled by horizontal drains and flat drains behind the shotcrete. We have done many walls in the Pacific Northwest in very very wet conditions. Preferable to the soil nail would be tensioned anchors where the anchor has a free stress length and a bond length. This helps to prevent the development of strain as the excavation proceeds.

 

[born2drill]

jdmm:

I'm sorry, but I can't agree with the statement:
"Preferable to the soil nail would be tensioned anchors where the anchor has a free stress length and a bond length. This helps to prevent the development of strain as the excavation proceeds."

A shotcrete & soil nail wall does not work the same way a tiedback pile & lagging wall does. For soil nails, a major post-tensioning effort defeats the purpose. Soil nails also should not be designed with a free length.
Soil nails are passive elements - they become tensioned as the excavation proceeds and the retained soil deforms laterally. Soil nails and tiebacks are fundamentally different. What is appropriate to specify for one is not necessarily appropriate for the other.

 

[DP103]

Are you still considering soil nailing for one of your projects? I am one of the authors of the FHWA manual and am based out of Phoenix. I also have design a soil nail wall under a bridge abutment for scour protection up near Prescott. It is possible.

 

[DP103]

born2drill:

Fundamentally you are correct. However, there such an animal as a prestressed soil nail. With type of nail is used where it is desireable to limit deformation under existing structure. It is constructed using corrugated sheathing. The inside of the sheathing is only filled with grout up to the top of the "bond" zone. The outside is filled to the surface. After the shotcrete is cured the nail is prestressed and the "unbonded" part of the sheathing is filled with grout. This provides for a fully bonded nail after prestressing. This technique was recently used on a project in the Phoenix area.

 

[cvg]

DP103
we were writing a proposal for a project in Glendale on the New River and were toying with different methods to protect the abutment. However, this is a potential alternative for another bridge we are looking at in Buckeye as well as a flood control channel in Lake Havasu.

Is your manual available online?

 

[DP103]

cvg

I don't think FHWA SA 96-069 or FHWA SA 96-069R are available on line but Circular No. 7 is. It looks substantially complete to me although I have not reviewed completed. I believe they left the LRFD methodology out Circ 7. They may hove modified the facing design as well. Not sure yet. Here is the site:

http://isddc.dot.gov/olpweb.asp?WCI...title) LIKE '%CIRCULAR%'&fr=1&lr=15&rc=15&WCU

you can get there from this site too:

http://www.fhwa.dot.gov/bridge/geopub.htm

 

[DP103]

cvg:

There are several contractors in the Valley that specialize in this type of work. I design for most of them. I have design over 150 projects in Arizona over the last 10 years. There are certain drilling techniques we use in the Southwest that provide substantial savings. The scour protection project we constructed last year near Prescott was on Fain Road. Unfortunately, we were forced to build during the rainy season. The most experienced contractor in the Valley is Blindheim Southwest, Inc. dba Earth Reinforcing Specialists. You can find them on the web AZ SOS web site.

The manuals are useful but there is no substitution for the type of experience we have. We can provide preliminary budgeting and design for your projects if you like.

 

[cvg]

what project did you do on Fain Road? Was it the bridge over Lynx Creek?

 

[DP103]

cvg

I think it was Agua Fria River actually as I look at Lynx Creek (see link). It seem too close to the 69. Yavapai County was the owner. As I recall they had planned for a Gabion Wall but that required temporary shoring so we proposed to eliminate the temp shoring under the bridge(which would have been a soil nail wall anyway)and replace both walls with a permanent soil nail wall. It survived this winter and you know how that was.

If you want to make a field trip I could meet you there on my way back from Vegas next week.

http://www.mapquest.com/maps/map.ad...Ji+6LuMd0iufzELZZunbDMRt54lDco+QaOGCDs/+zrtw=

I haven't worked as far out as Buckeye yet. Did a project at the Tolleson WTP last year. There is an excavation at 83rd & 101 for a pump station 35 foot deep that was soil nailed and is still open. I walked away from it because the boring were only 12 deep before they hit refusal and I had heard that at depth there might be running sands and I hadn't done any work on that side of town. An out of state contractor took it on. Everything went well as far as I know. But you could look at that as well.

http://www.mapquest.com/maps/map.ad...Ji+6LuMd0iufzELZZunbDMRt54lDco+QaOGCDs/+zrtw=