Innovative ideas for underground tunnel repair 1

[pbc825]

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.

 

[r13]

Where is the source of chloride, point of attack, how bad the concrete lining is, what is the depth of the tunnel from ground, type of soil, or rock?

 

[LittleInch]

"but we may not be able to consume that much room internally" - how much is "that much"?

What's causing the requirement for replacement?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[pbc825]

Thank you for your quick replies.

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.

 

[r13]

Is this tunnel under a structure. Is there a way to install piling on its sides? At this point, I am thinking underground excavation for construction access to repair from outside, that is less intrusive to the mining operation.

 

[BUGGAR]

It sounds like you could drill and grout around the tunnel from above to stop the water intrusion. Could you grout sufficiently to demo and replace the existing tunnel in sections?

 

[pbc825]

retired 13:
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?

 

[pbc825]

BUGGAR:

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?

 

[miningman]

It wont be cheap but ground freezing might be part of the ultimate solution. If the KCl is really originating as ground water flow ( which I doubt) then dewatering wells would minimize future ground water flow. If you have zero mining or grouting experience , you probably have to admit this to the client. This is not a project for inexperienced personnel , and that is why it WILL be expensive.

 

[LittleInch]

Well it sounds like the structural integrity of the current concrete is shot so all the grouting etc options are worthless.

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.

 

[r13]

The above is correct, you need help from the mining group, as well as geotechnical deep soil improvement technics. Provide a cross section (from ground down), if it is available.

 

[miningman]

LI, several million dollars is chickenfeed compared to the value of lost production during repair efforts. Nearly all the potash mines in Sask are now 40-60 years old and the infrastructure is failing inm the same modes. These repairs will likely be co ordinated with other departments so the downtime can be minimised. Probably the project will run into Billions$$. The last similiar one I was on ended up at around $4.5 billion over 4 years.

 

[r13]

Yes, the key is minimize the downtime.

 

[pbc825]

MM:
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.

 

[amlinerrichard]

What are they using the tunnel for? Can you expose one end of the tunnel?

 

[pbc825]

amlinerrichard:
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?

 

[amlinerrichard]

You may be able to slip line the tunnel with a pipe that is not attacked by the chlorine. Then pressure grout the annular space between the pipe and old tunnel.

 

[pbc825]

amlinerrichard:
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.

 

[miningman]

Well a D11 could almost certainly push the pipe , but where end of the pipe ends up , or what condition the pipe would be in , might be somewhat problematic. Thats the trouble with ideas originating from outside of the mining industry from persons with zero knowledge or understanding of the specifics of mining.

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??

 

[amlinerrichard]

I thought you said the tunnel was about 11 ft. tall and 9 ft. wide.