What are the principal requirements of a filter in soil mechanics?

In soil mechanics, a filter must meet several principal requirements: it should allow water to pass through while retaining soil particles, preventing clogging of drainage systems. The filter's particle size must be appropriate to ensure that it effectively retains coarser soil while allowing finer particles to drain. Additionally, the filter material should be chemically stable and durable to withstand environmental conditions and maintain its functionality over time. Proper gradation and thickness are also crucial to ensure effective filtration and prevent soil migration.