Corrugated Pipe Reclaim Tunnel 1

[ldeem]

I am designing a corrugated (Multi-Plate) pipe escape tunnel and conveyor tunnel. The new pipes are larger than the existing. I am comfortable with sizing the pipe and the hopper opening. My question is regarding the end of the pipe where it meets the existing concrete vault. The pipe will slope downward at 17 degrees so the end of the pipe is cut on the same 17 degree angle so it matches the vertical wall of the vault (see attached sketch). The result is the circular corrugations are cut off.

Does anyone have a design reference for the bevel cut support under heavy cover loads (67')? I was thinking of building a large concrete frame around the end of the corrugated pipe to support the soil loads until one or two full cooriguations. The loads are large so the result is a really massive concrete frame. Similar results when I use Roark to design a series of concentric rings capable of resisting the loads. Both of these options ignore the soil support that forms the basis of corrugated pipe design.

Another idea I had was to design a circular angle based on 5' of thrust load. The ring would be bigger than the pipe OD and the pipe would be grouted into the ring. The idea has intuitive sense.

Any papers or reference book suggestions would be helpful.

 

[BUGGAR]

This sounds similar to tunnel liner plates we used in tunneling. We always wound up making special transition fabrications at skewed shaft connections and pouring appropriate concrete portal transitions to support everything. We used the brute heavy structural steel force method (there is nothing that is too strong in a tunnel) and that worked. Maybe contact a liner plate manufacturer to see what they do.

 

[dhengr]

ldeem:
I’ve never considered this exact problem before, but I would be tempted to pour about a 5- 6' thick tunnel portal type structure around the pipe and integral with the conc. vault. This portal structure would have a radius of about 9.5 or 10.5' around the pipe (pipe radius plus several ft.) on the sides and over the top, and then go straight down to a footing. Then I would just leave the 13' corrugated (Multi-Plate) pipe straight cut (sq. cut) on the end, and cast into this portal. The ends of these corrugated pipes, or openings in them are always fairly fragile unless reinforced or supported in some way by a much stiffer element or structure. As BUGGAR suggests, I would talk with the corrugated pipe people, material handling, conveyor, mining or tunneling people about some typical details for these types of situations.

 

[ldeem]

I am waiting on a response from a pipe manufacturer to see what they recommend. I was hoping to find a paper dealing with this issue so I can independently verify whatever I am told by the manufacturers.

 

[robyengIT]

I agree with Dhengr. Final solution should be like in the file attached

 

[BUGGAR]

Launching a tunnel from a shaft is very common. You should be able to find a write-up in tunneling books/periodicals. Basically, you have the reverse of this but the support is similar.
In my tunneling experience, we seldom finessed such details, we just threw iron and concrete at it. Crude, yes, but at the cost of sandhogs in NYC, it was, by far, still the cheapest way.

 

[ldeem]

I got some feedback from the pipe manufacturer's and brute force idea is the way to go. Basically a steel of concrete frame around the end of the tunnel that can support all the external loads. I'll try and post some details of the final design for anyone in the future who runs across this thread.

 

[ldeem]

Attached is the concrete frame to support the bevel cut end of the tunnel. This is just for reference by anyone who happens to run across this thread in the future.

 

[BUGGAR]

Looks good to me! These things are always so much easier after you figure them out.

 

[BUGGAR]

Your drain detail reminded me of one I saw: "PVC pipe drain filled with drain rock packed into athletic sock".

 

[aggman]

Ideem,
Just an FYI, I have been involved with several pipe to bunker transitions like you have designed for stockpile reclaims. I have seen failures in your design where you see excessive diagonal cracking in the corners. This is prevalent on larger diameter tunnels because of the amount of deflection in them compared to the smaller diameter tunnels. What I was able to come up with was that pipes tend to settle (or squat and become egg-shaped) as the load is put on them. I believe they expect them to settle up to 1%, i.e. a 12' diameter tunnel will squat up to 1 1/2" and still be acceptable. As they do this the tunnel presses against the back fill on the sides of the tunnel to gain support. This deflection adds significant loads to the bunker as it fights to maintain the perfect round pipe. Usually this leads to corner cracking (negative moment in the corners and shear) on what would be your vertical wall areas at the tunnel to concrete interface. You can think of it as the ring compression at location becomes two horizontal point loads on the vertical concrete walls at the center of the tunnel. The load is more than just what is at the location of the concrete because the tunnels will transition from perfectly round to egg-shaped after it comes out of the bunker area and part of that load is carried on the concrete as well. That being said, we moved towards a sleeve design (oversized rolled plate form) that was over sized enough to allow for the pipe to slide into the concrete bunker. We would then fill the voids with expanding foam and have had great success with this.

Obviously your concern is the removal of the rings, but this shouldn't be a problem in this area because the concrete will be above the tunnel in this area and will carry the vertical load. From the theory the load is applied at the top of the tunnel and where the top of the tunnel exits the bunker and picks up the vertical load the rings are complete.

 

[ldeem]

Aggman, thank you for the comments.
I thought about the lateral forces but from a different approach. I set the width of the frame such that midline (springline) would meet the face of concrete frame. This way the first ring with a full vertical load (the one just past the face of the concrete frame) has its spring line about 24" away from the frame. Then as you move closer to the frame the top of each ring is increasingly covered by the frame (so less vertical load and less side thrust). Then I designed the frame for full vertical and At Rest lateral loads. The at rest lateral earth load moments are more than the tunnel side thrust load moments in the frame (evaluated independently so they don't cancel each other out).

As you point out the tunnel will squat down some amount to compress the surrounding backfill. So allowing some movement makes sense. Did you use any particular expanding foam?

 

[aggman]

I don't recall exactly what the foam was but it seems that we were looking for something that expanded but didn't harden. My mind is saying like foam insulation but that stuff hardens so it's not ideal. We were worried about rock pushing it back and allowing leakage but it never was a problem.