Foundation for waste tank

[Delijosi]

I am designing foundation for a waste steel tanks. The ground condition is 2m soft made ground , over 14m clay (Cu = 40 + 15z) over incompressible limestone. I am thinking of using pile foundation but also considering pad foundation.

Can I design the pile foundation as normal for other civil structures or is there still a risk of edge failure? Is pad foundation feasible in this condition? The base of the largest tank is 1.25 mbgl.

 

[BigH]

what is 1.25 mbgl in m? When you say base of tank I want to know the diameter and also the height and what what type of liquid will you store.
I would go for a tank pad foundation of granular - say 3 m or so larger than the diameter of your tank. If the soft made ground is fill and poorly placed, I would dig it out and replace with a compacted river sand - then the foundation pad above ground for 2 to 2.5 m or so.
There is always a risk of edge failure - especially if the clay has a bit of a desiccated crust (while you don't show it with your undrained shear strength profile, it might) But you design to mitigate such risk - in this case, check out Bjerrum's paper in the 1956 London ISSMFE conference. He talks specifically about a tank edge failure.
40 kPa is about 800 psf - I remember several storage tanks we did in years gone by with approximately this Su value. Youi might want to put in wick drains to speed up the settlement (and increase the Su values) - you can take advantage of initial water test loading - load in stages. Would work fine. . .
Don't forget to have flexible connections - you will get long term consolidation settlements (and secondary)

 

[Delijosi]

Thanks BigH. 1.25 mbgl means 1.25 metre below groung level. The base of some the tanks will be below ground level. Say 1.25 m to 1.4m. The tank diameter is 20m, height is 16m with liquid vertical level of 14m. Liquid type is waste water.

If I have to replace the fill with compacted sand, we are talking of excavation to about 4m depth with gwl at gl. Although that means founding on Su = 70kPa. The cost of excavation and excavation in dry ground may suggest using piles. Using pile means small and regular settlement which should prevent any differential settlement and damage to tank..

 

[BigH]

Don't think you have to worry about damaging steel tank. If you do decide to use piles (I doubt that I would) are you thinking equally spaced piles and pile cap? If so that would be a pretty thick pile cap. In this case, I would consider driving the piles, put a good 5 ft or more of sand and gravel across the top and found the steel tanks on the gravel pad - this is highlighted in Zeevaert's book on Foundation Design for Difficult Soils and you can see another in Deep Foundation Magazine - the big Greek Bridge that Ralph Peck worked on just a few years before he died (I think it is the Summer 2007 issue). You really don't want to go there - i.e. thick concrete base. Wick drains might very well be the best choice without piles.

 

[Delijosi]

I have done two designs - pile foundation and shallow foundation. The pile foundation founded on Limestone have high capacity but large distance between the piles. For the shallow foundation, I excavated to 3m depth and replaced the soil with gravel. The FoS >> 3 and total settlement (in cLAY) in the order of 100mm. The settlement profile is also expected to be profile B as in D'Orazio and Doncan.

The question is do I still need to check for edge failure? Does anyone have a copy of the work by D'Orazio and Doncan? Please pass it to me.