Contacted the manufacturer or supplier? I'm sure they'd be able to direct you.
I'm assuming if you work out the section properties then you can simply apply normal code provisions. The only thing that might require some research is how things behave with the joints between the sheets, but I'd assume someone must have some method developed to account for any effects related to this aspect. Never done any sheet piling myself.
Is it a temporary or permanent load situation? If permanent perhaps individual sheets can be welded so they act together to improve the sections buckling capacity.