Cracks to soil nailed concrete slope stabilization wall 2

[Ingenuity]

We have been engaged by a contractor who recently finished construction of a embankment/erosion/slope stabilization wall.

Max 1:1 sloped wall, about 100 feet long, and height of 20 feet. max.

Excavated face is basalt rock boulders, loose surrounding soil.

One single row of soil nails 6 feet from top, at 8 ft c/c.

Minimum concrete design thickness is 6", however, the excavated earth/rock race was highly irregular, so thickness probably varies to 18"+. Application via single and permanent shotcrete facing.

Lightly reinforced facing with #4@18", EW single layer.​

Shortly after construction (not sure of time line) there was considerable rainfall and cracks to the wall face are evident, with efflorescence staining etc. Relatively narrow cracks, described by the contractor as "fine". Cracks are typically diagonal, and no evidence of cold joints.

EoR is now requiring the contractor to fix the deficiencies - the cracks. Primarily so that efflorescence does not continue, and to reduce the occurrence of corrosion of the reinforcement. No mention of trying to structurally re-instate the integrity of the cracks.

We are 'arguing' that epoxy injection of cracks are not appropriate as, whilst injection into soil-backed concrete is problematic, any attempt to structurally re-connect the concrete will likely cause cracking at adjacent locations.

Both the highly textured evacuation face and the soil nails provide significant restraint to shrinkage and temperate effects of the wall. With only #4@18" (<0.18% reinforcement on a 6" design thickness) cracking was inevitable, and continued cracking is possible.

Polyuerthane injection has been raised, but depending on the technique used for PU injection, it may 'mud jack' the concrete face from the soil substrate, and also fill up the drainage board etc.

ADSC-IAFD has a Position Paper on the subject of b]Joints on Soil Nail Walls[/b], and it discusses joint spacing, minimum reinforcement, and crack expectation etc.

What is the typical treatment/repair of such cracks to concrete facing of soil nail walls?

There is no aesthetic requirement, except the wall face will be 'color-stained' to blend it to adjacent nature-scaping.

 

[oldestguy]

What about slump, W/C ratio, etc? Any curing steps? Any "sliding", "slumping"?

 

[Ingenuity]

Concrete supplier has been involved and concrete supplied met/exceeded specification (f^'_c, w/c, slump etc). Cores are being taken in case this "goes legal".

No sliding, slumping or sloughing of the shotcrete during placement. Experienced personnel applied the shotcrete.

Curing - not sure - most probable it was a liquid-applied membrane.

 

[Ingenuity]

I did find FHWA Soil Nail Walls Reference Manual Link and it states min reinforcement ratio of - assuming 4,000 psi ocncrete and grade 60 rebar:

So as-designed #4@18" does NOT meet this minimum criteria.

 

[Phil1934]

A single row of anchors is a tied back wall, not a soil nailed one. The rebar is inadequate by design, not minimums based on volume.

 

[MTNClimber]

Ingenuity - I agree that the reinforcement seems to be too light. I have a feeling more will continue to develop as time goes on. Do you have an idea of the crack widths, besides "fine"? Are talking hairline, 1/8", or more?

Phil1934 - This sounds like its not a wall but a reinforced slope. I wouldn't really agree with your definitions. It doesn't matter how many rows there are. Tiebacks are pre-stressed and locked off while soil nails are passively engaged. I would guess these are soil nails since they are typically used in these scenarios.

 

[PEinc]

Phil1934 made an important comment about this not being a soil nailed wall or slope. A 20' high wall or slope with only one row of widely spaced "nails" seems to me to be pushing soil nail technology and slope stabilization a bit too far. Slope stabilization with tieback anchors and no soldier beams or sheet piling usually involves large, reinforced concrete anchor blocks at each tieback anchor. I am not surprised that the described concrete facing has cracks. I am surprised that the cracks are fine and not gaping. Are the nails encased inside the concrete or are they exposed and bearing against the facing? I would be surprised if the wall/slope does not need more nails.

http://www.PeirceEngineering.com

 

[Ingenuity]

Thank you all for the responses.

I have a conference call this afternoon, so hopefully I obtain some answers to my questions like crack width, etc.

The single row of nails are definitely "passive" and not stressed.

The soil nail anchorage plate and nut is encased in concrete, therefore not exposed. Which makes it even more troublesome as there is no local thickening around the nail head to transfer forces from the facing to the nail.

The FHWA Ref Manual states an interesting way of calculating reinforcement ratio, ρ, it uses half of the facing thickness, h, and not the gross area of concrete, as traditionally used by Codes and Standards for reinforced concrete min reinforcement requirements:

Does anyone know the background to using 0.5h?

 

[GeoPaveTraffic]

Were any provisions made for drainage behind the shotcrete facing? If not, that is another possible source of the current and future cracking.

Mike Lambert

 

[PEinc]

It sounds like there is drainage behind the concrete facing. "Polyuerthane injection has been raised, but depending on the technique used for PU injection, it may 'mud jack' the concrete face from the soil substrate, and also fill up the drainage board etc."

http://www.PeirceEngineering.com

 

[Ingenuity]

Yes, 12" wide drainage board strips at 8' c/c, vertically spaced.

 

[hokie66]

6" thick, 20' tall, one row of soil nails near the top, This cannot be considered to be a structural wall. The shotcrete will provide a degree of erosion control, but only until it breaks up.

If there effective diversion drainage at the top of the slope?

 

[MTNClimber]

I'm dying to see the plans on this. This thing sounds ridiculously underdesigned.

 

[Ingenuity]

Here is a section from EoR:

I assume the soil nails are hollow-bar rods, but not completely sure. I am only involved with the facing cracking issue.

 

[ATSE]

Instead of using the term wall, it may be better to design this construction as Facing.
The "wall" shown in the figure above has only minor out-of-plane strength and stiffness.
The Facing (cementitious liner) will control raveling / surface erosion. If the excavated face is competent rock, sloped 1:1,with a robust drain system, stability is not a concern, and the construction may function as intended. It just won't be pretty.
Fixing the efflorescence will be very challenging.
Is it too late to back out? I don't think I would want my finger prints on this project.

 

[MTNClimber]

We obviously don't have all of the information here but the overall stability of this slope may be fine. It's a little light on nails (IMO) and slab reinforcement but it doesn't mean it won't work. The only thing I can think of for addressing the cracking issue is to dowel and pour a new concrete surface, which I know is the contractor's last resort. My gut is telling me that doing area specific fixes won't address the overall issue or being under-reinforced and more cracks may develop. I would ask to review the reinforcement calculations to see where this engineer got their value from.