Innovative ideas for underground tunnel repair
Innovative ideas for underground tunnel repair
(OP)
I'm working on a project for a mining operator. They have a reinforced concrete tunnel under a product storage structure, and the tunnel is highly chloride impregnated. Replacement in-kind of the tunnel is an option, but the price tag is rather high and indirect costs from lost production make it extremely unattractive to the client. The tunnel was constructed by excavating to the floor elevation, prepping for a floor, placing a floor, then walls, then roof (all with shear keys at the interfaces) to make a rectangular opening for several hundred feet then back-filling on top then constructing the storage structure above the tunnel. All tunnel elements are about 20" thick with two layers of reinforcing steel. Inside dimensions are about 11' tall and 9' wide.
I've had the ideal of constructing an internal tunnel, but we may not be able to consume that much room internally.
I've had the idea of encapsulating the tunnels, then raising the internal gauge pressure (as was done for caisson construction, Brooklyn bride towers for example), but I don't like the safety challenges associated with it (I think the/a Brooklyn bridge engineer died of the bends).
If anyone has experience with innovative repair/replacements of underground tunnels in-situ (i.e. without excavating to the top), I am interested in discussing with you.
Thank you in advance.
I've had the ideal of constructing an internal tunnel, but we may not be able to consume that much room internally.
I've had the idea of encapsulating the tunnels, then raising the internal gauge pressure (as was done for caisson construction, Brooklyn bride towers for example), but I don't like the safety challenges associated with it (I think the/a Brooklyn bridge engineer died of the bends).
If anyone has experience with innovative repair/replacements of underground tunnels in-situ (i.e. without excavating to the top), I am interested in discussing with you.
Thank you in advance.
RE: Innovative ideas for underground tunnel repair
RE: Innovative ideas for underground tunnel repair
What's causing the requirement for replacement?
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RE: Innovative ideas for underground tunnel repair
retired13:
The source of the chlorides is ground water with high chloride content. The client is mining potash. The weeping-tile drain is clearly not working, and groundwater is draining into the tunnel through wall and roof cracks rather than through the drain. I understand the same mechanism is pervasive through several hundred feet of tunnels.
I haven't seen the chloride report yet, but from the photographs I've viewed it appears as though the existing concrete is ruined and unsalvageable. Primary bars are corroded through. Concrete looks like porridge in some areas (specifically the floor to wall interface).
The top of the tunnel concrete is 12' from the top of slab above. There is fill between the roof and slab above. It may be well-graded granular fill or clay fill (TBC), but certainly soil.
LittleInch:
I'm not sure how much inside room the client has to spare. I'll confirm during a site visit. If they have lots, I'm sure I can design something that's thinner than the existing elements using steel (carbon or stainless).
The walls are beginning to show large horizontal cracks. The concrete looks horrible at the bottom (I suspect there may be a wall to floor shear failure beginning, which would be easy enough to fix with a new floor). Bars are exposed and corroded through in some cases. There are clearly some figurative dead canaries in the area.
RE: Innovative ideas for underground tunnel repair
RE: Innovative ideas for underground tunnel repair
RE: Innovative ideas for underground tunnel repair
Yes, the tunnel is under a long structure. To my knowledge, there's a slab-on-grade directly above the tunnel. The mining operation stores finished product on the slab on grade with an arch building above the slab. The arches reach well away from the tunnel and slab.
I don't have any new-tunnel/UG excavation experience, but that's a good suggestion. I'll see if that's an option.
Any other experiences out there?
RE: Innovative ideas for underground tunnel repair
I don't have any experience with grouting. Should I reach out to a grouting contractor to see if this is an option?
I suspect you would need lots of grout thickness for a replace-in-sections approach if the existing concrete is 20" thick. Would the grout tend to consolidate the surrounding soils or bulge the overlying slab-on-grade, or some combination?
RE: Innovative ideas for underground tunnel repair
RE: Innovative ideas for underground tunnel repair
So it's a 9ft segmental circular tunnel section or a bespoke rectangular pre cast segment that you can push into position.
Several hundred feet sounds like several million dollars to me.
Most steels won't stand high chlorides either.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Innovative ideas for underground tunnel repair
RE: Innovative ideas for underground tunnel repair
RE: Innovative ideas for underground tunnel repair
RE: Innovative ideas for underground tunnel repair
That's good advice, and I agree I will defer to those with the right experience if ground freezing, or grouting is contemplated. I'll see if I can follow up on my chlorinated-groundwater theory as well. There are certainly chlorides in the concrete, but perhaps they are coming from the inside rather than the outside.
LI:
I'm hopeful we can extend the life to a downtime in which replacement is feasible and attractive to the client. If we have room for steel, I'm thinking a waterproof membrane, new steel wall/roof (likely bolted together), then place concrete for another floor with a slope and trough (for chlorides originating on the inside surface) will get them to a place that replacement will work.
For the most part, mining operators are willing to shut-down for maintenance and repairs, but it needs to be for a reasonable duration and coordinated across the entire operation. I'll try to follow up after the site visit.
RE: Innovative ideas for underground tunnel repair
RE: Innovative ideas for underground tunnel repair
The tunnel is part of material handling system. A conveyor and small adjacent walkway populate the tunnel. We may be able to expose one end of the tunnel. I'll know more following the site visit. Would that be a benefit?
RE: Innovative ideas for underground tunnel repair
RE: Innovative ideas for underground tunnel repair
The pipe would need to be 19' in diameter. Is it realistic to find equipment to push that? We could build the pipe, but I'm interested if there's a machine able to push it in Western Canada.
RE: Innovative ideas for underground tunnel repair
With my very limited knowledge of the site specific cirumstances , I doubt that this task can be undertaken without a complete shut down of the material handling system within the tunnel, the ripping out the entire system and the rebuilding the tunnel by means presently unknown. Say 12-15 months minimum with the mine essentially shut down. What is this, around 2 million tonnes per year??
RE: Innovative ideas for underground tunnel repair
RE: Innovative ideas for underground tunnel repair
RE: Innovative ideas for underground tunnel repair
If all you really need to stop the thing falling down is a whole set of acrow props and a bunch or either timber or steel support frames then you should be able to erect those in situ around what ever is there.
I can't see anything external to the current tunnel being strong enough if the current structure is as bad as is being reported.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Innovative ideas for underground tunnel repair
RE: Innovative ideas for underground tunnel repair
RE: Innovative ideas for underground tunnel repair
RE: Innovative ideas for underground tunnel repair
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