cofferdam 1

[rhymn]

Hello all,

you guys do great work!

Contractor has lift station to place with in 60' of river. Water table at 4.9'. Very dense sand and gravel 50/4" at 5'and 15' remainder is 25-35 blow count sand/gravel with one clay layer at 20-23'.

Lift station 96" to be placed 19' deep.

any thoughts on alternative to sheetpile as not driveable through the hard layers.

Thank you.

Ted

 

[PEinc]

Could the contractor possibly probe and loosen the top hard layer (50/4" materials) using a backhoe? Then, the sheets could be driven.

 

[rhymn]

I don't think that will work as last layer at 15.5' depth, and the edge of this hole is located within 5' of existing liftstation.

Only other thoughts have been to erect a caisson and dig from within to allow to drop with excavation, as it is not felt soldier pile walls could seal water well enough.

 

[DirtDr]

Maybe you should consider a soil mixed wall or a jet grout wall. They are a little more expensive, but will penetrate most obstacles and will form a relatively tight seal. Either of these options may need tiebacks, but with the jet grout you might be able to insert a wide flange that is stiff enough to cantilever 19-ft.

 

[PEinc]

This excavation seems to me to be much too small and too shallow for any sophisticated excavation support system. The most likely solution may be to install a drilled sump or deep well, dewater, and then use vertical timber shoring and timber wales - you know, do it the old fashioned way, the way sewer contractors do it.

 

[Focht3]

Why not a "sunk caisson" approach? Use open-end RCP, mechanically and hydraulically mine from within. Pipe weight will advance the pipe. Use a convenient pipe segment length. When you get it to grade, allow the water table to reach equilibrium; clean the bottom, if needed - then plug it up with a high slump concrete.

There are risks: loss of ground, out-of-plumb installation, leaks. But these are things an experienced contractor can deal with. This is one hell of a lot cheaper than sheetpiling, grouting, dewatering, in-place improvement, etc.

Good luck!

 

[geomo]

What about vibrating the piles in place? There are some really powerful vibrators, some excavator mounted, that might work nicely in your saturated, very dense, cohesionless soils. The vibrator would create a temporary quicksand condition at the pile tip, considerably reducing the penetration resistance. I'm not sure how well it works in the extremely dense stuff, but the N=25-35 stuff should be no problem.

 

[rhymn]

Thank you all for your helpful insight-

PEInc- They may try soldier pile, will depend on river level, and their dewatering system.

Focht3- That was a thought of ours as well, but may be difficult to find that large of a diameter pipe in our area. When you talk about hydraulic excavation do you mean water jet?

DrDirt- I don't have a cost of that method, but think PEinc is correct that maybe to expensive in Montana.

geomo- Sheet pile manufacutres say they wouldn't even try to drive 1" thick pile in this mat'l, although may utilize combination of all methods- sheetpile, that is excavated from within to allow to drop, and drive though soft material.

I'll let you know what happens.

thanks again

 

[BigHarvey]

You should use a CFA piling rig with a 400 mm auger to loosen the soil down to the last resistant layer and then drive your sheetpiles as planned. Just screw the auger and then unscrew it so as to loosen the material without trying to remove it.

 

[Focht3]

While BigHarvey's approach might work, it's pretty risky - particularly if there are any large gravels or cobbles in the mix. Predrilling won't be of any benefit in those materials.

By hydraulically mining, I had in mind a combination of water jetting and air lifting.

Let us know what you do, what works - and what doesn't.

 

[layfieldman]

I was wondering if anyone here had heard of AquaDam. It is a large geosynthetic tube filled with water and can be placed in a cofferdam configuration by using 3 of them. They do leak a little but placing a trench with a pump just inside will handle that.

http://www.aquadam.com

for more details.

 

[Focht3]

I haven't heard of that product, but don't think it would be practical for [blue]rhymn[/blue]'s problem. After all, you couldn't just "wish" AquaDam into place at his site...

 

[Focht3]

Steel casing could work, but it wouldn't be temporary unless it was large enough to permit a 96-inch lift station to be built inside. And what would the construction sequence look like?

And regarding the "sunk caisson" approach: surely trucking in 96-inch RCP would be a lot cheaper than open cut excavation, jet grouting, diaphragm walls, etc. Lots of truckers out of work; although diesel is "expensive" right now, the haul costs ought to be pretty low in comparison to mob/demob costs of the equipment needed to complete some of the other approaches you have been considering...