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Buoyancy

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I am designing a retaining wall which will form the edge of a man made lake. There will be water in front of and behind the wall.

 

I asked the geotechnical engineer for specific gravities of proposed backfill consisting of either earth or gravel. I told him that I needed values both above the water and submerged. This is what he gave me:

Earth 115 PCF above and 130 PCF submerged

Gravel 130 PCF above and 140 PCF submerged

 

Does this sound right? I know that earth or gravel is different than submerging a solid object in that the water fills the voids normally occupied by air, but can someone confirm that there is no buoyancy effect at all or if the numbers above look about right.

 

Regarding the wall itself (precast concrete), I'm assuming I will have to subtract the displaced water (net = 150-62.4 PCF) when calculating it's weight and effect on soil bearing pressure as well as the effective frictional resistance.

 

Am I missing anything?

 

TIA,

 

 

T. William (Bill) Allen, S.E.

ALLEN DESIGNS

Consulting Structural Engineers
 
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