What are the two primary, sometimes conflicting, objectives that must be balanced when designing a granular filter to protect an underlying soil layer?

When designing a granular filter to protect an underlying soil layer, two primary and often conflicting objectives must be carefully balanced: retention and permeability. The first objective, retention, means the filter must effectively prevent the movement or erosion of particles from the protected soil layer. If soil particles are allowed to migrate into or through the filter, the underlying soil can lose mass, leading to phenomena such as piping (the progressive erosion of soil by seeping water, forming channels), settlement, or a reduction in the soil's strength and stability. To achieve adequate retention, the filter material must have sufficiently small pore spaces, which are the voids between ....