Tunnel Seal 1

[WARose]

This is one of those times you need a sanity check from someone else..........this one has been driving me nuts thinking about it.

A project I am working on has a situation where some pre-cast tunnels are going to be placed next to each other and sealed up using a attached angle and threaded rod assembly. I've represented the situation (simply) in a FBD (see link).

My concern is: when we go to seal up blocks 2&3.....will we unseal the seal from blocks 1 & 2?

So my question is: would the (min.) required force to unseal the joint from block #1&2 have to be in excess of the "threaded rod force" in the diagram plus the additional sliding force of one more block?

In equation form this would be: 3W(µ_k)+seal force

I realize that in order to get it moving, you would have to overcome the static coefficient first.....but I want to check the min. force.

My worry here is: as we go to "block" #3, 4 ,etc.....we unseal what has already been done. Just want to be sure here.

 

[miningman]

Well its driving me nuts as well , but probably for totally different reasons.. I cant begin to get my head around this design. Is this being designed to prevent human acccess ( or perhaps egress???), What are the approx tunnel dimensions?? Is this a plan view or section?? Approx block dimensions , which are obviously related to W might help as well

 

[WARose]

I cant begin to get my head around this design. Is this being designed to prevent human acccess ( or perhaps egress???)

No, it's a seal to prevent water infiltration.

What are the approx tunnel dimensions?? Is this a plan view or section?? Approx block dimensions , which are obviously related to W might help as well

let's skip all that for now. I don't want this thing to go down a rabbit hole and start talking about alternatives and so on. (I started to say that in my OP.) The focus needs to be 100% on the question at hand. If it can't be solved with that.....then we move on.

 

[GC_Hopi]

The connection would need to resist the load you are applying, i.e. for every force there is an equal and opposite reaction. If you are connecting the blocks by pulling 1 block then the force is W(µk)+seal force. If you are connecting the blocks by pulling 2 blocks then the force is 2W(µk)+seal force. This problem is dependent on the sequencing you use to seal the blocks.

 

[WARose]

If you are connecting the blocks by pulling 1 block then the force is W(µk)+seal force. If you are connecting the blocks by pulling 2 blocks then the force is 2W(µk)+seal force. This problem is dependent on the sequencing you use to seal the blocks.

That's a excellent point. The sequence I had envisioned was sealing block 1 & 2 first, then 2 & 3 and so on.

So basically starting from left to right. The threaded rod wouldn't even be present between (say) blocks 3 & 4 when 2 & 3 are being sealed.

 

[GC_Hopi]

In the scenario described, sealing the blocks sequentially, the force would be W(µk)+seal force as you pull the new block and add it to the group. I could see a scenario where you could work on sealing pairs of block simultaneously then pull them together in a group of four then use closure pours to seal between the groups. I guess in the end the possibilities for sequencing are quite large.

 

[CANPRO]

I don't think you have an issue here. I'm not sure it matters what order you work in either. I'm assuming the blocks need to move a very small amount to provide the seal. Look at it step by step.

[ol 1]
[li]The threaded rod between block 1 and 2 is tensioned to a force F. Block 1 and 2 both move slightly providing the seal, both are restrained by friction and the weak seal spring.[/li]
[li]The threaded rod between blocks 2 and 3 is tensioned to a force F. Block 2 now has equal and opposing forces applied - it remains static and does not require friction. Block 3 moves slightly providing the seal, and is restrained by friction and the weak seal spring. The tension in the rod between blocks 1 and 2 is unchanged, and block 1 is unaffected.[/li]
[li]The threaded rod between blocks 3 and 4 is tensioned to a foce F. Block 2 and 3 now have equal and opposing forces, no fricton required, block 3 doesn't move. Block 4 moves slightly providing the seal, and is restrained by friction and the weak seal spring.[/li]
[/ol]

As long as an adjacent rod isn't over tensioned, I think you're good. To unseal an adjacent tunnel I think you'd have to overcome the tension in the adjacent rod plus the static friction provided by a single block for the outer rods, and by two blocks for the interior rod. In your diagram, I think your tension is wrong for the rod between 1 and 2 - if the tension in the rod was more than 2x the friction provided by block #1, that sucker would be accelerating.

 

[WARose]

1. The threaded rod between block 1 and 2 is tensioned to a force F. Block 1 and 2 both move slightly providing the seal, both are restrained by friction and the weak seal spring.

2. The threaded rod between blocks 2 and 3 is tensioned to a force F. Block 2 now has equal and opposing forces applied - it remains static and does not require friction. Block 3 moves slightly providing the seal, and is restrained by friction and the weak seal spring. The tension in the rod between blocks 1 and 2 is unchanged, and block 1 is unaffected.

3. The threaded rod between blocks 3 and 4 is tensioned to a foce F. Block 2 and 3 now have equal and opposing forces, no fricton required, block 3 doesn't move. Block 4 moves slightly providing the seal, and is restrained by friction and the weak seal spring.

That's kind of how I see it. But I worry about one block having more sliding friction than the other. Coefficients of friction can be highly variable.

Thanks for the feedback.

 

[CANPRO]

I don't think varying friction values will matter much either - provided they're both greater than the required sealing force. It might make a difference for the first rod to be tensioned - if the adjacent blocks have different friction values, one will move more than the other...but I'm assuming the movements required to provide a tight seal are small, and any differential movement would be insignificant.

 

[racookpe1978]

No, the sealing force (compression of the seal against its mating surfaces) will be much, much less than the friction force.

But, even if each section only moves a tiny bit (<1/4 inch, 1/16 or so) then the accumulated movement will keep increasing as each section is added to the front of the string. The "string" of connected sections will always have more friction than the individual (last section) to be added - although! rocks or debris or accumulated "gunk" will pile up on the faces as they get pulled together.

So, assume 8 sections, each only needing 1/16 inch to compress the seal. The last section needs to move 1/2 inch. The static friction (prior to movement beginning) needs to be overcome by the bolts and their driver, then the running friction, then the compression seal forces. All assuming there is no gunk or rocks being squished or sheared.