MB4 242b: Ditches or Piles of Straw Between Two Chatzeiros (372:17-19)

Vis-a-vis hilchot eruvin, ditches have the same status and requirements as a wall.

Merging two courtyards that have a ditch between them via a plank or by filling it with straw. When would they have to make one eruv?

The dimensions of the plank.

If the ditch is filled with straw can the straw be used on Shabbos?