Consolidation Test to verify if consolidated 1

[Gimbli]

People; I have an old surcharge placed over soft soils and we would like to know, if possible, how can we determine de consolidation degree from consolidation tests?

 

[BigH]

Presumably the old surcharge exceeded the "then" preconsolidation pressure. Do your consolidation test - get the 'preconsolidation pressure' from the test and compare to the actual effective overburden pressure. (take into account the location of the sample within the stratum thickness - remember that the upper and lower exremeties of the layer will be normally conolidated first - and the middle lags - see your isochrones). If the old surcharge fill has not fully consolidated the layer, then the test's preconsolidation pressure should lie to the left of the existing overburden pressure (the difference being related to the still excess porewater pressure). If the preconsolidation pressure is equivalent ot the existing effective pressure - it is "fully consolidated" (for practical purposes). If you show some overconsolidation, then this is due to aging effects (see Bjerrum on this). I suggest that you might wish to plot log p' vs log e rather than on a semi-log paper - the preconsolidation pressure is better defined. There are a number of similar postulations and you can do a search - I once found a paper with some 8 such 'suggestions' on getting a good handle on the preconsolidation pressue.

 

[Gimbli]

Dear BigH:

When you say existing overburden, you mean including the surcharge?

 

[TDAA]

Yes, the overburden is based on the pressure with the surcharge. You are looking at the soils under the condition they are currently in.

You are basically trying to establish the relationship between the measured preconsolidation pressure (from the current test), the existing overburden pressure (with the surcharge fill), and the proposed pressure on the soil (surcharge removed, building foundation (?) added).

From there you can perform settlement calculations and determine if the settlement is within tolerable limits. Be mindful of where you obtain the sample, as BigH has said.

 

[BigH]

TDAA is correct - the surcharge (being old) is now part of the existing effective stress (even if young it would be too).