Foundations on mine spoil material 5

[dwumfour]

I am designing a silo and a storage tank on a mine spoil. The total thickness of the mine spoil is about 55m and about 10 years old.the unfactored load of the silo is about 70kPa and that of the storange tank is about 80kPa.
Can anybody help me with the design of the foundation especially the settlement.

 

[fattdad]

Do a DMT survey, look at the modulus values with depth and figure out the change in vertical effective stress with depth. Characterize the soil by "modulus" layers and superimpose the change in stress on each modulus layer. Integrate the stress area for each layer (i.e., tsf*depth) and divide that by the modulus value. That'll then give you the elastic soil compression for each layer. Add up the result for the whole system.

You really need to properly characterize the soil modulus.

p.s., where's the ground water table? My reply doesn't account for primary consolidation, which needs to be looked at also, but may ultimatly not be a factor.

f-d

¡papá gordo ain’t no madre flaca!

 

[oldestguy]

A simple way to be relatively sure of a good final outcome is to overload the site with a surcharge, providing at least the same pressure per unit area that the foundation would apply and measure the settlement with settlement platforms. If this is a waste disposal area,there should be plenty of waste that can be used for the surcharge and then returned to its original location. The readings of the platforms over a time period will tell if you have to leave surcharge there longer or need a higher surcharge.

Some corners can be cut on the required surcharge height, since the "bulb of pressure' from the tank foundation is likely smaller than that of the surcharge.

 

[aeoliantexan]

I would want to know a lot more about the nature and placement of the mine spoil material before supporting any structure on it. Many types of fill, even though well compacted, can compress significantly (say 1 or 2 percent) as they absorb moisture with time. One percent of 55M is a couple of feet of settlement. Settlement can be much greater if the fill is not well compacted.

Ten years may not be enough for the fill to have reached a stable moisture content. Preloading with a surcharge can be effective if the soil is saturated or will never see an increase in moisture content, but it can forget its stress history if it absorbs more water.

Your project sound like a challenging one.

 

[davep043]

We've done dynamic compaction on mine spoil sites also. The dynamic compaciton is typically limited to the upper 30 feet or so, but within that zone is more effective than preloading in compressing the material. Depending on the size of the footprint of the new facilities, you might consider preloading to address deep-seated settlement, plus dynamic compaction to create a semi-rigid surface layer to "bridge" over deeper differential movements.

No matter what, it will be important to properly characterize the geotechnical properties of the mine spoil.

 

[iandig]

It may also be worth sourcing the following book, BR 424: Building on Fill: 2nd Edition. This was written by Charles and it contains masses of useful information on deep fill exactly as described in your post. The book is published by the BRE [Building Research Establishemnt] in th UK and widely used as a refernce and guidance document. I don't have it to hand at the moment so can't provide the ISBN number but will look it up for you.