GeoPaveTraffic, gandersen, thanks for the replay.
I concur that any possible shallow soil is more likely to be reinforced (by specific species of grasses). With this type of vegetation, I can understand that the vegetated soil behaves like a composite material because the roots are relatively small and heavily distributed over the shallow depth.
However, using different plant (e.g. willow) to reach deeper locations, apparently the previous mechanism might partially still valid or at least the big root may work as a soil’ nail?