Foundation on mine's rock waste

[Canadaian]

An industrial plant location have been chosen by the client. The selceted location is a disposal area of mine's rock waste. The material is heterogenous uncontrolled crushed rock of sand to boulder sizes up to 600mm in diameter. Some of the waste is proceesed product and is "oxidized" which result in high to moderate weathering for some of the rock fragments with strong sulfuric odor. Few test pits was opened in the site I discovered some debries are there as well the thickness of the rock waste fill can not be established . Clay and organic was encountered in other test pits. Rock bed should be shallow however no sign for bedrock within the excavated depth of 3m "backhoe max reach" in summery I do not know how the rock waste fill is dumped and what is the thickness or what is the natural ground looks like, apparently no uniformity can be assumed. My next step is to recommned more detailed and careful site investigation.I do have the following questions:
1- what drilling method would be suitable to pass through the rock fill of boulder size?
2- If the bedrock is relativly deep to be used as bearing layer... is subcutting the exisitng rock fill to certain depth and backfilling it again in controlled manner will provide proper foundation. Plant will include some large tanks 75 ft dia, 20 ft high.


Any thoughts....thanks in advance

 

[msucog]

the drillers i know would probably say that there's too much of a chance of getting the augers hung up in the rock fill. i'd stick with pits.
as far as foundation support, from my experience with rock fill, i would not recommend building on it if its placement is undocumented, especially not for the size tank you describe. even if the material is relatively small in size and the chance for large voids is minimal, there is still the potential for settlement (it sounds like you have more of a possibility of larger voids spaces). of course, if you ask some contractors about #57 sized aggregate, they'll claim it is self-compacting and doesn't need to be densified...those funny contractors and the stuff they come up with.
if i were in your shoes, i would perform test pits with a large tracked excavator to try and find the bottom of it. even if you can't find the bottom with say a 20' reach, the owner is aware that he's got a lot of excavation to do...at that point does it really matter if it's 25' vs 20' (not really in my mind). if they really want to find a bottom deeper than that, they can put up the money to get a ridiculously large excavator for pits...at some point, it'll be too dangerous to dig deeper while sitting at the edge of the hole.
after that, i would recommend that it all come out (as i suspect the owner is not willing to accept the risk of putting a chemical tank on something that might settle significantly with no way of being able to estimate the settlement of that in place fill) and go back in as a controlled fill with particular discussion about the acceptable fill material criteria and placement procedures.
good luck.

 

[civilperson]

Excavation and replacement with a controlled fill on top of crushed rock columns is a reliable method of establishing competent base for tanks and such. The insertion of the stone columns densifies the surrounding soil and using a grid of columns with spacing equal to the controlled fill will produce satifactory bearing capacity.

 

[valleyboy]

Civilperson has suggested the use of Vibro-compaction. If the site is large enough to accomodate the plant, you could also consider the use of dynamic compaction.

 

[Patgeotech]

Undertake the trial holes using a bigger excavator to assess the consistency of the rock waste at depth. Dynamic compaction could prove to be the solution to the problem should the waste be found to be loose. The one problem with dynamic compacting rock waste is that the weight tends to 'bounce' and the compaction effort becomes questionable. A buffer layer would need to be placed on the rock mass - this could act as a plug, which you could place engineered soil layers on.

Depending on the size of the area concerned, one could use rapid impact compaction that could prove to be faster than dynamic methods, that is if time is a factor?

 

[jdonville]

Canadaian,

Suggest that you consider incorporating geophysics into the site recon. It could save you much time and trouble if the site is large enough.

Jeff

 

[Canadaian]

Thanks all for your valued contribution.

I am in the opinoin to proceed with test hole drilling to identify bedrock depth, rock fill and overburden thickness. Rock fill apparently did not placed in controlled manner I ll subcut to some depth, I am estimating bout 2.0m and backfill the same material in layers, compact with loaded construction equipment, place a foot or so of 3 inch down material as leveling pad on top of the rock fill...initially an allowable bearing capacity in the range of 250 to 300 kpa seems safe and acceptable to me, settlement estimate to be provided once thickness and propoerties of the overburden are made known to me .

How is that sound?

 

[casimmons]

How is a 6ft (2.0 m) undercut going to help with the area load from a 75 ft diameter tank, expecially when you don't know how deep the fill is, or what it is like below the depth you explored (only 3 m)? Depending on the depth of fill dynamic compaction could work. I don't think that Vibro-compaction will work- the large boulders may be hard to penetrate by either driller or vibrating a hole.

Hollow stem auger drilling will probably not penetrate this fill. You could have borings done using some percussive methods. That will penetrate the boulders, but will not result in any samples. So you will have to carefully observe the drilling to determine the thickness of each layer.

You need to have more information to make a good decision with respect to foundation support options.

 

[Canadaian]

Casimmons, thanks

True, no information is yet availble regarding the depth of fill and nature of mateial underneath....thats way I am heading for detailed investigation.

Vibrofloat , we have some experince with it in densifiying material up to 6 inches I am not sure if it would help in the fill nature that we have in the site ( boulders >2 ft).

large tank of 75 ft dia and 20 ft hieght filed with water what is the load...about 100 kpa. Do not you think that 2 m of preapred rock fill will help in distribute and reduce the stress intensity? Do you think that influnce of a dynamic compaction could reach 2 m below the surface and preapre the rock fill in equivelant way to 2m subcut and recompact?

I may consider some sort of dynamic compaction ...just a bulk load attached to crane or excavator to prepare the base of the sub cut.

Thanks again

 

[dgillette]

Two things come to mind:

If the boulders are not very hard, you might try a Becker drill, basically a diesel pile hammer with double-wall casing and compressed air that lifts the cuttings out to a cyclone for collection and logging. It won't get you good samples, but it can get you something, and can be used as casing for sampling when you get to softer material. The Becker drill can also act as a crude penetrometer of sorts. (Use as a penetrometer for liquefaction assessment is fairly common and standardized, but that requires a plugged bit, which would be harder to drive through the boulders.

For compaction, look at:

http://www.rapidimpact.ca/index.htm

I have no experience with it, but it looks cool and might be useful. Probably not too expensive to mob to your site since the equipment is small (relative to DC equipment). Its depth is limited to something less than the 10 m you can get with large dynamic compaction equipment.

Both are Canadian inventions, I think.