Soil Nail Wall

[DRB71]

I am involved in a project where a permanent soil nail wall was recently constructed, wall height ranges up to 20 feet, the permanent facing is trowled shotcrete. The retained soils are c-f Sand, some to trace gravel, some to trace silt, cobbles and boulders are numerous. Groundwater is within the cut depth.

There a two potential performance issues; 1) seamingly random hairline cracking of the permanent facing; and 2)moisture appearing on the face of the wall.

The cracking looks like shrinkage cracking. Could this be caused by improper control joint spacing? Could there be another cause of the cracking?

The design engineer claims that osmosis is the reason that moisture is coming through the wall as it is pulled from the always wet drainage strips, and this should have been expected. I'm of the mindset that a wall designed for permanent use should not have moisture coming through the permanent facing for fear of spalling during freeze/thaw.

Any thoughts or comments are certainly appreciated.

 

[dcarr82775]

Could be the mix was too wet. Another potential cause is the detail between the temporary and permanent facings. The permanent facing could have bonded or be hung up on the temporary facing effectively restraining the shrinkage. If that is the case the control joints are little more than wishful thinking (sometimes works sometimes doesn't).

The osmosis thing sounds a little hoaky to me, but exposed walls below water table will almost certainly leak. A couple of drain strips doesn't seem to me like it would cut it.

 

[USGeotech]

I have designed and constructed many permanent soil nail walls. Hairline cracks are very common in the permanent concrete facing. Assuming the reinforcing in the facing was designed per GEC 7, then I would expect some cracks. Assuming the reinforcing is designed correctly, it should be sufficient to hold the cracks "tight". The spacing of the control joints in a soil nail wall is no different than any other concrete wall.

Is the moisture on the surface due to condensation or are there wet spots on the facing?. Similar to slabs-on-grade in buildings, when the temperature of facing falls below the dew point, water vapor will condense on the facing. Since water vapor is almost always present in the soil, and sometimes present in the air, the facing can “sweat” on both the exposed surface and the surface in contact with the soil.

Wet spots are more troublesome. They can occur at the nail head even though the nail head is embedded in the facing. If the location of the wet spots is similar to the nail pattern and spacing, it is likely due to seepage around the nail grout following the "preferred" drainage path.

If wet spots are more wide spread, the drainage fabric may not be performing properly. I've also seen contractors install the drainage fabric properly but fail to connect the fabric to the collection pipe at the wall base or fail to connect the pipe to the site drainage system.

 

[PEinc]

I have also seen contractors install the drainage material backwards with the solid plastic side facing the soil - even on jobs with full time inspection.

http://www.PeirceEngineering.com

 

[FixedEarth]

You mentioned soils having numerous cobbles and gravel, but FHWA soil nail manual mentions it as not a suitable material. Were you able to pass the performance tests?

 

[DRB71]

Thank you for your posts, and validating my suspicion of the osmosis theory.

The soil is dense glacial till and the performance tests all passed. Also, no problems during materials testing of the shotcrete facing.

The areas with moisture on the facing appear random and localized, more like the wet spots you describe. Some wet spots could be at nail head locations where the moisture appears to eminate from a point and run down the wall. However, a few wet spots run horizontally.

If groundwater is seeping around the nail heads, is there a structural concern and if so, is there a fix?

I was concerned about the construction and proper functioning of the strip drains and toe drain. However, it sounds like the wet spots would likely be more wide spread if the drains were not functioning.

 

[USGeotech]

Provided there is no movement of "fines" along with the water, the seepage should not be a structural concern. The filter fabric backing on the drain mat will prevent most migration of fines. However, it is dfficult to check whether the fines are migrating along with the water. If possible, check the drain outlets or clean-outs for a build-up of "silt" in the bottom of the drain pipes. Since the volume of water typically coming out of the drain is small with only a low flow velocity, the silt tends to collect in the bottom of the pipe.

Are the horizontal wet spots at the same elevations as the joints in the temporary shotcrete facing?