Building on garbage

[minez]

I've got a 3 storey con't tilt up building to design.
The soil's engineeer says that the site has 15ft deep layer of garbage underneath and it is so loose that it wouldn't even resist the weight of 6" thick concrete slab without a significant settlement.(So the slab will be suspended)
We are thinking of having Frankie piles embedded into good soil underneath garbage layer for gravity loads. But for seismic, all I can think of is to have piles fixed at the top(bottom of building) and bottom(embedded enough into good soil) so piles can resist seimic forces with bending.

This building will be located in an high seismic zone and it is pretty heavy, so I am not sure at this moment that if I can make it work this way. Also, my boss is telling me that the garbage layer would be acting as a weight on piles in seismic design.

Is there any other way to resolve the seismic problem?

 

[civilperson]

Battered piles at the exterior rows will have some horizontal component of strength to resist lateral loads. Might be cheaper to remove the garbage and put in a controlled structural fill, (add a basement to lessen the replacement volume).

 

[jike]

If you build over the garbage, how do you intend to vent the methane that will be generated?

I agree with the comment about using batter piles.

 

[dik]

If heavy, relatively rigid and in a high seismic area, I'd seriously look at replacing the garbage with engineered fill and you may have to construct methane vents around the perimeter. If on engineered fill, can you create a diaphragm to transfer lateral loads to walls parallel to the force rather than use battered piles. For large forces, battered piles can be a problem with transferring horizontal forces.

Dik

 

[rb1957]

i'm really sorry, but i can't help but think of the sketch from monty python's "holy grail" about the guy who biult his castle in a swamp ...

the first fell down, so i build another, that fell down, so i built another ... and that's what you're going to inherit, the strongest castle in the land ...

fortunately he didn't sue his castle builders,
unfortunately he probably killed them (to show how tough he was)

 

[minez]

Thanks for the inputs.
As far as I know, the whole site will be pre-loaded and soil will be placed on top to make up the settlement. This will give us some bearing capacity but not enough to put a building on.(so we will still need piles)The mechanical engineer is working on the methane problem.

 

[mitchelon]

“This will give us some bearing capacity but not enough to put a building on” Then why are you going to pre-load it? I agree with removing the solid waste and replacing it with fill. Do a quick cost analysis comparing the cost of removing the waste vs providing moment-resistant piles/piers, the suspended slab, the methane vents, state permits, and everything else that comes with leaving the waste. Designing a system to capture/mitigate the methane is not that difficult, but I highly recommend you consult a civil engineer with an environmental (landfill construction and capping) background.

 

[minez]

It has to be pre-loaded to hold up concrete suspended slab while it's being cured. Otherwise we will have to use special forming. Also, there are parking lots and drive ways outside building that have to have some bearing capacities. Some trailers and trucks will be driving around the building, so the cost of replacing garbage with fill would be huge but I will look into that option as well.

 

[JLNJ]

At some point with a small diameter pile you might need to consider buckling (if you are getting no support from the "soil").

 

[twinnell]

We have designed many 4 story condos on bad soil; most of these are around the North Carolina coast. The geotechnical engineer should provide you with the recommended foundation. If he says the foundations should be piles than use piles. He should have provided (in his report) the pile capacities (downward, uplift, and lateral). Calculate the number of piles needed for gravity loads...see if that number works for the lateral loads...if not...add more piles. If you have to add too many, then go with battered.

It is the geotechs scope to determine which foundation is suitable and the capacites for that foundation (bearing capacity or load capacities for piles). He should also recommend the type of pile.