consider an oil storage tank 30 me

huangyhg

consider an oil storage tank 30 met
consider an oil storage tank 30 metres diam. supported on a consolidated stone pad bearing on vibrofloatation stone piles through alluvial sand and silt and gravel.
the vibro piles will be 2.5 metres apart.
the stone pad will be retained by a reinforced concrete ringwall.  the ringwall and stone pad will rise 1 metre above ground level (to lift the tank to the required height for process reasons) and may extend below gl if required forstructural reasons.
question: what is the overall depth of stone pad required to prevent the vibro piles punching through the stone pad?
using an angle of internal friction for the consolidated crushed stone = 30 degrees and spreading the load to the piles i have:
d = (2.5 /2)*cotan30 = 2.165 metres
this seems to be somewhat simplistic and yields a very deep pad.  
can i obtain some advantage from the restraining effect of the ringwall on the stone pad and increase the angle of internal friction?  is there something i have missed?
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for this particular application where both the piles and pad are both engineered controlled you may use a safe yet higher angle of inner friction, say 40 to 45 deg. in this model it is surmised the stone piles keep the integrity. hence if the piles do no punch the pad the adaptation will come lower down unto the ground (squashing of the stone columns, for which the number and size need be strong enough).
use of modest angles of inner friction is only required where the variability proper of the soils is expected, and not where the soils are proven extremely homogeneous or apport engineered materials.
thanks ishvaaag; it is always good to hear another engineers opinion.