Restraining Pipe Dead End with Dead Pipe Left Behind Cap

[JGov]

I am trying to calculate the amount of dead pipe required to be left in the ground behind a cut/capping of an existing water main. The main is 24" D.I. and will be capped temporarily until we come back to tie into the main with new pipe. I've attached two different calculations I've done to determine the amount of pipe required to be left in the ground behind the cap to resist potential thrust forces via the friction resistance between the dead pipe and the soil: one done through a DIPRA exercise and one done through an exercise found in ASCE Buried Flexible Steel Pipe textbook.

Why are the two analyses giving me such disparate results? Is the force exerted on the pipe by the soil in the Buried Flexible Steel Pipe textbook analysis correct?

 

[JGov]

(2nd post to attach textbook exercise)

 

[bimr]

The values are just estimates. You would have to know the soil properties plus the methods that were used to install the pipe in the original installation.

I recommend you use the more conservative approach. It would be prudent to install a temporary thrustblock.

 

[rconner]

While neither figure represents exactly how soil pressures act on buried pipes (the latter that is a quite complex depiction with many variables), I am aware the second at least loading/force depiction (e.g. from DIPRA) is a simplification that has worked quite well for general thrust restraint purposes for many decades. I am not as familiar with the first i.e. it appears assuming an equal soil pressure (similar to hydrostatic around a point) all around the pipes, but I am curious what exactly is the "ASCE Buried Flexible Steel Pipe textbook" you refer to (what is the full bibliographic reference?) In any reasonable thrust restraint analysis, however, I do not believe you would want to assume that BOTH the loading figures are at play to develop helpful friction in additive fashion at the same time, as that would seem non-conservative and not intended by either reference.
That being said, and while ductile iron pipe does indeed have great columnar strength (as even do even many unrestrained joints thereof, at least when assembled straight and tight to each other, and also laterally well-supported by backfill soil), I agree with bimr that some effective scheme of temporary external buttressing is likely more foolproof/dependable for periodically testing otherwise ordinary unrestrained joint piping. [This may be particularly true if you are not sure of the existing pipe-to-pipe alignment, and how tight together are the existing/unseen buried pipeline joints - e.g. if a pushed on joint has space in the rear, meaning essentially really only water behind say a somewhat not completely inserted, withdrawn or deflected pipe spigot end, one or more existing joints with only water behind same is obviously a less stiff column than if straight and tight against metal, and thrust on the cap could invite accumulated sort of telescoping movement to uncap the new test pipe end or the first buried unrestrained joint back on the new test section?]
All have a good weekend.

 

[cvg]

check your weight per lineal foot for 24 inch pipe. it appears to be quite low. make sure you include the weight of lining and coating. your unit weight for silt is also on the high side. also, you are using two different angles for internal friction of the silt. I would assume that it is the same silt for each calculation, it should have the same angle of internal friction.

 

[BigInch]

One takes 3.14 x the equivalent soil hydraulic pressure + Wp
while the other only takes 2 x the equivalent soil hydraulic pressure + Wp
3.1416 * D₀ * [γ] * H + W_p
2 * D₀ * [γ] * H + W_p

I believe both are attempts to estimate factors to obtain the effective width of the soil prism above the pipe. One thinks it should be 2 and the other says nearer to 3. Corp of Engineer methods assume that the effective width of that soil prism above the pipe has more to do with the effect of backfilled soil above the pipe bridging across the width of the trench, which is better described by the width of the trench in relation to the pipe diameter. If a narrow trench, there would be more bridging of backfill against the trench and the factor would be closer to 1, with a wide trench and reduced bridging of backfill from trench side to side of the trench, there would be more soil weight imparted to the pipe and the factor would be higher, 3.

This is the link to the COE engineering manual 1110 Conduits, Culverts and Pipes

http://www.publications.usace.army.mil/Portals/76/Publications/EngineerManuals/EM_1110-2-2902.pdf