water diversion/detention structures 2

[scherry]

I got tossed a set of drawings purporting to show a detention pond with concrete walls on all four sides and a rectangular wier at the low end. the pond itself has a natural soil bed. the single structural wall section showed a dimension of 2'-6" with about 13 horizontal rebars in it. and of course all of the rebar was on the wrong face of the wall. there should be, in my opinion, at least three sections and an elevation at the wier structurally speaking. I know how to design the retaining wall, or I know where to look to find the info to relearn it, for the most part. What my concern is, is that a one foot cover at the base of the toe of the wall in a detention basin may not adequately protect the toe of the wall. furthermore, since it is just listed as "topsoil" (i did change it to compacted soi) it will be saturated and there could be a bouyant force on the bottom of the wall. the walls on two sides are not drained, theoretically, because they are below impervious surfaces and to drain the soils below an impervious surface into a water bearing channel seems counterintuitive. I have no information on the natural water table. But I am unsure that it is conc=servative to design these walls as if they were drained, when they are not. the third side wall should be drained since it has landscaping and water will drain into the soil behind the wall. At the wier, there is 6" of six inch riprap, which seems pointless and a one foot compacted soil layer, which I can live with. The anticipated maximum water height is 2'6 above the elevation of the lip of the wier, so for the wier wall, the pressure on the wall is direct water at the upper part and soil or saturated soil at the lower part of the wall. I know how (theoertically) to determine the active pressure of the saturated soil at the heel, and I assume that the water just exerts 62.4 PSF (lateral) over the height of the wall it is behind, but I assume that being saturated affects the passive pressure at the toe of the wall too, (at the back wall of the structure). i have no idea where to go to look for any standards on the (soil)cover for this situation. It almost seems to me that there should be rip rap at the toes of the walls inside the retention pond, or a clay liner to prevent the soils below the pond area from being saturated, but this is not my area of expertise, and i am more concerned with the effects on the structures than on the drainage. And of course this is late going out and i am going to be unpopular for bringing up any more issues, especially if they are non-issues. If any one knows of somewhere I could look for minumum standards for this type of structure or any insight on the saturated soils issues i mentioned, I would appreciate it.

thanks!

 

[ishvaaag]

I am not sure if it may be helpful to you or not but

Handbook of Hydrology
David R. Maidment, editor in chief
Mc Graw Hill

has sections

28.6.6. Design of (Dry ) Extended Detention Basins.
28.6.7. Design of Retention Ponds.
28.6.8. Design of Multipurpose Detention Basins for Quantity and Quality Control

4 pages in all and nimble on structural aspects.

Respect a modest wall strength you look armed enough to deal with, so I assume it is the filtration problem what disturbs you. If you feel unsure about the structural performance of the thing, you must raise the question anyway, even to the extent of forcing someone knowledgeable on the filtration risks warrant what to be built is a sound design. The question is that some other engineers (not very careful seeing the section detail) think that the design is acceptable, maybe usual, and except you prove by analysis or example your doubts are warranted you are in uneasy position. You may argue the carelessness as origin to your doubt and then start asking for the analyses by what the designers came to the conclusion of that a wall like this would be correct.

I certainly would extend the concrete protection in front and behind of the weir (to underground soft embedment) to maybe 4 to 5 times its bigger dimension to diminish the likelihood of scour damaging the outlet.

 

[scherry]

Thanks, anything is helpful. I will get the book. but also your thoughts on the protection from scour are helpful. The problem is that the "other engineers" are not structurally oriented, and do not see the same issues that I do, and are my very exhausted, very pregnant, overwhelmed boss.

thanks again for the pointers and concurring opinion

 

[Focht3]

I don't think that six inches of 6" rip rap stones is an adequate design from an erosion protection standpoint. I recall a requirement that the depth of the stones should be at least three times the minimum dimension of the stones - so that the individual stones can interlock to resist the flow forces. But that 3:1 "rule" may have been a "rule of thumb", too.

I'm also concerned about the apparent absence of a footing beneath the walls. (Perhaps there is one, and you didn't think it was relevant to the discussion.)

Where is the site? Soil conditions? What has the geotech had to say about all this?



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[jdmm]

scherry

communication....

you have been provided with some design parameters. If you are unhappy with them (as you apparently are) express them to those that presented them to you. Design should not be a series of single steps but an interation of possibilities. You are one step in the process. Go back to the original designer and find out what goes on. You may find that these were only preliminary estimates to be "confirmed by the engineer".

Current practice is to behave as single companies but we really must try to be on the same team to achieve the same goal.

 

[scherry]

focht3 - thanks for the input, again this is consistent with the concerns I have. jdmm, i am afraid you hit a nerve. There is no geotech info, and no one around here seems to think that is odd or dangerous. The last set of drawings I saw go out had no technical specs calling out any soils, concrete, or rebar criteria. (My stamp is not on these drawings) and I am getting asked to look at these types of structures the day they go out of the office, because "they are civil jobs" and so I (as a structural)am not involved in the intial work or planning. I am about at the point of considering a job change if I cannot get through to management that this is unacceptable practise. On the last one, I asked a question about a sawtooth curb that was called out...I was told "well it could be cast in place, or precast, it just depends on how they decide to construct it..." Ummm, I always thought that was the engineers job, to TELL them how to build it, (well, ok, to tell them what should be built (so as not to start a discussion on means and methods)). I would be more than happy to communicate with a geotech if there was one involved. When I brought that up the answer was, well we usually can count on having black clay...that is the point exactly, my dear, and the code says if you are at risk for or suspect the presence of expansive soils then you have to have a soils report.... and the one I am working on now, which is oddly shaped box culvet inlet and outlet structures with a pipe culvert inlet on one side of the box culvert outlet has less than two feet of cover to the surface of the street, which is not highway traffic but will see the occasional semi, firetruck etc.... sorry. I realize I am ranting. But I spent this weekend reflecting on whether it would be easier to quit than fight, and that is a rotten way to start a monday. I do appreciate the input. I plan to print some of the responses and use them for additional ammunition if necessary, should I decide to fight. Which I will. at least initially.

thanks for your comments and help

 

[Focht3]

A really tough situation. But it sounds like you have a good head on your shoulders...

Don't leave yourself in a tough spot - start the job search immediately. Before you have any confrontations or make any demands. Try to calmly look at your personal situation carefully and realistically. You need to plan - be methodical. (If you are married, remember to include your wife in the process.) Remember, you have to look out for yourself because it's very unlikely that the company is looking out for your best interests -

And be very aware of the licensing issue. Your posts suggest that you are licensed. If you aren't, tread carefully - you don't want to burn any bridges that may be needed at a future date. Even if you are, you may need references for future licensing in other states.

In my experience, leopards don't change their spots - don't expect the company to change just because you stamp your foot and say, "If you don't stop - I'll quit!" Unfortunately, a senior manager may just laugh and show you the door. I'm not trying to be cruel or hurt your feelings, just trying to be realistic and help you see what's probably coming -

One final thing: you can complain about the lack of adequate review time. Simply tell your boss that you need to have adequate time to review the drawings, and that your review needs to take place early enough so that any errors can be addressed. Ask to participate in the selection and hiring of subconsultants - even the bidding and budgeting portions of the work. Seek to be more involved. If she brushes you off, it's time to move. But don't telegraph your intentions...



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.