I imagine that what you are trying to do is to minimise the risk of corrosion inherent in the traditional anchor system.
Geogrid is unlikely to provide the amount of resistance that you need to provide an effective prop in the top third of the sheet pile wall. Additionally you'll have a hell...
The short answer is that it depends of what equations you are using there isn't just one.
It is probably better to break the issue down into two parts:
1.What is the vertical stress, at various depths, under the strip footing? You can use Boussinesq for this analysis, but if the strip load...