Concrete Mine Shaft Cap

[cuels]

I have an interesting request from a client that we have worked with for years. They operate a mine. They follow veins of gilsonite which leave 3' wide "trenches" below the surface. They have always had the guideline of leaving a 15' thick bridge of gilsonite in place to hold the overburden and anything that passes over it in place. However, this is where the best quality gilsonite is typically located. They would like to investigate the cost of replacing that bridge with a much less thick bridge of concrete. Their plans include placing the concrete on top of the gilsonite, and then mining the gilsonite from below. It seems as though the method they would like to use to stabilize the surface would require a knowledge of the shear strength of the rock walls and of the concrete (almost a friction surface, if you will) to support the overburden. Has anyone else encountered this situation, and if so, what analyses have you investigated to correctly model the strength of the concrete bridge/rock walls. We thought about comparing the strength of the 15' thick gilsonite to the strength of the concrete and replacing an equivalent thickness of concrete. This would have to assume that the rock walls are much stronger than the concrete and gilsonite.

If anyone has any experience or guidance, please provide your input.

 

[ishvaaag]

Even with expectedly stable rock walls suppresion of significant of material in between may mean some movement and action over th slab that could prove detrimental to the idea. If really there's othing to fear from such viewpoint, dimensioning some slab to bridge some quantity of material above it would be not much deal (even arcing action could be taking unto account - if remains enough material to act as key, of course). The other relevant item would be the actual ability to mine in proper way as to be able to place the concrete slab atop the material to be exploited without untolerable risks.

SO for your calculation assumed lateral walls stable you would need the span (3 ft) tha amount of material to be supported, some expected actual compressive strength of the concrete once put in place, and from this derive its capacity on flexural tension according to the code. You may want use quick gain of strength concrete so no lees f'c than 35 MPa typically, and then you should have reliable values of the modulus of rupture in flexure of the concrete in between 1.5 and 1.75 MPa between 2 and 3 days after placement, or check with the supplier after making test of the quality of the placed concrete.

 

[ishvaaag]

Well, something more than 3 ft nominal span, to account for supports, say 1 ft more.

 

[ishvaaag]

And all this of course if the rules for mining allow such slab without reinforcement, that at such small span may be the convenient solution; if not simply change the way of dimensioning the slab as a reinforced one, in any case counting with proper safety factors.

 

[ishvaaag]

A much divided answer, but not forget any loads at the surface that can become of interest, what will become true if close to the surface.

 

[dhengr]

cuels:
Your’s is a mighty big question, with very little relevant info. to go on. How close to the surface is the top of these gilsonite seams? Can you trench down to that elev. to do the conc. work and then backfill? Is the elev. of the top of the gilsonite essentially the same as the top of the confining rock material? What is the variation in the mechanical properties of these various materials? Variation in seam width, overburden depth and make-up, any surcharge loads over the trench, depth of seams (left empty, I assume) after gilsonite is removed? Earthquake design considerations in your area? I assume the new conc. bridge is to protect the miners during their work in the seam and when they are done to prevent subsidence cave-ins above.
Your client is running a mining operation, I would think they would already have a good deal of this info. Their 15' gilsonite bridge is apparently conservative if they have not had any failures, but leaves a valuable resource in place. I would get a local GeoTech consultant involved in developing this scheme and monitoring its progress. They will have to do enough soil borings and testing to develop some understanding of the variations in conditions, and then some planning for changes in design and procedures as a function of these variations. I don’t think I would rely on a friction surface btwn. the confining rock and the new conc. bridge. I would key into the rock as if to take the thrust from an arch or for a horizontal bearing surface for the conc. slab to react on and slide on.
If you have to do this all from below the 15' bridge of gilsonite, you have a whole different animal. If the overburden is of the right type you might use some form if injection grouting to make your new conc. bridge. The local conditions and your friendly local soils engineer, will be able to offer various solution better than I can from here.

 

[Prepakt1]

Grouting would depend on the material above the gilsonite. The
more porous the better the grout bond. Fractured rock would be best. Any type of soil would cause problems with getting
high strengths. Basically the more porous and rock in the material the better. You can drill down or up and then grout into the material.

Intrusion Prepakt /marineconcrete.com