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# SO CONFUSED!!!

## SO CONFUSED!!!

(OP)
Hey guys, I've been struggling over a consolidation settlement problem. The problem has to do with the lowering of the water table.

"A 10-m depth of sand overlies an 8-m layer of clay, below which is a further depth of sand. For the clay, mv=0.83m2/MN and cv=4.4m2/year. The water table is at surface level but is to be lowered permanently by 4 m. Calculate the final settlement due to consolidation of the clay, assuming no change in the weight of the sand."

Here is their solution:

http://files.engineering.com/getfile.aspx?folder=7...

What I don't understand is why the change at the top of the clay layer is different from that at the bottom of the clay layer...? And why does the pore water pressure at the bottom of the layer stay constant?

### RE: SO CONFUSED!!!

I didn't try to check calculations, but you made an assumption that is not correct. Dropping the water surface level changes the water pressures all the way down to the center of the earth.

If this is a homeward problem, this is not the forum for that.

### RE: SO CONFUSED!!!

(OP)
Thanks for your help!! Actually this is not my answer but the answer given in a textbook! It's in Craig's Soil Mechanics...

### RE: SO CONFUSED!!!

Dear OP,

Please recognize that this the internet. . .

Dropping the water does not change the pressures all the way down to the center of the earth. We rarely consider perching effects at these great depths. They no doubt exist.

Within the realm of most civil engineering projects, we need to consider both geology in conjunction with elevation head and pressure head. Clearly you can normalize elevation head.

If I have a water table in a layer of sand and the underlying clay stratum does not support any pressure head, the water in the sand layer is perched. If I lower the water table in the sand layer, the effective stresses in the underlying clay will be REDUCED! If the lower clay layer shares the same pressure head as the sand layer, then lowering the water table in the sand will INCREASE the effective stresses.

It's not worth it to try to further simplify these discussions. In all honestly, most people don't even know how to discriminate a perched water table and make simplifying assumptions that are incorrect all the time.

f-d

ípapß gordo ainÆt no madre flaca!

### RE: SO CONFUSED!!!

Hi Mehdi
I got the final settlement 0.27m
There should be no inter-connection between the pore water pressure below and above the clay layer. You may check by installing the piezometers.

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