Foundation on loose sand 3

[vohai]

Hi all!
We are designing a three-story building in a site
with 17m depth of fine grained loose sand. Ground water table
fluctuate by monsoon at 7<->3m below surface. We are considering
many methods of deep compaction and method of compaction/replacement
surface sand with medium grained sand. Please share your opinion
about which method is best for this case and how to control work in
construction site. Thanks.

 

[Focht3]

I think that [blue]vohai[/blue] has missed a key part of the analysis of the settlement of a large building. S/he has focused exclusively on individual footings singly, and has not looked at the structure in total. It's a fairly common mistake, particularly for those unaccustomed to these types of evaluations, or who have not had the benefit of working with very experienced engineers.

[blue]vohai[/blue]:
You need to do a settlement analysis of the combined effects of the building footings. If you don't have access to a computer program to perform these analyses, use the stress distribution method proposed by Boussinesq. Do not use Westergaard, since loose sands will not provide the lateral restraint assumed in the development of Westergaard's charts.

A very quick way to do the analysis is to take the total building load and apply it to the footprint of the building, then look at the Boussinesq stress distribution for a depth equal to twice the building's width. It isn't a precise answer, but it will put you in the right ballpark.

To quote the 1992 Terzaghi lecturer, &quot;It is better to be approximate and correct than precise and wrong.&quot;



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