Precast Headwalls 2

[Alpine88]

I am a contractor working on a basic storm sewer project installing rcp pipe, sizes 18"-72", to replace failed metal pipes for multiple road crossings. In my area most people use precast reinforced headwalls that have an opening a couple inches larger than the pipe with 45 degree wing walls on both sides. The design was calling for half the head walls to be poured in place and the other half to be precast. The poured in place walls are fine, but the problem comes with the precast walls. The engineer said they should be in accordance with a design standard that is showing gravity non reinforced walls. Once I explained that it is impossible to lift non reinforced headwalls since they would most likely fall apart even if you could get a crane big enough to pick them up. He agreed to use the standard headwalls. Here is where I need some help. The structural engineer said the precast walls will not work and increased the wall thickness or added additional toe depth. That is a problem because all the precast manufactures that I know of basically have same headwall with 6" thick wall. I'm no geotech or structural engineer, but from seeing hundreds of these around for years I know they don't fall over, at least in my experience. I think the engineer is young and not used to headwalls, but treating it like a normal wall. It says in the report it was using quick r wall. I believe the difference is a majority of the wall is taken up by the pipe opening and the soil is not exerting pressure where the pipe is and the wall spreads at 45 degrees with shorter wingwalls and a fanned footing. Just looking for suggestions on ways to better calculate this or maybe I'm completely wrong and these precast headwalls are problems waiting to happen. I don't want to put in a bad product or make the engineer look bad, but I don't want to waste 100k if the design is based on incorrect assumptions.

 

[r13]

I see this is a design problem. Usually when we specify something, the available of such product should be investigated first. And, it seems to me the designer made change (addition toe depth) after the bid, isn't there a cost escalation clause in the contract? It is difficult to talk to an engineer with your "experience" on your back, rather make them learn from cost increases. Someone up will ask why.

 

[oldestguy]

For what it is worth see if there is a local consulting engineer with experience both in soil mechanics a well as structures. Perhaps they can make a quick review and write a report on their evaluation of the "standard" head wall you will use. Sometimes one has to bring in some help to "convince" a know-in-all engineer you are right. That PE designation helps a lot when talking to another engineer.

 

[Ron]

As usual, OG is on target. His experience is invaluable. Listen to him.

 

[r13]

Question here, what was the thickness of the unreinforced precast wall? I wonder why the designer felt the necessity to increase the wall thickness, and increase the toe depth, if your suggested substitution is the same thickness with reinforcement, which is much better than the original design. If the later change wasn't prompted by your substitution with a product deemed not meeting the design (thinner than the design), then the engineer is at fault. Otherwise, you might have to comply, and negotiate a cost adjustment for the additional work, and customized product. But you might not get it though.

In either way, my opinion differs from the advice above, I don't think a contractor shall initiate/invite expert review until the negation is broken and no satisfactory settlement in sight. Even then, if you are confident on your judgement, the owner shall be made aware of, through internal communication channel, the whole issue, your opinion, your intent and the follow up action in effort to resolve the technical matters. Remember you have a contract with the owner, not the engineers.

 

[Alpine88]

Thanks for the replies. That probably is the best idea to get somebody local to get another opinion from. I don't like second guessing the engineers judgement, but in this case with literally everyone else using these without problems I believe the math will back up what is seen in the field if the model is correct. If it doesn't and I'm wrong, that would be valuable for me and others to know.

 

[r13]

I still believe inviting a third party review is the owner's interest and privilege/discretion. I'm sad to see the unnecessary fight/prove ongoing, but its your pride and willingness to learn. Therefore, no comment, just wish you prevail ultimately.

 

[r13]

Note that the engineer's change maybe not economically desirable but technically wrong.

 

[oldestguy]

A little comment for retired 13. If you hit the "edit" button n your posts you can go back and change anything, delete, etc. Saves space,etc.

 

[r13]

OG,

Sorry, I don't quite follow. But I think you are technically right, it should be better to "cross out" the text to be edited before making corrections, if this is what you meant. I'll keep in mind (and sorry for the lost chance picking on my words .

 

[oldestguy]

OK here goes. I've occasionally run into this. After posting something, my old mind comes up with something that should have been in the text that now is posted. So I hit the "edit" button down on the lower right. Then up comes that former text in two places. One as it was posted and another that I can edit. So I edit the old post and that is entered by hitting "submit" button. No matter if other posts have come along, this old one gets edited. The idea is to bunch all ideas on one post. Forgot something. No need to cross out anything, just delete it as if it has yet to be posted. Second edit: The main thought I wanted to get across is: There is no need to add a post when one can just bring up the older post and add it there. Saves three postings.

 

[r13]

??? Yes, that's what I did. Otherwise, how to make change on posted response? Occasionally I forgot to hit "submit" though. Then, the revised response will never get through, just like nothing have happened (the old one will stay).

 

[oldestguy]

Back on the subject. Depending on your country the local or district highway departments likely have standard head wall designs that they use on their jobs. If that is the case your consultant on this likely can make use of them.

 

[r13]

The engineer said they should be in accordance with a design standard that is showing gravity non reinforced walls.

A gravity wall will general be thicker than 6", will it not? Now I got it, why OP does not want to address original wall thickness. Thanks for the mentioning "standard head wall designs".

 

[BigH]

Just a note that if you have another engineer review the design, your engineer should inform the designer that he is reviewing the design.