Erosion in Median

[wolfhnd]

We are building a 40 mile long guard cable project. I'm concerned about the possible erosion around the post during the period before grass has been established. The Soil is a silty clay with a liquid limit around 50. Standard ditch checks will be used. I had a previous project that required the use of blankets to prevent the erosion of loess soil which is highly erodible. We do not anticipate that this soil will erode as readily but any erosion that does happen will be difficult to repair with the post installed. Give me a hand here I want you thoughts. I plan on doing some calculations but normal procedures do not really apply. I'm assuming that the ditch checks work as detention storage but I don't have time to make this a research project and there are a host of things to consider. The actual physical properties of the soil are not even readily available from the standard soil data such as liquid limit and plastic limit.

 

[Focht3]

Is erosion common in the area? Look at any slopes and ditches along your alignment for long-term erosion problems. That's a cheap, fast initial check.

Soils that are easily eroded sometimes include dispersive soils, particularly if the soil is a moderately plastic clay. You can run a quick check on the soils to see if they may be dispersive. Get the soil samples already taken - run crumb tests on them. The crumb test checks for dispersive behavior. If you get even 20 percent positive, then you should expect problems. The test isn't perfect, but if it says you have a problem - you have a problem. No false positives -

Read this document to get a handle on testing dispersive soils - includes the crumb test:

http://www.usace.army.mil/inet/usace-docs/eng-manuals/em1110-2-1906/a-XIII.pdf

Standard lab tests are of no use in identifying dispersive soils. Crumb tests are cheap - you need the soil samples, distilled water and smooth, clear plastic cocktail glasses.

 

[cvg]

you could expect clay to withstand velocities up to about 5 feet/sec. Increase this to maybe 6 or 7 feet/sec with the grass (depending on the type of grass and thickness of cover).

How long will it take to establish grass, and how many times do you expect to see rain with runoff and erosive velocities before then?

do a risk analysis

1) determine probability of failure ie: erosive flow occurs causing erosion damage

2) determine consequence of that erosion - what will it cost to repair?

risk = probability X consequence

compare that risk to the cost to provide the blankets or other erosion protection

 

[wolfhnd]

The project will be let in the fall so most of the work will be done in the winter and spring. That means no grass until spring. The agrominous wants to plant native grass but I don't see how that will work. The mulch will have to be placed in the winter and by summer there could be temporary wheat and mulch in the way of the native grass. If we install mats they would have to be removed to plant the native grass defeating there purpose. There also will be rils and rivelets that could make reseeding next to the guard cable difficult. Mowing cost are almost double for guard cable sections of highway so that is a definite consideration in the selection of native grasses. Sod may be the only way to get a good stand of native grasses but the cost could be close 15% of the project for just a narrow strip next to the cable. Other areas have used rock but we are afraid that the contract mowers will throw even more rock at cars if we go that route.

 

[cvg]

It sounds like there will be a period of 1-2 months where the ground is unprotected. Assume remove mats or temporary erosion protection as soon as the ground temperature warms up sufficiently to plant. Apply pre-emergent weed killer to kill anything already growing. Then hydroseed with native grass mixture. (Rills wont matter if you hydroseed.) Mulch and fertilizer can also be applied with the hydroseed mixture. Wait for grass to grow.