Uncompacted rock fill 1

[sgsibob]

Does anyone have any data on the compressibility or settlement of tall (>50 ft) end-dumped and uncompacted rock fill? This occurs in surface mining applications when waste dumps have to be used for some thing else.

Information on the shear strength (related to some measure of particle durability or hardness) would also be helpful.

 

[concretemasonry]

Totally unpredictable on a surface mining operation. The rock is shot down to a size that can be loaded, hauled away and dumped. Absolutely no control over the size and only a general idea of the shape of the rock. If it was only 50' high, it could have been dumped overnight by the operations people and not contractors operating under a contract with specifications. The shape of the rock (flat or cubic) also has an effect.

I was on a project at a mining site and the plans were for the operations to place a dumped rock fill from strippings that was up to 150' high (into an old mine pit) and about 400' long.

Low and behold there was a change in the operations schedule and the entire fill (4' and down rock) was placed on a 24/7 operation over a long week end and it was in place on Monday morning 3 weeks before the scheduled date. Since it was to be a access road for 2500 vehicles/day (in and out), we chocked the rock, compacted a base and paved it.

For quantification numbers, about 2 monthes later the entire fill settled about 15' enmass overnight, so we just built on top of that with a new base and asphalt pavement. That was about 25 years ago and the road was in fine condition with no noticeable alignmentvariations.

Bottom line is you can try to do your best at making assumptions, but really do not have control of the effects if the owner paying the engineering fees has control over the machinery, materials and schedule, you can always show your design and leave the rest to the operations people.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[cvg]

a significant part of the settlement will be of the subgrade. you should at least be able to calcualte that part. the rest should occur during construction or shortly thereafter.

 

[sgsibob]

Dick,

Thanks for the information. Wow. 15 ft. That is just the kind of behavior that a lot of mine owners will swear will not happen. Sounds like a coal stripping operation. Terry West from Purdue has started studying settlements in subdivisions platted on top of old reclaimed strip mine spoil piles and the results are all over the place. There have been some damaging differential settlements at places and at other places everything is OK. In your case it sounds like spoil material that may have had an appreciable component of non-durable rock (claystone or siltstone) in it was probably affected by material degradation which evidenced itself overnight (although it may probably was progressing in the subsurface since placement). There may have been a major contribution from the expulsion of water from pit bottom sediments though.

The present interest is in a hard rock mining setting, which if anything is even tougher to pin down because of the variation in material types -- heavily altered and weathered at places and stronger at other places. The issue does, as you mentioned, have to do with particle size, particle size distribution, and particle durability, as well as the dimensions of the dump and the rate of placement. I was just looking for any empirical data or experience to get a general idea.

CVG, in most cases the subgrade deflections would be a negligible percentage of the self-weight compaction of the loose rock fill. However, as you may have been thinking, there may be instances where placement has occurred over thick, soft horizons and that could present possibility for delayed and significant settlements.

 

[concretemasonry]

sqsibob -

The rock dumped was very hard and durable rock (taconite, gabbro(?) and some 2" seams of hard granite in the inplace materials). The mine pit it cut across was on older (5 to 20 year old) iron ore mine with and hard taconite rock at the lower 50'. - All the materials were very tough and durable to a fault.

The material being dumped was from a rock excavation up to 70' high for a railroad line. There was about 1' of topsoil (down to nothing in places). The upper portions of the native rock had some fissures that made blasting tough because of the energy absorbed and the shot rock was about 6" to 12" in size. The lower portion was very solid and easy to blast, and there were a lot of 6' to 8' shot rock that had to be mudcapped or drilled and then blasted down to 3' to be loaded and moved.

From my eye, the maximum settlement was about 15% of the maximun fill depth and the settlement came within 6 months or so of the placement and was due to the physical loads, shifting and consolidation of the fill material and not from decomposition or working into the 5' of crystal clear water, typical of the abandoned open pit mines with very little sediment i the bottom.

The whole fill was designed, but because the owner had an excess of equipment available and needed a way to keep the busy and get rid of the rough rock excavation that they were responsible for. When the fill was brought up to plan grade, the top of the fill for the access road was widened by about 40' since the equipment and sound fill material was readily available. - Not the "by-the-book" way to do things, but M-K and the other contractors could not respond quick enough since the access road served all truck deliveries plus the car traffic for the 2500 workers on site 24/7.

This information may not be applicable to your purpose, but it is an example of one situation.

The whole several years on the job was an eye-opener when I was also an owners representative on the contruction of a 2 mile long tailings dam through a swamp, connecting silty sand hills that required about 3,000,000 yards of clay and sand. Seeing clay and sand being excavated and hauled 2 miles at 0F to -40F on a 24/7 basis was also a "not-by- the book" operation, but the dam through up to 35' of muskeg swamp could not be started until the middle of January when the swamp surface could be made stable enough for dragline access.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[miningman]

Dont be too hard on the mine operators or the blasters in cases like this. I have lots of experience in building waste dumps on mining properties 50-150 feet high, typically by simple end dumping from whatever elevation seems appropriate.

One of the benefits of this is the coarse rock naturally forms a french drain at the toe of the dump which is usefull for stabilitiy purposes anywhere but especially when dumping into a swamp area. And yes at 10,000 tons per day, with a strip ratio of 3 or 4:1 , I can generate and place a lot of rock in a long weekend.

For a hard rock to settle 15 feet I would expect there to have been a failure of the slope face. There are warning signs of this being imminent , but you'll never see them in the office or text books.

And remember , its the profits that the mine operators make that generate the revenue to pay the invoices of consulting engineers etc. Production is the name of the game.

 

[sgsibob]

Concretemasonry,

Thanks muchly for the post. That is relevant to our issue, as I was referring to hard rock mining more so than coal stripping. The point is that thick, rapidly-placed rock fills adjust to the stresses induced by edge and point contact in complex ways and cannot be assessed using traditional consolidation mechanics. Rock fills constructed in lifts and compacted using rolling equipment are a completely different animal. In my experience the "compaction" that occurs as the incidental consequence of rolling over the fill surface with loaded haul trucks and dozers is not adequate to alleviate settlement on highways. At any rate, I was hoping to rely on empirical experience. It sounds like in your case there wasn't noticeable settlement except for one relatively sudden event that may actually have been an internal failure. Thanks for the very interesting post.

Miningman,

Thanks for the post. I sympathize with your defense of the mining industry -- I work in that industry a lot and it is amazing what we do. This came up due to the proposed relocation of a public highway through an area that will be built up with uncompacted, end-dumped rock fill and before the area is paved and put to public use a better estimate of settlement would be helpful. Because the fill will at places be above 100 ft thick even 1% settlement will noticeably deflect the highway. As long as the strains due to settlement are not too great this can be accommodated by flexible pavement. If the settlement gets to 5% then there are substantial impacts to the roadway profile. Previous rock fills in the vicinity have settled noticeably. Some time back I had occasion to specify a similar pit backfill that would would buttress a tall slope and also serve as the foundation for a conveyor. In that case we specified a conveyor alignment that avoided the thickest fills, because of settlement concerns, but we had no good way to calculate the settlement.

 

[fattdad]

refer to CPGR#41 at the following link. You'll have to buy it, but it has very good discussion about valley fills (typical for mining and other thick fills).

http://www.cgpr.cee.vt.edu/index.php?do=read&content=publication

I work for Virginia Department of Transportation and we have recently completed a 2.5 mile section of the "Coalfield Expressway," which will ultimately be a long through road in coal country. We are getting our rough grading performed by the coal company who is building fills (up to 250 or 300-ft thick) using the valley fill method - end dumped 50-ft thick fills. Large rocks race to the bottom of the slope, smaller rocks to the middle and such. The fill placement is basically an inclined surface ala the "angle of repose" (yeah, I kind of hate that term, but you get the idea, eh?). The publication I've referenced discusses this placement method.

In our case, we get the rough grading and then wait for a spell.

On a contrasting, but similar note. Back as a consultant (i.e., before joining the public sector), I was involved in two damage claims where owners' buildings suffered damages from thick rock fills. Frozen weather placement, and too-rapid of construction following placement were big factors!

Check out the "Settlement of Valley Fills." It'll give you good information for making your own decisions.

f-d

¡papá gordo ain’t no madre flaca!

 

[msucog]

there's a lot of very complicated variables involved with this but you might as well call it "unpredictable". one might be able to pull a number out of thin air but the accuracy of that number will be very poor i'm quite certain. it's hard enough to "accurately" predict settlement and time in residual soils with known fill placement parameters and reasonably accurate insitu/lab testing. based on what i know, the most practical approach is to take broad assumptions to develop a wide envelope of settlement potential, construct the thing then play the waiting game with the understanding that there's always going to be elevated risk associated with building on/around such fills.