Protecting the MSE retaining walls from Ultraviolet light 6

[AlaaElnahas]

I am designing the wing walls of a 2.5m*2.5m square culvert below a road in the northwest of the UK. The road side slopes are 2:1. The wing walls has a height that start at 4m and goes down to 0.0m at the end of the embankment slope. The wing wall inclination to the horizontal is 70 degrees, and the client want it to be made from MSE with a rape araound facing. That means I will use geogrid layers. The culvert life period is 120 years. So, the wing wall life period should be the same, but may be reduced to 60 years because with slopes not steeper than 70 degrees, it is still considered a reinforced earth by the British Highway Agency (HA).

I am worried about the effect of the ultraviolet light on the raparaound part of the geogrid. The makers of the geogrid (e.g. Tensar and Huesker) say that the geogrid layers include black carbon which protect the layers from degradation together with the vegetation. Prof. Das book "Principles of foundation engineering" says that the exposed wall face MUST be covered to protect the layers, and the wall face should be sprayed by bitumin emulsion or gunite, with a wire mesh anchored to the geotextile to keep the coating on. My questions are:
1- Do you have an advice about how to protect the reinforcement in the long term (60-120 years)?

2- Do you think that vegetation can grow on a 70 degree slope and protect it (together with the black carbon in the reinforcement layers)from the Ultraviolet rays? and if the answer is yes, what would the situation be during relatively dry seasons

Best regards
Ala'a

 

[GabionGuy]

I suggest you contact Linear Composites for their input.

As a British manufacturer of geogrid, they have substantial experience with the type of structure you are designing.

I hope this helps.

 

[AlaaElnahas]

Thanks Gabion Guy. I will contact them. Two points I forgot to mention. They are:
1- the culvert will work like an underpass for sheeps below a road. So, the sheeps can eat the vegetation.
2- Due to some reasons, the ground water level is at the foundation level for the culvert at one end and 0.45m above its foundation level on the other end of the inclined culvert. Therefore, unless someone is brave enough to change the height or the foundation level of the culvert, the MSE wall bottom reinforcement layer will be 0.95m below the permanent ground water level in a sand and gravel soil layer.

 

[BigH]

First - I don't think that sheep root out the vegetation roots when they graze. Second, there should be no problem with the grid being underwater. You can confirm this with your industry contact.

 

[TDAA]

Your initial post discusses two different materials: geogrid and geotextile. These materials can be quite different in composition and behavior.

Talk with local suppliers to get recommendations on materials that are suited to your project. Just be sure to get complete technical information on the product so you can better understand the properties you are dealing with.

I also recommend you download the following:
Reinforced Soil Structures Mechanically Stabilized Earth Walls and Reinforced Soil Slopes Design and Construction Guidelines 2000 FHWA-NHI-00-043

and

Reinforced Soil Structures Corrosion/Degradation of Soil Reinforcements for Mechanically Stabilized Earth Walls and Reinforced Slopes 2000 FHWA-NHI-00-044

at:

http://www.fhwa.dot.gov/engineering/geotech/library_listing.cfm

 

[PEinc]

Does anyone actually think a wrap-around geotextile or geogrid facing will last 120 years? Besides UV deterioration, the wall could be especially susceptible to damage from other applied forces. A wrap-around facing is not very rugged. I'd look for a different type of facing and tell the owner to expect a shorter design life.

 

[cvg]

it sounds like only 60 years is required for the walls, however I wouldn't count on exposed fabric or grid even lasting that long. Even the gunite with wire mesh may not last that long, especially with freeze thaw and if drainage behind the gunite is not controlled. Gabions might work, if corrosion of the wire can be controlled. Dry stacked rock might be more foolproof if done properly.

 

[GabionGuy]

While starting somewhat of a tangent to this thread, PVC coated galvanized wire double twisted mesh has been successfully used in normal applications for over 50 years now. If there is any questions what so every about corrosion, it would be irresponsible for any engineer to use anything other than PVC coated wire. I have seen galvanized structures last less than 2 years in a watercourse because someone wanted to save a few pennies.

You pay a slight premimum for the material, but you get a lifespan easily 75 years+

 

[AlaaElnahas]

Thanks Gabion Guy for your contribution. Is the PVC coated galvanized mesh for making reinforced earth or gabions? If for reinforced earth can you tell me who are the suppliers.

 

[GabionGuy]

Contact Maccaferri. PVC mesh is available for all double twist wire materials (rockfall mesh, gabions, Reno mats, MSE systems).

These systems are also routinely combined with geogrid for MSE and RSS applications. Whenever the mesh is not strong enough, geogrids make for very cost effective structures.

 

[blueoak]

I've enjoyed the Maccaferri presentation, but I am a little wary of a coating providing significant corrosion resistance, especially when you are filling it with rock and putting it in in a waterway with large debris floating by. I think claiming a 75+ year life is really site dependent. Sometimes in shipping the pvc coating gets ripped and if you don't see it right away chances are it will get buried or a little paint makes it almost impossible to find on a large site. Then again, many (if not most) sites aren't that corrosive. We checked for old culverts to see what shape they were in.

 

[cvg]

blueoak

it sounds like this is a sheep crossing, not a drainage culvert. Unless I have misinterpreted the posting, no water will flow through this.

 

[AlaaElnahas]

Thanks CVG. The bottom of the sheep crossing underpass (Culvert) is below the ground water level. This is a mistake that was not created by me, but I have to leave with it. The first reinforcement layer is 0.5m below the ground level at the underpass exit. Therrefore it is 0.95m below the ground water level.

 

[cvg]

so you may have some water in the underpass. But will it carry surface water runoff during a rainstorm? If so, then erosion control and floating debris will need to be managed by your structure.

 

[blueoak]

cvg,

I should have referenced my post. I was commenting on GabionGuy's comments on Macceferri's PVC coated products.
"You pay a slight premimum for the material, but you get a lifespan easily 75 years+"

Thanks,

 

[GabionGuy]

blueoak,

Every project should be looked at on it's merits. In some instances, the PVC coating may be insufficient on its own and the Engineer/owner may want to look enhanced polymer coatings or perhaps a PVC + galfan wire material.

The original PVC coated wire structures were built in the early 1950's and are permanently exposed to salt water and wave action. These structure are still intact and performing satisfactorily.

The 75 year life is a general expectation based upon ongoing monitoring of in situ structures, testing of material properties of built structures and accelerated testing in order to determine expected lifespan of a structure.

A typical galvanized wire structure siting in freshwater in Northamerica will be lucky if it survives 15 years. I know structures that are much older in that situation and are in good condition, but I am also aware that in other parts of Northamerica, people are lucy to get 10 years out of the structure.

 

[johnthewallguy]

The facing of a reinforced earth slope (batter less than 72 degrees) is a compaction form-a non-structural element only. The structural integrity of these wall is in the geosynthetic reinforcement within the fill. Again there is nothing structual about the face of any MSE structure. Design the wall with a secondary wrap and a primary reinforcement. Remember, the Great Wall of China was reinforced with horizontal layers of straw, and its doing fine!

WallGuy770