Determining Restraint Length for a 24-inch Ductile-Iron Water Main Encased in Concrete

[nberkholz]

Hi all,

I am new to the forum and I tried to use the search at the top of the page to find what I am looking for but could not come across anything useful. If I missed it please kindly direct me to the source. Also, HI!

I am currently working on a design of a 24-inch ductile-iron water main. The water main is to be no less than 5 feet below ground surface. The soils are very stiff to hard clay. Test pressure is to be 150 psi. Factor of safety is to be 1.5. I need to determine restraint lengths for various angles (22.5, 45, and 90 degrees).I have been messing around with the calculators provided by EBAA Iron and DIPRA to get a feel for the restraint length required however I am not sure if these can be applied to an concrete encased water main (per client request). I also tried a hand calculation using AWWA M41, and got in the same ballpark with my length as compared to the programs. However this is my first time really trying to do these calculations and would appreciate a reality check.

Basically can I apply equations for normally restrained joints (slip on / mechanical) to that which will be encased with concrete?

Thank you in advance for any help!

Nick

 

[rconner]

Am curious is the entire main to be encased, or just some or all of the restrained joint pipes?

 

[nberkholz]

The client wants it encased where ever a bend of 22.5 degrees or greater is used.

Also, I talked with a few other engineers I work with and agree that my approach is okay because it will yield a more conservative restraint length.

 

[rconner]

Concrete encasement will likely provide greater resistance to thrust than any soil embedment condition described by DIPRA or AWWA Manual M41, and stiff clay has normally good bearing resistance. Consequently, I suspect your determinations are adequate. It may not be a bad idea to locate the transition between encased and non-encased pipe with a flexible joint at or just outside the concrete encased structure.

 

[nberkholz]

Thanks rconner I will make a note of this. I appreciate your input

Nick

 

[greggy]

Just a suggestion: encase the pipe in concrete, but leave the pipe joints exposed for future inspection for leaks and future maintenance or disassembly. If possible, instead pour thrust blocks on the backside of the bends, with the block surface area bearing on undisturbed earth. Calculate thrust force vector and magnitude. Bearing area (sqft) should be equal to soil resistance (lbs/sqft) needed to equal thrust force (lbs).

 

[cvg]

better yet, wrap everything in poly; encase with 1/2 sack mix which is "diggable". blockout just the valves. no need for thrust blocks, but if you must, is probably better to restrain the joints near the bends, valves, tees and blinds than pour more concrete.

 

[nberkholz]

Unfortunately the Client has been very adamant about the encasements for all bends 22.5 degrees and greater. We, the Consultant, tried to use typical restrained joints but they said no and if the Contractor proposes the change during construction they may accept it.

I agree with everyone that there are better ways of doing this, I was more or less seeing if my theory was sound for the calculation of the lengths.

I appreciate everyone input and will consider these issues during the design and meetings.

Nick