mountain top mining fill settlement 1

[NASC]

Anyone on the mining end have literature about the settlement of blast rock spoils from mountaintop mining used to fill in some deep valleys?

The fill soils consist of end-dumped gravel, sand, silt and clay from blasted sandstone and shale with lots of cobbles and boulders. No structural loads to be applied, just self-weight.

Thanks.

 

[Backfill]

NASC,
Self compaction won't get you far quickly, but if some tension cracks and settlement depressions are OK, check out your original material void ratio or bulk density. Either will give you a handle on unit weight or unit volume, which you can then use to back into a target densification or volume. Of it's own, it will take forever for end-dumped material to consolidate back to that original condition. Shallow fills may get most of their consolidation across one full year (i.e. wet and dry seasons), especially with reclamation shaping or other traffic loading.

How high (deep) are these fills? Will you have diversions above them? Do you expect to have the phreatic surface to deal with down the road? Will there be drainage or springs underneath them?

How much settlement can you live with? Without engineered compaction, you're probably going to get several percent settlement, maybe above ten. That may happen even with a mixed gradation if there is flow below in the valley bottom. Depending on the fill rate, you may be able to make up that settlement as you continue the end dumping. If you are relatively fast, you may complete the fill before the settlement occurs. In that case, overfill by that several percent and life will end up good.

Contour or shape the dumps to minimize erosion, plant some trees or shrubs, sell it for a view, or at least a new dispatch trailer.

We did a 500,000 ton canyon pre-fill to prepare a leach pad base. We ran the dump material in (shot limestone, sandstone, shale, with sand and clay soils also) and layed it down on advance in 10-ft lifts. That was using 85-ton fixed frame end-dumps, and tracked dozers to tend dump. We got nearly 95% modified Proctor field densities, and virtually no settlement. That was important, as we used that fill to smooth out an area over which we subsequently laid down a composite liner. I need to make clear that at its deepest, this fill was only about 70 feet (i.e. seven lifts).

Try:
McCarter, M.K., ed.,1985, 'Design of Non-Impounding Mine Waste Dumps,' AIME-SME, NY ISBN 0-89520-445-2

Hartman, H.L, ed.,1992, 'SME Mining Engineering Handbook,' SME, Vols. 1-2, Littleton, CO, ISBN 0-87335-100-2

You also can get Hartman's texts on CD from

http://www.smenet.org.

Hope this helps. Like always in dirt, shed the water and life will be kinder, unless you're farming.

Good luck. Please keep us posted on your decisions.

 

[NASC]

Thanks for the references, I'll check them out.

The fill was end-dumped 5 yrs ago before this project was even considered so we are out of luck on requiring compaction, but at least some of the initial settlement will have occurred. Fill heights are up to 230 ft in the valleys, so deep ground improvement is more or less ruled out. There will be surface water diversions and lined SWM ponds to prevent surface infiltration. The deep borings showed no buildup of water in the backfill from any springs or ephemeral streams, but for slope stability calcs we assumed a phreatic surface anyway.

The client wants to route utilities across the deep fill without appreciable settlement needing flexible connections. We are requiring deep foundations but I thought that I should check and see if there is some basis to show that my settlement values are too conservative. We have asked the client to do settlement monitoring so we can get an idea of rate of consolidation/settlement.

Thanks again.