levee construction 1

[cvg]

I have a levee/river bank protection project in the southwestern United States where a gabion bank lining will be constructed on a normally dry river. This will protect against erosion during periodic flows.

There is some base flow and ground water is maybe a few feet below the surface. The bank lining requires a "toe-down" below the surface to protect against scour. Construction will require excavation, compaction and placement of fabric and gabions or some other type of lining material below the water table. We are trying to determine methods of carrying out this construction, possible costs and other implications.

Existing ground is a mixture of sand, gravel and silt with very little cohesion. Very few boulders exist. All is deposited in layers by periodic flows in the river. Bank slope will probably be 2H:1V or flatter. I prefer not to cutoff or prevent subsurface flow from entering the river, as this is considered "Waters of the US", and doing so would have an adverse effect on the environment.

Ideas we have considered:
1. Dig trench and install sump pumps - cheap but how effective?

2. Install sheet piling, dig trench and install sump pumps - expensive

3. Install wells - expensive, but how effective?

4. Launch the gabions into a wet trench with crane - expensive, difficult?

5. Install sheet pile cutoff at the toe and no excavation - not environmentally friendly

This is a planning study at this point, and I would appreciate any input on cost, feasibility, etc.

 

[Focht3]

A 2(H):1(V) in sands, gravels and silts is very steep. Excavating at the toe of this slope involves considerable risk; you need a competent, experienced geotechnical engineer to assist you with this project.

A wet excavation will be the cheapest, but may be difficult because of the silts clouding the water and obstructing the contractor's / inspector's view of the work in progress. Not to mention the risk of triggering a slope failure.

Sump pumps are pretty risky to me - I've been burned by this approach. Go this route only if you have a bonded, experienced contractor willing to do the job on a performance basis (i.e. he will dewater to a certain elevation for a certain period of time.)

Permanent sheet piling will work, but expensive. Temporary sheet piling can be risky - sheets can damage gabions during extraction.

 

[Prepakt1]

The problems I see with launching gabions into a wet trench
is getting a suitable even base in a slurry type river bottom. Possible solution is to use fabric formwork as your scour toe or base and partly up or all the way up the bank and then cover with gabions or just use sufficiently thick concrete mattress. Fabric formwork will eliminate any water pumping requirements.

 

[BigH]

With respect to the sump approach - this may be problematic as Focht3 says if the excavation is made all at once . I might suggest you could try a leap-frog approach. Open up only 10 to 15m of the channel - sump it out while you get in your gabions, then close it up. At the same time, you would do this at other locations say starting at 40m away. After this initial excavation/placement, you then open up another 10 to 15m sections, finish it off, then the remaining section. This way the "slope" isn't opened for much width and the overall slope may not even "see" this excavation. I've done this with tailings dam stabilizating berms.

 

[DRC1]

I would not try to do the gabions in the wet esp. if you have to do compaction. To better answer your questions, a few details are required. One how deep are you going below the water? how much silt is in the soil? If you are only going below the water a couple of feet, you may be able to over excavate and place crushed stone to avoid compation. Then just work in the wet. I have used sumps a lot with success. The trick is not to go too far with your excavation. the problem with sump pumps is that on some projects they need to run 24 hours so you need electricity. Some require 480 3-phase. I would consider wells if you are going fairly deep with the excavation. I don't see why intalling sheeting in back of the wall is enviornmentaly unfriendly. It may solve your other problems but I would also caution you that pulling them may cause some settlemet of the gabion wall.
Let us know how you make out.

 

[GabionGuy]

cvg,

Installing gabion linings into wet channels is done on a routine basis. The type of equipment available will dictate the amount of gabion installed per cycle - large capacity cranes will be expensive, but allow you to install more m2 per cycle. Smaller cranes are cheaper, but not as productive. Gabion/Reno mattresses are assembled and prefilled in the dry and placed into position.

Because the gabions are a flexible structure, they will conform to the subgrade.

Depending upon the depth of water, divers are also frequently part of the equation.

Contact the Technical Department at Maccaferri, Inc. They will be able to provide you with additional design assistance.