a design question about consolidation of clays

[ONENGINEER]

I have some misconceptions about consolidation of clays. Thanks to the dash-pot model which helps understanding of the consolidation process easy. But Lets say we have 10 m of clay thickness and groundwater is 3m below the ground level. I understand that we have air-water phase instead of only water phase. However, how would the Cc and Cv values change? What parameters do designers take into account for clay consolidation above the water table. Lets say we have 90% saturation. Thank you.

 

[GeoPaveTraffic]

In my experience unsaturated soil mechancis is seldom considered by engineers and I'm included in this statement.

I believe that there would be very little consolidation settlement of the 3m of clay above the water table and normally it would be ignored. I would generally calculate elastic settlements for that 3m of clay.



Mike Lambert

 

[fattdad]

You can either look at the elastic compression or use 1-d consolidation theory for unsaturated clay. For the former, you need soil modulus. For the latter you'll be on the recompression curve as the unsaturated clays are overconsolidated. It's likely that the overconsolidation ratio will be less with depth, so the terms you use below the water table may be different.

f-d

¡papá gordo ain’t no madre flaca!

 

[ONENGINEER]

Many thinks GeoPaveTrafic and Fattad for your valuable feedback. As Fattad suggested, may you explain what overconsolidation ratio (or ressure) should I assume for design purposes above the ground water as the clay is already overconsolidated with a overconsolidation pressure of 50 kPa.

 

[fattdad]

it would be impossible to give you some value. Just be sure to acknowledge that the loss of pore water in a saturated soil leads to capillary tension in the soil grains, which acts as further consolidation pressure. If the entire soil mass (i.e., below the water table) is overconsolidated, then your value of Cr (or Cer) would form the basis for your settlement calculations. You could extract the Cc value from the time plots for your stress level of interest.

f-d

¡papá gordo ain’t no madre flaca!