Preloading

[PhilippeMayu]

We know that when loading-unloading a sample of overconsolidated clay in the oedometer, the path on the e-logp' is about the same when loading and when unloading.
Experience on preloading overconsolidated clay with a fill shows substential improvement in the sense that rebound when tremoving the fill is about 1/3 of the settlement obtained under the fill load.
I would like to find info on this matter and satisfactory explanation. If anyone could help me would be very grateful.

 

[jdonville]

phillipe,

You appear to say that the field deformation (in this case swelling) characteristics of overconsolidated clays are different than the oedometer results under the same pressures.

Recall that in the lab, there is an unlimited supply of water available to restore into the pore spaces of the clay. This may not be the case under field conditions.

Secondly, in the field, the time for the swelling to occur will be longer as the size of the affected loaded area is much much larger than an oedometer sample. Also, the flow path of the pore water may be lateral rather than vertical for much of the in-situ soil. Thus, the amount of swelling may be the same, but the time rate for full rebound in the field (at the permeabilities involved) may be orders of magnitude longer than that required in the lab.

Just my thoughts,

Jeff

 

[jdmm]

If you exceed the preconsolidation pressure and cause settlement then you essentially change the soil to a stiffer one. This would make the unload flatter than the loading curve. If you load below the preconsolidation pressure then I would agree that the load-unload slope should be similar.

Remember also, that within the soil profile, there may be some variability of stress history such that some zones may exceed the preconsolidation stress and other zones may not. Beneath a preload, the distribution of stress and settlement is a 3-D problem. In an oedometer it is assumed to be 1-D.

ps: why would you be preloading an overconsolidated clay?