Overturning due to settlement

[rduke]

I'm looking for discussion on overturning for a mat footing. The site in question is south of Houston and has deep (30' investigated) saturated (H2O @ 3') clays with low strength. Torvane range 0.5 to 1.5 tsf., penetrometer range 0.75 to 2.5, UU's 0.6 to 1.8 tsf. The foundation is for a 50' to 75' tall (unknown at present), narrow vertical vessel. The footing will be lightly loaded, but I am concerned about overturning due to eccentricity. Any eccentricity could lead to differential settlements which would increase the eccentricity (and on and on) and I end up with the "leaning vessel of Houston". However, I don't want to specify deep foundations if it is not warranted. Should mat foundations be considered for this case?

 

[BigH]

The clays are not that soft. What do you have as far as consolidation data? This reminds me of Sarnia clays - but I'll need more data to confirm. To my knowledge we never had problems with our projects there (many oil refineries). Can you give some more data - consolidation, Atterberg's, expansive details, etc. of the clays? Thanks.

 

[rduke]

Just a little more info. clay: top 3'- LL=52, PL=18, PI=34. Below 3' - LL=34, PL=17, PI=17. Consolidation testing was beyond the scope, however, the secant modulus for 1/2 peak stress from the UU tests averages about 100 ksf. Also, the vessle may only be 40' tall, 8' diameter, 140 kips (fully loaded).