Extending the radiating pattern of the voussoirs

We would like to have the design of the bridge that we're building at next months bridge workshop, have abutments that continue the radiating pattern of the voussoirs over the bridge opening. It will be a fairly flat bridge.

Enough mass will be established below grade to support the longish abutment stones and also there will be large stones stacked at the ends of the bridge below grade to eliminate the tendency of the arch to spread. 

The problem area will be where we are laying the abutment stones nearest the point where the arch opening begins. These stones will want to slide until they are pinned by the entire weight of the voussoirs over the bridge. The opening below the form could get narrower but it could hinder taking out the form. I'm wondering if anyone has any suggestions.