Final answer:
The minimum size of the filter material for a subsurface drainage structure is a coarse-grained sand with a minimum particle size of 2 mm.
Step-by-step explanation:
The minimum size of the filter material for a subsurface drainage structure depends on various factors, such as the soil properties and the desired drainage capacity. However, generally, a coarse-grained sand with a minimum particle size of 2 mm is recommended as the filter material.
This size ensures that the filter material allows for efficient water flow while preventing the migration of finer particles that could clog the drainage system. Additionally, the filter material should have good permeability and be resistant to degradation to ensure its long-term effectiveness.