Elevation Stake

[Monkey]

Can anyone give me a suggestion to the attached problem. And what the appropriate answer may be. Thanks

 

[Monkey]

Can anyone give me a suggestion to the attached problem. And what the appropriate answer may be. Thanks

Forgot to attach the file.

EL-Stake.pdf

 

[ptatohed]

I can tell this is not a very professionally written problem. The answer choices are not in ascending order, the significant digits of answer C don't match the others and, there is an apparent conflict in the slope - in the graphic, it shows a downward/up-station slope of 2.5% but in the wording, it states the pipe slopes .25" for every 1'. Solving the problem with either of the slopes does not yield any of the answer choices.

I guess we have to assume the written word trumps the graphic so, I'll assume the pipe slope is .25 inch per 1 foot. Also, I guess one could reasonably argue that the -2.5% is for the slope of the ground above the pipe, and not for the pipe itself (although the ground slope is unnecessary information). Thus, the pipe slope = ((1/12)/1)(100) = 2.083%

Starting with the known elevation of 160' at the ground surface at station 1+10 and working our way to the IE at Sta 3+20 gives:

160' [surface Elev @ Sta 1+10] - 3' [Ground Cover] - (1/12) [Pipe thickness] - 3' [Pipe ID] - .02083(Sta 320 - Sta 110) = 149.54 [iE Elev of pipe @ Sta 3+20] = None of the available answers.

Ahhhh.... I was just about to hit the button and then it hit me. I know what they are doing. It is completely wrong because RCP pipe size always refers to the Inside Diameter (ID), not the Outside Diameter (OD) but what they are doing is.... they are assuming the 36" RCP pipe size is the OD measurement. This gives:

160' [surface Elev @ Sta 1+10] - 3' [Ground Cover] - 3' [Pipe OD] - .02083(Sta 320 - Sta 110) + (1/12) [Pipe thickness] = 149.71 [iE Elev of pipe @ Sta 3+20] = Answer D

 

[Monkey]

So, the ground slope does not matter at all. The stake set at sta 3+20, should indicate the elevation in reference to Original Ground Elev 160' at the start point.

What if it were and 8" PVC Water main, with 0.5" wall thickness, same slope and same ground slope, what would the invert elevation be? What would be your thoughts on that.

Thanks

 

[rc10gt]

The calc'd answer is Inv.El. 149.54. The "best" or "closest" or "most near" answer would be 149.71 "D".

Slope of the surface grade is irrelevant. The slope of the pipeline (0.25"/LF)*110' = 4.375' of fall. Ground elevation (160.0) - 3' cover - 1/12' wall thk - 3' pipe ID = Inv.El. 153.92 at Sta. 1+10. Subtract the overall slope pipeline length from the beginning invert elevation to get the invert elevation at the desired station.

FYI reinforced concrete pipe (RCP) is designated by its internal diameter. The wall thickness varies in thickness this is called out as the pipe class or "wall" (i.e. A-wall, B-wall, C-wall, etc.). The wall thickness determines the maximum depth of cover the pipeline can safely be installed. A typical 36" RCP has an internal diameter (ID) of 36" (3') and a wall thickness from about 3" to 4.25" depending on its class. Rubber gasketed reinforced concrete pipe (RGRCP) may be called out in a Line List as "A50 pipe". This pipe would have an A-wall (depth of cover about 12') and a pressure rating of 50' of head (how much internal water pressure the gaskets can seal against).

Hope this helps and/or makes sense, if it doesn't, I can explain it a different way.

 

[ptatohed]

So, the ground slope does not matter at all. The stake set at sta 3+20, should indicate the elevation in reference to Original Ground Elev 160' at the start point.

What if it were and 8" PVC Water main, with 0.5" wall thickness, same slope and same ground slope, what would the invert elevation be? What would be your thoughts on that.

Thanks

You are welcome Monkey.

The ground slope could matter in a different scenario but, in this case, since it conflicts with the pipe slope, use the pipe slope. In theory, if there was always 3' of cover over the pipe, then the ground slope would be the same as the pipe slope. Another gripe I have with this problem is that the ground slope is steeper than the pipe slope which would cause the cover over the pipe at Sta 3+20 to be less than the 3' Min Ground Cover required by the problem graphic. Lame.

Regarding your question about the 8" PVC, nothing would change regarding the fundamentals of this problem. I think the only slightly tricky part of this question is for us to know if the pipe diameter is ID or OD. In the case of RCP (reinforced concrete pipe), it's my experience that the stated pipe size is always ID. That's why I think the correct answer to this question is 149.54 and not 149.71. However, with your PVC example, I think PVC pipe sizes always refer to the OD, right? So, the answer to your question would be:

160' [surface Elev @ Sta 1+10] - 3' [Ground Cover] - (8/12)' [Pipe OD] - .02083x(Sta 320 - Sta 110) + (.5/12)' [Pipe thickness] = 152.00 [iE Elev of 8" PVC pipe @ Sta 3+20]

But, I don't think you need to memorize which pipe material sizes are measured in ID and which in OD, I suspect the exam will state if the pipe size given in the problem is ID or OD.

 

[Monkey]

Thanks ptatohed,

Yes you are right, the slope is steeper than the pipe slope. I was trying to make a judgement call. If the problem mentioned to maintain a 3' ground cover, then that has to be considered and I believe the best choice in that regard would be the elevation to maintain that minimum cover. What call would you make, considering 36" OD, 1" Wall Thickness, and the stake is placed at station 3+20? In that case what would be the elevation?. The problem does not sound realistic, and also the setup is questionable.

Appreciate your explaination.

 

[bradlelf]

This is a very poorly researched problem and narrative.. Ptatohed is absolutely correct in his explanation and math.

First of all 3' dia RCP is 4" thick walls for an OD of 44" ... this author has no clue.

Cranky rant over.

 

[ptatohed]

Thanks ptatohed,

Yes you are right, the slope is steeper than the pipe slope. I was trying to make a judgement call. If the problem mentioned to maintain a 3' ground cover, then that has to be considered and I believe the best choice in that regard would be the elevation to maintain that minimum cover. What call would you make, considering 36" OD, 1" Wall Thickness, and the stake is placed at station 3+20? In that case what would be the elevation?. The problem does not sound realistic, and also the setup is questionable.

Appreciate your explaination.

Yeah, I totally see what you are saying Monkey. You are right, if the design requirement is to always have three feet of cover, then there is no reason to ignore that constraint (except for the conflicting pipe slope info. in the problem statement, of course). So, if we held the 3' of cover (and ignore the given pipe slope), the solution/answer would be:

160' [surface Elev @ Sta 1+10] - .025 x (Sta 320 - Sta 110) - 3' [Ground Cover] - (1/12)' [Pipe thickness] - (36/12)' [Pipe ID] = 148.67 [iE Elev of 36" PVC pipe @ Sta 3+20] = no available answers

And, if we treat the given 36" RCP pipe diameter as OD (which, again, is not correct per convention), we'd get:

160 - .025(210) - 3 - 3 + (1/12) = 148.83' = Answer A

Where did you get this problem?

 

[Monkey]

From somebody's notes. May be the problem, was noted down wrong. Anyway, I appreciate your help in clarifying the solutions to me.

Thanks

 

[Jayman_PE]

Root cause - that survey problem was written by an engineer. Dangerous.