I have a project where we will be creating a wetland by filling in an old canal that is currently submerged with up to 10 to 15 feet of free draining fill. In the canal, there is approximately 10' of extremely weak silts (c~40 - 80 psf) before hitting stiff clay. We will likely place a retaining wall to contain the fill since the material does not have much bearing capacity.

Question: I know that when we place the material, there will be bearing capacity failures unless we have careful placement of thin lifts. With soil that weak maybe the best option is to place the fill material in larger lifts an allow it to fail into the weak material and create a solid base. IT would likely result in higher fill volumes, but a shorter construction time.

It seems that method may leave some pockets of weak material that would still have long term consolidation? The equations for estimating the depth of failure are based on a foundation width, but I don't really have a foundation here.

There are no critical structures here since it is a wetland project, just trying to get good numbers for fill volumes and consolidation times. Excessive settlement after planting would have an impact on plant survival.

Any thoughts on the failure mode would be helpful.