Secondary Compression Calpha

[UP830]

Is it standard to determine secondary compression settlement in soil with say 40% fine and 60% granular. Atterberg classification shows that the fines are clay.

Even a 2m thick layer of clay ( C alpha Of 0.012, e of 1 and t2 of 50 year and t1 of 1 year) will show a lot of secondary compression settlement (50mm+).

Is this routinely considered when doing a settlement calculation?

I just find it hard to believe as it means that before any structure or load is applied the souls will settle 50mm+ during its life time!

 

[HENRYZAU]

Your Ca of 1.2% is likely for normally consolidated soft clay/silt. Natural soil normally has a yield stress/preconsolidation more than its initial effective vertical stress (YSR/OCR > 1.0 depending on its stress history/ageing/cementation etc.), which means without loading the creep rate of natural soil is most likely in its reloading range, much lower than that under primary loading (or normally consolidated)

 

[r13]

Without significant change in water content and external surcharge, the secondary settlement may never get to that level.

 

[EireChch]

Henry, can you expand on that comment on creep rate in its reloading stage?

Creep is independent on load, as creep occurs with no increase in effective stress.

A material with 40% clay could definitely undergo secondary compression.

You need to determine your OCR, if you are normally consolidated then 0.012 seems right. What is your Cc values as Calpha should be approx 3% of this value.

 

[HENRYZAU]

EireChch,
See attached paper by Ewers & Allman in 2000.

Key conclusion from this paper: "Cα was found to be dependent on the current vertical effective stress and the preconsolidation pressure. The magnitude of Cα was relatively small at pressures below the preconsolidation pressure and reached a maximum at a pressure of approximately twice the preconsolidation pressure. The ratio of Cα/Cc
was reasonably constant over the stress range tested, and compared well to the published value of 0.04 for
soft inorganic clays."