interpreting spt in diabase, curacao 1

[Bartje]

Our company is structural engineer for a project on curacao, a geo-consultancy firm made spt-tests but no longer exist.

spt's show diabase-sand that's a little silty and the worst one has following spt-log, 18, 52, 74 and at last 122.
These are respectively the 1st, to 4th foot that are tested; and we want to design a footing above the layer with 52 blows per foot.

Sub a: "we use pression-meters on non-rocky clayy dutch soil; so we can't intepret these things."

Sub b: "We can calculate the Mpa-value of the spt's but we don't know how these can be transformed to allowable footing-pressure"

Sub c: "We have column footings with value of 1500-2500 kN for each column"

Question: does anybody know an estimation of allowable stress and how to get there, we designed it to be 0.5 Mpa; but i think it can be much more - a document with formulae should also be terrific

If there's someone who can deliver an footing-advise for approx. 800 US-dollars based on 4 SPT's than that would be an outcome, too

I've got scans of spt's and drawing, so that's no problem


Thank you very much