Surchange required for low embankment ?

[qhtony]

Hi guys, have a problem recently in relation to a low embankment (~2m in height) road construction on highly plastic soft peaty clay (3m thickness).

As the 2m fill will only induce less than 40Kpa on the existing road, and the allowable settlement is 20mm, is there a need to surcharge the peaty clay?

If settlement will be a problem, can we apply some treatment to the clay (say mixing with sand)?? Are there any other ways we can tackle this problem instead of surcharge the area? Or would it not be a problem at all without surcharge/treatment?

thanks guys...

 

[fattdad]

If it's truly an organic soil (i.e., dry method LL <75% of the wet method LL), then you should be concerned about the secondary compression that'll follow primary consolidation. The only way to properly address this is to use a surcharge to "drive out" the secondary compression in advance of putting the road into service.

It would help this discussion to provide data on the soil layer (i.e., thickness, Atterberg limits, and consolidation terms) as well as the position of the water table. Then again, maybe all you have is SPT N-values and some moisture contents. . . .

f-d

¡papá gordo ain’t no madre flaca!

 

[STVU]

Peat is a problem. As a general guide, peat settles 10 to 20% of its thickness. So the 3 m thick could possible settle as much as 0.6m. It has little to do with the induced load. Peat decomposes and is constantly consolidating.

We have two choices-You could remove all 3 m and replace with suitable soil as fill and compact it in place in thin lifts. Or you could remove all 3 m of peat and place an EPS product. EPS is made of similar materals as your coffee cup but is very light weight. See link below:

http://www.geofoam.com/applications.asp

You will need help from a geotechnical firm to limit differential settlement.

 

[fattdad]

Option 3 is consistent with the original post - design a proper surcharge. If the peat is below the water table decay is not likely a factor. Secondary compression is the big factor and the time-line for secondary compression will be reset under the application of the new load. A consolidation test will provide you with C-alpha values for the proposed state of stress, which can guide the level of this concern.

Again, you can design a surcharge to deal with this potential, which would be perfectly appropriate for road construction. I would not pre-determine the need to hog out all the peat just to construct the road.

Then again, you could. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[civilperson]

Best results for modest expenditure will come from over excavating the organics from beneath your embankment and construct as a controlled fill. Scraper excavation borrow to fill can be as low as 2 dollars a cubic yard if your borrow is close.