riggly - what is different from your thread of a month ago? thread256-191870: Compaction of Fly-Ash?.  It was discussed in some detail there - do you have additional information?  Perhaps you should give us some details:  What is grain size distribution?; what is specific gravity?; what is compaction test results (ASTM D698 or D1557)?; have you done pin-hole dispersion test? etc.  I don't think that we can offer any more than we did without some real input from you.

BigH, I was not trying to figure by calculations whether the soil is collapsible- I just wanted to know whether anyone has encountered collapse in flyash used as a backfill

BigH, actually, it is the same situation, but I can't recall anyone explaining from the perspective that the soil might be collapsible-although one person mention that some flyash when wet can sort of liquify.  Anyway, to be more specific, about 85% of the particles passes the #200 sive, and ther is less than 5% gravel and sand.  The maximum density was about 100pcf, at about 12% moisture using AASHTO T-180.  The density of the undisturbed sample was about 76pcf.  A pin-hole dispersion test was not perform. GS= 2.2, PL = 26, LL = 30, and PI = 4.

Big, I will recant my earlier statement.  Although, 'collapsible' wasn not mentioned specifically, you did raise the point about loess structure  in the provious post.

I will close my case :)

We've used Pulverised Fly Ash as MSE back fill in a few projects.  The fill you describe would not be suitable for use as an engineered fill, in effect it is a silt and I would guess is very susceptible to collapse.  Do you have any knowledge of how it was deposited?

MSEMan, I do not have the details of how it was placed, but I believe the placed the moist flyash and tried compacting it.  With the tight area that they had to work,I doubt they were placing in small lifts.

Riggly,

Given the limited information we have, I think your instincts are likely spot on.  If the fill was deposited wet of optimum and in relatively thick lifts the loads would have been primarily supported by the pore water pressures.  However, you still have to work our why it happened now?  What circumstances caused the PFA fill to become drained and collapse? An alternative explanation could be that the fill was deposited dry of optimum and became wet?  That would definately lead to a collapse.

MSE man, that sounds like a reasonable explanation, and may help to explain why the soils was inadequately compacted, and in a collapsable state.