concrete pipe encasement 1

[cvg]

I am designing a reach of large diameter, reinforced concrete storm drain pipe which is quite deep. Pipe ranges from Class IV to Class V. About a 1/4 mile is too deep for Class V and required D-load goes as high as 4,062 (0.01" crack). I am thinking of giving the contractor an option to encase the pipe (Class IV or V) in concrete to provide the necessary extra strength instead of providing the special D-Load pipe. Has anybody done this before and will it reduce the contractor's bid? I would expect the contractor to provide calcs / shop drawings for any proposed encasement.

 

[civilperson]

Try 50-100 psi flowable fill for the ultimate high class bedding and encasement. Works good for railroad loading.

 

[cvg]

that's what I was thinking. If I used a 1 1/2 sack mix with 425 psi @ 28 days - how much of an increase in D-Load could I get? I have 72" pipe at 20 feet deep and in silty sand material. Should I expect the encasement to come about a foot above the top of the pipe?

 

[BarryEng]

Pipe encasements have been used extensively by USBR (United States Bureau of Reclamation) usually thru dams. They have a design guide for culverts under dams - I think that it uses the Beggs deformeter method of analysis. It gives the compressive, tensile & shear coeficients for a variety of conduit configurations. It may be still available on their web site - I think in the area of old monographs that they still make available on the web site.

Failing that, I usually (for small jobs) just assume that the lower half of the encasement will remain in compression & the top half becomes a simple arch. In the design, you usually ignore the pipe & just use it for a form for the encasement.

In most cases, reinforcement may not be necessary to carry the load but I usually use about 0.5% reinf for half the conc thickness (or 250 mm whichever is the smallest).

What is your pipe dia & how deep is it?

 

[RWF7437]

BarryEng,

Couldn't resist quoting this:

" About a 1/4 mile is too deep ..."

 

[bpattengale]

4,000 Seems high for 20 feet of fill. What type of bedding (bedding factor) are you calculating your D-loads with now? What is the unit weight of the soil over the pipe?

A Type 4 Standard installation (Lowest Bedding class)has a minimum bedding factor of 1.5 for a trench installation. If you use a Type 1 Standard installation (Highest Bedding Class) a minimum bedding factor of 2.3 can be used for a trench installation.

 

[cvg]

I'm using type 1 installation in a trench, bedding factor of 2.3 and using Design Data 40 for the methodology. Soil types are quite variable - SC, SM, SW mostly with some CL, GC, GM and SP. The material is mostly deep alluvial deposits I'm assuming unit weight of 120.

 

[bpattengale]

I don't know how wide the trench is, so assuming it's wider than the transition width the installation will behave as an enbankment condition. The prism load of the soil based on 20 feet of fill above the top of pipe and a unit weight of 120 I get approximately 18,000 for soil load then using the a VAF of 1.35 for the Type 1 installation the total load is approximately 25,000. At a depth of 20 feet live load is neglitible for normal highway traffic so I'm not including it.

D=(weight of soil/bedding factor)X(Safety Factor/Inside Pipe Diameter)

Based on a safety factor of 1.5(Conservative, you can use 1) and a 72 inch pipe I get a D load of approximately 2,700. Class 5 pipe has a D Load of 3000 with 0.01 inch crack. If you use a saftey factor of 1 you can use class 4 pipe.

 

[cvg]

thanks for the help - I discovered an error in my spreadsheet which was increasing the calculated loads. I am now at class IV maximum for the project and no need for the encasement.