Construction on old mine tailings

[GilBor]

I am trying to decide about the purchase of a land property (5000 M^2)
located in an area used by a mining company to dump tailings
about 50 yrs ago. I hired a soil engineer to help me make the decision
and am seeking farther advice (second opinion) from the experts in
this forum. Details follow.

The property is located in Guanajuato (Mexico). The tailings were
layered on a slope of a hill, forming a big flat terrace sized about
200 by 100 meter. The soil is mostly fine powder, with some thin layer
of top soil (max 30cm) with some vegetation (trees up to 3 meters
high). I intend to use this land to construct 3 brick houses (for
myself and 2 friends). A soil engineer from a nearby university, who
also works as a consultant for foundations, looked at it, took some
superficial samples (the owners wouldnt let us make holes deeper then
1 meter) and is supposed to send me a lab report within several days
with recommendations for foundations. He said I will probably need to
build the house on a concrete slab located on top of a treated layer
of imported material (soil moved from some other site, i guess).

Specific questions:

1. Is it reasonable to build a (modest) house on such a soil? I mean,
can you make not-too-expensive foundations that will support the house
for many yrs?

2. In case the answer is yes, is the aformentioned method the right one
or I should look into other methods?

 

[tincan]

Are these the old silver mines to the north and east of the city? If so you will have one beautiful view. The removal of the top layer and its replacement of a more compatable layer (compactable granular soil)is an accepted method. I would construct a waffle slab as an added precution.

Good luck, Tincan.
P.S. We will build in San Miguel de Allende, GTO next year.

 

[sgsibob]

Sounds like the material is mill tailings (as opposed to waste rock, often erroneously called "tailings" by laypersons). Mill tailings consist of ground rock particles and are generally transported in slurry form and settle under water. As such the relative densities can be appreciable. With that said, the materials have not typically been subject to any controlled form of deposition, nor have they been in place long enough to densify themselves naturally. If water was scarce, it is entirely possible that portions of the tailings were dumped dry or in a loosened condition. This is, after all, Mexico. I would be very concerned about variable densities unless site investigations prove that the densities are both high and consistent. Settlements can originate from quite deep within a tailings pile -- certainly deeper than the 1 m that you are currently being permitted to investigate. Mill tailings can also retain considerable proportions of water which can cause foundation problems, not the least is liquefaction, in seismically active areas.

The philosophy of supporting the houses on mat foudations may be workable provided the foundations are extensively reinforced to guard against differential settlements.

I would definitely explore the full thickness of the tailings deposits with SPTs or cone penetrometer at a minimum. If the vendor has a problem with that, he (she) probably knows something you are not to be permitted to find out, and you ought to consider walking away from the project. Surface soil samples from 1 m depth just won't suffice.

 

[GilBor]

Thanks alot for the prompt replies and the inetersting and valueable information.

Answering tincan: yes, these tailings como from old silver mines (gold also, i think), to the north of the city of guanajuato, in the "valenciana" area. the view is spectacular and the price is attractive (else we wouldnt bother with the soil question).

to sgsibob: you are exactly right, these are mill tailings
pumped into the area in liquid form (slurry) during yrs, then when it dries it looks like fine powder (dust). i dont think the mine company is trying to hide something by forbidding deep exploration, they are just impatient
with lengthy negotiations and want the deal quickly done.

I appreciate yr suggestions for deeper samples and extra caution about variable densities and humidity. i guess the concern is that different parts of the house will sink in at different rates resulting in stress in the structure of the house and possible fracture, right? but, if you may excuse my laymen terminology, isnt the idea of "mat foundation" suppose to solve this problem, as it makes the whole structure of the house move rigidly like a ship on the waves?

when the lab tests of this soil come in a day or two i can report it and it can help you help me...








Gil Bor, Math Reserach Centre (CIMAT),
Guanajuato, Mexico.

 

[Focht3]

Be Careful! I'm with [blue]sgsibob[/blue] with regard to why the owner refused deep borings. It would only take an hour to go an extra 5 meters - so why say 'No'? Because you have something to hide -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[sgsibob]

The idea of a mat foundation is to have the structure float on the subgrade, true, which is why I mentioned adequate reinforcement. also the entire structure can tilt, and the wall-floor connections are not perfectly rigid, either.

Take the time to insist on some cone penetrometer testing at least. Would only take a few days if you can find a vendor in the area.

 

[tincan]

I concurr with tha above. It allways pays to investigate fully the foundation before construction. Better before than afterwards. A rigid foundation, either a thickened slab (mat) or a waffle type will better resist differential settlements. The removal of the top portion of the inplace soil and its replacement with imported material serves to remove any surface deficiencies and to provide a more uniform bearing surface for the floor slab.

?Are you going one floor or more with your houses?

Best, Tincan.

 

[GilBor]

tincan: yes, a 1 floor structure. if we buy it it's going to be lots of land and there is no need to make it 2 floors.

ok, thanks again all of you for the information and warnings.


Gil Bor, Math Reserach Centre (CIMAT),
Guanajuato, Mexico.

 

[BigH]

A few questions and a BIG CAUTION .

First, do you know where the spigot was at the time of slurrying in relation to your planned site? Sounds to me that you might be in the slimes region (dust powder). Second, did you encounter water in your shallow boring? - in other words what is the level of saturation in the tailings? Third, How far are you from the "face" of the dam? I, in general, agree with a raft foundation - if there is any movement it will be manifested by "total" tilting, etc.

Now: I would WORRY about seismic events !!!! Slurrying in tailings may have led to less than stellar relative densities and the whole of the old pond area may be subject to liquefaction.

Anyway - hope this helps.

Hasta la vista.

 

[GilBor]

BigH: its difficult to answer yr questions w/o a drawing but i'll try. i doubt there is any of the original humidity of the tailings left in the tailings. they stopped dumping tailing in the site around 1980. we didnt see any humidity
in our shallow probe. the dam face runs south-north and the property forms the southern part of the tailing area, to the west of the face dam, so the dam face forms the eastern limit of the property. along this limit runs a small canal (section is about 30cm by 30cm) sloping towards the midpoint of the dam face, which is north-east of the property, in which the slurry was led. about 80-100m from the dam face is the hill side, with solid ground, which is the western limit of the property. so near this edge we might be quite near solid ground (like 5m) but on most of the property it is way deeper.

is the "slimes region" good or bad for me?

i have to admit i dont understand the terminology in yr last comment about seismic events (liquefaction, less-then-stellar-relative-densities). well, you must excuse me, i am not a geotechnical engineer (i am a mathematician and amatuer woodworker...), so maybe i am in the wrong forum, but if you would be willing to explain in greater detail (2-3 sentences) what you are referring to it will greatly appreciated. thanks!



Gil Bor, Math Reserach Centre (CIMAT),
Guanajuato, Mexico.

 

[Focht3]

You are in the right forum - no need to look anywhere else for advice.

The problem is that soil deposited in a slurry mixture has lots of void space - think of a house made of playing cards. A seismic event would be ground shaking from an earthquake; the ground shaking would cause the house of cards to collapse. The presence of water could cause the collapse to happen more rapidly by causing the soil and water to turn into a slurry, or liquefy - the process is called liquefaction. Think of quicksand - that will give you a pretty good idea of what happens, even if quicksand has a different cause. And the 'slimes' region is bad - it is more likely to liquefy in an earthquake.

But even dry, loose soils would be quite likely to collapse in a strong earthquake, particularly if there is no cementing action between the grains of soil.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[GilBor]

BigH: thanks for the explanations (and the assurances regarding the right forum). so earthquack, something else to worry about... maybe i should just stay in my rented house forever?

most welcome to visit here, in case you pass by (spectacular view, as tincan noted above).

Gil Bor, Math Research Centre (CIMAT),
Guanajuato, Mexico.

 

[sgsibob]

Just get some subsurface exploration done. A geotech can evaluate the liquefaction potential. The tailings may indeed have dewatered over time, although you would be surprised at how slow a process that can be as evidenced by other, large mine tailings piles. Even if the surface crust seems dry it is likely that the deeper tailings are wet if not saturated. Think about it -- how would the water exit? it has to drain through all that fine material.

The construction of the dam is also something to consider. Hopefully it was built right and if there is a long wet period the dam will hold. Mines typically design tailings dams with minimal safety factors against base shear and rotational slip failures -- something like 1.3 is not uncommon -- because they are industrial facilities and not intended for public use. In civil construction the safety factors used in design would be much higher.

As for the slimes, the issue there is that the finer the size fraction, the more unstable if wet. This is because finer materials can hold a lot of water that does not readily move through the formation under a pressure gradient -- the pore spaces are too small. Hence the fluid component pressurizes, which creates a uniform force tending to counteract forces resisting failure (in other words, fluid pressure promotes failure). It's known as "pore pressure" and it is responsible for all kinds of geotechnical failures the world over.

Sorry to focus on problems rather than solutions. Just get the exploration done and see if all these concerns are realistic in your case.

 

[boo1]

Is there hazard material issues like mercury amalgamation or lead concentrations to consider?

 

[GilBor]

boo1: yes, of course, this is another concern... we have some evidence this is not a problem, but will have to check it also. this would be next step after the soil mechanics.


Gil Bor, Math Research Centre (CIMAT),
Guanajuato, Mexico.

 

[dicksewerrat]

Can you put the house on the original hill and use tailings deposit for yard? I don't think you want to invest a lot of money in a house that may move downhill in avery wet year or God forbid earthquake. If the mining company won't allow subsurface investigation, I would back away.

 

[GilBor]

dicksewerrat: no, we tried this idea, but the owner of the solid part wan't sell... i am pretty sure the mine company will allow cone penetrometer study, once i explain to them what it means, but i dont think we are ready to pay for such a study at the moment, esp given all the warnings in this forum.

btw, can anyone of you experts can give me an idea of how much such a study cost, say in the US, based on the information in this thread?





Gil Bor, Math Research Centre (CIMAT),
Guanajuato, Mexico.

 

[GilBor]

Well, for whatever it's worth, i got some technical data
from a soil engineer on the properties of the make up of
this site. it is translated (by me) from spanish, so you may need to use yr imagination a bit. also, it is
based solely on taking 4 supeficial samples from the site.

it is fine sand of type SP (according to the soil
classification system), that is, a fine sand with very regular granules size. humidity of order of 9%. (it was raining some recently). density in loose state is 1.24 Ton/m^3. in compacted state it can reach 1.65, and in sturated compact state can reach 2.05. fricction angle is 26.6 degrees. based on this he concludes that 5 Ton/M^2
is the upper limit (calculated with Terzaghi).

there are also experiemnts with the samples of "consolidation", concluding that we get very rapid consolidation (within minutes). this was done by applying pressures in the range 1.2 - 9.2 Ton/M^2. the compresibility index is 0.194.

his recommendation is to use this land for light construction with foundation design similar to the one
mentioned in this thread.

all comments are VERY welcome.



Gil Bor, Math Research Centre (CIMAT),
Guanajuato, Mexico.

 

[jdmm]

GilBor

Very good dialogue. All very good advice. There seems to be a lot of concern about your lot. You want to buy a lot in an area of mine tailings. You know that with these conditions there are potential problems with settlment of the buildings and catastrophic displacement in the result of earthquake. There are also potential problems associated with harmful contaminants (mercury etc). With time an energy (and probably a great deal of money) you can rate these potential problems. Some will be acceptible others not. Frequently, companies wanting to sell such properties will assess these risks for you in order to get the highest price in the event that the risk is not really a risk. I am not as familiar with Mexican law as with local but you might attempt to put some responsibility on the owner of the land with respect to "suitability for intended purpose". You want to build a house, and you buy the lot with the intended purpose of bulding a house and you include this in your purchase to buy. Should things turn out less than you expect that you might have recourse to the seller. this would (i think) in the u.s. and canada.

You might want to use the resources of the Math Research Centre to work out the probabilities of a good outcome.

 

[BigH]

5 tonne/m2 is about 50 kPa (kN/m2). This isn't very much! I would still be careful. Of course, you could always consider driving some wood piles or small dia. concrete piles to support a raft foundation. You've indicated that good ground should be relatively shallow. This is costlier than spread footings/raft, but might let you utilize a great piece of property. As for environmental, some good points have been raised. You should know the process by which the metal was extracted. You might wish to "remediate" - or have the mine do it - the upper 1.5 to 2m under your house. The yard could be graded with a few feet of good clean fill to put any metal concerns at depth.