Soil Units Weight Below Water Table For Analysis

[dcarr82775]

We are having a bit of a debate in the office about what loads to apply to a retaining wall below water table. One one side we have people who say take the dry unit weight of soil and add to it the unit weight of water. On the other side we have people who say take the buoyant weight of soil and add to it the unit weight of water. This obviously makes a sizable difference.

What do you do in practice for this condition?

 

[PEinc]

Below the water table, any increased lateral soil pressure is based on the buoyant weight of the soil (= saturated unit weight of soil minus unit weight of water). In addition, water pressure should be added below the water surface at its unit weight times the depth. Dry unit weight of soil plus unit weight of water does not equal saturated unit weight. For example, if the dry unit weight of a soil is about 115 pcf, the saturated unit weight would normally be about 125 pcf. However, adding 62.4 pcf to 115 pcf would be 177.4 pcf which is way too high for a saturated unit weight. If you have a 1 cubic foot box filled with totally dry soil, the weight of water for saturation is based on the amount of water that can fit into the soil voids. Certainly there is not 1 cubic foot of voids in a 1 cubic foot box full of dry soil.

http://www.PeirceEngineering.com

 

[dcarr82775]

Thanks PEinc. That was my opinion, water+buoyant weight. You can't have (2) components occupying the same space at the same time.

 

[fattdad]

The horizontal forces for a retaining wall that's free to rotate 1 inch for each 10-ft height (i.e., active earth pressure conditions) are based on effective unit weight times the Rankine coefficient of at-rest earth pressure plus the hydrostatic pressure.

So, below the water table the equivalent fluid density be [(gammaSAT-gammaWATER)*(Ka)]+gammaWATER.

Let's say the water table is at 10 ft, Ka is 0.333, the moist unit weight is 115 pcf and the saturated unit weight is 120 pcf. The horizontal pressure at 10 ft would be 383 pcf and the horizontal pressure at 15 ft would be 791 pcf.

f-d

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