Settlement Estimation 1

[nbr1]

Two part question:

1. Should both immediate (elastic settlement) and consolidation settlement be added together where estimating settlement in a moist (stiff to soft) clay above groudwater; assuming stresses induced are below preconsolidation pressures? Or should one or the other method be used?

2. If elastic settlement is considered applicable and significant (in above soil profile), would pressuremeter and/or dilatometer testing prove beneficial for settlement estimates or would a series of consolidation tests with depth be more useful? Are pressuremeter/dilatometer methods applicable (for modulus/settlement estimation) where the soil profile is similar to above (stiff to soft clay) with occassional water table at deeper (60ft+) depths?

 

[fattdad]

For unsaturated soils above the water table, I'd just use elastic setlement. I like the dilatometer. I have less experience with the pressuremeter though. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[BigHarvey]

Pressuremeter is certainly the best tool for estimating settlement except in soft clays where it gives optimistic values. In this case oedometric method should be prefered. But as fattdad pointed out, above groundwatertable an elastic approach should prove satisfactory and it can be with pressuremeter values provided you are at ease with this approach !

 

[jrm49]

Regarding immediate and/or consolidation settlement, if you are below the pre-consolidation pressure, you should not have consolidation settlement even below the watertable.

 

[nbr1]

I'm curious what the basis/mechanism is for not having consolidation settlement (whether above or below the water table) if induced stresses are below the pre-consolidation pressure.

Was curious if others could to comment on this statement.

 

[moe333]

If the loading was below the preconsolidation pressure, and the soil was below groundwater table, I believe the settlement would still be termed "consolidation settlemet" since the settlement is occuring by pore water squeezing out of the soil due to applied load. The settlement would be through recompression, so the settlement would be much smaller than if the pressure exceeded the preconsolidation pressure, and the rate would be quicker.

 

[Mccoy]

My opinions:
elastic and consolidation settlements should be both calculated as a rule, although often one of'em governs and since we are dealing with but rough estimates it all remains within the error range.

In your case dilatometer is good, it yields a drained edometric modulus which in the hypothesis of saturation<100% can be related to the elastic modulus by simple elastic relationships, or can be equated to the elastic modulus since in unsuturated soil Poisson's modulus is pretty low as research suggests.

beware: sometimes in soft clays elastic modulus may be relatively high whereas edometric modulus may be low, that is consolidation may govern (when clays retain water) so calculating only the elastic settlements may grossly underestimate total settlements.
That's why the safest procedure is to calculate always elastic and consolidation settlements.