Permanent Vertical Shoring with surface water 1

[cmpadres]

I am designing a flood channel that will outlet in an extremely environmentally sensitive marsh area. The portion of my channel that must extend into the marsh will be a trench approximately 10 to 15 feet wide and 300-feet long. Do to the environmental concerns, I want to minimize my footprint within the habitat areas. The trench will be excavated/dredged 4 to 7 feet deep, and surface (or near surface) water is expected during construction and through out the design life. The soils investigation is not complete yet, but a generally clayey soil is expected.

I am looking for a way to permanently retain a vertical excavation for this trench. The requirements are:
1) The trench must be soft-bottom (e.g no channel linings like concrete)
2) Disturbance outside the ~10-15' base width must be minimal (i.e. no reinforced earth strips extending behind the retaining system)
3) Need to be constructable in a manner that is minimally invasive to the habitat area.
4) The entire retaining system will likely be underwater - at least partially - for its entire life.
5) Because of the environmental sensitivity of the area, the area will be considered unmaintainable, so I am looking for a durable, maintenance free installation.

My first thought was a Soldier pile and concrete lagging type installation, but have read that these don't work well for installations with Ground Water.

Does anyone have any recommendations?

 

[cvg]

how about gabions, possibly aluminized or pvc coated? Can be stacked vertically or near vertical and can withstand the expected settlement and movement of the clay. Also can be backfilled, planted so that it will appear part of the environment.

 

[civilman72]

Have you thought about using a three-sided steel-arch pipe? Once you cut your trench, you can pour small footers, build pipe and then backfill over the pipe; the finished bottom would be soft. So long as you can fulfill your capacity reqs., I'm almost sure you can get these steel arch pipes dimensioned at 15' wide by 7' high. I also don't imagine it would require any maintenance, so long as erosion/sediment control is dealt with upstream.

I think the idea of gabions would also be feasible, but from my experience gabions placed along turbulent water do require maintenance down the road.