(Geotech Depth) NCEES Prob 507 and Prob 14 from Civil PE Sample Examination

[Pelambre]

Hey guys,

I plan to take the PE exam, geotech depth, on October 2008.

So far, I have been trying to solve a couple of problems from several books. I have some questions regarding Prob 507 from the NCEES and prob 14 from the "Civil PE sample examination" book (By Linderburgh).

In the NCEES practice book, in prob 507 (geotech depth), I think there is something wrong with the solution.

I know that we need to solve it by using the chart that is given (DM7.1) and using the consolidation formula as well.

Since the consolidation formula involves both Initial vertical stress and stress increment, we need to find the stress increment from the chart. What I notice is that after the author computed the stress increment (delta sigma), he did not take into account that the water table was above the point of interest (the NC clay layer), so he was supposed to substract the water pressure from the stress increment, wasn;t he?

I know he did so when he computed the initial vertical stress, but I think he had to do it for computing the stress increment as well.

Anyone had the same problem?, please let me know

Regarding the "Civil PE sample examination" book (By Linderburgh), in prob 14, when computing the FS for the saturated clay slope, accordgin to Eq 40.28, you are supposed to work with gamma effective, But i notice that in the solution, the author uses gamma saturaded instead of the effective.

Anyone had the same problems?

Please let me know.

 

[sehad]

lgarcia..............I'll have to check again in my DAS textbook when I get back from the office but, if I am correct, the initial effective vertical stress is the stress added by all the soil layers & thickness minus the weight of water of what is below the water table. As far as I can remember, and I'm going off of memory, the pore pressure does not get subtracted from an applied force above the water table that causes a change in pressure in the layer below the water table. The consolidation is mainly affected by the initial stress in the soil and whether or not the clay is overconsolidatd or normally consolidated. A sumberged soil has less initial stress, therefore the same change in pressure will cause a submerged soil to have a higher consolidation that a dry soil.

For the second question.......the reason for subtracting the weight of water is due to the water level being high enough to cause buoyancy. Meaning the water is actually trying to lift up the soil. If the soil weighs 130 lb/ft^3 and is submerged, the water is trying to push up the soil at 62.4 lb/ft^3. Meaning that the effective weight of the soil is 130 minus 62.4. However, if the soil is not submerged, there is no uplift force. Meaning if the problem states that the clay is saturated but does not indicated that the clay is sumberged, you would use gamma saturated because the water is adding to the weight of the soil and not trying to lift it. If it says that it is submerged, you would use gamma effective because the water is trying to lift the soil.

Hope this helps. You can PM me if I didn't make something clear.