Average Degree of Consolidation, U (%) 1

[pelelo]

Engineers,

I have few questions about the "Average Degree of Consolidation, U(%)", provided by NAVFAC DM 7.01 (See attached).

1 - We all know that if a clay layer is between to pervious layers (e.g sands) then there is a two way drainage path (H/2). On the other hand, if clay layer is between a Impervious material at the bottom (e.g rock) and a pervious one on top (sand), then the drainage path is only one way, therefore H = H.

My question is, if there is a situation in which I have a clay layer with a sand layer (Pervious) at the bottom and on top is just the surface, do you consider that as a 2 way drainage condition?. My understanding is it is, as flow tends to travel towards the surface.


2 - Per the plot attached, the there are alpha values for conditions when you have 2 way drainage and 1 way drainage. For the 1 way drainage the plot presents 2 boundaries of alpha values, an upper (alpha =0) and a lower (alpha=infinite). How do know which one is the appropriate one to use?


3 - If my strata is (from top to bottom):

Sand (0 - 10 ft)
Clay #1 (10- 25 ft)
Sand/Gravel (25-35 ft)
Clay #2 (35 - 45 ft)
BEDROCK (At 45 ft)

I am required to compute the time after reaching 25% of settlement on both clay layers after construction, do you understand that for this strata there are going to be different alpha values in order to compute the time factor T or (Tv)?

Clay #1 is double drained, therefore i would use the plot that corresponds to alpha =1.

On other hand, Clay #2 is single drained, therefore i will need to use either alpha =infinite or alpha =0. Do you agree?


4 - Under which conditions do i know when to use alpha =5 and alpha = 0.5 curves?

Thanks

 

[geomane]

It looks like alpha is equal to U1/U2. U1 and U2 are the initial (excess) pore water pressures, which are influenced by the effect of pressure distribution. So, determine the excess pore pressures at the top and bottom of your stratum, and calculate alpha. For one way drainage, the value of zero or infinite depends on if your drainage is in the downward or upward direction. Increasing U2 will move alpha towards zero, increasing U1 will move alpha towards infinite. An increase in U2 would represent upward drainage, and an increase in U1 would represent downward drainage.

Question #1 - yes.

Question #2 - see my first paragraph.

Question #3 - if the boundary conditions are different, I would expect different alpha values.

Question #4 - depends on your stress distribution with depth.

 

[geomane]

EM 1110-1-1904 Settlement Analysis also has discussions on this. I would give this document a look-see.