PE practice problem (retaining wall)

[workingweekends]

Hello,

See attachment. The provided solution shows the specific weight of water subtracted from the specific weight of the soil (highlighted) in determining the active soil pressure below the water table. Should it not be the specific weight of water subtracted from the saturated soil specific weight.

Is the provided solution incorrect or am I confused? Thank you in advance.

 

[geomane]

It looks like they only gave you the dry density of the soil. They should have given you the saturated unit weight of the soil below the water table, then you would have subtracted the unit weight of water from that. Or they could have given you the dry density of the soil, and the water content for above and below the water table then you go from there.

 

[CarlB]

The provided solution is incorrect.
The problem provided the void ratio; the original author expected with this you should apply soil relationships to calculate the volume of solids & volume of voids, then the saturated unit weight.
I find that the specific gravity of solids is 2.64, and the saturated unit weight is 124.6 pcf. That value rather than 100 pcf should have been used to determine P2.