Permanent Vertical Shoring with Near-Surface G.W.

[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?

 

[BigH]

Why not use concrete U-channels - drop them into place and backfill behind with local "marshy material"? Depth is not great - you can construct in short segments and "piece together" like hume pipes.

 

[cmpadres]

The marsh land is federally regulated and it is highly doubtful that they would accept a concrete channel bisecting their sacred land. It will be much more likely to be permitted keeping it a soft-bottom ditch.

Thanks though!

 

[oldestguy]

You say clay soil?

In my area marshes generally are overlain with peat of one type or another. I have seen dredged peat situations where the internal fiber structure of the peat holds it together nicely with a steep slope of cut, relatively permanently. Since the material is under water, its submerged unit weight is very low, especially if high in organic content (some peats float). Then, not much tendendy to slough in.

Consider leaving a steep natural slope under water. Certainly the restrictions can't be so bad as to require vertical side slopes

Can natural vegitation be used to reinforce this slope?

For excavation, I'd look at a dragline sitting on wood mats. Start out at end and re-handle the stuff to get to shore.

If cold climate, best time is in the winter. Much easier then.

 

[civilperson]

Aluminum sheet piling vibrated into place, capped and then excavate between for the channel. Very high class, permanent and used at lakeside home shoreline boundaries. Dig channel with crane and clamshell bucket, (70 foot reach with 110' stick

 

[GeoPaveTraffic]

civilperson's idea would seem to have merit, expensive, but with all of your constraints; nothing is going to be cheap.

I think you should reconsider the concrete U sections. You could create a soft bottom by placing the bottoms of the structers several feet deeper than the flowline and placing soil in the structure. These may be easier to transport to the site and place than the sheetpiles or solider piles and lagging.

Can the materials be brought in by barge from the discharge side?

 

[BigHarvey]

I would go for civilperson's solution, may be using PVC sheetpiles which will be less expensive and work with these heights. Look for C-LOC products from Crane Plastics Company or

http://www.c-loc.com.

Similar products exist in the Netherlands with cofra.

 

[fattdad]

. . . and do a flow net to make sure that you don't have a critical gradient at the base of the excavation. Otherwise, you'll have a disturbed subgrade and likely have instability.

f-d

¡papá gordo ain’t no madre flaca!

 

[reddog99o]

All above are good options. How about stone filled gabions? Easy to install and soft on the eyes. Very accepted by a variety of Environmental Groups.

R.A. Hassett, P.E.