I don’t understand what you’re trying to achieve. Character won’t fall through ground in walking mode (collides with it) or navmesh walking (stands on navmesh poly).
You can tell if it’s enabled when you have AI agent moving in places where navmesh poly cuts into the ground. This happens usually at upper ends of ramps, especially for more coarse generation settings (bigger voxel grid: CellSize, CellHeight). Without additional traces AI will just stick to navmesh poly and go through ground’s geometry, with them AI will move on ground level, slightly above its navmesh poly.
Interval is defined in seconds, so I’d suggest keeping it in 0.1 - 0.5 range for most cases. Default values for trace offsets are quite big, so AI should be able to work with navmesh - ground height differences up to capsule height downward and 2/3 height upwards.