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Preloading-Unloading Rebound

Preloading-Unloading Rebound

Preloading-Unloading Rebound

I have a site with soft soil. The site needs preloading with wick drains before building the MSE wall. I am recommending 90% consolidation before unloading and the construction of the MSE wall. Since there will be preloading and unloading at the site my thinking that the rebound at the site will be negligible since the time between the unloading and the construction of the MSE wall will be short (say less than 40 days with 90% prior consolidation). In addition, I think that during the construction of the MSE wall, most of the rebound will transfer to instant (elastic) settlement and will not show as "consolidation" settlements after building the MSE wall.
Am I thinking right? Any thoughts in this regard? How to calculate the rebound if you have 90% consolidation? Thanks all.  

RE: Preloading-Unloading Rebound

Why don't you search for other threads - this has been discussed in a number of them.  I know of one project where wick drains were put in (6 m of very soft clay) and the MSE wall was built in stages.  The walls were about 9 m in total height.  Walls settled more than 900mm; still stayed vertical - looked good.  Did have a few problems with a few of them but not because of the agreed methods. You can estimate the rebound after unloading (as per coefficient of rebound) - but remember these kind of computations are not "exact" - within 30% is usually considered "Wow".

RE: Preloading-Unloading Rebound

Please provide some clarifation:  Are you making this post in the slope stability section as the primary concern is slope stability?  If this is the case, then you are preloading the wall for the benefit of strength gain - not for the benefit of controlling settlement.

Can you describe the site conditions (i.e., LL, PI, Wn, percent sand and where the water table is located)?  How thick is the layer of soft cohesive soil?

Irrespective of the advice you get on this forum, what is your game plan for measuring the degree of consolidation and the corresponding strength gain?  Is your intention to install piezometers in the center of the wick network to monitor the relief of excess pore pressure?  Will you take shelby tube samples at some point for UU testing?

I'd think if you truely had acheived 90 percent consolidation and fully characterized the Su/P ratio, you'd be in good shape for construction.  That said, I'd analyze settlement using the re-compression ratio, just to understand how much to anticipate.  This is not likely to have any affect on the slope stability however.

Just one man's opionion.  Good luck.


¡papá gordo ain’t no madre flaca!

RE: Preloading-Unloading Rebound

The thickness of the soft cohesive soil is 55 feet. Cc=0.24, Cr=0.06. The degree of consolidation will be checked using piezometers. Shelby tubes may be considered (working on the approval) in the future.

RE: Preloading-Unloading Rebound

Is that the consolidation index or consolidation ratio?  If the former, you'd have to provide the initial void ratio for further input from me (well along with some of the other items I mentioned).

Just out of curiosity, what is the undrained shear strength and how does it vary with depth?

Any reason you didn't characterize the clay in the above reply (i.e., LL, PI, Wn, percent sand, etc.)?


¡papá gordo ain’t no madre flaca!

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