settlement of decommisioned tailingsdam

[ROCKSPIDER]

Hi All,

I am involved with an interesting project.....
A client wants to build light structures for energy generation on large 40+ hectare area 60m high decommisioned gold tailingsdams (slimesdams).

The dams have only recently been decommisions and although the surface is dry the dam below is still very wet.

How would one go about calculating the settlement over time of the structure?

I first want to determine if the project is feasable before I allow detailed investigation.

Anny and all feedback will be appreciated.

 

[fattdad]

You need to install some piezometers to see if there are excess pore pressures. I'd think vibrating wire piezometers at 5m depth intervals would be telling.

I'd also jam CPT and DPT profiles at a few locations.

Then I'd run some numbers.

f-d

¡papá gordo ain’t no madre flaca!

 

[ROCKSPIDER]

I am going to do piezo cone probing later but for now i want to determine whether the differential settlement is a fatal flaww as the structures can only handle 5-8mm differential settlement

 

[fattdad]

You can always pile support the buildings, but if the ground is still going to settle a few feet, you'll need to reconstruct the stoops as time passes. . .

I'd consider using the DPT and I'd also install a few real piezometers.

f-d

¡papá gordo ain’t no madre flaca!

 

[MiketheEngineer]

Move the building

 

[ROCKSPIDER]

The structures are solar panel frames. 60+m deep pilling for a few hundred foundations is not an option.

 

[miningman]

You do realise that tailings dams are typically constructed so as to have permanent seepage around the toe area so as to prevent any rise in the phreatic surface which can lead to piping and catastrophic failure I hope??

The dam itself should ALWAYS be wet and any settlement must not change this condition. I would be very hesitant to disturb the core of this dam with anything resembling a single pile.

 

[cvg]

are you building on the tailings impoundment or on the actual embankment? The embankment is probably much more stable than the slimes impoundment.

tailings placement is uncontrolled and not compacted. method of placement (cyclone, spigot, point discharge) can vary, pulp density, water content, specific gravity and gradation of the tails and slimes can vary from time to time. The result is unpredictable variability in the density, gradation, voids and water content of the in-place tailings. I would say there is a high possibility of differential settlement and 8 mm is not much room for error. the end result is that O&M will be required frequently to monitor settlement and adjust / level the structures in order to account for the settlement. This could be a deal killer.

 

[fattdad]

except the fact that there will be lots of total settlement and design to limit differential settlement. If you are installing solar panels, it'd seem they could tolerate settlement! You could reconstruct the upper few feet to include some aggregate and geogrid to make a soil raft. Then use spread footings.

I'd still have a cone or dilatometer for soundings of soil strength and such.

f-d

¡papá gordo ain’t no madre flaca!

 

[ROCKSPIDER]

Miningman, CVG

The dam has been decomossiioned and are going into rehabilitation so the only fluids that wil be added to it is rainfall. over time it will settle and compact and the settlement will decrease. This is why I want to determine the settlemant for over say the next 5 years to determine when it will be safe to construct the solar farm on the surface. The client wants to start ASAP but I feel it will have to wait a few years. I just need to work out roughly how long.

This is a LARGE tailings dam ~440 000 sq m. and the generating potential planned for is 40 MWp from PV, so reconstructing a soil raft is not an option.

 

[fattdad]

Piles are not an option, settlement is not an option, soil raft is not an option.

Maybe this is not the site for the project?

f-d

¡papá gordo ain’t no madre flaca!

 

[cvg]

depending on the type of tailings dam construction (upstream, downstream or centerline) as well as the geometry (in valley, side hill, ring or pit) there may be significant areas of the impoundment that have variable depths of tailings in addition to variable gradations (sand vs slimes). These areas are guaranteed to have longterm differential settlement. This could seriously reduce your buildable area so that you can only construct in the middle of the impoundment, not near the edges. You will need to generate well over 400 kw per acre to fit on your site, which seems a bit high to me

 

[aeoliantexan]

You have had a lot of good advice so far. You need to gather data on the profile and the material properties to make any meaningful calculations. You may also want to start a settlement monitoring program, say conventional level circuits to grids of stakes placed at several locations across the site. Observation often beats lab testing and analyses, and certainly can provide a good check.

It will probably be desirable to grade the site to drain the rainfall off rather than maintaining ponds. This will accelerate settlement and facilitate development.

You could consider using a rolling surcharge to hasten settlement, eliminating the large early settlements from consideration.

Be sure the dam and pond are stable before placing a big investment on it.

 

[moe333]

You should also check seismic loading for both settlement and stability.

 

[dgillette]

Hello Rockspider. Hate to say this, but if the structures can only tolerate 5-8 mm of diff settlement, you may need a different site. (This is the appurtenant structures for inverters and switching and such, or the PV panels themselves?) Or, maybe you need different STRUCTURES, perhaps ones that can be adjusted with jacks periodically to accommodate the settlement. That might be cheaper than a new site, assuming this one doesn't have the seismic issue moe333 mentions. [Hard rock tailings are about the worst material you can find for seismic loading, and if this one is in an active seismic area, there may not be an economically viable solution, unless it is risk tolerance.]

Regards,
DRG