Shotcrete Wall over Rough Rock Surface - Chain Link Fence Reinforcemen 1

[palmahouse]

Poll:

Has anyone used galvanized chain link fence, as opposed to welded wire fabric, as reinforcement for an engineered anchored shotcrete wall placed over a rough bedrock surface?

I figure that one advantage of using fence is that it is more "conformable" to the rough surface compared to stiff WWF. In comparison, WWF is stiff, and you need to go with two lifts of shotcrete if it is used.

However, the practice of using chain link fence seems "old school" to me.

Any other thoughts or comments would be appreciated.

 

[cvg]

it seems that the flexible nature of chain link may limit it's effectiveness in stopping cracks. However, I have not seen it used before.

 

[GabionGuy]

Look at hexagonal double twist woven wire mesh.

 

[DMcGrath]

Chain link fence fabric can "unravel" if broken, whereas hexagonal double twist woven wire mesh, a.k.a. "rockfall netting" won't. I would also think there would be a lack of consistency in the fence fabric manufacturing process such that it would be hard to know if it will perform the way you wish. Rockfall netting is manufactured to known standards.

 

[palmahouse]

Thank you all for your responses.

I spoke with a knowledgeable contractor and one engineering peer after I first posted. I have the following additional thoughts:

-Use of chain link fence as reinforcement can lead to poor quality in-place shotcrete, because you get "shadows" behind any reinforcement in shotcrete, and with chain link, you get lots of shadow compared to welded wire mesh.

-I confirmed from the contractor that in the past, engineers and contractors regularly designed/constructed using chain link fence as reinforcement for shotcrete, but engineers typically do not design with it for this purpose these days. cvg's response supports this.

-Chain link fence will only provide significant tensile resistance in one direction. There is little structural continuity in the orthogonal direction to give you significant resistance in that orthogonal direction. At this time, I am not convinced that double twist wire would have a significant advantage in this department (but I would need to research this belief further), and it would probably result in significant shotcrete shadow.

-Actually, the manufacturer of the material the contractor on my project intends to use advertises that their material meets ASTM standard.

-I am not sure that the unraveling issue is a big one for this application, because there in little chance of wire breaking and unraveling if a concrete panel against a rock surface completely embeds it. Although I agree with the implication that chain link fence should not be used to catch rockfall.

-Either way, I have some ammunition for discussions with my client. We will recommend that their contractor use a more conventional welded wire fabric for reinforcement, and that if necessary, the shotcrete should be placed in two lifts.

 

[henders1]

have you considered using steel fibre reinforcement as an alternative to mesh reinforcement or chain link, you would be able to design the sprayed concrete structure to span between anchor heads and make major savings in amount of concrete used due to the structure following the shape of the rock surface. it would also be much quicker to construct

 

[palmahouse]

Yes,
We did consider steel fiber, but ended up using synthetic fiber. Either way, some mesh reinforcement was warranted for this project because it was very difficult to get dowels in because the rock is intensely fractured (making it difficult to drill without shaking the whole slope down) and the site was not accessible to air track equipment (we used a jack-leg), so the bolts ended up pretty far apart. They were spot bolts as opposed to pattern bolts, and ended up an average of about 12 feet apart in both directions.

 

[SolidGroundEngineer]

I've used both chain link and rock fall netting. Chain link for temporary cut and netting for permanent cut. The netting works well for sculpted facings on nail walls.

 

[Patgeotech]

I have just conducted my own investigation into the properties of shotcrete. In a nutshell, I looked at normal shotcrete, fibre-reinforced shotcrete and double twist mesh in shotcrete and conbination of mesh-fibres in shotcrete.

The fibre-reinfored shotcrete and mesh shotcrete showed higher tensile strength attributes (as expected) - the best performance was from the fibre-reinforced shotcrete. The fibres were added in quite a low concentration into the mix (20mm steel fibres). The mesh contributed very little due to its flexibility (as cvg stated).

 

[PEinc]

Patgeotech, sounds like good stuff. Do you have a report available? Can we get a copy?

 

[Patgeotech]

PEinc, as soon as the project is completed, I will post the results in a table form on this post. The idea was to try convince my colleagues on the attributes of fibre-reinforced shotcrete in a cut rock slope scenario, rather than the conventional (time consuming) mesh application. We have been undertaking several lateral support jobs that have involved 3/4 basement construction in solid blue shale - Cape Town getting ready for the 2010 football world cup.

 

[PEinc]

Patgeotech, I look forward to your findings. Usually, I design with welded wire mesh unless I'm shotcreting over a rough rock surface. Then I switch to fibers.