geotech depth (probs 42, 49, 63 from 6 mins solution)

[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 about some problems that a least the don;t make sense to make, solutionwise.

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.

From the 6 mins solution book (2nd edition), I already checked the errata from "the other board" and still I keep seeing weird things:

In prob 42,

In order to compute the stress increment, i believe the author was supposed to compute the stress increment at the mid depth of the clay layer instead of in the sand layer (18 ft). I say so, because when he uses the settlement formula later, of course, he computes the settlement with the clay layer properties.

I know we are being asked to compute the settlement of the backfill, and since sands have little settlement, the clay layer will represent the total settlement of the profile. Or maybe other way of computing this is by using the settlement formula for sands (the one that involves the poisson and the Yound modulus) and then we might used this formula at the 18 ft deep.

Also, by solving this problem using the settlement formula for clays that we all know, it would make the problem a little bit harder i think, because by the time you wish to compute the stress increment either by using the Bousiness charts or the 1H:2V rule in the clay layer, you don;t know what is the length of the 148 psf surcharge, and you can not consider only the wall base, because obviusly the backfill material acts as a load.

In prob 49, when computing the surcharge, wasn;t the author supposed to compute the moist unit weight above and below the WT and multiply those by their respectives depths?, instead of using the dry unit weight? (I believe for some reason we were given the %w above and below).

IN problems 63 and 72, I notice that in order to compute the effective stresses, the author substracts the total water height (including the one that is above the ground surface) times 62.4 from the total stress. I think that in order to compute the effective stress you need to substract to the total stress the water height until the ground surface. Am I clear? Am i Right?

Please let me know.

Thanks

 

[sehad]

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 about some problems that a least the don;t make sense to make, solutionwise.

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.

I left a comment in the other thread about this one.

From the 6 mins solution book (2nd edition), I already checked the errata from "the other board" and still I keep seeing weird things:

In prob 42,

In order to compute the stress increment, i believe the author was supposed to compute the stress increment at the mid depth of the clay layer instead of in the sand layer (18 ft). I say so, because when he uses the settlement formula later, of course, he computes the settlement with the clay layer properties.

I know we are being asked to compute the settlement of the backfill, and since sands have little settlement, the clay layer will represent the total settlement of the profile. Or maybe other way of computing this is by using the settlement formula for sands (the one that involves the poisson and the Yound modulus) and then we might used this formula at the 18 ft deep.

Also, by solving this problem using the settlement formula for clays that we all know, it would make the problem a little bit harder i think, because by the time you wish to compute the stress increment either by using the Bousiness charts or the 1H:2V rule in the clay layer, you don;t know what is the length of the 148 psf surcharge, and you can not consider only the wall base, because obviusly the backfill material acts as a load.

You should compute the settlement for both layers if the problem asks for total settlement and only for the clay if it asks for it in the clay. Remember too as well that the change in pressure is different for the clay layer than it is for the sand layter. As you get deeper, the change is not as great. I would need to see this problem before I could answer exactly what you are talking about. If you can PM me and scan/email me the problem, I might be able to help you further.

In prob 49, when computing the surcharge, wasn;t the author supposed to compute the moist unit weight above and below the WT and multiply those by their respectives depths?, instead of using the dry unit weight? (I believe for some reason we were given the %w above and below).

If given gamma d and the % moisture you would have to solve for the moist density. Again, I would have to see the problem to be able to be of more help.

IN problems 63 and 72, I notice that in order to compute the effective stresses, the author substracts the total water height (including the one that is above the ground surface) times 62.4 from the total stress. I think that in order to compute the effective stress you need to substract to the total stress the water height until the ground surface. Am I clear? Am i Right?

I can't see the drawing, BUT I'm thinking that the water level is above ground level as in the bottom of a river or lake? If so, you would subtract the full height above the ground level as well. Think of it this way........if you are in a swimming pool with no water and weigh yourself you would weigh your normal weight. If you filled the pool up to waist level, you would way less. If you filled it up to the top of your head, you would weight evel less than at waist level. If you filled up to where it was 4 feet above your head, you would weight even less than if the water level was at the top of your head.

Please let me know.

Thanks

I hope this helps. I took the Geotech depth in April '08 and passed it on my first try

 

[Pelambre]

I hope this helps. I took the Geotech depth in April '08 and passed it on my first try

Hey Sehad,

Thanks for your reply, I really appreciate it.

Regarding prob 14, "you left a comment in the other thread about this one", do you know the name of the other threat?, i haven;t found it yet.

In prob 42, I am attaching the prob and the solution by the author, do you agree with that?. My problem is, Since he asks for the settlement of the backfill, i believe you are supposed to compute the settlement both in the sand layer and in the clay layer. I haven;t seen in any PE book/ manual that gives the formula for computing the settlement in sands, (even though there are few formulas). As you can see, the author in the solution uses the one for computing the settlement in the clay layer BUT when he computes the stress increment I believe he was supposed to used either boussiness chart or any other in order to see what is the stress increment in the CLAY layer, as you can see he computes the stress cinrement in the sand layer (SANd(18 ft)) which i think is not correct, what do you think about that?

Do you have an email, i thought that i could upload the files, but they are bigger than the max capacity size., Sorry for the inconvenience.

Regarding prob 63, I agree with what you said about the effective stress and using the lake example and the body in a pool and also i agree with the author in this prob.

Regarding prob 72, Do you agree the way how the author computed the initial effective stress?, i believe there is a mistake in the depth. Anyways.. just in case I got:

(at 40 ft, b/c the piles will be driven 30 ft below the bottom of the reservoir, so the settlement needs to be computed in the midpoint between the tip of the pile and the top of the gravel layer)

-----> 5 x 62.4 + 35 * 101.2 - 62.4 * 40 = 1358 psf

in another way:

------> (101.2 - 62.4) * 35 = 1358 psf

REgarding prob 49,

When computing the surcharge member of Qultimate, instead of using 105, I think he was supposed to used ((105 x (1.20))* 2.5 + ((105 x (1.19)) - 62.4)* 2. This is surcharge ONLY.

Other questions,

Also, when dealing with the ultimate capacity of footing, since there are many formulas, one uses the depth correction factor, order uses the shape correction factor, other uses both, other is the original formula. Will they ask in the ask in the exam to use a especific formula for computing the beargin capacity?

Also will they give the factors, you know.. since there are different factors (terzagui, Meyerhoff, Vesic, etc), it might be risky.

Can I take with me to the exam any books, including these solutuion books?

Once again, thanks a lot.