bearing capacity for fill over muskeg

[gwkwong]

I wonder how to estimate the bearing capacity for clay fill over muskeg with woven geotextile in between. Clay fill is between 4 to 9 feet and muskeg is about 3 to 6 feet. Liner is Nilex woven geotextile W300.

 

[oldestguy]

Quite likely the unit pressure from the fill will be such that compression of the muskeg will control the overall situation as to total settlement taking place. Thus,the bearing pressure from a footing on the fill is more the capability of the fill than how this pressure affects the lower muskeg.

In short, settlement caused by the fill would be your main concern, rather than a "bearing capacity" of a footing on the fill.

In these n stances, I usually neglected any increase in vertical pressure in the "muskeg" resulting from a footing on the fill, since I routinely surcharged the site, over and above the loadings from the long term fill alone. The slight excess vertical pressure from the footing in the top of the muskeg had incidental effect on the overall situation.

 

[MiketheEngineer]

What is "muskeg"???

 

[PST09]

FYI

'an acidic soil type common in Arctic and boreal areas'

http://lmgtfy.com/?q=muskeg

 

[gwkwong]

oldestguy,

My Client plans to put some frac tank on the site for about 2 months only. There would be a lot of flexiblity to the piping. Settlement is not a concern, which they did that before without engineer involve. I am trying to find articles about the bearing capacity for fill over muskeg with liner reinforcement. Somehow, I am out of luck. Any suggestions?

 

[dik]

PST09... you might have added that it is highly organic and highly compressible. Any loading would likely have to accommodate large differential movement. The depth of muskeg can vary from a few feet to 10' or 20' or more.

Dik

 

[PST09]

Dik,
Noted, thats quite an important bit of information to leave out, sorry about that MiketheEngineer!

PST

 

[gwkwong]

They put all together 1.5 feet thick of wood matting on the surface. The clay fill is about 4 to 9 feet, the muskeg underneather is about 3 to 6 feet. The soils are partially frozen because they construct the site during winter time and frozen fill was placed.

 

[oldestguy]

I'd suggest that you have a consultant who is familiar with the compression properties of muskeg using some basic tests, such as loss on ignition and moisture content to estimate the settlement you will get. Apparently this is in Canada and you have much information on muskeg in publications from your government. Perhaps BigH can steer you to a source. My old info shows the "Muskeg Engineering Handbook" as far back as 1969. They must have a newer edition now with good info in it. I did not find it with a Google search. However, thread 260-28620 has some info.

I'd also ask the consultant to give some evaluation to the mat you will have as to whether or not it will prevent a local slip of the edge of the work. Placing the fill over a considerable distance out from the tank is a way to minimize that slip potential, but will contribute some to the total settlements.

In this case, with your loads at least a rough estimate of the total and differential settlements that may occur and the approximate timing of them.

Of course if you have the time for a pre-load, if only for a few weeks, you may find a significantly better situation.

 

[dik]

You need a good geotekkie. For 3-6' of muskeg, I'd probably have excavated it using a backhoe and front loader working in conjunction with each other and filling the area with large stone. As far as frozen fill... not a good thing, generally, but depends on the material... again a good geotekkie.

How old is the oldestguy...

Dik

 

[oldestguy]

Oldest guy just turned 81

 

[BigH]

And he's a Big Red fan!!! Happy Birthday! I'm turning a new year in a couple of weeks.

One of the things that was not indicaed was whether the peat is fibrous (containing copius amounts of woody fibres) or amorphous (silt like). This does make a difference on the shear strength of the peat and on the settlement. Fibrous peat would have higher shear strengths. Also, if you wait a couple of months for the peat to settle under the fill, you will get an increase in shear strength due to the consolidation. I would estimate that you should have Su about 10 kPa after the fill has been placed. You didn't say if you were in permafrost or not. If you have muskeg, much of the muskeg should be frozen most of the year.

You also have the corduroy strengthening of the wood planks. Old time road construction through muskeg and paludal deposits (swamps) used trees laid down for stabilizing.

http://web.mst.edu/~rogersda/umrcourses/ge342/Alcan Highway-revised.pdf

See also:

http://www.roadex.org/Publications/docs-RII-EN/2_5 Roads on Peat_l.pdf

Su of 10 kPa should be stable for about 2.5 to 3m of fill which is more than you have put on. Given that the tank will be of short term use and you will construct the tank well after the fill is put on (as I presume) and, as indicated you have flexibility with the piping, my gut reaction is that you should be okay with respect to edge shear failure - you could always put on a 5 ft wide stabilizing berm half way up the clay fill if you are worried. The Muskeg book alluded to by OldestGuy is still suitable - true and tried throughout Canada's north.

 

[BigH]

Another reference:

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=AD434077&Location=U2&doc=GetTRDoc.pdf

 

[oldestguy]

BigH:

I knew you would come thru with good info.

I particularity like the Roadex II info. I surely would have liked to have something like that in hand when I first got into surcharging and related work. I notice that some of the info is very similar to my experiences. One modification to the construction of roads thru swamps with surcharge and excavation as compared to the example shown. We (when I was in Wisconsin DOT)really used a high surcharge at the head end of the fill rather than holding the fill at final grade. Got much more in the way of an intended slip that way.

Thanks for your great source of these things.

 

[BigH]

oldestguy - that must be a "rolling surcharge" (your WIDOT)- forming a slip to displace the organic out of the way. (It is referenced in Tschebotarioff's 1953 book). I used the same for displacing tailing slimes - we built up a few feet of sand fill - the dozer ran up on it and caused a failure - displaced 39 ft of slimes that way. My mentor used dynamite to remould silt to permit similar to be done for the Rainey Lake Causeway in Northern Ontario.

 

[oldestguy]

Yup, rolling surcharge. Actually it was copied from Michigan's methods. I got it started on WDOT jobs by urging a contractor and the DOT job engineer to use it on a 26 ft. deep peat area and it worked. It probably would have been adopted anyway, but "taking the bull by the horns" and not asking "higher ups" first did the job.

That "rolling" procedure for surcharging on building sites also means re-handling the earth, but at least the full quantity for surcharging the whole site at one time is not needed. The highest I've used is 25 ft. on a 70 ft. deep site. 10 ft. is more common.

 

[ecanuck]

Gwkwong just wants to put a tank, no problems with settlements. Some geotextile, a few feet of fill using light equipment - the biggest concern is support for equipment, I should think. The actual weight of the tank and footprint was not mentioned, but it would be nice to comment on that.

BigH: I wonder if your mentor is the same as mine? I was involved with a rolling fill displacement in Saint John in the early 70's. A 30 ft high fill displacing some organics for a marine terminal landfill.

 

[gwkwong]

the tank is about 8.5' wide x 10' high x 45' long . About 6 of them sitting beside each other with 3' apart. Muskeg handbook mentioned that the "safe" bearing capacity is about 1500psf with softclay/muskeg total thickness being less than 15' thick. For greater than 15', the "safe" bearing could be about 1000psf. I was trying to see if there is any empirical method to calculate a complex system like what I mentioned.

 

[ecanuck]

Well Gwkwong the bearing capacities mentioned by others verify that you would have no problem and I think you are o.k. IF:

1 The muskeg/fill combination can support a decent dozer and loaded trucks.
2 The clay fill is good - not soft can be placed, and it also supports the dozer OK. (If the clay is placed frozen and then it thaws, and then it rains, you will have a messy situation.)
3 The tanks, connections and piping can support some inches of settlments, including differential settlements, for the time of construction which I assume would be only some months. I assume you are fracking with mud? The loads under the tanks would be 1000psf or so. That means the peat will compress rapidly once it thaws. But under the clay it may not thaw for some time.

Hope this helps.