Excavation for 1M barrel tank producing soils with gypsum content 7-26% by weight, water soluble sulphate content is 0.72-2.81%. Spec, probably mistakenly, prohibits more than 5% gypsum in subgrade beneath tank. I suspect the intent of the clause was to capture soluble sulphate content of 5% and not gypsum (its been as much as admitted).

GWL (already hyper-saline) is 4-5m below the founding level and the subgrade will be isolated from the tank by means of welded 2mm HDPE liner,500mm of clean blinding sand and the tank base structure (TBC but possibly steel plate or asphalt).

Isolation from water body should reduce risk of secondary settlements by collapse of soil fabric due to washout of soluble soil fabric, in addition the settlement values from a 2m thick layer of this subgrade i'm thinking will be minor and tolerable in what is a very large structure (100m dia.).

Heave from precipitation crystalline growth may be a risk despite the lack of water but likely increased temperatures however again can those expansion forces be significant enough to be of concern?

In summary I'm of the opinion to follow convention (ICE Manual/Foundation Engineering Handbook) and allow soils with less than 2% water solubles into the subbase.

Bit of a wordy rant I suppose but was looking to hear from other professionals who might have worked in these conditions and faced these issues.

TIA