Tank Loading on Fill and Preconsolidated Peat 1

[claytill]

Scenario: A 4.3 m diameter and 3 m high tank is to be founded on the following soil profile: 4.5 m of sand fill (N=16), 1.5 m of silty clay fill (remolded till N=10), 1.1 m of peat (N=8), 4 m silty clay (N=5), and silty clay till below. The fill has been in place for ~15 to 20 years with an unknown history. The boreholes indicated "clean fill" with essentially no deleterious materials.

Questions: 1)For a shallow foundation (i.e., gravel pad or concrete slab), how much additional settlement could be expected from the peat layer? 2)What would be an appropriate Es value to use in determining elastic settlement?

 

[BigH]

Tank is for what material? Oil, water, ??? N values seem high in the peat - but then again it is overlain by some 100 kPa pressure - assuming that the groundwater level is at the top of the peat - another question - where is the gwl?

Not a high tank - even full of water only some 60 kPa bearing pressure including slab and tank dead load - not much. I think that I would use a square of strip footings or a ring footing properly spaced so as not to overlap or create large moments on a supported slab then put the tank on it. The footings in N=16 sand fill would likely be some 600 to 800 mm in width. Depth of influence of the footings would be, say, 3x800 or 2.4 m (+ 400 or so for frost although it would be argued that clean sand fill is non-frost susceptible) - which means that the pressure bulb of interest is inside your fill - so tank pressure wouldn't be seen by the peat. I'd not worry too much about immediate settlements based on this scenario - you should likely have less than 25mm or so. If you think that the sand is not uniformly compacted, then dig out a metre or so and pack it. May have continued secondary compression of the peat - but after 15 some odd years, you would be in a very long log cycle.

 

[claytill]

Thanks BigH!

The tank is for a cautic solution to treat acid water. Also, the GWT is ~3.5 mbgs. Any other comments would be appreciated.

 

[BigH]

mbgs? - oh, m below ground surface. As I said, check out your frost depth. But besides what I alluded to earlier which is fine, you might want to put on a pad of good well graded sand and gravel, say 1 m thick on top or as a replacement of the sand fill. Again, the size is quite small. Different scenario if the tank had diameter of 10 m (more or little less).