40 ft of peat at tip of landfill

[cz]

The 40 ft deep of peat is between the toe of a landfill and a small creek. The landfill slope is about 200 feet long and parallel to the creek. The distance between the tip and creek bank is about 30 to 50 feet. The outslope of the landfill is 4H:1V.

10 ft deep x10 wide hardfill (concrete debris) with clay trench behind the hardfill was placed between the landfill and creek to cutoff any leachate path (based on preliminary knowledge that the peat was only 2 to 10 feet deep). After the installation of the hardfill trench, it is observed that the hardfill sunk about 3 feet and the creek bottom heaved up. The creek has to be excavated to maintain flow.

Now we need to build the contamination trench about 40 ft deep with hardfill in front or behind to cutoff any leachate flow and to save the creek runing. How to build the trench and berm in such depth and scope in a landfill site. I am considering two issues, one is constructibility, the other is safety of construction. Also, geotech issues such as displacement or in-situ operation methods other than removal and excvavation earthwork.

Any suggestions or thoughts or inputs will be very appreciated.

CSZ

 

[Focht3]

Wow! Forty feet of peat - now that's a problem!

A steel sheet pile wall looks to me to be the best overall solution; there are lots of things I don't like about this approach, but it seems to solve a lot of your problems while addressing the environmental and site safety issues. Removal and replacement of the 40 feet of material is a non-starter - too risky. The same goes for a concrete diaphragm wall.

A sheet pile wall has one big advantage - that can also be a liability. It's flexible.

What options have you considered?

 

[cz]

Based on two boring logs, the 40 feet of peat are underlaid by about 10 feet of very soft oganic clay over silty clay. The clay layer is under about 75 feet artesian pressure. As you may expect that the peat is alaways saturated with water which my also come from the landfill.

Focht3:
Thanks for your quick response of this posting. We do consider sheet pile wall as one of the options. However, our other concerns over the options also include the stability issues of the landfill. Current phase construction is increasing the height and push the toe of the landfill a litle further towards the creek direction. The multiples issues related to slope stability, settlement and movement of the peat materials, uplift of the creek bottom, leachate containment and collection, constructibility and cost, etc, lead us to examine any other engineering solutions.

 

[Focht3]

The situation you have described reinforces my belief that sheet piling - with "something" to increase the lateral stability of the wall - is the best answer. The artesian condition generally precludes the use of a diaphragm wall or drilled piers/caissons/bored piles. Frankly, I don't see any other viable approaches.

Lateral stability can be increased by installing some sheet pile sections perpendicular to the main 200 foot long alignment; they will act something like anchors or braces (depending on which side of the wall they are installed.) Or you can install one or more walers and large H-piles to increase the lateral stiffness.

There is no cheap fix for this one -

 

[BigH]

My first thought was of sheet pile wall, too. Thinking of it, I remember, somewhere back in the recesses of my mind, that there are some chemicals that can be mixed with poor soils (e.g. peat) which can make a wall. The name that comes to mind is fuicon (or something like that - it is Japanese I think from a couple of decades ago). I'll rack my brain (if I can get off it) a little more and see if I can think up the actual name - but this may be enough for some. You could then mix this with the peat (better if amorphous) and make a wall - but in situ with the peat.

Another "wierd" way is to make a structural channel for the stream bed in the 200 ft. whereby the stream bed elevation is supported on the underlying material. Use wood piles (always under water so won't rot), then a concrete slab atop with sidewalls. Do architectural stuff to make it look like a stream again. Sounds expensive but may not be - wood piles are not too expensive, normally. But, expensive - so is sheetpiles and it is only 200ft long - not that big. They do this kind of work for large rainwater sewer outflows in built-up areas (Rambo Hagar in Hamilton Ontario).

 

[VAD]

Hello csz: Interesting problem that calls for some thought.

BiG H: The Japenese product is fujibeton. I had discussions with representatives over twelve years ago. At the time we were do do a trial section in muskeg but it did not materialize.


Just a thought. Is it possible to place a culvert (csp)(steel) in the channel and berm over. This would have the effect of reducing the likelihood of upheval of the creek and would allow the creek flow to be maintained. Berm will also cause consolidation of peat.

 

[Focht3]

I thought about a culvert, too - but decided against recommending that option because of the likely movement magnitudes - vertical and horizontal - that would screw up the flow line, and probably breach the pipe. Remember that the landfill is still growing - and getting closer to the creek. (Current phase construction is increasing the height and push the toe of the landfill a litle further towards the creek direction.) And I have the same concern for BigH's timber pile and concrete channel approach, too. Both approaches would work fine if this cell of the landfill were effectively closed. But it isn't.

Having said all that, the owner may elect to try one of these cheaper approaches. After all, they are probably more concerned with cost than "fix it once and for all."

csz: If the owner elects to use one of the "cheap" methods instead of a steel sheet pile wall, be sure that you thoroughly document all the potential problems that could arise. You will need to cover your backside.

 

[VAD]

A welded steel pipe conduit would be satisfactory if installed properly.

 

[Focht3]

That would address the breaching problem. How would yousuggest addressing the heave/settlement/horizontal deflection issue?

 

[ANTHAE]

USER ASSUMES ALL LIABILITY FOR USE OF THESE OBSERVATIONS.

You might find a bentonite:cement:soil slurry wall is
cheaper than sheet pile, more flexible and leak-tight.
Figure about $200 a running foot, and no call backs.
As the landfill pushes closer, might use a geotextile-
wrapped native borrow berm on the slurry wall (trench),
you might have to excavate down the creek thalweg if it
heaves up after the berm is placed. Without a drainage
gallery in the advancing landfill, leachate will just
build up until it overtops the berm, so pencil that in.
No rigid structures will work in those soil conditions.

USER ASSUMES ALL LIABILITY FOR USE OF THESE OBSERVATIONS.

 

[Focht3]

A CB (cement-bentonite) wall is attractive; but I'd be concerned about a breach. I don't think the "self healing" characteristics could handle 3+ feet of differential displacement...

 

[BigH]

Focht3 - I thought of the heave bit after I had posted - thanks for pointing it out. Still, one could work up a system that would leave a void beneath the slab to accomodate later heave. Details could be worked out - but in the end, it is cost cost cost, eh? As a note, my old mentor recently wrote a paper about a landfill dyke failing - might be interesting reading. I will look up the reference and get back to you all.

VAD - thanks for help on the name; I was in the right ball park for a 17 year "lapse".

 

[Focht3]

[blue]BigH[/blue]:
Yes, a "movement independent" design could be developed - but as you point out, the issue is cost. And risk of failure is a concern to me. I look forward to hearing how [blue]csz[/blue] solves the problem -