Secondary settlement / heave
Secondary settlement / heave
(OP)
I have a question concerning secondary compression and secondary heave. I have a deep structure (approximately 35 ft. deep) to be built on a site with 20 ft. of sand and gravel over up to 80 ft. of compressible soils. This will result in a net unloading of the soil.
Based on the consolidation test results, I have calculated that during excavation, the soil will heave past the point where it will end up, once the structure is built and backfilled. I call this the "final resting place". At some point during construction, the soil will stop heaving and start settling back the "final resting place".
One fellow engineer argues that since the construction will result in a net unloading, once primary settlement is finished, the secondary movement will be a heave.
I argue that since the last primary movement is a settlement, the secondary movement will be a settlement.
Any thoughts?
Based on the consolidation test results, I have calculated that during excavation, the soil will heave past the point where it will end up, once the structure is built and backfilled. I call this the "final resting place". At some point during construction, the soil will stop heaving and start settling back the "final resting place".
One fellow engineer argues that since the construction will result in a net unloading, once primary settlement is finished, the secondary movement will be a heave.
I argue that since the last primary movement is a settlement, the secondary movement will be a settlement.
Any thoughts?





RE: Secondary settlement / heave
RE: Secondary settlement / heave
It may be that at the field scale, the primary consolidation is slow enough that the secondary gets lost in the decimal dust. That is, the dissipation of pore pressure is slow enough that the effective stress is applied to the soil very slowly (over months, rather than minutes or hours as in the oedometer), so the secondary can, in a sense, keep up with the increase in effective stress. Is this a real concern for the final resting place with an overconsolidated clay, or is it intellectual curiosity? With "forgiving" structures, like highway embankments, secondary is generally neglected (quite reasonably), except sometimes with organics.
You got me curious, too.
DRG
RE: Secondary settlement / heave
It would be my guess any "heave" occurs before the structure is built
Then too, once backfill is in place the structure is held down and, in this case, that backfill may cause settlement due to loosening of subsoils due to dewatering.
The closest I have seen with heave is unloading a site that has been surcharged and some "rebound" has been measured.
RE: Secondary settlement / heave
RE: Secondary settlement / heave
Now that I've said all of this, I'd also look at the 1-d reconsolidation index and calculate settlement that way also. You may want to look at strain distribution; however if you do this depending on how limited the size of the structure. Mike Duncan's settlement manual (Virginia Tech) has information on this.